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Träfflista för sökning "WFRF:(Varenius Eskil 1986) "

Sökning: WFRF:(Varenius Eskil 1986)

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1.
  • Albentosa, Ezequiel, et al. (författare)
  • Current Status of the EU-VGOS Project
  • 2023
  • Ingår i: International VLBI Service for Geodesy and Astrometry 2022 General Meeting Proceedings. ; NASA/ CP–20220018789, s. 85-89
  • Konferensbidrag (refereegranskat)abstract
    • The EU-VGOS project began in 2018 with the aim of using the VGOS infrastructure in Europe to investigate methods for VGOS data processing. The project is now structured into Working Groups dealing with operations (stations), e-transfer, correlation and post-processing, and analysis. This is a report on the status of the project.
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2.
  • Alef, Walter, et al. (författare)
  • Geodetic data analysis of VGOS experiments
  • 2021
  • Ingår i: 2021 34th General Assembly and Scientific Symposium of the International Union of Radio Science, URSI GASS 2021.
  • Konferensbidrag (refereegranskat)abstract
    • Very Long Baseline Interferometry (VLBI) serves as one of the common geodetic methods to define the global reference frames and monitor Earth's orientation variations. The technical upgrade of the VLBI method known as the VLBI Global Observing System (VGOS) includes a critical re-design of the observed frequencies from the dual band mode (S and X band, i.e. 2 GHz and 8 GHz) to observations in a broadband (2-14 GHz). Since 2019 the first VGOS experiments are available for the geodetic analysis in free access at the International VLBI service for Geodesy and Astrometry (IVS). Also regional-only subnetworks such as European VLBI stations have succeeded already in VGOS mode. Based on these brand-new observations we review the current geodetic data analysis workflow to build a bridge between geodetic observed delays derived from different bands.
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3.
  • Xu, Minghui, et al. (författare)
  • Baseline Vector Repeatability at the Sub-Millimeter Level Enabled by Radio Interferometer Phase Delays of Intra-Site Baselines
  • 2023
  • Ingår i: Journal of Geophysical Research: Solid Earth. - 2169-9356 .- 2169-9313. ; 128:3
  • Tidskriftsartikel (refereegranskat)abstract
    • We report the results of position ties for short baselines at eight geodetic sites based on phase delays that are extracted from global geodetic very-long-baseline interferometry (VLBI) observations rather than dedicated short-baseline experiments. An analysis of phase delay observables at X band from two antennas at the Geodetic Observatory Wettzell, Germany, extracted from 107 global 24-hr VLBI sessions since 2019 yields weighted root-mean-square scatters about the mean baseline vector of 0.3, 0.3, and 0.8 mm in the east, north, and up directions, respectively. Position ties are also obtained for other short baselines between legacy antennas and nearby, newly built antennas. They are critical for maintaining a consistent continuation of the realization of the terrestrial reference frame, especially when including the new VGOS network. The phase delays of the baseline WETTZ13N–WETTZELL enable an investigation of sources of error at the sub-millimeter level. We found that a systematic variation of larger than 1 mm can be introduced to the Up estimates of this baseline vector when atmospheric delays were estimated. Although the sub-millimeter repeatability has been achieved for the baseline vector WETTZ13N–WETTZELL, we conclude that long term monitoring should be conducted for more short baselines to assess the instrumental effects, in particular the systematic differences between phase delays and group delays, and to find common solutions for reducing them. This will be an important step toward the goal of global geodesy at the 1 mm level.
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4.
  • Aalto, Susanne, 1964, et al. (författare)
  • The hidden heart of the luminous infrared galaxy IC 860: I. A molecular inflow feeding opaque, extreme nuclear activity
  • 2019
  • Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 627
  • Tidskriftsartikel (refereegranskat)abstract
    • High-resolution (0.'03-0.'09 (9-26 pc)) ALMA (100-350 GHz (λ3 to 0.8 mm)) and (0.'04 (11 pc)) VLA 45 GHz measurements have been used to image continuum and spectral line emission from the inner (100 pc) region of the nearby infrared luminous galaxy IC 860. We detect compact (r ∼ 10 pc), luminous, 3 to 0.8 mm continuum emission in the core of IC 860, with brightness temperatures TB > 160 K. The 45 GHz continuum is equally compact but significantly fainter in flux. We suggest that the 3 to 0.8 mm continuum emerges from hot dust with radius r ∼ 8 pc and temperature Td ∼ 280 K, and that it is opaque at millimetre wavelengths, implying a very large H2 column density N(H2)≥ 1026 cm-2. Vibrationally excited lines of HCN v2 = 1f J = 4 - 3 and 3-2 (HCN-VIB) are seen in emission and spatially resolved on scales of 40-50 pc. The line-to-continuum ratio drops towards the inner r = 4 pc, resulting in a ring-like morphology. This may be due to high opacities and matching HCN-VIB excitation- and continuum temperatures. The HCN-VIB emission reveals a north-south nuclear velocity gradient with projected rotation velocities of v = 100 km s-1 at r = 10 pc. The brightest emission is oriented perpendicular to the velocity gradient, with a peak HCN-VIB 3-2 TB of 115 K (above the continuum). Vibrational ground-state lines of HCN 3-2 and 4-3, HC15N 4-3, HCO+ 3-2 and 4-3, and CS 7-6 show complex line absorption and emission features towards the dusty nucleus. Redshifted, reversed P-Cygni profiles are seen for HCN and HCO+ consistent with gas inflow with vin ≤ 50 km s-1. Foreground absorption structures outline the flow, and can be traced from the north-east into the nucleus. In contrast, CS 7-6 has blueshifted line profiles with line wings extending out to -180 km s-1. We suggest that a dense and slow outflow is hidden behind a foreground layer of obscuring, inflowing gas. The centre of IC 860 is in a phase of rapid evolution where an inflow is building up a massive nuclear column density of gas and dust that feeds star formation and/or AGN activity. The slow, dense outflow may be signaling the onset of feedback. The inner, r = 10 pc, IR luminosity may be powered by an AGN or a compact starburst, which then would likely require a top-heavy initial mass function.
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5.
  • Amanullah, Rahman, et al. (författare)
  • Diversity in extinction laws of Type Ia supernovae measured between 0.2 and 2 μm
  • 2015
  • Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 453:3, s. 3300-3328
  • Tidskriftsartikel (refereegranskat)abstract
    • We present ultraviolet (UV) observations of six nearby Type Ia supernovae (SNe Ia) obtained with the Hubble Space Telescope, three of which were also observed in the near-IR (NIR) with Wide-Field Camera 3. UV observations with the Swift satellite, as well as ground-based optical and NIR data provide complementary information. The combined data set covers the wavelength range 0.2-2 mu m. By also including archival data of SN 2014J, we analyse a sample spanning observed colour excesses up to E(B - V) = 1.4 mag. We study the wavelength-dependent extinction of each individual SN and find a diversity of reddening laws when characterized by the total-to-selective extinction R-V. In particular, we note that for the two SNe with E(B - V) greater than or similar to 1 mag, for which the colour excess is dominated by dust extinction, we find R-V = 1.4 +/- 0.1 and R-V = 2.8 +/- 0.1. Adding UV photometry reduces the uncertainty of fitted R-V by similar to 50 per cent allowing us to also measure R-V of individual low-extinction objects which point to a similar diversity, currently not accounted for in the analyses when SNe Ia are used for studying the expansion history of the Universe.
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6.
  • Asadi, Saghar, et al. (författare)
  • The case against gravitational millilensing in the multiply-imaged quasar B1152+199
  • 2020
  • Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 492:1, s. 742-748
  • Tidskriftsartikel (refereegranskat)abstract
    • Previous very long baseline interferometry (VLBI) observations of the quasar B1152+199 at 5 GHz has revealed two images of a strongly lensed jet with seemingly discordant morphologies. Whereas the jet appears straight in one of the images, the other exhibits slight curvature on milliarcsecond scales. This is unexpected from the lensing solution and has been interpreted as possible evidence for secondary, small-scale lensing (millilensing) by a compact object with a mass of 10(5)-10(7) M-circle dot located close to the curved image. The probability for such a superposition is extremely low unless the millilens population has very high surface number density. Here, we revisit the case for millilensing in B1152+199 by combining new global-VLBI data at 8.4 GHz with two data sets from the European VLBI Network (EVN) at 5 GHz (archival), and the previously published 5 GHz Very Long Baseline Array (VLBA) data. We find that the new data with a more circular synthesized beam, exhibits no apparent milliarcsecond-scale curvature in image B. Various observations of the object spanning similar to 15 yr apart enable us to improve the constraints on lens system to the point that the only plausible explanation left for the apparent curvature is the artefact due to the shape of the synthesized beam.
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7.
  • Baldi, R. D., et al. (författare)
  • LeMMINGs - II. The e-MERLIN legacy survey of nearby galaxies. The deepest radio view of the Palomar sample on parsec scale
  • 2021
  • Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 500:4, s. 4749-4767
  • Tidskriftsartikel (refereegranskat)abstract
    • We present the second data release of high-resolution (≤0.2 arcsec) 1.5-GHz radio images of 177 nearby galaxies from the Palomar sample, observed with the e-MERLIN array, as part of the Legacy e-MERLIN Multi-band Imaging of Nearby Galaxies Sample (LeMMINGs) survey. Together with the 103 targets of the first LeMMINGs data release, this represents a complete sample of 280 local active (LINER and Seyfert) and inactive galaxies (H ii galaxies and absorption line galaxies, ALG). This large program is the deepest radio survey of the local Universe, ≳1017.6 W Hz-1, regardless of the host and nuclear type: we detect radio emission ≳0.25 mJy beam-1 for 125/280 galaxies (44.6 per cent) with sizes of typically ≲100 pc. Of those 125, 106 targets show a core which coincides within 1.2 arcsec with the optical nucleus. Although we observed mostly cores, around one third of the detected galaxies features jetted morphologies. The detected radio core luminosities of the sample range between ∼1034 and 1040 erg s-1. LINERs and Seyferts are the most luminous sources, whereas H ii galaxies are the least. LINERs show FR I-like core-brightened radio structures while Seyferts reveal the highest fraction of symmetric morphologies. The majority of H ii galaxies have single radio core or complex extended structures, which probably conceal a nuclear starburst and/or a weak active nucleus (seven of them show clear jets). ALGs, which are typically found in evolved ellipticals, although the least numerous, exhibit on average the most luminous radio structures, similar to LINERs.
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8.
  • Cumming, Robert, 1967, et al. (författare)
  • Radioteleskop för alla
  • 2021
  • Ingår i: Fysikaktuellt. - 0283-9148. ; 2021:2, s. 5-7
  • Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
    • Vid Onsala rymdobservatorium finns två små radioteleskop som är tillgängliga för alla. SALSA-antennerna används flitigt av gymnasiestudenter från Sverige och hela världen. Ett nytt projekt på Chalmers ska göra det möjligt för fler och yngre studenter att själv styra ett eget teleskop, och få närkontakt med vår galax och annat i rymden.
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9.
  • Dall` Olio, Daria, 1981, et al. (författare)
  • Methanol masers reveal the magnetic field of the high-mass protostar IRAS 18089-1732
  • 2017
  • Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 607
  • Tidskriftsartikel (refereegranskat)abstract
    • © ESO, 2017. Context. The importance of the magnetic field in high-mass-star formation is not yet fully clear and there are still many open questions concerning its role in the accretion processes and generation of jets and outflows. In the past few years, masers have been successfully used to probe the magnetic field morphology and strength at scales of a few au around massive protostars, by measuring linear polarisation angles and Zeeman splitting. The massive protostar IRAS 18089-1732 is a well studied high-mass-star forming region, showing a hot core chemistry and a disc-outflow system. Previous SMA observations of polarised dust revealed an ordered magnetic field oriented around the disc of IRAS 18089-1732. Aims. We want to determine the magnetic field in the dense region probed by 6.7 GHz methanol maser observations and compare it with observations in dust continuum polarisation, to investigate how the magnetic field in the compact maser region relates to the large-scale field around massive protostars. Methods. We reduced MERLIN observations at 6.7 GHz of IRAS 18089-1732 and we analysed the polarised emission by methanol masers. Results. Our MERLIN observations show that the magnetic field in the 6.7 GHz methanol maser region is consistent with the magnetic field constrained by the SMA dust polarisation observations. A tentative detection of circularly polarised line emission is also presented. Conclusions. We found that the magnetic field in the maser region has the same orientation as in the disk. Thus the large-scale field component, even at the au scale of the masers, dominates over any small-scale field fluctuations. We obtained, from the circular polarisation tentative detection, a field strength along the line of sight of 5.5 mG which appeared to be consistent with the previous estimates.
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10.
  • Diamantidis, Periklis, 1988, et al. (författare)
  • Combining VGOS, legacy S/X and GNSS for the determination of UT1
  • 2022
  • Ingår i: Journal of Geodesy. - : Springer Science and Business Media LLC. - 0949-7714 .- 1432-1394. ; 96:8
  • Tidskriftsartikel (refereegranskat)abstract
    • We perform a combination on the observation level (COL) between VLBI and co-located GNSS in the context of VLBI intensive sessions. Our approach revolves around an estimation procedure which uses 3 h of GNSS data that uniformly encapsulate the 1-h VLBI data, in order to provide consistent troposphere information. We test this approach on both VGOS and Legacy S/X using the VGOS-B and concurrently observed INT1 sessions. The COL strategy is found to increase the precision by 15 % over both session types and leads to an increase of 65 % in the agreement between the sessions when estimating tropospheric gradients every 3 h. A more frequent estimation of the gradients every 1 h, which can be rigorously pursued with the utilization of multi-GNSS, results in a further convergence of the two session types by 30 %. The COL-aided length-of-day (LOD) products also show a 55 % better agreement to external GNSS-derived LOD. In the light of the increasing precision of broadcast GNSS orbits and clocks, this COL strategy can be used to derive rapid UT1-UTC products.
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11.
  • Elgered, Gunnar, 1955, et al. (författare)
  • Radiometry performance of the VGOS receivers of the Onsala twin telescopes
  • 2023
  • Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
    • With the introduction of the VLBI Global Observing System (VGOS) the parallel use of the VGOS receiver as radiometer in order to estimate the wet propagation delay was recognised as a future possibility. That is when observations can be carried out at higher frequencies, closer to the water vapour emission line at 22.2 GHz. An advantage of having the radiometer in the VLBI telescope, compared to the use of a stand-alone Water Vapour Radiometer (WVR), is that the radiometer will observe the same atmospheric volume that is causing the signal propagation delay. We have assessed this method using simulations and arrived at the following two important conclusions: (1) the receiver’s measurements of the sky brightness temperature is likely to be the main error source, rather than the algorithm error introduced when calculating the wet delay from the observed sky brightness temperatures; (2) the method requires an extension of the frequency range of the receiver well beyond 14 GHz in order to increase the sensitivity for water vapour. The radiometric measurements shall be made within a couple of GHz from the emission line at 22.2 GHz. In spite of the fact that the present VGOS receivers observe at too low frequencies we find it meaningful to assess the radiometric stability of these receivers at the higher end of the frequency band. We have used one of the Onsala Twin Telescopes for this purpose, which is able to observe both polarizations in the frequency band 15.36–15.58 GHz. The system temperature has been observed at different elevation angles in order to separate the atmospheric sky brightness temperature and the receiver noise temperature. The observations are carried out during different atmospheric conditions and the estimated sky temperatures are compared to the observations done with one of our stand-alone WVRs. By using one-frequency algorithms we may also, during cloud-free conditions, compare the wet propagation delays using 20.7 GHz observations from the stand-alone WVR and 15 GHz observations from the VGOS receiver.
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12.
  • Elgered, Gunnar, 1955, et al. (författare)
  • Radiometry performance of the VGOS receivers of the Onsala twin telescopes
  • 2023
  • Ingår i: Proceedings of the 26th European VLBI Group for Geodesy and Astrometry Working Meeting. ; , s. 45-49
  • Konferensbidrag (refereegranskat)abstract
    • We have assessed to stability of the present VGOS receivers in the Onsala twin telescopes (OTT) in order evaluate the possibility to use them to estimate the wet propagation delay of the atmosphere. As expected the highest possible frequencies that can be used in the present OTT receivers, 15.3 to 15.6 GHz, are too far from the centre of the water vapour emission line at 22.2 GHz in order to be meaningful for critical assessments of the wet delays estimated from the VLBI data themselves. However, we do find clear correlations between the wet delays estimated from the VGOS receivers with those provided by a traditional stand-alone microwave radiometer.
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13.
  • Fogasy, Judit, 1988, et al. (författare)
  • VLA detects CO(1 0) emission in the z = 3.65 quasar SDSS J160705+533558
  • 2022
  • Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 660
  • Tidskriftsartikel (refereegranskat)abstract
    • We present CO(1 0) observations of the high-redshift quasar SDSS J160705+533558 (z=3.653) using the Karl G. Jansky Very Large Array (VLA). We detect CO emission associated with the quasar and at ∼16.8 kpc projected distance from it, separated by ∼800kmas-1 in velocity. The total molecular gas mass of this system is ∼5×1010 M˙. By comparing our CO detections with previous submillimetre (submm) observations of the source, an offset between the different emission components is revealed: the peak of the submm emission is offset from the quasar and from the CO companion detected in our VLA data. To explain our findings, we propose a scenario similar to that for the Antennae galaxies: SDSS J160705+533558 might be a merger system in which the quasar and the CO companion are the merging galaxies, whose interaction resulted in the formation of a dusty, star-forming overlap region between the galaxies that is dominant at the submm wavelengths.
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14.
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15.
  • Haas, Rüdiger, 1966, et al. (författare)
  • Observing UT1‑UTC with VGOS
  • 2021
  • Ingår i: Earth, Planets and Space. - : Springer Science and Business Media LLC. - 1880-5981 .- 1343-8832. ; 73:1
  • Tidskriftsartikel (refereegranskat)abstract
    • We present first results for the determination of UT1-UTC using the VLBI Global Observing System (VGOS). During December 2019 through February 2020, a series of 1 h long observing sessions were performed using the VGOS stations at Ishioka in Japan and the Onsala twin telescopes in Sweden. These VGOS-B sessions were observed simultaneously to standard legacy S/X-band Intensive sessions. The VGOS-B data were correlated, post-correlation processed, and analysed at the Onsala Space Observatory. The derived UT1-UTC results were compared to corresponding results from standard legacy S/X-band Intensive sessions (INT1/INT2), as well as to the final values of the International Earth Rotation and Reference Frame Service (IERS), provided in IERS Bulletin B. The VGOS-B series achieves 3–4 times lower formal uncertainties for the UT1-UTC results than standard legacy S/X-band INT series. The RMS agreement w.r.t. to IERS Bulletin B is slightly better for the VGOS-B results than for the simultaneously observed legacy S/X-band INT1 results, and the VGOS-B results have a small bias only with the smallest remaining standard deviation.
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18.
  • Handirk, Rebekka, 1994, et al. (författare)
  • Obtaining Local-Tie Vectors from Short-Baseline Interferometry
  • 2023
  • Ingår i: IVS 2022 General Meeting Proceedings. ; 1:NASA/CP-20220018789, s. 134-138
  • Konferensbidrag (refereegranskat)abstract
    • With the VLBI Global Observing System (VGOS) being the next step in the development of geodetic VLBI, it is necessary to connect the new VGOS network to the existing legacy S/X telescopes. At the Onsala Space Observatory (OSO), this is being done by short-baseline interferometry between the VGOS Onsala twin telescopes ONSA13SW and ONSA13NE and the legacy antenna ONSALA60. The main aim of these sessions, referred to as ONTIE, is to obtain local-tie vectors between these three OSO telescopes that all take part in regular geodetic VLBI observations. Each ONTIE session is about 24 h long, during which all three telescopes observe simultaneously the same sources at X-band. A total of 37 ONTIE sessions have been observed since April 2019. In November 2021, the ONTIE sessions were for the first time observed with alternative observation frequency setups in order to mitigate the influence of known RFI. Additionally, scheduling was done — also for the first time — with VieSched++ instead of sked. Interesting findings of the ONTIE sessions include unexpected offsets in the results of group and phase delays, jumps in the coordinates of the twin telescopes, and apparent yearly trends that might be an artifact of unmodeled thermal expansion of the telescopes that is left in the data. Future ONTIE sessions are envisioned to happen on a regular basis and could, as a by-product, also serve as quasar flux-monitoring sessions by investigation of the recorded system temperatures during observation. This paper summarizes the current status and results of the ONTIE sessions.
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19.
  • Heald, G. H., et al. (författare)
  • The LOFAR Multifrequency Snapshot Sky Survey (MSSS) : I. Survey description and first results
  • 2015
  • Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 582, s. 1-22
  • Tidskriftsartikel (refereegranskat)abstract
    • We present the Multifrequency Snapshot Sky Survey (MSSS), the first northern-sky Low Frequency Array (LOFAR) imaging survey. In this introductory paper, we first describe in detail the motivation and design of the survey. Compared to previous radio surveys, MSSS is exceptional due to its intrinsic multifrequency nature providing information about the spectral properties of the detected sources over more than two octaves (from 30 to 160 MHz). The broadband frequency coverage, together with the fast survey speed generated by LOFAR’s multibeaming capabilities, make MSSS the first survey of the sort anticipated to be carried out with the forthcoming Square Kilometre Array (SKA). Two of the sixteen frequency bands included in the survey were chosen to exactly overlap the frequency coverage of large-area Very Large Array (VLA) and Giant Metrewave Radio Telescope (GMRT) surveys at 74 MHz and 151 MHz respectively. The survey performance is illustrated within the MSSS Verification Field (MVF), a region of 100 square degrees centered at (α,δ)J2000 = (15h,69°). The MSSS results from the MVF are compared with previous radio survey catalogs. We assess the flux and astrometric uncertainties in the catalog, as well as the completeness and reliability considering our source finding strategy. We determine the 90% completeness levels within the MVF to be 100 mJy at 135 MHz with 108″ resolution, and 550 mJy at 50 MHz with 166″ resolution. Images and catalogs for the full survey, expected to contain 150 000–200 000 sources, will be released to a public web server. We outline the plans for the ongoing production of the final survey products, and the ultimate public release of images and source catalogs.
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20.
  • Hobiger, Thomas, 1978, et al. (författare)
  • Hard and Software Tools for the Education of Geodetic VLBI
  • 2016
  • Ingår i: IVS 2016 General Meeting Proceedings "New Horizons with VGOS". ; :NASA/CP-2016-219016, s. 234-238
  • Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
    • The Onsala Space Observatory hosts two 2.3 m radio telescopes called SALSA (”Such a lovely small antenna”) which are utilised to bring front-line interactive astronomy to the classroom. Until now SALSA has been used for astronomical educational purposes solely, in particular demonstrating the concept of single dish measurements. However, it is possible to combine both SALSAs to an interferometer by making use of hardware which has been developedfor software-defined radio. In doing so, one can utilise the SALSA antenna pair as a student demonstrator for geodetic Very Long Baseline Interferometry. Itis discussed which COTS hardware components are necessary to turn the SALSA installation into an interferometer. A simple Octave-based correlator hasbeen written in order to process SALSA data. Results from a test run during which the Sun was tracked are presented and discussed here.
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21.
  • Hobiger, Thomas, 1978, et al. (författare)
  • Hard- and software tools for the education of Geodetic VLBI
  • 2016
  • Ingår i: 9th IVS General Meeting, March 2016, Johannesburg, South Africa.
  • Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
    • Onsala Space Observatory hosts two 2.3 m radio telescopes called SALSA ("Such a lovely small antenna") which are utilised to bring front-line interactive astronomy to the classroom. Until now SALSA has been used for astronomical educational purposes solely, in particular demonstrating the concept of single dish measurements. However, it is possible to combine both SALSAs to an interferometer by making use of hardware which has been developed for software-defined radio. In doing so, one can utilise the SALSA antenna pair as a student demonstrator for geodetic Very Long Baseline Interferometry. We will discuss the COTS hardware components that are necessary to turn the SALSA installation into an interferometer. Moreover, we will show how a simple correlator has been used to detect fringes and provide single-band delays. Such delays were then processed with our analysis software c5++. We are going to discuss how it is possible to mimic the complete processing chain of geodetic VLBI and how this can be used for training of students and other interested parties.
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22.
  • Jackson, N., et al. (författare)
  • LBCS: The LOFAR Long-Baseline Calibrator Survey
  • 2016
  • Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 595, s. Art no A86-
  • Tidskriftsartikel (refereegranskat)abstract
    • We outline the LOFAR Long-Baseline Calibrator Survey (LBCS), whose aim is to identify sources suitable for calibrating the highest-resolution observations made with the International LOFAR Telescope, which include baselines > 1000 km. Suitable sources must contain significant correlated flux density (greater than or similar to 50 - 100 mJy) at frequencies around 110-190 MHz on scales of a few hundred milliarcseconds. At least for the 200-300-km international baselines, we find around 1 suitable calibrator source per square degree over a large part of the northern sky, in agreement with previous work. This should allow a randomly selected target to be successfully phase calibrated on the international baselines in over 50% of cases. Products of the survey include calibrator source lists and fringe-rate and delay maps of wide areas-typically a few degrees-around each source. The density of sources with significant correlated flux declines noticeably with baseline length over the range 200-600 km, with good calibrators on the longest baselines appearing only at the rate of 0.5 per sq. deg. Coherence times decrease from 1-3 min on 200-km baselines to about 1 min on 600-km baselines, suggesting that ionospheric phase variations contain components with scales of a few hundred kilometres. The longest median coherence time, at just over 3 min, is seen on the DE609 baseline, which at 227 km is close to being the shortest. We see median coherence times of between 80 and 110 s on the four longest baselines (580-600 km), and about 2 min for the other baselines. The success of phase transfer from calibrator to target is shown to be influenced by distance, in a manner that suggests a coherence patch at 150-MHz of the order of 1 deg. Although source structures cannot be measured in these observations, we deduce that phase transfer is affected if the calibrator source structure is not known. We give suggestions for calibration strategies and choice of calibrator sources, and describe the access to the online catalogue and data products.
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23.
  • Jaron, Frédéric, et al. (författare)
  • Cross-Polarization Gain Calibration of Linearly Polarized VLBI Antennas by Observations of 4C 39.25
  • 2024
  • Ingår i: Radio Science. - 0048-6604 .- 1944-799X. ; 59:4
  • Tidskriftsartikel (refereegranskat)abstract
    • Radio telescopes with dual linearly polarized feeds regularly participate in Very Long Baseline Interferometry. One example is the VLBI Global Observing System (VGOS), which is employed for high-precision geodesy and astrometry. In order to achieve the maximum signal-to-noise ratio, the visibilities of all four polarization products are combined to Stokes I before fringe-fitting. Our aim is to improve cross-polarization bandpass calibration, which is an essential processing step in this context. Here we investigate the shapes of these station-specific quantities as a function of frequency and time. We observed the extra-galactic source 4C 39.25 for 6 hours with a VGOS network. We correlated the data with the DiFX software and analyzed the visibilities with PolConvert to determine the complex cross-bandpasses with high accuracy. Their frequency-dependent shape is to first order characterized by a group delay between the two orthogonal polarizations, in the order of several hundred picoseconds. We find that this group delay shows systematic variability in the range of a few picoseconds, but can remain stable within this range for several years, as evident from earlier sessions. On top of the linear phase-frequency relationship there are systematic deviations of several tens of degrees, which in addition are subject to smooth temporal evolution. The antenna cross-bandpasses are variable on time scales of ∼1 hr, which defines the frequency of necessary calibrator scans. The source 4C 39.25 is confirmed as an excellent cross-bandpass calibrator. Dedicated surveys are highly encouraged to search for more calibrators of similar quality.
  •  
24.
  • König, Sabine, 1983, et al. (författare)
  • Major impact from a minor merger The extraordinary hot molecular gas flow in the Eye of the NGC 4194 Medusa galaxy
  • 2018
  • Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 615
  • Tidskriftsartikel (refereegranskat)abstract
    • Context. Minor mergers are important processes contributing significantly to how galaxies evolve across the age of the Universe. Their impact on the growth of supermassive black holes and star formation is profound - about half of the star formation activity in the local Universe is the result of minor mergers. Aims. The detailed study of dense molecular gas in galaxies provides an important test of the validity of the relation between star formation rate and HCN luminosity on different galactic scales - from whole galaxies to giant molecular clouds in their molecular gas-rich centers. Methods. We use observations of HCN and HCO+ 1-0 with NOEMA and of CO 3-2 with the SMA to study the properties of the dense molecular gas in the Medusa merger (NGC 4194) at 1" resolution. In particular, we compare the distribution of these dense gas tracers with CO 2-1 high-resolution maps in the Medusa merger. To characterize gas properties, we calculate the brightness temperature ratios between the three tracers and use them in conjunction with a non-local thermodynamic equilibrium (non-LTE) radiative line transfer model. Results. The gas represented by HCN and HCO+ 1-0, and CO 3-2 does not occupy the same structures as the less dense gas associated with the lower-J CO emission. Interestingly, the only emission from dense gas is detected in a 200 pc region within the "Eye of the Medusa", an asymmetric 500 pc off-nuclear concentration of molecular gas. Surprisingly, no HCN or HCO(+ )is detected for the extended starburst of the Medusa merger. Additionally, there are only small amounts of HCN or HCO+ associated with the active galactic nucleus. The CO 3-2/2-1 brightness temperature ratio inside "the Eye" is similar to 2.5 - the highest ratio found so far - implying optically thin CO emission. The CO 2-1/HCN 1-0 (similar to 9.8) and CO 2-1/HCO+ 1-0 (similar to 7.9) ratios show that the dense gas filling factor must be relatively high in the central region, consistent with the elevated CO 3-1/2-1 ratio. Conclusions. The line ratios reveal an extreme, fragmented molecular cloud population inside the Eye with large bulk temperatures (T > 300 K) and high gas densities (n(H-2) >10(4) cm(-3) ). This is very different from the cool, self-gravitating structures of giant molecular clouds normally found in the disks of galaxies. The Eye of the Medusa is found at an interface between a large-scale minor axis inflow and the central region of the Medusa. Hence, the extreme conditions inside the Eye may be the result of the radiative and mechanical feedback from a deeply embedded, young and massive super star cluster formed due to the gas pile-up at the intersection. Alternatively, shocks from the inflowing gas entering the central region of the Medusa may be strong enough to shock and fragment the gas. For both scenarios, however, it appears that the HCN and HCO+ dense gas tracers are not probing star formation, but instead a post-starburst and/or shocked ISM that is too hot and fragmented to form new stars. Thus, caution is advised in taking the detection of emission from dense gas tracers as evidence of ongoing or imminent star formation.
  •  
25.
  • Lucatelli, Geferson, et al. (författare)
  • The PARADIGM project I: a multiscale radio morphological analysis of local U/LIRGS
  • 2024
  • Ingår i: Monthly Notices of the Royal Astronomical Society. - 0035-8711 .- 1365-2966. ; 529:4, s. 4468-4499
  • Tidskriftsartikel (refereegranskat)abstract
    • Disentangling the radio flux contribution from star formation (SF) and active-galactic-nuclei (AGNs) activity is a long-standing problem in extragalactic astronomy, since at frequencies of 10 GHz, both processes emit synchrotron radiation. We present in this work the general objectives of the PARADIGM (PAnchromatic high-Resolution Analysis of DIstant Galaxy Mergers) project, a multi-instrument concept to explore SF and mass assembly of galaxies. We introduce two novel general approaches for a detailed multiscale study of the radio emission in local (ultra) luminous infrared galaxies (U/LIRGs). In this work, we use archival interferometric data from the Very Large Array (VLA) centred at ∼6 GHz (C band) and present new observations from the e-Multi-Element Radio-Linked Interferometer Network (e-MERLIN) for UGC 5101, VV 705, VV 250, and UGC 8696. Using our image decomposition methods, we robustly disentangle the radio emission into distinct components by combining information from the two interferometric arrays. We use e-MERLIN as a probe of the core-compact radio emission (AGN or starburst) at ∼20 pc scales, and as a probe of nuclear diffuse emission, at scales ∼100-200 pc. With VLA, we characterize the source morphology and the flux density on scales from ∼200 pc up to and above 1 kpc. As a result, we find deconvolved and convolved sizes for nuclear regions from ∼10 to ∼200 pc. At larger scales, we find sizes of 1.5-2 kpc for diffuse structures (with effective sizes of ∼300-400 pc). We demonstrate that the radio emission from nuclear extended structures (∼100 pc) can dominate over core-compact components, providing a significant fraction of the total multiscale SF output. We establish a multiscale radio tracer for SF by combining information from different instruments. Consequently, this work sets a starting point to potentially correct for overestimations of AGN fractions and underestimates of SF activity.
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26.
  • Mattila, S., et al. (författare)
  • A dust-enshrouded tidal disruption event with a resolved radio jet in a galaxy merger
  • 2018
  • Ingår i: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 361:6401, s. 482-485
  • Tidskriftsartikel (refereegranskat)abstract
    • Tidal disruption events (TDEs) are transient flares produced when a star is ripped apart by the gravitational field of a supermassive black hole (SMBH). We have observed a transient source in the western nucleus of the merging galaxy pair Arp 299 that radiated >1.5 × 1052erg at infrared and radio wavelengths but was not luminous at optical or x-ray wavelengths. We interpret this as a TDE with much of its emission reradiated at infrared wavelengths by dust. Efficient reprocessing by dense gas and dust may explain the difference between theoretical predictions and observed luminosities of TDEs. The radio observations resolve an expanding and decelerating jet, probing the jet formation and evolution around a SMBH.
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27.
  • Moldón, J., et al. (författare)
  • The LOFAR long baseline snapshot calibrator survey
  • 2015
  • Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 574
  • Tidskriftsartikel (refereegranskat)abstract
    • Aims. An efficient means of locating calibrator sources for international LOw Frequency ARray (LOFAR) is developed and used to determine the average density of usable calibrator sources on the sky for subarcsecond observations at 140 MHz.Methods. We used the multi-beaming capability of LOFAR to conduct a fast and computationally inexpensive survey with the full international LOFAR array. Sources were preselected on the basis of 325 MHz arcminute-scale flux density using existing catalogues. By observing 30 different sources in each of the 12 sets of pointings per hour, we were able to inspect 630 sources in two hours to determine if they possess a sufficiently bright compact component to be usable as LOFAR delay calibrators.Results. More than 40% of the observed sources are detected on multiple baselines between international stations and 86 are classified as satisfactory calibrators. We show that a flat low-frequency spectrum (from 74 to 325 MHz) is the best predictor of compactness at 140 MHz. We extrapolate from our sample to show that the sky density of calibrators that are sufficiently bright to calibrate dispersive and non-dispersive delays for the international LOFAR using existing methods is 1.0 per square degree.Conclusions. The observed density of satisfactory delay calibrator sources means that observations with international LOFAR should be possible at virtually any point in the sky provided that a fast and efficient search, using the methodology described here, is conducted prior to the observation to identify the best calibrator.
  •  
28.
  • Moldón, J., et al. (författare)
  • The LOFAR long baseline snapshot calibrator survey
  • 2014
  • Ingår i: Proceedings of Science. - 1824-8039. ; 2014
  • Konferensbidrag (refereegranskat)abstract
    • With a current maximum baseline of 1300 km, the International LOFAR array is capable of attaining an angular resolution of 0.4 arcsec at a frequency of 140 MHz, opening for the first time the possibility of true subarcsecond imaging at wavelengths longer than 1 m. We used the multibeaming capability of LOFAR to conduct a fast and computationally inexpensive survey to inspect 630 sources in two hours to determine if they possess a sufficiently bright compact component to be usable as LOFAR delay calibrators. Here we summarize the characteristics of the survey, and its main results. In particular we have obtained the density of calibrators on the sky that are sufficiently bright to calibrate dispersive and non-dispersive delays for the International LOFAR. © Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence.
  •  
29.
  • Morabito, L. K., et al. (författare)
  • Discovery of Carbon Radio Recombination Lines in M82
  • 2014
  • Ingår i: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8213 .- 2041-8205. ; 795:2, s. Art. no. L33-
  • Forskningsöversikt (refereegranskat)abstract
    • Carbon radio recombination lines (RRLs) at low frequencies (less than or similar to 500 MHz) trace the cold, diffuse phase of the interstellar medium, which is otherwise difficult to observe. We present the detection of carbon RRLs in absorption in M82 with the Low Frequency Array in the frequency range of 48-64 MHz. This is the first extragalactic detection of RRLs from a species other than hydrogen, and below 1 GHz. Since the carbon RRLs are not detected individually, we cross-correlated the observed spectrum with a template spectrum of carbon RRLs to determine a radial velocity of 219 km s(-1). Using this radial velocity, we stack 22 carbon-alpha transitions from quantum levels n = 468-508 to achieve an 8.5 sigma detection. The absorption line profile exhibits a narrow feature with peak optical depth of 3x10(-3) and FWHM of 31 km s(-1). Closer inspection suggests that the narrow feature is superimposed on a broad, shallow component. The total line profile appears to be correlated with the 21 cm Hi line profile reconstructed from Hi absorption in the direction of supernova remnants in the nucleus. The narrow width and centroid velocity of the feature suggests that it is associated with the nuclear starburst region. It is therefore likely that the carbon RRLs are associated with cold atomic gas in the direction of the nucleus of M82.
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30.
  • Morabito, L. K., et al. (författare)
  • LOFAR VLBI studies at 55 MHz of 4C 43.15, a z=2.4 radio galaxy
  • 2016
  • Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 461:3, s. 2676-2687
  • Tidskriftsartikel (refereegranskat)abstract
    • The correlation between radio spectral index and redshift has been exploited to discover high-redshift radio galaxies, but its underlying cause is unclear. It is crucial to characterize the particle acceleration and loss mechanisms in high-redshift radio galaxies to understand why their radio spectral indices are steeper than their local counterparts. Low-frequency information on scales of similar to 1 arcsec are necessary to determine the internal spectral index variation. In this paper we present the first spatially resolved studies at frequencies below 100 MHz of the z = 2.4 radio galaxy 4C 43.15 which was selected based on its ultrasteep spectral index (alpha
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31.
  • Muxlow, T. W. B., et al. (författare)
  • The e-MERGE Survey (e-MERLIN Galaxy Evolution Survey): overview and survey description
  • 2020
  • Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 495:1, s. 1188-1208
  • Tidskriftsartikel (refereegranskat)abstract
    • We present an overview and description of the e-MERGE Survey (e-MERLIN Galaxy Evolution Survey) Data Release 1 (DR1), a large program of high-resolution 1.5-GHz radio observations of the GOODS-N field comprising similar to 140 h of observations with enhanced-Multi-Element Remotely Linked Interferometer Network (e-MERLIN) and similar to 40 h with the Very Large Array (VLA). We combine the long baselines of e-MERLIN (providing high angular resolution) with the relatively closely packed antennas of the VLA (providing excellent surface brightness sensitivity) to produce a deep 1.5-GHz radio survey with the sensitivity (similar to 1.5 mu Jy beam(-1)), angular resolution (0.2-0.7 arcsec) and field-of-view (similar to 15x15 arcmin(2)) to detect and spatially resolve star-forming galaxies and active galactic nucleus (AGN) at z greater than or similar to 1. The goal of e-MERGE is to provide new constraints on the deep, sub-arcsecond radio sky which will be surveyed by SKA1-mid. In this initial publication, we discuss our data analysis techniques, including steps taken to model in-beam source variability over an similar to 20-yr baseline and the development of newpoint spread function/primary beam models to seamlessly merge e-MERLIN and VLA data in the uv plane. We present early science results, including measurements of the luminosities and/or linear sizes of similar to 500 galaxies selected at 1.5 GHz. In combination with deep Hubble Space Telescope observations, we measure a mean radio-to-optical size ratio of r(e-MERGE)/r(HST) similar to 1.02 +/- 0.03, suggesting that in most high-redshift galaxies, the similar to GHz continuum emission traces the stellar light seen in optical imaging. This is the first in a series of papers that will explore the similar to kpc-scale radio properties of star-forming galaxies and AGN in the GOODS-N field observed by e-MERGE DR1.
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32.
  • Nilsson, Tobias, et al. (författare)
  • The Current and Future Performance of VGOS
  • 2023
  • Ingår i: International VLBI Service for Geodesy and Astrometry 2022 General Meeting Proceedings. ; NASA/CP–20220018789, s. 192-196
  • Konferensbidrag (refereegranskat)abstract
    • In this work we investigate the performance of the 24-hour VGOS sessions observed in 2019–2021. We look at the station positions and the Earth Orientation parameters (EOP), and we compare them with the results from the legacy S/X VLBI sessions as well as with simulations. We find that the station position repeatabilities obtained from the VGOS sessions are significantly better than what is obtained from the legacy S/X VLBI sessions. However, the EOP from the VGOS sessions are less accurate than those from the legacy S/X sessions, a consequence of the low number and poor global coverage of the currently operational  VGOS stations.
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33.
  • O Gorman, Eamon, 1984, et al. (författare)
  • A search for radio emission from exoplanets around evolved stars
  • 2018
  • Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 612
  • Tidskriftsartikel (refereegranskat)abstract
    • The majority of searches for radio emission from exoplanets have to date focused on short period planets, i.e., the so-called hot Jupiter type planets. However, these planets are likely to be tidally locked to their host stars and may not generate sufficiently strong magnetic fields to emit electron cyclotron maser emission at the low frequencies used in observations (typically >= 150 MHz). In comparison, the large mass-loss rates of evolved stars could enable exoplanets at larger orbital distances to emit detectable radio emission. Here, we first show that the large ionized mass-loss rates of certain evolved stars relative to the solar value could make them detectable with the LOw Frequency ARray (LOFAR) at 150 MHz (lambda = 2 m), provided they have surface magnetic field strengths >50 G. We then report radio observations of three long period (>1 au) planets that orbit the evolved stars beta Gem, iota Dra, and beta UMi using LOFAR at 150 MHz. We do not detect radio emission from any system but place tight 3 sigma upper limits of 0.98, 0.87, and 0.57 mJy on the flux density at 150 MHz for beta Gem, iota Dra, and beta UMi, respectively. Despite our non-detections these stringent upper limits highlight the potential of LOFAR as a tool to search for exoplanetary radio emission at meter wavelengths.
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34.
  • Palliyaguru, N. T., et al. (författare)
  • VLBI Observations of Supernova PTF11qcj: Direct Constraints on the Size of the Radio Ejecta
  • 2021
  • Ingår i: Astrophysical Journal. - : American Astronomical Society. - 1538-4357 .- 0004-637X. ; 910:1
  • Tidskriftsartikel (refereegranskat)abstract
    • We present High Sensitivity Array and enhanced Multi-Element Remotely Linked Interferometer Network observations of the radio-loud broad-lined Type Ic supernova PTF11qcj obtained similar to 7.5 yr after the explosion. Previous observations of this supernova at 5.5 yr since explosion showed a double-peaked radio light curve accompanied by a detection in the X-rays, but no evidence for broad H alpha spectral features. The Very Long Baseline Interferometry (VLBI) observations presented here show that the PTF11qcj GHz radio ejecta remains marginally resolved at the submilliarcsecond level 7.5 yr after the explosion, pointing toward a nonrelativistic expansion. Our VLBI observations thus favor a scenario in which the second peak of the PTF11qcj radio light curve is related to the strong interaction of the supernova ejecta with a circumstellar medium of variable density, rather than to the emergence of an off-axis jet. Continued VLBI monitoring of PTF11qcj in the radio may further strengthen this conclusion.
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35.
  • Ramirez-Olivencia, Naim, et al. (författare)
  • EVN imaging of the LIRGI sample
  • 2014
  • Ingår i: Proceedings of Science. - 1824-8039.
  • Konferensbidrag (refereegranskat)abstract
    • The central few hundred parsecs of Ultra/Luminous Infrared Galaxies (U/LIRGs) are hidden from view at most wavelengths due to the presence of large amounts of dust. Very Long Baseline Interferometry (VLBI) observations are a powerful tool to unveil the nature of the sources beyond dust clouds. In fact, radio observations are unaffected by dust, and thanks to the high angular resolution are able to pinpoint the compact components in the central regions of U/LIRGs. Thus, VLBI observations can potentially disentangle the dominant mechanism responsible for the dust heating in U/LIRGs, i.e., an Active Galactic Nucleus (AGN), or a starburst, or a combination of both. In this contribution we present preliminary results for 12 galaxies from the Luminous Infrared Galaxy Inventory (LIRGI) sample, obtained from observations with the European VLBI Network (EVN) at 6 and 18 cm. We show images from NGC 2623 and Mrk 331. Mrk 331 shows three relatively faint compact components at C-band, consistent with emission from young supernovae and/or supernova remnants. On the contrary, NGC 2623 shows a single, very bright compact component at both wavelengths and a very inverted spectral index, clearly indicating that this is the AGN. © Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence.
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36.
  • Ramirez-Olivencia, Naim, et al. (författare)
  • Sub-arcsecond imaging of Arp 299-A at 150 MHz with LOFAR: Evidence for a starburst-driven outflow
  • 2018
  • Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 610
  • Tidskriftsartikel (refereegranskat)abstract
    • We report on the first sub-arcsecond (0.44 × 0.41 arcsec 2 ) angular resolution image at 150 MHz of the A-nucleus in the luminous infrared galaxy Arp 299, from International Low Frequency Array (LOFAR) Telescope observations. The most remarkable finding is that of an intriguing two-sided, filamentary structure emanating from the A-nucleus, which we interpret as an outflow that extends up to at least 14 arcsec from the A-nucleus in the N-S direction (≈5 kpc deprojected size) and accounts for almost 40% of the extended emission of the entire galaxy system. We also discuss HST/NICMOS [FeII] 1.64 μm and H 2 2.12 μm images of Arp 299-A, which show similar features to those unveiled by our 150 MHz LOFAR observations, providing strong morphological support for the outflow scenario. Finally, we discuss unpublished Na I D spectra that confirm the outflow nature of this structure. From energetic arguments, we rule out the low-luminosity active galactic nucleus in Arp 299-A as a driver for the outflow. On the contrary, the powerful, compact starburst in the central regions of Arp 299-A provides plenty of mechanical energy to sustain an outflow, and we conclude that the intense supernova (SN) activity in the nuclear region of Arp 299-A is driving the observed outflow. We estimate that the starburst wind can support a mass-outflow rate in the range (11-63 M ⊙ yr -1 ) at speeds of up to 370-890 km s -1 , and is relatively young, with an estimated kinematic age of 3-7 Myr. Those results open an avenue to the use of low-frequency (150 MHz), sub-arcsecond imaging with LOFAR to detect outflows in the central regions of local luminous infrared galaxies.
  •  
37.
  • Ramirez-Olivencia, Naim, et al. (författare)
  • Sub-arcsecond LOFAR imaging of Arp 299 at 150 MHz: Tracing the nuclear and diffuse extended emission of a bright LIRG
  • 2022
  • Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 658
  • Tidskriftsartikel (refereegranskat)abstract
    • Context. Arp 299 is the brightest luminous infrared galaxy (LIRG) within 50 Mpc, with IR luminosity log(LIR/L⊙) = 11.9. It provides a unique laboratory for testing physical processes in merging galaxies. Aims. We study for the first time the low-frequency (∼150 MHz) radio brightness distribution of Arp 299 at subarcsecond resolution, tracing in both compact and extended emission regions the local spectral energy distribution (SED) in order to characterize the dominant emission and absorption processes. Methods. We analysed the spatially resolved emission of Arp 299 revealed by 150 MHz international baseline Low-Frequency Array (LOFAR) and 1.4, 5.0, and 8.4 GHz Very Large Array (VLA) observations. Results. We present the first subarcsecond (0.4″ ×100 pc) image of the whole Arp 299 system at 150 MHz. The high surface brightness sensitivity of our LOFAR observations (∼100 μJy beam-1) allowed us to detect all of the nuclear components detected at higher frequencies, as well as the extended steep-spectrum emission surrounding the nuclei. We obtained spatially resolved, two-point spectral index maps for the whole galaxy: the compact nuclei show relatively flat spectra, while the extended, diffuse component shows a steep spectrum. We fitted the radio SED of the nuclear regions using two different models: a continuous free-free medium model and a clumpy model. The continuous model can explain the SED of the nuclei assuming a population of relativistic electrons subjected to synchrotron, bremsstrahlung, and ionization losses. The clumpy model fits assuming relativistic electrons with negligible energy losses, and thermal fractions that are more typical of star-forming galaxies than those required for the continuous model. Conclusions. Our results confirm the usefulness of combining spatially resolved radio imaging at both MHz and GHz frequencies to characterize in detail the radio emission properties of LIRGs from the central 100 pc out to the kiloparsec galaxy-wide scales.
  •  
38.
  • Shimwell, T. W., et al. (författare)
  • The LOFAR Two-metre Sky Survey: I. Survey description and preliminary data release
  • 2017
  • Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 598, s. Art no A104-
  • Tidskriftsartikel (refereegranskat)abstract
    • The LOFAR Two-metre Sky Survey (LoTSS) is a deep 120-168 MHz imaging survey that will eventually cover the entire northern sky. Each of the 3170 pointings will be observed for 8 h, which, at most declinations, is sufficient to produce ~5? resolution images with a sensitivity of ~100 ?Jy/beam and accomplish the main scientific aims of the survey, which are to explore the formation and evolution of massive black holes, galaxies, clusters of galaxies and large-scale structure. Owing to the compact core and long baselines of LOFAR, the images provide excellent sensitivity to both highly extended and compact emission. For legacy value, the data are archived at high spectral and time resolution to facilitate subarcsecond imaging and spectral line studies. In this paper we provide an overview of the LoTSS. We outline the survey strategy, the observational status, the current calibration techniques, a preliminary data release, and the anticipated scientific impact. The preliminary images that we have released were created using a fully automated but direction-independent calibration strategy and are significantly more sensitive than those produced by any existing large-Area low-frequency survey. In excess of 44 000 sources are detected in the images that have a resolution of 25?, typical noise levels of less than 0.5 mJy/beam, and cover an area of over 350 square degrees in the region of the HETDEX Spring Field (right ascension 10h45m00s to 15h30m00s and declination 45°00?00? to 57°00?00?).
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39.
  • Shimwell, T. W., et al. (författare)
  • The LOFAR Two-metre Sky Survey: II. First data release
  • 2019
  • Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 622
  • Forskningsöversikt (refereegranskat)abstract
    • The LOFAR Two-metre Sky Survey (LoTSS) is an ongoing sensitive, high-resolution 120-168 MHz survey of the entire northern sky for which observations are now 20% complete. We present our first full-quality public data release. For this data release 424 square degrees, or 2% of the eventual coverage, in the region of the HETDEX Spring Field (right ascension 10h45m00s to 15h30m00s and declination 45°00′00″ to 57°00′00″) were mapped using a fully automated direction-dependent calibration and imaging pipeline that we developed. A total of 325 694 sources are detected with a signal of at least five times the noise, and the source density is a factor of ∼10 higher than the most sensitive existing very wide-area radio-continuum surveys. The median sensitivity is S144 MHz = 71 μJy beam -1 and the point-source completeness is 90% at an integrated flux density of 0.45 mJy. The resolution of the images is 6″ and the positional accuracy is within 0.2″. This data release consists of a catalogue containing location, flux, and shape estimates together with 58 mosaic images that cover the catalogued area. In this paper we provide an overview of the data release with a focus on the processing of the LOFAR data and the characteristics of the resulting images. In two accompanying papers we provide the radio source associations and deblending and, where possible, the optical identifications of the radio sources together with the photometric redshifts and properties of the host galaxies. These data release papers are published together with a further ∼20 articles that highlight the scientific potential of LoTSS.
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40.
  •  
41.
  • Varenius, Eskil, 1986, et al. (författare)
  • Broad band flux-density monitoring of radio sources with the Onsala twin telescopes
  • 2022
  • Ingår i: Experimental Astronomy. - : Springer Science and Business Media LLC. - 0922-6435 .- 1572-9508. ; 54, s. 137-155
  • Tidskriftsartikel (refereegranskat)abstract
    • The Onsala twin telescopes (OTT) are two 13 m telescopes located at the Onsala Space Observatory in Sweden. With dual linear polarized broad-band (3-14 GHz) receivers, they are part of the next generation Very Long Baseline Interferometry (VLBI) Global Observing System (VGOS) for geodesy and astrometry. In addition to purely geodetic data products, VGOS will regularly produce full-polarisation images of hundreds of radio sources. These rich monitoring data will be valuable for both astronomy and geodesy. In this pilot study we aimed at monitoring ten bright radio sources to search for flares or similar activity, and at testing the instrument calibration on long (months) and short (hours) time scales. Using the OTT as a standalone instrument, we observed and analysed 91 short (<30 min) sessions spanning seven months. We monitored seven potentially variable radio sources (0059+581, 0552+398, 1144+402, 1156+295, 1617+229, 3C418, OJ287) and three reference calibrators (3C147, 3C286, 3C295). We used the Common Astronomy Software Applications (CASA) package to fringe-fit, bandpass-correct and scale the data to obtain flux densities in the four standard VGOS bands: 3.0-3.5 GHz (band 1), 5.2-5.7 GHz (band 2), 6.3-6.8 GHz (band 3), and 10.2-10.7 GHz (band 4). We obtained simultaneous multi-frequency light curves for ten radio sources. A bright multi-frequency flare is observed in the radio source 0059+581. OJ287 and 1156+295 show significant long-term variability in their light curves. After correcting for instrumental biases, we determine the empirical flux density uncertainty as similar to 5 %.
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42.
  • Varenius, Eskil, 1986 (författare)
  • High-resolution radio imaging of galaxy nuclei
  • 2014
  • Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • Star formation and galaxy evolution are intimately linked together. A detailed understanding of the environment where stars form may help us explain the differences we see between galaxies today and at earlier times in the history of the Universe. Galaxy nuclei are often heavily obscured by dust and cannot be observed in detail at optical wavelengths. Since radio waves are almost unaffected by dust, they can tell us about the physics in dense star forming regions. To achieve high enough image resolution at radio wavelengths, we use interferometry to synthesize a large aperture using data from multiple radio telescopes. We obtain detailed images of radio sources associated with star formation, such as core-collapse supernovae and their corresponding remnants. Using series of images at different observing wavelengths and different times, we can model the evolution of the radio sources and estimate densities, magnetic fields and star formation rates.We have studied the luminous nuclei of three different galaxies; M82, NGC 4418 and Arp 220. M82 and Arp 220 are well known starburst galaxies, meaning they have rates of star formation so high that they cannot be sustained over the history of the Universe. Both galaxies also follow the well-known FIR/radio correlation. In this thesis, I present ongoing work to model in detail the environment of Arp 220 and M82, based on high-resolution imaging of their nuclei. Our image of M82 is a new record in terms of image resolution at low frequencies. NGC 4418 is different; it does not follow the FIR/radio correlation. The high observed excess infrared luminosity of this galaxy has been suggested to come instead from an accreting black hole. We argue that although there might be an accreting black hole in NGC 4418, it cannot account for all the complex structure revealed by high-resolution radio imaging, indicating a significant starburst component.
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43.
  • Varenius, Eskil, 1986 (författare)
  • Investigating the origin of radio emission in nearby starburst galaxies via high-resolution metre and centimetre observations
  • 2016
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • Star formation and galaxy evolution are intimately linked together. A detailed understanding of the physics of star formation can help us explain how galaxies evolve into, for example, our present Milky Way. Using Very Long Baseline Interferometry (VLBI) techniques it is possible to achieve high enough angular resolution to study radio emission from other galaxies in great detail. This thesis presents observations of three nearby Luminous Infrared Galaxies (LIRGs): NGC 4418, M 82 and Arp 220. While the centres of these galaxies are all heavily obscured in optical wavelengths, radio observations can be used to probe star formation properties in the centres of these galaxies.The galaxy NGC 4418 is radio weak with respect to the far infrared (FIR)-radio correlation for star forming galaxies. We present evidence for a young starburst in the centre, likely fuelled by gas falling in from a recent interaction with the nearby galaxy VV 655. We argue that this scenario can explain the low radio luminosity of this galaxy, and possibly also of other galaxies which appear to be radio weak.M 82 and Arp 220 follow the FIR-radio correlation and are excellent laboratories to study the physics of star formation in extreme environments. In this thesis we report on new groundbreaking subarcsecond resolution observations of M 82 and Arp 220 with the international LOFAR telescope, where we for the first time spatially resolve the radio emission from their nuclei at metre wavelengths. We report on previously unknown steep-spectrum radio structures and study effects of free-free absorption. We conclude that high angular resolution is essential for a correct interpretation of the radio emission from these complex objects. Furthermore, this work demonstrates that LOFAR can be used to obtain subarcsecond resolution images at metre wavelengths, a capability which can be used in multiple areas of astronomy in the future.Finally, we present results from new and archival global VLBI observations of Arp 220 spanning 17 years. We show that a self-consistent approach is essential to understand the nature of the radio emission. We find the data rich in details: we detect more than 80 compact objects, many with luminosities and sizes measured at multiple times and frequencies. We present a first analysis of the data where we discuss the general properties of the source population.
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44.
  • Varenius, Eskil, 1986, et al. (författare)
  • Short-baseline interferometry local-tie experiments at the Onsala Space Observatory
  • 2021
  • Ingår i: Journal of Geodesy. - : Springer Science and Business Media LLC. - 0949-7714 .- 1432-1394. ; 95:5
  • Tidskriftsartikel (refereegranskat)abstract
    • We present results from observation, correlation and analysis of interferometric measurements between the three geodetic very long baseline interferometry (VLBI) stations at the Onsala Space Observatory. In total, 25 sessions were observed in 2019 and 2020, most of them 24 h long, all using X band only. These involved the legacy VLBI station ONSALA60 and the Onsala twin telescopes, ONSA13NE and ONSA13SW, two broadband stations for the next-generation geodetic VLBI global observing system (VGOS). We used two analysis packages: nu Solve to pre-process the data and solve ambiguities, and ASCOT to solve for station positions, including modelling gravitational deformation of the radio telescopes and other significant effects. We obtained weighted root mean square post-fit residuals for each session on the order of 10-15 ps using group-delays and 2-5 ps using phase-delays. The best performance was achieved on the (rather short) baseline between the VGOS stations. As the main result of this work, we determined the coordinates of the Onsala twin telescopes in VTRF2020b with sub-millimetre precision. This new set of coordinates should be used from now on for scheduling, correlation, as a priori for data analyses, and for comparison with classical local-tie techniques. Finally, we find that positions estimated from phase-delays are offset similar to+3 mm in the up-component with respect to group-delays. Additional modelling of (elevation dependent) effects may contribute to the future understanding of this offset.
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45.
  • Varenius, Eskil, 1986, et al. (författare)
  • Subarcsecond international LOFAR radio images of Arp 220 at 150 MHz: A kpc-scale star forming disk surrounding nuclei with shocked outflows
  • 2016
  • Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 593, s. A86-
  • Tidskriftsartikel (refereegranskat)abstract
    • Context. Arp 220 is the prototypical ultra luminous infrared galaxy (ULIRG). Despite extensive studies, the structure at MHz-frequencies has remained unknown because of limits in spatial resolution.Aims: This work aims to constrain the flux and shape of radio emission from Arp 220 at MHz frequencies.Methods: We analyse new observations with the International Low Frequency Array (LOFAR) telescope, and archival data from the Multi-Element Radio Linked Interferometer Network (MERLIN) and the Karl G. Jansky Very Large Array (VLA). We model the spatially resolved radio spectrum of Arp 220 from 150 MHz to 33 GHz.Results: We present an image of Arp 220 at 150 MHz with resolution 0.̋65 × 0.̋35, sensitivity 0.15 mJy beam-1, and integrated flux density 394 ± 59 mJy. More than 80% of the detected flux comes from extended (6''≈ 2.2 kpc) steep spectrum (α = -0.7) emission, likely from star formation in the molecular disk surrounding the two nuclei. We find elongated features extending 0.3'' (110 pc) and 0.9'' (330 pc) from the eastern and western nucleus respectively, which we interpret as evidence for outflows. The extent of radio emission requires acceleration of cosmic rays far outside the nuclei. We find that a simple three component model can explain most of the observed radio spectrum of the galaxy. When accounting for absorption at 1.4 GHz, Arp 220 follows the FIR/radio correlation with q = 2.36, and we estimate a star formation rate of 220 M⊙ yr-1. We derive thermal fractions at 1 GHz of less than 1% for the nuclei, which indicates that a major part of the UV-photons are absorbed by dust.Conclusions: International LOFAR observations shows great promise to detect steep spectrum outflows and probe regions of thermal absorption. However, in LIRGs the emission detected at 150 MHz does not necessarily come from the main regions of star formation. This implies that high spatial resolution is crucial for accurate estimates of star formation rates for such galaxies at 150 MHz.
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46.
  • Varenius, Eskil, 1986, et al. (författare)
  • Subarcsecond international LOFAR radio images of the M82 nucleus at 118 MHz and 154 MHz
  • 2015
  • Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 574, s. Art. no. A114-
  • Tidskriftsartikel (refereegranskat)abstract
    • Context. The nuclear starburst in the nearby galaxy M82 provides an excellent laboratory for understanding the physics of star formation. This galaxy has been extensively observed in the past, revealing tens of radio bright compact objects embedded in a diffuse free-free absorbing medium. Our understanding of the structure and physics of this medium in M82 can be greatly improved by high-resolution images at tow frequencies where the effects of free-free absorption are most prominent. Aims. The aims of this study are, firstly, to demonstrate imaging using international baselines of the Low Frequency Array (LOFAR), and secondly, to constrain low frequency spectra of compact and diffuse emission in the central starburst region of M82 via high resolution radio imaging at low frequencies. Methods. The international LOFAR telescope was used to observe M82 at 110-126 MHz and 146-1621\4Hz. Images were obtained using standard techniques from very long baseline interferometry. images were obtained at each frequency range: one only using international baselines, and one only using the longest Dutch (remote) baselines. Results. The 154 MHz image obtained using international baselines is a new imaging record in terms of combined image resolution (0.3") and sensitivity (sigma = 0.15 mIy/beath) at low frequencies (
  •  
47.
  • Varenius, Eskil, 1986, et al. (författare)
  • The population of SNe/SNRs in the starburst galaxy Arp 220 - A self-consistent analysis of 20 years of VLBI monitoring
  • 2019
  • Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 623
  • Tidskriftsartikel (refereegranskat)abstract
    • Context . The nearby ultra-luminous infrared galaxy (ULIRG) Arp 220 is an excellent laboratory for studies of extreme astrophysical environments. For 20 years, Very Long Baseline Interferometry (VLBI) has been used to monitor a population of compact sources thought to be supernovae (SNe), supernova remnants (SNRs), and possibly active galactic nuclei (AGNs). SNe and SNRs are thought to be the sites of relativistic particle acceleration powering star formation induced radio emission in galaxies, and are hence important for studies of for example the origin of the FIR-radio correlation. Aims . In this work we aim for a self-consistent analysis of a large collection of Arp 220 continuum VLBI data sets. With more data and improved consistency in calibration and imaging, we aim to detect more sources and improve source classifications with respect to previous studies. Furthermore, we aim to increase the number of sources with robust size estimates, to analyse the compact source luminosity function (LF), and to search for a luminosity- diameter (LD) relation within Arp 220. Methods . Using new and archival VLBI data spanning 20 years, we obtained 23 high-resolution radio images of Arp 220 at wavelengths from 18 cm to 2 cm. From model-fitting to the images we obtained estimates of flux densities and sizes of detected sources. The sources were classified in groups according to their observed lightcurves, spectra and sizes. We fitted a multi-frequency supernova light-curve model to the object brightest at 6 cm to estimate explosion properties for this object. Results. We detect radio continuum emission from 97 compact sources and present flux densities and sizes for all analysed observation epochs. The positions of the sources trace the star forming disks of the two nuclei known from lower-resolution studies. We find evidence for a LD-relation within Arp 220, with larger sources being less luminous. We find a compact source LF n(L) proportional to L-beta with beta = -2.19 +/- 0.15, similar to SNRs in normal galaxies, and we argue that there are many relatively large and weak sources below our detection threshold. The brightest (at 6 cm) object 0.2195+0.492 is modelled as a radio SN with an unusually long 6 cm rise time of 17 years. Conclusions. The observations can be explained by a mixed population of SNe and SNRs, where the former expand in a dense circumstellar medium (CSM) and the latter interact with the surrounding interstellar medium (ISM). Nine sources are likely luminous SNe, for example type IIn, and correspond to few percent of the total number of SNe in Arp 220. Assuming all IIns reach these luminosities, and no confusion with other SNe types, our data are consistent with a total SN-rate of 4 yr(-1) as inferred from the total radio emission given a normal stellar initial mass function (IMF). Based on the fitted luminosity function, we argue that emission from all compact sources, also below our detection threshold, make up at most 20% of the total radio emission at GHz frequencies. However, colliding SN shocks and the production of secondary electrons through cosmic ray (CR) protons colliding with the dense ISM may cause weak sources to radiate much longer than assumed in this work. This could potentially explain the remaining fraction of the smooth synchrotron component. Future, deeper observations of Arp 220 will probe the sources with lower luminosities and larger sizes. This will further constrain the evolution of SNe/SNRs in extreme environments and the presence of AGN activity.
  •  
48.
  • Varenius, Eskil, 1986, et al. (författare)
  • The radio core structure of the luminous infrared galaxy NGC4418 A young clustered starburst revealed?
  • 2014
  • Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 566
  • Tidskriftsartikel (refereegranskat)abstract
    • Context. The galaxy NGC4418 contains one of the most compact obscured nuclei within a luminous infrared galaxy (LIRG) in the nearby Universe. This nucleus contains a rich molecular gas environment and an unusually high ratio of infrared-to-radio luminosity (q-factor). The compact nucleus is powered by either a compact starburst or an active galactic nucleus (AGN). Aims. The aim of this study is to constrain the nature of the nuclear region (starburst or AGN) within NGC4418 via very-high-resolution radio imaging. Methods. Archival data from radio observations using the European Very Long Baseline Interferometry Network (EVN) and Multi-Element Radio Linked Interferometer Network (MERLIN) interferometers are imaged. Sizes and flux densities are obtained by fitting Gaussian intensity distributions to the image. The average spectral index of the compact radio emission is estimated from measurements at 1.4 GHz and 5.0 GHz. Results. The nuclear structure of NGC4418 visible with EVN and MERLIN consists of eight compact ( = 0.7 (S-nu proportional to nu(alpha)) for the compact radio emission. Conclusions. Brightness temperatures >10(4.8) K indicate that these compact features cannot be HII-regions. The complex morphology and inverted spectrum of the eight detected compact features is evidence against the hypothesis that an AGN alone is powering the nucleus of NGC4418. The compact features could be super star clusters with intense star formation, and their associated free-free absorption could then naturally explain both their inverted radio spectrum and the low radio-to-IR ratio of the nucleus. The required star formation area density is extreme, however, and close to the limit of what can be observed in a well-mixed thermal/non-thermal plasma produced by star formation, and is also close to the limit of what can be physically sustained.
  •  
49.
  • Wandia, Kelvin, et al. (författare)
  • An interferometric SETI observation of Kepler-111 b
  • 2023
  • Ingår i: Monthly Notices of the Royal Astronomical Society. - 0035-8711 .- 1365-2966. ; 522:3, s. 3784-3794
  • Tidskriftsartikel (refereegranskat)abstract
    • The application of very long baseline interferometry (VLBI) to the search for extraterrestrial intelligence (SETI) has been limited to date, despite the technique offering many advantages over traditional single-dish SETI observations. In order to further develop interferometry for SETI, we used the European VLBI network (EVN) at 21 cm to observe potential secondary phase calibrators in the Kepler field. Unfortunately, no secondary calibrators were detected. However, a VLBA primary calibrator in the field, J1926+4441, offset only ∼1.88 arcmin from a nearby exoplanet Kepler-111 b, was correlated with high temporal (0.25 s) and spectral (16384 × 488 Hz channels) resolution. During the analysis of the high-resolution data, we identified a spectral feature that was present in both the auto and cross-correlation data with a central frequency of 1420.424 ± 0.0002 MHz and a width of 0.25 MHz. We demonstrate that the feature in the cross-correlations is an artefact in the data, associated with a significant increase in each telescope's noise figure due to the presence of H I in the beam. This would typically go unnoticed in data correlated with standard spectral resolution. We flag (excluded from the subsequent analysis) these channels and phase rotate the data to the location of Kepler-111 b aided by the Gaia catalogue and search for signals with SNR > 7. At the time of our observations, we detect no transmitters with an equivalent isotropically radiated power ≳4 × 1015 W.
  •  
50.
  • Yang, Jun, 1979, et al. (författare)
  • Intermediate-mass black holes: finding of episodic, large-scale, and powerful jet activity in a dwarf galaxy
  • 2023
  • Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 520:4, s. 5964-5973
  • Tidskriftsartikel (refereegranskat)abstract
    • Dwarf galaxies are characterized by a very low luminosity and low mass. Because of significant accretion and ejection activity of massive black holes, some dwarf galaxies also host lo w-luminosity acti ve galactic nuclei (AGNs). In a few dwarf AGNs, very long baseline interferometry (VLBI) observations have found faint non-thermal radio emission. SDSS J090613.77 + 561015.2 is a dwarf AGN owning an intermediate-mass black hole (IMBH) with a mass of M-BH = 3 . 6 (+5.9) (-2.3 )x 10 (5) M(? )and showing a rarely seen two-component radio structure in its radio nucleus. To further probe their nature, i.e. the IMBH jet activity, we performed additional deep observations with the European VLBI Network (EVN) at 1.66 and 4.99 GHz. We find the more diffuse emission regions and structure details. These new EVN imaging results allow us to reveal a two-sided jet morphology with a size up to about 150 mas (projected length similar to 140 pc) and a radio luminosity of about 3 x 10( 38) erg s( -1). The peak feature has an optically thin radio spectrum and thus more likely represents a relatively young ejecta instead of a jet base. The EVN study on SDSS J090613.77 + 561015.2 demonstrates the existence of episodic, relatively large-scale, and powerful IMBH jet activity in dwarf AGNs. Moreo v er, we collected a small sample of VLBI-detected dwarf AGNs and investigated their connections with normal AGNs. We notice that these radio sources in the dwarf AGNs tend to have steep spectra and small linear sizes, and possibly represent ejecta from scaled-down episodic jet activity.
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