Ref HAL: in2p3-00175791_v1
Ref Arxiv: 0709.4584
DOI: 10.1051/0004-6361:20078462
Ref. & Cit.: NASA ADS
Exporter : BibTex | endNote
162 citations
Abstract:

AIMS: To investigate the very high energy (VHE: >100 GeV) gamma-ray emission from the high-frequency peaked BL Lac 1ES 0229+200. METHODS: Observations of 1ES 0229+200 at energies above 580 GeV were performed with the High Energy Stereoscopic System (HESS) in 2005 and 2006. RESULTS: 1ES 0229+200 is discovered by HESS to be an emitter of VHE photons. A signal is detected at the 6.6 sigma level in the HESS observations (41.8 h live time). The integral flux above 580 GeV is (9.4 +- 1.5 {stat} +- 1.9 {syst}) x 10^{-13} cm^{-2} s^{-1}, corresponding to ~1.8% of the flux observed from the Crab Nebula. The data show no evidence for significant variability on any time scale. The observed spectrum is characterized by a hard power law (Gamma = 2.50 +- 0.19 {stat} +- 0.10 {syst}) from 500 GeV to ~15 TeV. CONCLUSIONS: The high-energy range and hardness of the observed spectrum, coupled with the object's relatively large redshift (z=0.1396), enable the strongest constraints so far on the density of the Extragalactic Background Light (EBL) in the mid-infrared band. Assuming that the emitted spectrum is not harder than Gamma_{int} ~ 1.5, the HESS data support an EBL spectrum ~ lambda^{-1} and density close to the lower limit from source counts measured by Spitzer, confirming the previous indications from the HEGRA data of 1ES 1426+428 (z=0.129). Irrespective of the EBL models used, the intrinsic spectrum of 1ES 0229+200 is hard, thus locating the high-energy peak of its spectral energy distribution above a few TeV.

Comments: 5 pages, 4 figures;

Ref HAL: in2p3-00177454_v1
DOI: 10.1051/0004-6361:20078412
Exporter : BibTex | endNote
94 citations
Abstract:

Aims.Our aim is to study the production mechanism for very-high-energy (VHE; >100 GeV) ?-rays in distant active galactic nuclei (AGN) and use the observed VHE spectrum to derive limits on the Extragalactic Background Light (EBL). We also want to determine physical quantities through the modeling of the object's broad-band spectral energy distribution (SED). Methods: VHE observations (~25 h live time) of the BL Lac 1ES 0347-121 (redshift z = 0.188) were conducted with the High Energy Stereoscopic System (HESS) between August and December 2006. Contemporaneous X-ray and UV/optical observations from the SWIFT satellite are used to interpret the SED of the source in terms of a synchrotron self Compton (SSC) model. Results: An excess of 327 events, corresponding to a statistical significance of 10.1 standard deviations, is detected from 1ES 0347-121. Its photon spectrum, ranging from ~250 GeV to ~3 TeV, is well described by a power law with a photon index of ? = 3.10 ± 0.23stat ± 0.10sys. The integral flux above 250 GeV corresponds to ~2% of the flux of the Crab Nebula above the same threshold. No VHE flux variability is detected within the data set. Conclusions: Constraints on the EBL density at optical to near-infrared wavelengths derived from the photon spectrum of 1ES 0347-121 are close to the strongest limits derived previously. The strong EBL limits confirm earlier findings, that the EBL density in the near-infrared is close to the lower limits from source counts. This implies that the universe is more transparent to VHE ?-rays than previously believed. An SSC model provides a reasonable description of the contemporaneous SED. Affiliations are only available in electronic form at http://www.aanda.org

Ref HAL: in2p3-00152627_v1
Ref Arxiv: 0706.0797
DOI: 10.1086/520635
Ref. & Cit.: NASA ADS
Exporter : BibTex | endNote
553 citations
Abstract:

The high-frequency peaked BL Lac PKS 2155-304 at redshift z=0.116 is a well-known VHE (>100 GeV) gamma-ray emitter. Since 2002 its VHE flux has been monitored using the H.E.S.S. stereoscopic array of imaging atmospheric-Cherenkov telescopes in Namibia. During the July 2006 dark period, the average VHE flux was measured to be more than ten times typical values observed from the object. This article focuses solely on an extreme gamma-ray outburst detected in the early hours of July 28, 2006 (MJD 53944). The average flux observed during this outburst is I(>200 GeV) = (1.72$\pm$$0.05_{\rm stat}$$\pm$$0.34_{\rm syst}$) $\times$ 10$^{-9}$ cm$^{-2}$ s$^{-1}$, corresponding to ~7 times the flux, I(>200 GeV), observed from the Crab Nebula. Peak fluxes are measured with one-minute time scale resolution at more than twice this average value. Variability is seen up to ~600 s in the Fourier power spectrum, and well-resolved bursts varying on time scales of ~200 seconds are observed. There are no strong indications for spectral variability within the data. Assuming the emission region has a size comparable to the Schwarzschild radius of a ~10$^9 M_\odot$ black hole, Doppler factors greater than 100 are required to accommodate the observed variability time scales.

Comments: Accepted (June 5, 2007) for publication in ApJ Letters 8 pages, 3 figures

Ref HAL: in2p3-00148778_v1
Ref Arxiv: 0705.2946
DOI: 10.1051/0004-6361:20077057
Ref. & Cit.: NASA ADS
Exporter : BibTex | endNote
92 citations
Abstract:

The blazar 1ES 1101-232 was observed with the High Energy Stereoscopic System (H.E.S.S.) of Atmospheric Cherenkov Telescopes (ACT) in 2004 and 2005, for a live time of 43 hours. VHE (E > 10^11 eV) gamma-rays were detected for the first time from this object. VHE observations of blazars are used to investigate the inner parts of the blazar jets, and also to study the extragalactic background light (EBL) in the near-infrared band. Observations in 2005 were conducted in a multiwavelength campaign, together with the RXTE satellite and optical observations. In 2004, simultaneous observations with XMM-Newton were obtained. 1ES 1101-232 was detected with H.E.S.S. with an excess of 722 photons, at a significance of 12 sigma. The measured VHE gamma-ray flux amounts to dN/dE = (5.63 +- 0.89) x 10^-13 (E/TeV)^-(2.94 +- 0.20) cm^-2 s^-1 TeV^-1, above a spectral energy threshold of 225 GeV. No significant variation of the VHE gamma-ray flux on any time scale was found. 1ES 1101-232 exhibits a very hard spectrum, and at a redshift of z=0.186, is the blazar with the highest confirmed redshift detected in VHE gamma-rays so far. The data allow the construction of truly simultaneous spectral energy distributions of the source, from the optical to the VHE band. Using an EBL model with \nu F_\nu = 14 nW m^-2 sr^-1 at 1.5 \mu m as presented in Aharonian et al. (Nature, 2006, 440, 1018) suggests an intrinsic VHE power output peak of the source at above 3 TeV.

Comments: 18 Pages, 6 Figures, accepted for publication in A&A

Ref HAL: in2p3-00148471_v1
Ref Arxiv: 0705.1605
DOI: 10.1051/0004-6361:20077280
Ref. & Cit.: NASA ADS
Exporter : BibTex | endNote
54 citations
Abstract:

We present the discovery of two very-high-energy gamma-ray sources in an ongoing systematic search for emission above 100 GeV from pulsar wind nebulae in survey data from the H.E.S.S. telescope array. Imaging Atmospheric Cherenkov Telescopes are ideal tools for searching for extended emission from pulsar wind nebulae in the very-high-energy regime. H.E.S.S., with its large field of view of 5 degrees and high sensitivity, gives new prospects for the search for these objects. An ongoing systematic search for very-high-energy emission from energetic pulsars over the region of the Galactic plane between -60 degrees < l < 30 degrees, -2 degrees < b < 2 degrees is performed. For the resulting candidates, the standard H.E.S.S. analysis was applied and a search for multi-wavelength counterparts was performed. We present the discovery of two new candidate gamma-ray pulsar wind nebulae, HESS J1718-385 and HESS J1809-193. H.E.S.S. has proven to be a suitable instrument for pulsar wind nebula searches.

Ref HAL: in2p3-00147240_v1
Ref Arxiv: 0704.0171
DOI: 10.1051/0004-6361:20077299
Ref. & Cit.: NASA ADS
Exporter : BibTex | endNote
87 citations
Abstract:

The complex Monoceros Loop SNR/Rosette Nebula region contains several potential sources of very-high-energy (VHE) gamma-ray emission and two as yet unidentified high-energy EGRET sources. Sensitive VHE observations are required to probe acceleration processes in this region. The H.E.S.S. telescope array has been used to search for very high-energy gamma-ray sources in this region. CO data from the NANTEN telescope were used to map the molecular clouds in the region, which could act as target material for gamma-ray production via hadronic interactions. We announce the discovery of a new gamma-ray source, HESS J0632+058, located close to the rim of the Monoceros SNR. This source is unresolved by H.E.S.S. and has no clear counterpart at other wavelengths but is possibly associated with the weak X-ray source 1RXS J063258.3+054857, the Be-star MWC 148 and/or the lower energy gamma-ray source 3EG J0634+0521. No evidence for an associated molecular cloud was found in the CO data.

Comments: 5 pages, 4 figures, to appear in A&A

Ref HAL: in2p3-00142313_v1
Ref Arxiv: astro-ph/0703427
DOI: 10.1051/0004-6361:20066950
Ref. & Cit.: NASA ADS
Exporter : BibTex | endNote
99 citations
Abstract:

Results from gamma-ray observations by the H.E.S.S. telescope array in the direction of the young stellar cluster Westerlund 2 are presented. Stereoscopic imaging of Cherenkov light emission of gamma-ray induced showers in the atmosphere is used to study the celestial region around the massive Wolf-Rayet (WR) binary WR 20a. Spectral and positional analysis is performed using standard event reconstruction techniques and parameter cuts. The detection of a new gamma-ray source is reported from H.E.S.S. observations in 2006. HESS J1023-575 is found to be coincident with the young stellar cluster Westerlund 2 in the well-known HII complex RCW 49. The source is detected with a statistical significance of more than 9 sigma, and shows extension beyond a point-like object within the H.E.S.S. point-spread function. The differential gamma-ray spectrum of the emission region is measured over approximately two orders of magnitude in flux. The spatial coincidence between HESS J1023-575 and the young open cluster Westerlund 2, hosting e.g. the massive WR binary WR 20a, requires a look into a variety of potential models to account for the observed very-high-energy (VHE) gamma-ray emission. Considered emission scenarios include emission from the colliding wind zone of WR 20a, collective stellar winds from the extraordinary ensemble of hot and massive stars in the stellar cluster Westerlund 2, diffusive shock acceleration in the wind-blown bubble itself, and supersonic winds breaking out into the interstellar medium (ISM). The observed source extension argues against a single star origin of the observed VHE emission.

Comments: 7 pages, 5 figures, accepted for publication in A&A