The extreme synchrotron BL Lac object H 2356-309, located at a redshift of $z = 0.165$, was observed from June to December 2004 with a total exposure of ≈40 h live-time with the HESS (High Energy ...Stereoscopic System) array of atmospheric-Cherenkov telescopes (ACTs). Analysis of this data set yields, for the first time, a strong excess of 453 γ-rays (10 standard deviations above background) from H 2356-309, corresponding to an observed integral flux above 200 GeV of I(>200 GeV) = (4.1 ± 0.5) $\times$ 10-12 cm-2 s-1 (statistical error only). The differential energy spectrum of the source between 200 GeV and 1.3 TeV is well-described by a power law with a normalisation (at 1 TeV) of N0 = (4.1 ± 0.5) $\times$ 10-13 cm-2 s-1 TeV-1 and a photon index of Γ = $3.09\,\pm\,0.24_\mathrm{stat}\,\pm\,0.10_\mathrm{sys}$. H 2356-309 is one of the most distant BL Lac objects detected at very-high-energy γ-rays so far. Results from simultaneous observations from ROTSE-III (optical), RXTE (X-rays) and NRT (radio) are also included and used together with the HESS data to constrain a single-zone homogeneous synchrotron self-Compton (SSC) model. This model provides an adequate fit to the HESS data when using a reasonable set of model parameters.
The extreme synchrotron BL Lac object H2356-309, located at a redshift of z = 0.165, was observed from June to December 2004 with a total exposure of approx. 40 h live-time with the H.E.S.S. (High ...Energy Stereoscopic System) array of atmospheric-Cherenkov telescopes (ACTs). Analysis of this data set yields, for the first time, a strong excess of 453 gamma-rays (10 standard deviations above background) from H2356-309, corresponding to an observed integral flux above 200 GeV of I(>200GeV) = (4.1+-0.5) 10^12 cm^-2.s^-1 (statistical error only). The differential energy spectrum of the source between 200 GeV and 1.3 TeV is well-described by a power law with a normalisation (at 1 TeV) of N_0 = (3.00 +- 0.80_stat +- 0.31_sys) 10^-13 cm^-2.s^-1.TeV^-1 and a photon index of Gamma = 3.09 +- 0.24_stat +- 0.10_sys. H2356-309 is one of the most distant BL Lac objects detected at very-high-energy gamma-rays so far. Results from simultaneous observations from ROTSE-III (optical), RXTE (X-rays) and NRT (radio) are also included and used together with the H.E.S.S. data to constrain a single-zone homogeneous synchrotron self-Compton (SSC) model. This model provides an adequate fit to the H.E.S.S. data when using a reasonable set of model parameters.
We report on a survey of the inner part of the Galactic Plane in very high energy gamma-rays, with the H.E.S.S. Cherenkov telescope system. The Galactic Plane between +-30deg in longitude and +-3deg ...in latitude relative to the Galactic Centre was observed in 500 pointings for a total of 230 hours, reaching an average flux sensitivity of 2% of the Crab Nebula at energies above 200 GeV. Fourteen previously unknown sources were detected at a significance level greater than 4 sigma after accounting for all trials involved in the search. Initial results on the eight most significant of these sources were already reported elsewhere. Here we present detailed spectral and morphological information for all the new sources, along with a discussion on possible counterparts in other wavelength bands. The distribution in Galactic latitude of the detected sources appears to be consistent with a scale height in the Galactic disk for the parent population smaller than 100 pc, consistent with expectations for supernova remnants and/or pulsar wind nebulae.
The diffuse Extragalactic Background Light (EBL) contains unique information about the epochs of formation and the history of evolution of galaxies. Unfortunately, direct measurements are subject to ...large systematic uncertainties due to the difficulties in the accurate model-based subtraction of the bright foregrounds. An alternative approach is based on the detection and identification of EBL absorption features in high-energy spectra of objects of known redshift. Here we exploit this method on the blazars H 2356-309 (z=0.165) and 1ES 1101-232 (z=0.186), newly discovered at TeV energies by the H.E.S.S. Collaboration. They are the most distant sources with measured spectra known so far at these energies. Their hard spectra provide the most stringent upper limit to date on the EBL in the Opt--NIR band, which appears significantly lower than expected from the current "direct" estimates and very close to the absolute lower limit represented by the integrated light of resolved galaxies. In addition to important cosmological implications, this result shows that the intergalactic space is more transparent to gamma-rays than previously thought, expanding the horizon of the TeV Universe.
The detection of fast variations of the TeV (10^12 eV) gamma-ray flux, on time-scales of days, from the nearby radio galaxy M 87 is reported. These variations are ~10 times faster than that observed ...in any other waveband and imply a very compact emission region with a dimension similar to the Schwarzschild radius of the central black hole. We thus can exclude several other sites and processes of the gamma-ray production. The observations confirm that TeV gamma-rays are emitted by extragalactic sources other than blazars, where jets are not relativistically beamed towards the observer.
We report on a possible association of the recently discovered very high-energy γ-ray source HESS J1825–137 with the pulsar wind nebula (commonly referred to as G 18.0–0.7) of the $2.1\times 10^{4}$ ...year old Vela-like pulsar PSR B1823–13. HESS J1825–137 was detected with a significance of 8.1σ in the Galactic Plane survey conducted with the HESS instrument in 2004. The centroid position of HESS J1825–137 is offset by 11´ south of the pulsar position. XMM-Newton observations have revealed X-ray synchrotron emission of an asymmetric pulsar wind nebula extending to the south of the pulsar. We argue that the observed morphology and TeV spectral index suggest that HESS J1825–137 and G 18.0–0.7 may be associated: the lifetime of TeV emitting electrons is expected to be longer compared to the XMM-Newton X-ray emitting electrons, resulting in electrons from earlier epochs (when the spin-down power was larger) contributing to the present TeV flux. These electrons are expected to be synchrotron cooled, which explains the observed photon index of ~2.4, and the longer lifetime of TeV emitting electrons naturally explains why the TeV nebula is larger than the X-ray size. Finally, supernova remnant expansion into an inhomogeneous medium is expected to create reverse shocks interacting at different times with the pulsar wind nebula, resulting in the offset X-ray and TeV γ-ray morphology.
We report the discovery of very-high-energy (VHE) γ-ray emission of the binary system PSR B1259-63/SS 2883 of a radio pulsar orbiting a massive, luminous Be star in a highly eccentric orbit. The ...observations around the 2004 periastron passage of the pulsar were performed with the four 13 m Cherenkov telescopes of the HESS experiment, recently installed in Namibia and in full operation since December 2003. Between February and June 2004, a γ-ray signal from the binary system was detected with a total significance above $13\sigma$. The flux was found to vary significantly on timescales of days which makes PSR B1259-63 the first variable galactic source of VHE γ-rays observed so far. Strong emission signals were observed in pre- and post-periastron phases with a flux minimum around periastron, followed by a gradual flux decrease in the months after. The measured time-averaged energy spectrum above a mean threshold energy of 380 GeV can be fitted by a simple power law $F_0(E/1\,\rm TeV)^{-\Gamma}$ with a photon index $\Gamma = 2.7\pm0.2_\mathrm{stat}\pm0.2_\mathrm{sys}$ and flux normalisation $F_0 = (1.3 \pm 0.1_\mathrm{stat} \pm 0.3_\mathrm{sys}) \times 10^{-12}\,\rm TeV^{-1}\,\rm cm^{-2}\,\rm s^{-1}$. This detection of VHE γ-rays provides unambiguous evidence for particle acceleration to multi-TeV energies in the binary system. In combination with coeval observations of the X-ray synchrotron emission by the RXTE and INTEGRAL instruments, and assuming the VHE γ-ray emission to be produced by the inverse Compton mechanism, the magnetic field strength can be directly estimated to be of the order of 1 G.
The region around PSR B1706–44 has been observed with the HESS imaging atmospheric Cherenkov telescopes in 2003. No evidence for γ-ray emission in the TeV range was found at the pulsar position or at ...the radio arc which corresponds to the supernova remnant G 343.1–2.3. The 99% confidence level flux upper limit at the pulsar position is $F_{{\rm ul}}$($E>350~{\rm GeV}) = 1.4 \times 10^{-12}~ {\rm s}^{-1}\,{\rm cm}^{-2}$ assuming a power law (${\rm d}N/{\rm d}E \propto E^{-\Gamma}$) with photon index of $\Gamma = 2.5$ and $F_{{\rm ul}}$($E > 500\,{\rm GeV}) = 1.3 \times 10^{-12} ~ {\rm s}^{-1}\, {\rm cm}^{-2}$ without an assumption on the spectral shape. The reported upper limits correspond to 8% of the flux from an earlier detection by the CANGAROO experiment.
X-ray binaries are composed of a normal star in orbit around a neutron star or stellar-mass black hole. Radio and X-ray observations have led to the presumption that some X-ray binaries called ...microquasars behave as scaled down active galactic nuclei. Microquasars have resolved radio emission that is thought to arise from a relativistic outflow akin to active galactic nuclei jets, in which particles can be accelerated to large energies. Very high energy gamma-rays produced by the interactions of these particles have been observed from several active galactic nuclei. Using the High Energy Stereoscopic System, we find evidence for gamma-ray emission >100 GeV from a candidate microquasar, LS 5039, showing that particles are also accelerated to very high energies in these systems.
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