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| 3. |
- Klionsky, Daniel J., et al.
(författare)
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Guidelines for the use and interpretation of assays for monitoring autophagy
- 2012
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Ingår i: Autophagy. - : Informa UK Limited. - 1554-8635 .- 1554-8627. ; 8:4, s. 445-544
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Forskningsöversikt (refereegranskat)abstract
- In 2008 we published the first set of guidelines for standardizing research in autophagy. Since then, research on this topic has continued to accelerate, and many new scientists have entered the field. Our knowledge base and relevant new technologies have also been expanding. Accordingly, it is important to update these guidelines for monitoring autophagy in different organisms. Various reviews have described the range of assays that have been used for this purpose. Nevertheless, there continues to be confusion regarding acceptable methods to measure autophagy, especially in multicellular eukaryotes. A key point that needs to be emphasized is that there is a difference between measurements that monitor the numbers or volume of autophagic elements (e.g., autophagosomes or autolysosomes) at any stage of the autophagic process vs. those that measure flux through the autophagy pathway (i.e., the complete process); thus, a block in macroautophagy that results in autophagosome accumulation needs to be differentiated from stimuli that result in increased autophagic activity, defined as increased autophagy induction coupled with increased delivery to, and degradation within, lysosomes (in most higher eukaryotes and some protists such as Dictyostelium) or the vacuole (in plants and fungi). In other words, it is especially important that investigators new to the field understand that the appearance of more autophagosomes does not necessarily equate with more autophagy. In fact, in many cases, autophagosomes accumulate because of a block in trafficking to lysosomes without a concomitant change in autophagosome biogenesis, whereas an increase in autolysosomes may reflect a reduction in degradative activity. Here, we present a set of guidelines for the selection and interpretation of methods for use by investigators who aim to examine macroautophagy and related processes, as well as for reviewers who need to provide realistic and reasonable critiques of papers that are focused on these processes. These guidelines are not meant to be a formulaic set of rules, because the appropriate assays depend in part on the question being asked and the system being used. In addition, we emphasize that no individual assay is guaranteed to be the most appropriate one in every situation, and we strongly recommend the use of multiple assays to monitor autophagy. In these guidelines, we consider these various methods of assessing autophagy and what information can, or cannot, be obtained from them. Finally, by discussing the merits and limits of particular autophagy assays, we hope to encourage technical innovation in the field.
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- van Haarlem, M. P., et al.
(författare)
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LOFAR : The LOw-Frequency ARray
- 2013
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 556, s. 1-53
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Tidskriftsartikel (refereegranskat)abstract
- LOFAR, the LOw-Frequency ARray, is a new-generation radio interferometer constructed in the north of the Netherlands and across europe. Utilizing a novel phased-array design, LOFAR covers the largely unexplored low-frequency range from 10–240 MHz and provides a number of unique observing capabilities. Spreading out from a core located near the village of Exloo in the northeast of the Netherlands, a total of 40 LOFAR stations are nearing completion. A further five stations have been deployed throughout Germany, and one station has been built in each of France, Sweden, and the UK. Digital beam-forming techniques make the LOFAR system agile and allow for rapid repointing of the telescope as well as the potential for multiple simultaneous observations. With its dense core array and long interferometric baselines, LOFAR achieves unparalleled sensitivity and angular resolution in the low-frequency radio regime. The LOFAR facilities are jointly operated by the International LOFAR Telescope (ILT) foundation, as an observatory open to the global astronomical community. LOFAR is one of the first radio observatories to feature automated processing pipelines to deliver fully calibrated science products to its user community. LOFAR’s new capabilities, techniques and modus operandi make it an important pathfinder for the Square Kilometre Array (SKA). We give an overview of the LOFAR instrument, its major hardware and software components, and the core science objectives that have driven its design. In addition, we present a selection of new results from the commissioning phase of this new radio observatory.
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- Heald, G. H., et al.
(författare)
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The LOFAR Multifrequency Snapshot Sky Survey (MSSS) : I. Survey description and first results
- 2015
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 582, s. 1-22
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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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- Jelic, V., et al.
(författare)
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Initial LOFAR observations of epoch of reionization windows II. Diffuse polarized emission in the ELAIS-N1 field
- 2014
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 568, s. A101-
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Tidskriftsartikel (refereegranskat)abstract
- Aims. This study aims to characterise the polarized foreground emission in the ELAIS-N1 field and to address its possible implications or extracting of the cosmological 21 cm signal from the LOw-Frequency ARray - Epoch of Reionization (LOFAR-EoR) data Methods. We used the high band antennas of LOFAR to image this region and RM-synthesis to unravel structures of polarized emission at high Galactic latitudes. Results. The brightness temperature of the detected Galactic emission is on average similar to 4 K in polarized intensity and covers the range from -10 to +13 rad m(-2) in Faraday depth, The total polarized intensity and polarization angle show a wide range of morphological features. We have also used the Westerbork Synthesis Radio Telescope (WSRT) at 350 MHz to image the same region. The LOFAR and WSRT images show a similar complex morphology at comparable brightness levels, but their spatial correlation is very low. The fractional polarization at 150 MHz, expressed as a percentage of the total intensity, amounts to approximate to 1.5%. There is no indication of diffuse emission in total intensity in the interferometric data. in line with results at higher frequencies Conclusions. The wide frequency range. high angular resolution, and high sensitivity make LOFAR an exquisite instrument for studying Galactic polarized emission at a resolution of similar to 1-2 rad m(-2) in Faraday depth. The different polarized patterns observed at 150 MHz and 350 MHz are consistent with different source distributions along the line of sight wring in a variety of Faraday thin regions of emission. The presence of polarized foregrounds is a serious complication for epoch of reionization experiments. To avoid the leakage of polarized emission into total intensity, which can depend on frequency, we need to calibrate the instrumental polarization across the field of view to a small fraction of 1%.
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| 7. |
- Pilia, M., et al.
(författare)
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Wide-band, low-frequency pulse profiles of 100 radio pulsars with LOFAR
- 2016
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 586
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Tidskriftsartikel (refereegranskat)abstract
- Context. LOFAR offers the unique capability of observing pulsars across the 10−240 MHz frequency range with a fractional bandwidth of roughly 50%. This spectral range is well suited for studying the frequency evolution of pulse profile morphology caused by both intrinsic and extrinsic effects such as changing emission altitude in the pulsar magnetosphere or scatter broadening by the interstellar medium, respectively.Aims. The magnitude of most of these effects increases rapidly towards low frequencies. LOFAR can thus address a number of open questions about the nature of radio pulsar emission and its propagation through the interstellar medium.Methods. We present the average pulse profiles of 100 pulsars observed in the two LOFAR frequency bands: high band (120–167 MHz, 100 profiles) and low band (15–62 MHz, 26 profiles). We compare them with Westerbork Synthesis Radio Telescope (WSRT) and Lovell Telescope observations at higher frequencies (350 and 1400 MHz) to study the profile evolution. The profiles were aligned in absolute phase by folding with a new set of timing solutions from the Lovell Telescope, which we present along with precise dispersion measures obtained with LOFAR.Results. We find that the profile evolution with decreasing radio frequency does not follow a specific trend; depending on the geometry of the pulsar, new components can enter into or be hidden from view. Nonetheless, in general our observations confirm the widening of pulsar profiles at low frequencies, as expected from radius-to-frequency mapping or birefringence theories.
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| 8. |
- Stewart, A. J., et al.
(författare)
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LOFAR MSSS : detection of a low-frequency radio transient in 400 h of monitoring of the North Celestial Pole
- 2016
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 456:3, s. 2321-2342
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Tidskriftsartikel (refereegranskat)abstract
- We present the results of a four-month campaign searching for low-frequency radio transients near the North Celestial Pole with the Low-Frequency Array (LOFAR), as part of the Multifrequency Snapshot Sky Survey (MSSS). The data were recorded between 2011 December and 2012 April and comprised 2149 11-min snapshots, each covering 175 deg2. We have found one convincing candidate astrophysical transient, with a duration of a few minutes and a flux density at 60 MHz of 15–25 Jy. The transient does not repeat and has no obvious optical or high-energy counterpart, as a result of which its nature is unclear. The detection of this event implies a transient rate at 60 MHz of 3.9−3.7+14.7×10−4" style="position: relative;" tabindex="0" id="MathJax-Element-1-Frame" class="MathJax">3.9+14.7−3.7×10−4 d−1 deg−2, and a transient surface density of 1.5 × 10−5 deg−2, at a 7.9-Jy limiting flux density and ∼10-min time-scale. The campaign data were also searched for transients at a range of other time-scales, from 0.5 to 297 min, which allowed us to place a range of limits on transient rates at 60 MHz as a function of observation duration.
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| 9. |
- Sobey, C., et al.
(författare)
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LOFAR discovery of a quiet emission mode in PSR B0823+26
- 2015
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 451, s. 2493-2506
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Tidskriftsartikel (refereegranskat)abstract
- PSR B0823+26, a 0.53-s radio pulsar, displays a host of emission phenomena over time-scales of seconds to (at least) hours, including nulling, subpulse drifting, and mode-changing. Studying pulsars like PSR B0823+26 provides further insight into the relationship between these various emission phenomena and what they might teach us about pulsar magnetospheres. Here we report on the LOFAR (Low-Frequency Array) discovery that PSR B0823+26 has a weak and sporadically emitting ‘quiet’ (Q) emission mode that is over 100 times weaker (on average) and has a nulling fraction forty-times greater than that of the more regularly-emitting ‘bright’ (B) mode. Previously, the pulsar has been undetected in the Q mode, and was assumed to be nulling continuously. PSR B0823+26 shows a further decrease in average flux just before the transition into the B mode, and perhaps truly turns off completely at these times. Furthermore, simultaneous observations taken with the LOFAR, Westerbork, Lovell, and Effelsberg telescopes between 110 MHz and 2.7 GHz demonstrate that the transition between the Q mode and B mode occurs within one single rotation of the neutron star, and that it is concurrent across the range of frequencies observed.
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| 10. |
- Coenen, T., et al.
(författare)
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The LOFAR pilot surveys for pulsars and fast radio transients
- 2014
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 570, s. 1-16
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Tidskriftsartikel (refereegranskat)abstract
- We have conducted two pilot surveys for radio pulsars and fast transients with the Low-Frequency Array (LOFAR) around 140 MHz and here report on the first low-frequency fast-radio burst limit and the discovery of two new pulsars. The first survey, the LOFAR Pilot Pulsar Survey (LPPS), observed a large fraction of the northern sky, ~ 1.4 × 104 deg2, with 1 h dwell times. Each observation covered ~75 deg2 using 7 independent fields formed by incoherently summing the high-band antenna fields. The second pilot survey, the LOFAR Tied-Array Survey (LOTAS), spanned ~600 deg2, with roughly a 5-fold increase in sensitivity compared with LPPS. Using a coherent sum of the 6 LOFAR “Superterp” stations, we formed 19 tied-array beams, together covering 4 deg2 per pointing. From LPPS we derive a limit on the occurrence, at 142 MHz, of dispersed radio bursts of < 150 day-1 sky-1, for bursts brighter than S> 107 Jy for the narrowest searched burst duration of 0.66 ms. In LPPS, we re-detected 65 previously known pulsars. LOTAS discovered two pulsars, the first with LOFAR or any digital aperture array. LOTAS also re-detected 27 previously known pulsars. These pilot studies show that LOFAR can efficiently carry out all-sky surveys for pulsars and fast transients, and they set the stage for further surveying efforts using LOFAR and the planned low-frequency component of the Square Kilometer Array.
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