| 1. |
- Acharya, B. S., et al.
(författare)
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Introducing the CTA concept
- 2013
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Ingår i: Astroparticle physics. - : Elsevier BV. - 0927-6505 .- 1873-2852. ; 43, s. 3-18
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- The Cherenkov Telescope Array (CTA) is a new observatory for very high-energy (VHE) gamma rays. CTA has ambitions science goals, for which it is necessary to achieve full-sky coverage, to improve the sensitivity by about an order of magnitude, to span about four decades of energy, from a few tens of GeV to above 100 TeV with enhanced angular and energy resolutions over existing VHE gamma-ray observatories. An international collaboration has formed with more than 1000 members from 27 countries in Europe, Asia, Africa and North and South America. In 2010 the CTA Consortium completed a Design Study and started a three-year Preparatory Phase which leads to production readiness of CTA in 2014. In this paper we introduce the science goals and the concept of CTA, and provide an overview of the project. (C) 2013 Elsevier B.V. All rights reserved.
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| 2. |
- Abdalla, H., et al.
(författare)
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Sensitivity of the Cherenkov Telescope Array for probing cosmology and fundamental physics with gamma-ray propagation
- 2021
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Ingår i: Journal of Cosmology and Astroparticle Physics. - : Institute of Physics Publishing (IOPP). - 1475-7516. ; :2
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Tidskriftsartikel (refereegranskat)abstract
- The Cherenkov Telescope Array (CTA), the new-generation ground-based observatory for gamma-ray astronomy, provides unique capabilities to address significant open questions in astrophysics, cosmology, and fundamental physics. We study some of the salient areas of gamma-ray cosmology that can be explored as part of the Key Science Projects of CTA, through simulated observations of active galactic nuclei (AGN) and of their relativistic jets. Observations of AGN with CTA will enable a measurement of gamma-ray absorption on the extragalactic background light with a statistical uncertainty below 15% up to a redshift z = 2 and to constrain or detect gamma-ray halos up to intergalactic-magnetic-field strengths of at least 0.3 pG. Extragalactic observations with CTA also show promising potential to probe physics beyond the Standard Model. The best limits on Lorentz invariance violation from gamma-ray astronomy will be improved by a factor of at least two to three. CTA will also probe the parameter space in which axion-like particles could constitute a significant fraction, if not all, of dark matter. We conclude on the synergies between CTA and other upcoming facilities that will foster the growth of gamma-ray cosmology.
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| 3. |
- Abramowski, A., et al.
(författare)
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The 2010 very high energy gamma-RAY flare and 10 years of multi-wavelength observations of M 87
- 2012
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 746:2, s. 151-
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Tidskriftsartikel (refereegranskat)abstract
- The giant radio galaxy M 87 with its proximity (16 Mpc), famous jet, and very massive black hole ((3-6) x 10(9) M-circle dot) provides a unique opportunity to investigate the origin of very high energy (VHE; E > 100 GeV) gamma-ray emission generated in relativistic outflows and the surroundings of supermassive black holes. M 87 has been established as a VHE gamma-ray emitter since 2006. The VHE gamma-ray emission displays strong variability on timescales as short as a day. In this paper, results from a joint VHE monitoring campaign on M 87 by the MAGIC and VERITAS instruments in 2010 are reported. During the campaign, a flare at VHE was detected triggering further observations at VHE (H.E.S.S.), X-rays (Chandra), and radio (43 GHz Very Long Baseline Array, VLBA). The excellent sampling of the VHE gamma-ray light curve enables one to derive a precise temporal characterization of the flare: the single, isolated flare is well described by a two-sided exponential function with significantly different flux rise and decay times of tau(rise)(d) = (1.69 +/- 0.30) days and tau(decay)(d) = (0.611 +/- 0.080) days, respectively. While the overall variability pattern of the 2010 flare appears somewhat different from that of previous VHE flares in 2005 and 2008, they share very similar timescales (similar to day), peak fluxes (Phi(>0.35 TeV) similar or equal to (1-3) x 10(-11) photons cm(-2) s(-1)), and VHE spectra. VLBA radio observations of 43 GHz of the inner jet regions indicate no enhanced flux in 2010 in contrast to observations in 2008, where an increase of the radio flux of the innermost core regions coincided with a VHE flare. On the other hand, Chandra X-ray observations taken similar to 3 days after the peak of the VHE gamma-ray emission reveal an enhanced flux from the core (flux increased by factor similar to 2; variability timescale <2 days). The long-term (2001-2010) multi-wavelength (MWL) light curve of M 87, spanning from radio to VHE and including data from Hubble Space Telescope, Liverpool Telescope, Very Large Array, and European VLBI Network, is used to further investigate the origin of the VHE gamma-ray emission. No unique, common MWL signature of the three VHE flares has been identified. In the outer kiloparsec jet region, in particular in HST-1, no enhanced MWL activity was detected in 2008 and 2010, disfavoring it as the origin of the VHE flares during these years. Shortly after two of the three flares (2008 and 2010), the X-ray core was observed to be at a higher flux level than its characteristic range (determined from more than 60 monitoring observations: 2002-2009). In 2005, the strong flux dominance of HST-1 could have suppressed the detection of such a feature. Published models for VHE gamma-ray emission from M 87 are reviewed in the light of the new data.
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| 4. |
- Acero, F., et al.
(författare)
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Prospects for Cherenkov Telescope Array Observations of the Young Supernova Remnant RX J1713.7-3946
- 2017
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Ingår i: Astrophysical Journal. - : Institute of Physics Publishing (IOPP). - 0004-637X .- 1538-4357. ; 840:2
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Tidskriftsartikel (refereegranskat)abstract
- We perform simulations for future Cherenkov Telescope Array (CTA) observations of RX J1713.7-3946, a young supernova remnant (SNR) and one of the brightest sources ever discovered in very high energy (VHE) gamma rays. Special attention is paid to exploring possible spatial (anti) correlations of gamma rays with emission at other wavelengths, in particular X-rays and CO/H I emission. We present a series of simulated images of RX J1713.7-3946 for CTA based on a set of observationally motivated models for the gamma-ray emission. In these models, VHE gamma rays produced by high-energy electrons are assumed to trace the nonthermal X-ray emission observed by XMM-Newton, whereas those originating from relativistic protons delineate the local gas distributions. The local atomic and molecular gas distributions are deduced by the NANTEN team from CO and H I observations. Our primary goal is to show how one can distinguish the emission mechanism(s) of the gamma rays (i.e., hadronic versus leptonic, or a mixture of the two) through information provided by their spatial distribution, spectra, and time variation. This work is the first attempt to quantitatively evaluate the capabilities of CTA to achieve various proposed scientific goals by observing this important cosmic particle accelerator.
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| 5. |
- Aliu, E., et al.
(författare)
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Long-term TeV and X-ray Observations of the Gamma-ray Binary HESS J0632+057
- 2014
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 780:2
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Tidskriftsartikel (refereegranskat)abstract
- HESS J0632+057 is the only gamma-ray binary known so far whose position in the sky allows observations with ground-based observatories in both thenorthern and southern hemispheres. Here we report on long-term observations of HESS J0632+057 conducted with the Very Energetic Radiation Imaging Telescope Array System and High Energy Stereoscopic System Cherenkov telescopes and the X-ray satellite Swift, spanning a time range from 2004 to 2012 and covering most of the system's orbit. The very-high-energy (VHE) emission is found to be variable and is correlated with that at X-ray energies. An orbital period of 315(-4)(+6) days is derived from the X-ray data set, which is compatible with previous results, P = (321 +/- 5) days. The VHE light curve shows a distinct maximum at orbital phases close to 0.3, or about 100 days after periastron passage, which coincides with the periodic enhancement of the X-rayemission. Furthermore, the analysis of the TeV data shows for the first time a statistically significant (> 6.5 sigma) detection at orbital phases 0.6-0.9. Theobtained gamma-ray and X-ray light curves and the correlation of the source emission at these two energy bands are discussed in the context of the recent ephemeris obtained for the system. Our results are compared to those reported for other gamma-ray binaries.
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| 6. |
- Acharyya, A., et al.
(författare)
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Monte Carlo studies for the optimisation of the Cherenkov Telescope Array layout
- 2019
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Ingår i: Astroparticle physics. - : Elsevier. - 0927-6505 .- 1873-2852. ; 111, s. 35-53
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Tidskriftsartikel (refereegranskat)abstract
- The Cherenkov Telescope Array (CTA) is the major next-generation observatory for ground-based veryhigh-energy gamma-ray astronomy. It will improve the sensitivity of current ground-based instruments by a factor of five to twenty, depending on the energy, greatly improving both their angular and energy resolutions over four decades in energy (from 20 GeV to 300 TeV). This achievement will be possible by using tens of imaging Cherenkov telescopes of three successive sizes. They will be arranged into two arrays, one per hemisphere, located on the La Palma island (Spain) and in Paranal (Chile). We present here the optimised and final telescope arrays for both CTA sites, as well as their foreseen performance, resulting from the analysis of three different large-scale Monte Carlo productions.
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| 7. |
- Abramowski, A., et al.
(författare)
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Discovery of VHE gamma-ray emission and multi-wavelength observations of the BL Lacertae object 1RXSJ101015.9-311909
- 2012
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 542, s. A94-
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Tidskriftsartikel (refereegranskat)abstract
- 1RXS J101015.9-311909 is a galaxy located at a redshift of z = 0.14 hosting an active nucleus (called AGN) belonging to the class of bright BL Lac objects. Observations at high (HE, E > 100 MeV) and very high (VHE, E > 100 GeV) energies provide insights into the origin of very energetic particles present in such sources and the radiation processes at work. We report on results from VHE observations performed between 2006 and 2010 with the H. E. S. S. instrument, an array of four imaging atmospheric Cherenkov telescopes. H. E. S. S. data have been analysed with enhanced analysis methods, making the detection of faint sources more significant. VHE emission at a position coincident with 1RXS J101015.9-311909 is detected with H. E. S. S. for the first time. In a total good-quality livetime of about 49 h, we measure 263 excess counts, corresponding to a significance of 7.1 standard deviations. The photon spectrum above 0.2 TeV can be described by a power-law with a photon index of Gamma = 3.08 +/- 0.42(stat) +/- 0.20(sys). The integral flux above 0.2 TeV is about 0.8% of the flux of the Crab nebula and shows no significant variability over the time reported. In addition, public Fermi/LAT data are analysed to search for high energy emission from the source. The Fermi/LAT HE emission in the 100 MeV to 200 GeV energy range is significant at 8.3 standard deviations in the chosen 25-month dataset. UV and X-ray contemporaneous observations with the Swift satellite in May 2007 are also reported, together with optical observations performed with the atom telescope located at the H. E. S. S. site. Swift observations reveal an absorbed X-ray flux of F(0.3-7) keV = 1.04(-0.05)(+0.04) x 10(-11) erg cm(-2) s(-1) in the 0.3-7 keV range. Finally, all the available data are used to study the multi-wavelength properties of the source. The spectral energy distribution (SED) can be reproduced using a simple one-zone Synchrotron Self Compton (SSC) model with emission from a region with a Doppler factor of 30 and a magnetic field between 0.025 and 0.16 G. These parameters are similar to those obtained for other sources of this type.
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| 8. |
- Abramowski, A., et al.
(författare)
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Probing the gamma-ray emission from HESS J1834-087 using HESS and Fermi LAT observations
- 2015
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 574
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Tidskriftsartikel (refereegranskat)abstract
- Aims. Previous observations with the High Energy Stereoscopic System (H.E.S.S.) have revealed an extended very-high-energy (VHE; E > 100 GeV) gamma-ray source, HESS J1834-087, coincident with the supernova remnant (SNR) W41. The origin of the gamma-ray emission was investigated in more detail with the H.E.S.S. array and the Large Area Telescope (LAT) onboard the Fermi Gamma-ray Space Telescope. Methods. The gamma-ray data provided by 61 h of observations with H.E.S.S., and four years with the Fermi LAT were analyzed, covering over five decades in energy from 1.8 GeV up to 30 TeV. The morphology and spectrum of the TeV and GeV sources were studied and multiwavelength data were used to investigate the origin of the gamma-ray emission toward W41. Results. The TeV source can be modeled with a sum of two components: one point-like and one significantly extended (sigma(TeV) = 0.17 degrees +/- 0.01 degrees), both centered on SNR W41 and exhibiting spectra described by a power law with index Gamma(TeV) similar or equal to 2.6. The GeV source detected with Fermi LAT is extended (sigma(GeV) = 0.15 degrees +/- 0.03 degrees) and morphologically matches the VHE emission. Its spectrum can be described by a power-law model with an index Gamma(GeV) = 2.15 +/- 0.12 and smoothly joins the spectrum of the whole TeV source. A break appears in the gamma-ray spectra around 100 GeV. No pulsations were found in the GeV range. Conclusions. Two main scenarios are proposed to explain the observed emission: a pulsar wind nebula (PWN) or the interaction of SNR W41 with an associated molecular cloud. X-ray observations suggest the presence of a point-like source (a pulsar candidate) near the center of the remnant and nonthermal X-ray diffuse emission that could arise from the possibly associated PWN. The PWN scenario is supported by the compatible positions of the TeV and GeV sources with the putative pulsar. However, the spectral energy distribution from radio to gamma-rays is reproduced by a one-zone leptonic model only if an excess of low-energy electrons is injected following a Maxwellian distribution by a pulsar with a high spin-down power (> 10(37) erg s(-1)). This additional low-energy component is not needed if we consider that the point-like TeV source is unrelated to the extended GeV and TeV sources. The interacting SNR scenario is supported by the spatial coincidence between the gamma-ray sources, the detection of OH (1720 MHz) maser lines, and the hadronic modeling.
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| 9. |
- Abramowski, A., et al.
(författare)
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Discovery of TeV gamma-ray emission from PKS 0447-439 and derivation of an upper limit on its redshift
- 2013
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 552, s. A118-
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Tidskriftsartikel (refereegranskat)abstract
- Very high-energy gamma-ray emission from PKS 0447-439 was detected with the H. E. S. S. Cherenkov telescope array in December 2009. This blazar is one of the brightest extragalactic objects in the Fermi bright source list and has a hard spectrum in the MeV to GeV range. In the TeV range, a photon index of 3.89 +/- 0.37 (stat) +/- 0.22 (sys) and a flux normalisation at 1 TeV, phi(1) (TeV) = (3.5 +/- 1.1(stat) +/- 0.9(sys)) x 10(-13) cm(-2) s(-1) TeV-1 were found. The detection with H. E. S. S. triggered observations in the X-ray band with the Swift and RXTE telescopes. Simultaneous UV and optical data from Swift UVOT and data from the optical telescopes ATOM and ROTSE are also available. The spectrum and light curve measured with H. E. S. S. are presented and compared to the multi-wavelength data at lower energies. A rapid flare is seen in the Swift XRT and RXTE data, together with a flux variation in the UV band, at a time scale of the order of one day. A firm upper limit of z < 0.59 on the redshift of PKS 0447-439 is derived from the combined Fermi-LAT and H. E. S. S. data, given the assumptions that there is no upturn in the intrinsic spectrum above the Fermi-LAT energy range and that absorption on the extragalactic background light (EBL) is not weaker than the lower limit provided by current models. The spectral energy distribution is well described by a simple one-zone synchrotron self-Compton scenario, if the redshift of the source is less than z less than or similar to 0.4.
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| 10. |
- Abramowski, A., et al.
(författare)
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Discovery of the Hard Spectrum VHE γ-Ray Source HESS J1641–463
- 2014
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Ingår i: Astrophysical Journal Letters. - 2041-8205 .- 2041-8213. ; 794:1, s. Article ID: L1-
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Tidskriftsartikel (refereegranskat)abstract
- This Letter reports the discovery of a remarkably hard spectrum source, HESS J1641-463, by the High Energy Stereoscopic System (H.E.S.S.) in the very high energy (VHE) domain. HESS J1641-463 remained unnoticed by the usual analysis techniques due to confusion with the bright nearby source HESS J1640-465. It emerged at a significance level of 8.5 standard deviations after restricting the analysis to events with energies above 4 TeV. It shows a moderate flux level of phi(E > 1TeV) = (3.64 +/- 0.44(stat)+/- 0.73(sys)) x 10(-13) cm(-2) s(-1), corresponding to 1.8% of the Crab Nebula flux above the same energy, and a hard spectrum with a photon index of Gamma = 2.07 +/- 0.11(stat)+/- 0.20(sys). It is a point-like source, although an extension up to a Gaussian width of sigma = 3 arcmin cannot be discounted due to uncertainties in the H.E.S.S. point-spread function. The VHE gamma-ray flux of HESS J1641-463 is found to be constant over the observed period when checking time binnings from the year-by-year to the 28 minute exposure timescales. HESS J1641-463 is positionally coincident with the radio supernova remnant SNR G338.5+0.1. No X-ray candidate stands out as a clear association; however, Chandra and XMM-Newton data reveal some potential weak counterparts. Various VHE gamma-ray production scenarios are discussed. If the emission from HESS J1641-463 is produced by cosmic ray protons colliding with the ambient gas, then their spectrum must extend close to 1 PeV. This object may represent a source population contributing significantly to the galactic cosmic ray flux around the knee.
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| 11. |
- Abramowski, A., et al.
(författare)
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HESS and Fermi-LAT discovery of gamma-rays from the blazar 1ES 1312-423
- 2013
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 434:3, s. 1889-1901
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Tidskriftsartikel (refereegranskat)abstract
- A deep observation campaign carried out by the High Energy Stereoscopic System (HESS) on Centaurus A enabled the discovery of gamma-rays from the blazar 1ES 1312-423, 2 degrees away from the radio galaxy. With a differential flux at 1 TeV of phi(1 TeV) = (1.9 +/- 0.6(stat) +/- 0.4(sys)) x 10(-13) cm(-2) s(-1) TeV-1 corresponding to 0.5 per cent of the Crab nebula differential flux and a spectral index Gamma = 2.9 +/- 0.5(stat) +/- 0.2(sys), 1ES 1312-423 is one of the faintest sources ever detected in the very high energy (E > 100 GeV) extragalactic sky. A careful analysis using three and a half years of Fermi Large Area Telescope (Fermi-LAT) data allows the discovery at high energies (E > 100 MeV) of a hard spectrum (Gamma = 1.4 +/- 0.4(stat) +/- 0.2(sys)) source coincident with 1ES 1312-423. Radio, optical, UV and X-ray observations complete the spectral energy distribution of this blazar, now covering 16 decades in energy. The emission is successfully fitted with a synchrotron self-Compton model for the non-thermal component, combined with a blackbody spectrum for the optical emission from the host galaxy.
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| 12. |
- Abramowski, A., et al.
(författare)
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HESS discovery of VHE gamma-rays from the quasar PKS 1510-089
- 2013
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 554, s. A107-
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Tidskriftsartikel (refereegranskat)abstract
- The quasar PKS 1510-089 (z = 0.361) was observed with the H.E.S.S. array of imaging atmospheric Cherenkov telescopes during high states in the optical and GeV bands, to search for very high energy (VHE, defined as E >= 0.1 TeV) emission. VHE gamma-rays were detected with a statistical significance of 9.2 standard deviations in 15.8 h of H. E. S. S. data taken during March and April 2009. A VHE integral flux of I(0.15 TeV < E < 1.0TeV) = (1.0 +/- 0.2(stat) +/- 0.2(sys)) x 10(-11) cm(-2) s(-1) is measured. The best-fit power law to the VHE data has a photon index of G = 5.4 +/- 0.7(stat) +/- 0.3(sys). The GeV and optical light curves show pronounced variability during the period of H.E.S.S. observations. However, there is insufficient evidence to claim statistically significant variability in the VHE data. Because of its relatively high redshift, the VHE flux from PKS 1510-089 should suffer considerable attenuation in the intergalactic space due to the extragalactic background light (EBL). Hence, the measured gamma-ray spectrum is used to derive upper limits on the opacity due to EBL, which are found to be comparable with the previously derived limits from relatively-nearby BL Lac objects. Unlike typical VHE-detected blazars where the broadband spectrum is dominated by nonthermal radiation at all wavelengths, the quasar PKS 1510-089 has a bright thermal component in the optical to UV frequency band. Among all VHE detected blazars, PKS 1510-089 has the most luminous broad line region. The detection of VHE emission from this quasar indicates a low level of gamma - gamma absorption on the internal optical to UV photon field.
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| 13. |
- Abramowski, A., et al.
(författare)
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HESS observations of the binary system PSR B1259-63/LS 2883 around the 2010/2011 periastron passage
- 2013
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 551, s. A94-
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Tidskriftsartikel (refereegranskat)abstract
- Aims. We present very high energy (VHE; E > 100 GeV) data from the gamma-ray binary system PSR B1259-63/LS 2883 taken around its periastron passage on 15th of December 2010 with the High Energy Stereoscopic System (H. E. S. S.) of Cherenkov Telescopes. We aim to search for a possible TeV counterpart of the GeV flare detected by the Fermi LAT. In addition, we aim to study the current periastron passage in the context of previous observations taken at similar orbital phases, testing the repetitive behaviour of the source. Methods. Observations at VHEs were conducted with H.E.S.S. from 9th to 16th of January 2011. The total dataset amounts to similar to 6 h of observing time. The data taken around the 2004 periastron passage were also re-analysed with the current analysis techniques in order to extend the energy spectrum above 3 TeV to fully compare observation results from 2004 and 2011. Results. The source is detected in the 2011 data at a significance level of 11.5 sigma revealing an averaged integral flux above 1 TeV of (1.01 +/- 0.18(stat) +/- 0.20(sys)) x 10(-12) cm(-2) s(-1). The differential energy spectrum follows a power-law shape with a spectral index Gamma = 2.92 +/- 0.30(stat) +/- 0.20(sys) and a flux normalisation at 1 TeV of N-0 = (1.95 +/- 0.32(stat) +/- 0.39(sys)) x 10(-12) TeV-1 cm(-2) s(-1). The measured light curve does not show any evidence for variability of the source on the daily scale. The re-analysis of the 2004 data yields results compatible with the published ones. The differential energy spectrum measured up to similar to 10 TeV is consistent with a power law with a spectral index Gamma = 2.81 +/- 0.10(stat) +/- 0.20(sys) and a flux normalisation at 1 TeV of N-0 = (1.29 +/- 0.08(stat) +/- 0.26(sys)) x 10(-12) TeV-1 cm(-2) s(-1). Conclusions. The measured integral flux and the spectral shape of the 2011 data are compatible with the results obtained around previous periastron passages. The absence of variability in the H.E.S.S. data indicates that the GeV flare observed by Fermi LAT in the time period covered also by H.E.S.S. observations originates in a different physical scenario than the TeV emission. Moreover, the comparison of the new results to the results from the 2004 observations made at a similar orbital phase provides a stronger evidence of the repetitive behaviour of the source.
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| 14. |
- Abramowski, A., et al.
(författare)
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Search for Photon-Linelike Signatures from Dark Matter Annihilations with H.E.S.S.
- 2013
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 110:4, s. 041301-
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Tidskriftsartikel (refereegranskat)abstract
- Gamma-ray line signatures can be expected in the very-high-energy (E-gamma > 100 GeV) domain due to self-annihilation or decay of dark matter (DM) particles in space. Such a signal would be readily distinguishable from astrophysical gamma-ray sources that in most cases produce continuous spectra that span over several orders of magnitude in energy. Using data collected with the H. E. S. S. gamma-ray instrument, upper limits on linelike emission are obtained in the energy range between similar to 500 GeV and similar to 25 TeV for the central part of the Milky Way halo and for extragalactic observations, complementing recent limits obtained with the Fermi-LAT instrument at lower energies. No statistically significant signal could be found. For monochromatic gamma-ray line emission, flux limits of (2 x 10(-7)-2 x 10(-5)) m(-2)s(-1)sr(-1) and (1 x 10(-8)- 2 x 10(-6)) m(-2)s(-1)sr(-1) are obtained for the central part of the Milky Way halo and extragalactic observations, respectively. For a DM particle mass of 1 TeV, limits on the velocity- averaged DM annihilation cross section (chi chi ->gamma gamma) reach similar to 10(-27)cm(3)s(-1), based on the Einasto parametrization of the Galactic DM halo density profile. DOI: 10.1103/PhysRevLett.110.041301
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| 15. |
- Abramowski, A., et al.
(författare)
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Search for very-high-energy gamma-ray emission from Galactic globular clusters with HESS
- 2013
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 551, s. A26-
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Tidskriftsartikel (refereegranskat)abstract
- Context. Globular clusters (GCs) are established emitters of high-energy (HE, 100 MeV < E < 100 GeV) gamma-ray radiation which could originate from the cumulative emission of the numerous millisecond pulsars (msPSRs) in the clusters' cores or from inverse Compton (IC) scattering of relativistic leptons accelerated in the GC environment. These stellar clusters could also constitute a new class of sources in the very-high-energy (VHE, E > 100 GeV) gamma-ray regime, judging from the recent detection of a signal from the direction of Terzan 5 with the H.E.S.S. telescope array. Aims. To search for VHE gamma-ray sources associated with other GCs, and to put constraints on leptonic emission models, we systematically analyzed the observations towards 15 GCs taken with the H. E. S. S. array of imaging atmospheric Cherenkov telescopes. Methods. We searched for point-like and extended VHE gamma-ray emission from each GC in our sample and also performed a stacking analysis combining the data from all GCs to investigate the hypothesis of a population of faint emitters. Assuming IC emission as the origin of the VHE gamma-ray signal from the direction of Terzan 5, we calculated the expected gamma-ray flux from each of the 15 GCs, based on their number of millisecond pulsars, their optical brightness and the energy density of background photon fields. Results. We did not detect significant VHE gamma-ray emission from any of the 15 GCs in either of the two analyses. Given the uncertainties related to the parameter determinations, the obtained flux upper limits allow to rule out the simple IC/msPSR scaling model for NGC6388 and NGC7078. The upper limits derived from the stacking analyses are factors between 2 and 50 below the flux predicted by the simple leptonic scaling model, depending on the assumed source extent and the dominant target photon fields. Therefore, Terzan 5 still remains exceptional among all GCs, as the VHE gamma-ray emission either arises from extra-ordinarily efficient leptonic processes, or from a recent catastrophic event, or is even unrelated to the GC itself.
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| 16. |
- Abramowski, A., et al.
(författare)
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Constraints on the gamma-ray emission from the cluster-scale AGN outburst in the Hydra A galaxy cluster
- 2012
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 545, s. A103-
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Tidskriftsartikel (refereegranskat)abstract
- Context. In some galaxy clusters, powerful active galactic nuclei (AGN) have blown bubbles with cluster scale extent into the ambient medium. The main pressure support of these bubbles is not known to date, but cosmic rays are a viable possibility. For such a scenario copious gamma-ray emission is expected as a tracer of cosmic rays from these systems. Aims. Hydra A, the closest galaxy cluster hosting a cluster scale AGN outburst, located at a redshift of 0.0538, is investigated for being a gamma-ray emitter with the High Energy Stereoscopic System (H.E.S.S.) array and the Fermi Large Area Telescope (Fermi-LAT). Methods. Data obtained in 20.2 h of dedicated H. E. S. S. observations and 38 months of Fermi-LAT data, gathered by its usual all-sky scanning mode, have been analyzed to search for a gamma-ray signal. Results. No signal has been found in either data set. Upper limits on the gamma-ray flux are derived and are compared to models. These are the first limits on gamma-ray emission ever presented for galaxy clusters hosting cluster scale AGN outbursts. Conclusions. The non-detection of Hydra A in gamma-rays has important implications on the particle populations and physical conditions inside the bubbles in this system. For the case of bubbles mainly supported by hadronic cosmic rays, the most favorable scenario, which involves full mixing between cosmic rays and embedding medium, can be excluded. However, hadronic cosmic rays still remain a viable pressure support agent to sustain the bubbles against the thermal pressure of the ambient medium. The largest population of highly-energetic electrons, which are relevant for inverse-Compton gamma-ray production is found in the youngest inner lobes of Hydra A. The limit on the inverse-Compton gamma-ray flux excludes a magnetic field below half of the equipartition value of 16 mu G in the inner lobes.
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| 17. |
- Abramowski, A., et al.
(författare)
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Discovery of gamma-ray emission from the extragalactic pulsar wind nebula N 157B with HESS
- 2012
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 545, s. Article ID: L2-
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Tidskriftsartikel (refereegranskat)abstract
- We present the significant detection of the first extragalactic pulsar wind nebula (PWN) detected in gamma rays, N 157B, located in the large Magellanic Cloud (LMC). Pulsars with high spin-down luminosity are found to power energised nebulae that emit gamma rays up to energies of several tens of TeV. N 157B is associated with PSR J0537-6910, which is the pulsar with the highest known spin-down luminosity. The High Energy Stereoscopic System telescope array observed this nebula on a yearly basis from 2004 to 2009 with a dead-time corrected exposure of 46 h. The gamma-ray spectrum between 600 GeV and 12 TeV is well-described by a pure power-law with a photon index of 2.8 +/- 0.2(stat) +/- 0.3(syst) and a normalisation at 1 TeV of (8.2 +/- 0.8(stat) +/- 2.5(syst)) x 10(-13) cm(-2) s(-1) TeV-1. A leptonic multi-wavelength model shows that an energy of about 4 x 10(49) erg is stored in electrons and positrons. The apparent efficiency, which is the ratio of the TeV gamma-ray luminosity to the pulsar's spin-down luminosity, 0.08% +/- 0.01%, is comparable to those of PWNe found in the Milky Way. The detection of a PWN at such a large distance is possible due to the pulsar's favourable spin-down luminosity and a bright infrared photon-field serving as an inverse-Compton-scattering target for accelerated leptons. By applying a calorimetric technique to these observations, the pulsar's birth period is estimated to be shorter than 10 ms.
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| 18. |
- Abramowski, A., et al.
(författare)
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DISCOVERY OF THE HARD SPECTRUM VHE gamma-RAY SOURCE HESS J1641-463
- 2014
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Ingår i: Astrophysical Journal Letters. - 2041-8205 .- 2041-8213. ; 794:1, s. L1-
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Tidskriftsartikel (refereegranskat)abstract
- This Letter reports the discovery of a remarkably hard spectrum source, HESS J1641-463, by the High Energy Stereoscopic System (H.E.S.S.) in the very high energy (VHE) domain. HESS J1641-463 remained unnoticed by the usual analysis techniques due to confusion with the bright nearby source HESS J1640-465. It emerged at a significance level of 8.5 standard deviations after restricting the analysis to events with energies above 4 TeV. It shows a moderate flux level of phi(E > 1TeV) = (3.64 +/- 0.44(stat)+/- 0.73(sys)) x 10(-13) cm(-2) s(-1), corresponding to 1.8% of the Crab Nebula flux above the same energy, and a hard spectrum with a photon index of Gamma = 2.07 +/- 0.11(stat)+/- 0.20(sys). It is a point-like source, although an extension up to a Gaussian width of sigma = 3 arcmin cannot be discounted due to uncertainties in the H.E.S.S. point-spread function. The VHE gamma-ray flux of HESS J1641-463 is found to be constant over the observed period when checking time binnings from the year-by-year to the 28 minute exposure timescales. HESS J1641-463 is positionally coincident with the radio supernova remnant SNR G338.5+0.1. No X-ray candidate stands out as a clear association; however, Chandra and XMM-Newton data reveal some potential weak counterparts. Various VHE gamma-ray production scenarios are discussed. If the emission from HESS J1641-463 is produced by cosmic ray protons colliding with the ambient gas, then their spectrum must extend close to 1 PeV. This object may represent a source population contributing significantly to the galactic cosmic ray flux around the knee.
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| 19. |
- Abramowski, A., et al.
(författare)
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HESS J1640-465-an exceptionally luminous TeV gamma-ray supernova remnant
- 2014
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 439:3, s. 2828-2836
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Tidskriftsartikel (refereegranskat)abstract
- The results of follow-up observations of the TeV gamma-ray source HESS J1640-465 from 2004 to 2011 with the High Energy Stereoscopic System (HESS) are reported in this work. The spectrum is well described by an exponential cut-off power law with photon index Gamma = 2.11 +/- 0.09(stat) +/- 0.10(sys), and a cut-off energy of E-2 = 6.0(-1.2)(+2.0) TeV. The TeV emission is significantly extended and overlaps with the northwestern part of the shell of the SNR G338.3-0.0. The new HESS results, a re-analysis of archival XMM-Newton data and multiwavelength observations suggest that a significant part of the gamma-ray emission from HESS J1640-465 originates in the supernova remnant shell. In a hadronic scenario, as suggested by the smooth connection of the GeV and TeV spectra, the product of total proton energy and mean target density could be as high as W(p)n(H) similar to 4 x 10(52)(d/10kpc)(2) erg cm(-3).
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| 20. |
- Abramowski, A., et al.
(författare)
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HESS observations of the Carina nebula and its enigmatic colliding wind binary Eta Carinae
- 2012
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 424:1, s. 128-135
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Tidskriftsartikel (refereegranskat)abstract
- The massive binary system Eta Carinae and the surrounding H ii complex, the Carina nebula, are potential particle acceleration sites from which very high energy (VHE; E= 100 GeV) ?-ray emission could be expected. This paper presents data collected during VHE ?-ray observations with the HESS telescope array from 2004 to 2010, which cover a full orbit of Eta Carinae. In the 33.1-h data set no hint of significant ?-ray emission from Eta Carinae has been found and an upper limit on the ?-ray flux of (99 per cent confidence level) is derived above the energy threshold of 470 GeV. Together with the detection of high energy (HE; 0.1 =E= 100 GeV) ?-ray emission by the Fermi Large Area Telescope up to 100 GeV, and assuming a continuation of the average HE spectral index into the VHE domain, these results imply a cut-off in the ?-ray spectrum between the HE and VHE ?-ray range. This could be caused either by a cut-off in the accelerated particle distribution or by severe ?? absorption losses in the wind collision region. Furthermore, the search for extended ?-ray emission from the Carina nebula resulted in an upper limit on the ?-ray flux of (99 per cent confidence level). The derived upper limit of 23 on the cosmic ray enhancement factor is compared with results found for the old-age mixed-morphology supernova remnant W28.
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| 21. |
- Abramowski, A., et al.
(författare)
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Identification of HESS J1303-631 as a pulsar wind nebula through gamma-ray, X-ray, and radio observations
- 2012
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 548, s. A46-
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Tidskriftsartikel (refereegranskat)abstract
- Aims. The previously unidentified very high-energy (VHE; E > 100 GeV) gamma-ray source HESS J1303-631, discovered in 2004, is re-examined including new data from the H. E. S. S. Cherenkov telescope array in order to identify this object. Archival data from the XMM-Newton X-ray satellite and from the PMN radio survey are also examined. Methods. Detailed morphological and spectral studies of VHE gamma-ray emission as well as of the XMM-Newton X-ray data are performed. Radio data from the PMN survey are used as well to construct a leptonic model of the source. The gamma-ray and X-ray spectra and radio upper limit are used to construct a one zone leptonic model of the spectral energy distribution (SED). Results. Significant energy-dependent morphology of the gamma-ray source is detected with high-energy emission (E > 10 TeV) positionally coincident with the pulsar PSR J1301-6305 and lower energy emission (E < 2 TeV) extending similar to 0.4 degrees to the southeast of the pulsar. The spectrum of the VHE source can be described with a power-law with an exponential cut-off N-0 = (5.6 +/- 0.5) x 10(-12) TeV-1 cm(-2) s(-1), Gamma = 1.5 +/- 0.2) and E-cut = (7.7 +/- 2.2) TeV. The pulsar wind nebula (PWN) is also detected in X-rays, extending similar to 2-3' from the pulsar position towards the center of the gamma-ray emission region. A potential radio counterpart from the PMN survey is also discussed, showing a hint for a counterpart at the edge of the X-ray PWN trail and is taken as an upper limit in the SED. The extended X-ray PWN has an unabsorbed flux of F2-10 (keV) similar to 1.6(-0.4)(+0.2) x 10(-13) erg cm(-2)s(-1) and is detected at a significance of 6.5 sigma. The SED is well described by a one zone leptonic scenario which, with its associated caveats, predicts a very low average magnetic field for this source. Conclusions. Significant energy-dependent morphology of this source, as well as the identification of an associated X-ray PWN from XMM-Newton observations enable identification of the VHE source as an evolved PWN associated to the pulsar PSR J1301-6305. This identification is supported by the one zone leptonic model, which suggests that the energetics of the gamma-ray and X-ray radiation are such that they may have a similar origin in the pulsar nebula. However, the large discrepancy in emission region sizes and the low level of synchrotron radiation suggest a multi-population leptonic nature. The low implied magnetic field suggests that the PWN has undergone significant expansion. This would explain the low level of synchrotron radiation and the difficulty in detecting counterparts at lower energies, the reason this source was originally classified as a "dark" VHE gamma-ray source.
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| 22. |
- Abramowski, A., et al.
(författare)
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Long-term monitoring of PKS2155-304 with ATOM and HESS:investigation of optical/gamma-ray correlations in different spectral states
- 2014
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 571
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Tidskriftsartikel (refereegranskat)abstract
- In this paper we report on the analysis of all the available optical and very high-energy gamma-ray (> 200 GeV) data for the BL Lac object PKS 2155-304, collected simultaneously with the ATOM and H.E.S.S. telescopes from 2007 until 2009. This study also includes X-ray (RXTE, Swift) and high-energy gamma-ray (Fermi-LAT) data. During the period analysed, the source was transitioning from its flaring to quiescent optical states, and was characterized by only moderate flux changes at different wavelengths on the timescales of days and months. A flattening of the optical continuum with an increasing optical flux can be noted in the collected dataset, but only occasionally and only at higher flux levels. We did not find any universal relation between the very high-energy gamma-ray and optical flux changes on the timescales from days and weeks up to several years. On the other hand, we noted that at higher flux levels the source can follow two distinct tracks in the optical flux-colour diagrams, which seem to be related to distinct gamma-ray states of the blazar. The obtained results therefore indicate a complex scaling between the optical and gamma-ray emission of PKS 2155 304, with different correlation patterns holding at different epochs, and a gamma-ray flux depending on the combination of an optical flux and colour rather than a flux alone.
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| 23. |
- Abramowski, A., et al.
(författare)
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Measurement of the extragalactic background light imprint on the spectra of the brightest blazars observed with HESS
- 2013
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 550, s. A4-
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Tidskriftsartikel (refereegranskat)abstract
- The extragalactic background light (EBL) is the diffuse radiation with the second highest energy density in the Universe after the cosmic microwave background. The aim of this study is the measurement of the imprint of the EBL opacity to.-rays on the spectra of the brightest extragalactic sources detected with the High Energy Stereoscopic System (H. E. S. S.). The originality of the method lies in the joint fit of the EBL optical depth and of the intrinsic spectra of the sources, assuming intrinsic smoothness. Analysis of a total of similar to 10(5) gamma-ray events enables the detection of an EBL signature at the 8.8 sigma level and constitutes the first measurement of the EBL optical depth using very-high energy (E > 100 GeV) gamma-rays. The EBL flux density is constrained over almost two decades of wavelengths [0.30 mu m, 17 mu m] and the peak value at 1.4 mu m is derived as lambda F-lambda = 15 +/- 2(stat) +/- 3(sys) nW m(-2) sr(-1).
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| 24. |
- Abramowski, A., et al.
(författare)
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Probing the extent of the non-thermal emission from the Vela X region at TeV energies with HESS
- 2012
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 548, s. A38-
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Tidskriftsartikel (refereegranskat)abstract
- Context. Vela X is a region of extended radio emission in the western part of the Vela constellation: one of the nearest pulsar wind nebulae, and associated with the energetic Vela pulsar (PSR B0833-45). Extended very-high-energy (VHE) gamma-ray emission (HESS J0835-455) was discovered using the H. E. S. S. experiment in 2004. The VHE gamma-ray emission was found to be coincident with a region of X-ray emission discovered with ROSAT above 1.5 keV (the so-called Vela X cocoon): a filamentary structure extending southwest from the pulsar to the centre of Vela X. Aims. A deeper observation of the entire Vela X nebula region, also including larger offsets from the cocoon, has been performed with H. E. S. S. This re-observation was carried out in order to probe the extent of the non-thermal emission from the Vela X region at TeV energies and to investigate its spectral properties. Methods. To increase the sensitivity to the faint gamma-ray emission from the very extended Vela X region, a multivariate analysis method combining three complementary reconstruction techniques of Cherenkov-shower images is applied for the selection of gamma-ray events. The analysis is performed with the On/Off background method, which estimates the background from separate observations pointing away from Vela X; towards regions free of gamma-ray sources but with comparable observation conditions. Results. The gamma-ray surface brightness over the large Vela X region reveals that the detection of non-thermal VHE gamma-ray emission from the PWN HESS J0835-455 is statistically significant over a region of radius 1.2 degrees around the position alpha = 08(h)35(m)00(s), delta = -45 degrees 36'00 '' (J2000). The Vela X region exhibits almost uniform gamma-ray spectra over its full extent: the differential energy spectrum can be described by a power-law function with a hard spectral index Gamma = 1.32 +/- 0.06(stat) +/- 0.12(sys) and an exponential cutoff at an energy of (14.0 +/- 1.6(stat) +/- 2.6(sys)) TeV. Compared to the previous H. E. S. S. observations of Vela X the new analysis confirms the general spatial overlap of the bulk of the VHE gamma-ray emission with the X-ray cocoon, while its extent and morphology appear more consistent with the (more extended) radio emission, contradicting the simple correspondence between VHE gamma-ray and X-ray emissions. Morphological and spectral results challenge the interpretation of the origin of gamma-ray emission in the GeV and TeV ranges in the framework of current models.
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| 25. |
- Abramowski, A., et al.
(författare)
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Search for dark matter annihilation signatures in HESS observations of dwarf spheroidal galaxies
- 2014
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Ingår i: Physical Review D. - 1550-7998 .- 1550-2368. ; 90:11, s. 112012-
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Tidskriftsartikel (refereegranskat)abstract
- Dwarf spheroidal galaxies of the Local Group are close satellites of the Milky Way characterized by a large mass-to-light ratio and are not expected to be the site of nonthermal high-energy gamma-ray emission or intense star formation. Therefore they are among the most promising candidates for indirect dark matter searches. During the last years the High Energy Stereoscopic System (H.E.S.S.) of imaging atmospheric Cherenkov telescopes observed five of these dwarf galaxies for more than 140 hours in total, searching for TeV gamma-ray emission from annihilation of dark matter particles. The new results of the deep exposure of the Sagittarius dwarf spheroidal galaxy, the first observations of the Coma Berenices and Fornax dwarves and the reanalysis of two more dwarf spheroidal galaxies already published by the H.E.S.S. Collaboration, Carina and Sculptor, are presented. In the absence of a significant signal new constraints on the annihilation cross section applicable to weakly interacting massive particles (WIMPs) are derived by combining the observations of the five dwarf galaxies. The combined exclusion limit depends on the WIMP mass and the best constraint is reached at 1-2 TeV masses with a cross-section upper bound of similar to 3.9 x 10(-24) cm(3) s(-1) at a 95% confidence level.
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