| 31. |
- Abramowski, A., et al.
(författare)
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Discovery of hard-spectrum gamma- ray emission from the BL Lacertae object 1ES 0414+009
- 2012
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 538, s. A103-
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Tidskriftsartikel (refereegranskat)abstract
- Context. 1ES 0414+009 (z = 0.287) is a distant high-frequency- peaked BL Lac object, and has long been considered a likely emitter of very-highenergy (VHE, E > 100 GeV) gamma-rays due to its high X-ray and radio flux. Aims. Observations in the VHE gamma-ray band and across the electromagnetic spectrum can provide insights into the origin of highly energetic particles present in the source and the radiation processes at work. Because of the distance of the source, the gamma-ray spectrum might provide further limits on the level of the extragalactic background light (EBL). Methods. We report observations made between October 2005 and December 2009 with H. E. S. S., an array of four imaging atmospheric Cherenkov telescopes. Observations at high energies (HE, 100 MeV-100 GeV) with the Fermi-LAT instrument in the first 20 months of its operation are also reported. To complete the multi-wavelength picture, archival UV and X-ray observations with the Swift satellite and optical observations with the ATOM telescope are also used. Results. Based on the observations with H.E.S.S., 1ES 0414+009 is detected for the first time in the VHE band. An excess of 224 events is measured, corresponding to a significance of 7.8 sigma. The photon spectrum of the source is well described by a power law, with photon index of Gamma(VHE) = 3.45 +/- 0.25(stat) +/- 0.20(syst). The integral flux above 200 GeV is (1.88 +/- 0.20(stat) +/- 0.38(syst)) x10(-12) cm(-2) s(-1). Observations with the Fermi-LAT in the first 20 months of operation show a flux between 200 MeV and 100 GeV of (2.3 +/- 0.2(stat)) x 10(-9) erg cm(-2) s(-1), and a spectrum well described by a power-law function with a photon index Gamma(HE) = 1.85 +/- 0.18. Swift/XRT observations show an X-ray flux between 2 and 10 keV of (0.8-1) x 10(-11) erg cm(-2) s(-1), and a steep spectrum Gamma(X) = (2.2-2.3). Combining X-ray with optical-UV data, a fit with a log-parabolic function locates the synchrotron peak around 0.1 keV. Conclusions. Although the GeV-TeV observations do not provide better constraints on the EBL than previously obtained, they confirm a low density of the EBL, close to the lower limits from galaxy counts. The absorption-corrected HE and VHE gamma-ray spectra are both hard and have similar spectral indices (approximate to 1.86), indicating no significant change of slope between the HE and VHE gamma-ray bands, and locating the gamma-ray peak in the SED above 1-2 TeV. As for other TeV BL Lac objects with the gamma-ray peak at such high energies and a large separation between the two SED humps, this average broad-band SED represents a challenge for simple one-zone synchrotron self-Compton models, requiring a high Doppler factor and very low B-field.
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| 32. |
- Abramowski, A., et al.
(författare)
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Discovery of high and very high-energy emission from the BL Lacertae object SHBL J001355.9-185406
- 2013
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 554, s. A72-
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Tidskriftsartikel (refereegranskat)abstract
- The detection of the high-frequency peaked BL Lac object (HBL) SHBL J001355.9-185406 (z = 0.095) at high (HE; 100 MeV < E < 300 GeV) and very high-energy (VHE; E > 100 GeV) with the Fermi Large Area Telescope (LAT) and the High Energy Stereoscopic System (H.E.S.S.) is reported. Dedicated observations were performed with the H. E. S. S. telescopes, leading to a detection at the 5.5 sigma significance level. The measured flux above 310 GeV is (8.3 +/- 1.7(stat) +/- 1.7(sys)) x 10(-13) photons cm(-2) s(-1) (about 0.6% of that of the Crab Nebula), and the power-law spectrum has a photon index of Gamma = 3.4 +/- 0.5(stat) +/- 0.2(sys). Using 3.5 years of publicly available Fermi-LAT data, a faint counterpart has been detected in the LAT data at the 5.5 sigma significance level, with an integrated flux above 300 MeV of (9.3 +/- 3.4(stat) +/- 0.8(sys)) x 10(-10) photons cm(-2) s(-1) and a photon index of Gamma = 1.96 +/- 0.20(stat) +/- 0.08(sys). X-ray observations with Swift-XRT allow the synchrotron peak energy in vF(v) representation to be located at similar to 1.0 keV. The broadband spectral energy distribution is modelled with a one-zone synchrotron self-Compton (SSC) model and the optical data by a black-body emission describing the thermal emission of the host galaxy. The derived parameters are typical of HBLs detected at VHE, with a particle-dominated jet.
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| 33. |
- 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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| 34. |
- 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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| 35. |
- Abramowski, A., et al.
(författare)
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Discovery of the source HESS J1356-645 associated with the young and energetic PSR J1357-6429
- 2011
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 533, s. A103-
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Tidskriftsartikel (refereegranskat)abstract
- Context. Several newly discovered very-high-energy (VHE; E > 100 GeV) gamma-ray sources in the Galaxy are thought to be associated with energetic pulsars. Among them, middle-aged (greater than or similar to 10(4) yr) systems exhibit large centre-filled VHE nebulae, offset from the pulsar position, which result from the complex relationship between the pulsar wind and the surrounding medium, and reflect the past evolution of the pulsar. Aims. Imaging Atmospheric Cherenkov Telescopes (IACTs) have been successful in revealing extended emission from these sources in the VHE regime. Together with radio and X-ray observations, this observational window allows one to probe the energetics and magnetic field inside these large-scale nebulae. Methods. H.E.S.S., with its large field of view, angular resolution of less than or similar to 0.1 degrees and unprecedented sensitivity, has been used to discover a large population of such VHE sources. In this paper, the H. E. S. S. data from the continuation of the Galactic Plane Survey (-80 degrees < l < 60 degrees, vertical bar b vertical bar < 3 degrees), together with the existing multi-wavelength observations, are used. Results. A new VHE gamma-ray source was discovered at RA (J2000) = 13(h)56(m)00(s), Dec (J2000) = -64 degrees 30'00 '' with a 2' statistical error in each coordinate, namely HESS J1356-645. The source is extended, with an intrinsic Gaussian width of (0.20 +/- 0.02)degrees. Its integrated energy flux between 1 and 10 TeV of 8 x 10(-12) erg cm(-2) s(-1) represents similar to 11% of the Crab Nebula flux in the same energy band. The energy spectrum between 1 and 20 TeV is well described by a power law dN/dE proportional to E-Gamma with photon index Gamma = 2.2 +/- 0.2(stat) +/- 0.2(sys). The inspection of archival radio images at three frequencies and the analysis of X-ray data from ROSAT/PSPC and XMM-Newton/MOS reveal the presence of faint non-thermal diffuse emission coincident with HESS J1356-645. Conclusions. HESS J1356-645 is most likely associated with the young and energetic pulsar PSR J1357-6429 (d = 2.4 kpc, tau(c) = 7.3 kyr and (E) over dot = 3.1 x 10(36) erg s(-1)), located at a projected distance of similar to 5 pc from the centroid of the VHE emission. HESS J1356-645 and its radio and X-ray counterparts would thus represent the nebula resulting from the past history of the PSR J1357-6429 wind. In a simple one-zone model, constraints on the magnetic field strength in the nebula are obtained from the flux of the faint and extended X-ray emission detected with ROSAT and XMM-Newton. Fermi-LAT upper limits in the high-energy ( HE; 0.1-100 GeV) domain are also used to constrain the parent electron spectrum. From the low magnetic field value inferred from this approach (similar to 3-4 mu G), HESS J1356-645 is thought to share many similarities with other known gamma-ray emitting nebulae, such as Vela X, as it exhibits a large-scale nebula seen in radio, X-rays and VHE gamma-rays.
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| 36. |
- Abramowski, A., et al.
(författare)
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Discovery of the VHE gamma-ray source HESS J1832-093 in the vicinity of SNR G22.7-0.2
- 2015
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 446:2, s. 1163-1169
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Tidskriftsartikel (refereegranskat)abstract
- The region around the supernova remnant (SNR) W41 contains several TeV sources and has prompted the HESS Collaboration to perform deep observations of this field of view. This resulted in the discovery of the new very high energy (VHE) source HESS J1832-093, at the position RA = 18(h)32(m)50(s) +/- 3(stat)(s) +/- 2(syst)(s), Dec = -9 degrees 22'36 '' +/- 32(stat)'' +/- 20(syst)'' (J2000), spatially coincident with a part of the radio shell of the neighbouring remnant G22.7-0.2. The photon spectrum is well described by a power law of index Gamma = 2.6 +/- 0.3(stat) +/- 0.1(syst) and a normalization at 1 TeV of Phi(0) = (4.8 +/- 0.8(stat) +/- 1.0(syst)) x 10(-13) cm(-2) s(-1) TeV-1. The location of the gamma-ray emission on the edge of the SNR rim first suggested a signature of escaping cosmic rays illuminating a nearby molecular cloud. Then a dedicated XMM-Newton observation led to the discovery of a new X-ray point source spatially coincident with the TeV excess. Two other scenarios were hence proposed to identify the nature of HESS J1832-093. Gamma-rays from inverse Compton radiation in the framework of a pulsar wind nebula scenario or the possibility of gamma-ray production within a binary system are therefore also considered. Deeper multiwavelength observations will help to shed new light on this intriguing VHE source.
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| 37. |
- Abramowski, A., et al.
(författare)
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Discovery of very high energy gamma-ray emission from the BL Lacertae object PKS0301-243 with HESS
- 2013
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 559, s. A136-
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Tidskriftsartikel (refereegranskat)abstract
- The active galactic nucleus PKS 0301-243 (z = 0.266) is a high-synchrotron-peaked BL Lac object that is detected at high energies (HE, 100 MeV < E < 100 GeV) by Fermi/LAT. This paper reports on the discovery of PKS 0301-243 at very high energies (E > 100 GeV) by the High Energy Stereoscopic System (H.E.S.S.) from observations between September 2009 and December 2011 for a total live time of 34.9 h. Gamma rays above 200 GeV are detected at a significance of 9.4 sigma. A hint of variability at the 2.5 sigma level is found. An integral flux I(E > 200GeV) = (3.3 +/- 1.1(stat) +/- 0.7(syst)) x 10(-12) ph cm(-2) s(-1) and a photon index Gamma = 4.6 +/- 0.7(stat) +/- 0.2(syst) are measured. Multi-wavelength light curves in HE, X-ray and optical bands show strong variability, and a minimal variability timescale of eight days is estimated from the optical light curve. A single-zone leptonic synchrotron self-Compton scenario satisfactorily reproduces the multi-wavelength data. In this model, the emitting region is out of equipartition and the jet is particle dominated. Because of its high redshift compared to other sources observed at TeV energies, the very high energy emission from PKS 0301-243 is attenuated by the extragalactic background light (EBL) and the measured spectrum is used to derive an upper limit on the opacity of the EBL.
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| 38. |
- Abramowski, A., et al.
(författare)
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Discovery of VHE emission towards the Carina arm region with the HESS telescope array : HESSJ1018-589
- 2012
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 541, s. A5-
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Tidskriftsartikel (refereegranskat)abstract
- The Carina arm region, containing the supernova remnant SNRG284.3-1.8, the high-energy (HE; E > 100 MeV) binary 1FGL J1018.6-5856 and the energetic pulsar PSRJ1016-5857 and its nebula, has been observed with the H. E. S. S. telescope array. The observational coverage of the region in very-high-energy (VHE; E > 0.1TeV) gamma-rays benefits from deep exposure (40 h) of the neighboring open cluster Westerlund 2. The observations have revealed a new extended region of VHE gamma-ray emission. The new VHE source HESS J1018-589 shows a bright, point-like emission region positionally coincident with SNRG284.3-1.8 and 1FGL J1018.6-5856 and a diffuse extension towards the direction of PSRJ1016-5857. A soft (Gamma = 2.7 +/- 0.5(stat)) photon index, with a differential flux at 1 TeV of N-0 = (4.2 +/- 1.1) x 10(-13) TeV-1 cm(-2) s(-1) is found for the point-like source, whereas the total emission region including the diffuse emission region is well fit by a power-law function with spectral index Gamma = 2.9 +/- 0.4(stat) and differential flux at 1 TeV of N-0 = (6.8 +/- 1.6) x 10(-1)3 TeV-1 cm(-2) s(-1). This H. E. S. S. detection motivated follow-up X-ray observations with the XMM-Newton satellite to investigate the origin of the VHE emission. The analysis of the XMM-Newton data resulted in the discovery of a bright, non-thermal point-like source (XMMU J101855.4-58564) with a photon index of Gamma = 1.65 +/- 0.08 in the center of SNRG284.3-1.8, and a thermal, extended emission region coincident with its bright northern filament. The characteristics of this thermal emission are used to estimate the plasma density in the region as n approximate to 0.5 cm(-3) (2.9 kpc/d)(2). The position of XMMUJ101855.4-58564 is compatible with the position reported by the Fermi-LAT collaboration for the binary system 1FGL J1018.6-5856 and the variable Swift XRT source identified with it. The new X-ray data are used alongside archival multi-wavelength data to investigate the relationship between the VHE gamma-ray emission from HESS J1018-589 and the various potential counterparts in the Carina arm region.
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| 39. |
- 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
-
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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| 40. |
- Abramowski, A., et al.
(författare)
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Flux upper limits for 47 AGN observed with HESS in 2004-2011
- 2014
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 564, s. A9-
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Tidskriftsartikel (refereegranskat)abstract
- Context. About 40% of the observation time of the High Energy Stereoscopic System (H.E.S.S.) is dedicated to studying active galactic nuclei (AGN), with the aim of increasing the sample of known extragalactic very-high-energy (VHE, E > 100 GeV) sources and constraining the physical processes at play in potential emitters. Aims. H.E.S.S. observations of AGN, spanning a period from April 2004 to December 2011, are investigated to constrain their gamma-ray fluxes. Only the 47 sources without significant excess detected at the position of the targets are presented. Methods. Upper limits on VHE fluxes of the targets were computed and a search for variability was performed on the nightly time scale. Results. For 41 objects, the flux upper limits we derived are the most constraining reported to date. These constraints at VHE are compared with the flux level expected from extrapolations of Fermi-LAT measurements in the two-year catalog of AGN. The H.E.S.S. upper limits are at least a factor of two lower than the extrapolated Fermi-LAT fluxes for 11 objects Taking into account the attenuation by the extragalactic background light reduces the tension for all but two of them, suggesting intrinsic curvature in the high-energy spectra of these two AGN. Conclusions. Compilation efforts led by current VHE instruments are of critical importance for target-selection strategies before the advent of the Cherenkov Telescope Array (CTA).
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