[en] H.E.S.S. (High Energy Stereoscopic System) is an array of four large imaging Cherenkov telescopes devoted to gamma-ray astronomy above 100 GeV. It has been fully operational since December 2003. Here, we report on a new analysis method based on a simple 3D-modeling of an electromagnetic air shower. This method allows to separate gamma-rays from background events in the field of view without any assumption on the morphology of the source or on the background distribution. This is a crucial point in the study of extended sources (e.g. Supernova Remnants) or in the search for unexpected sources in a sky survey. The performance and results of this method on galactic sources are presented

[en] Two new analysis methods for imaging atmospheric Cherenkov telescopes have been developed in the framework of the H.E.S.S. experiment. Both methods make use of the full stereoscopic information, beyond the Hillas parameters of each image. The first method is a simple 3D-modeling of an electromagnetic shower; the second is based on an analytical model of the light profile in the images. Both methods have confirmed the standard H.E.S.S. analysis for the currently observed sources. They further provide some improvement in angular resolution and are well-suited to background subtraction for extended sources

[en] Vela is the brightest pulsar known in the GeV gamma ray range, yielding a clear signal in less than a day of observations with the Large Area Telescope (LAT). The striking pulsed signature provides a rich opportunity to compare the real gamma ray response of the GLAST LAT to expectations from the highly-detailed Monte Carlo detector simulations. This is critical because all flux and spectral measurements with the LAT rely on the acceptances parametrized by the Instrument Response Functions (IRF), extracted from the simulations. We successively apply the analysis cuts used to identify gamma rays and to reject background, comparing at each step the observed and predicted yields. This procedure is repeated for gamma rays incident on different parts of the LAT and at different angles. Incidence angles will vary widely during normal observations in sky survey mode, and a 2-week pointed mode observation during the Launch and Early Operations phase (L and EO) favors yet a different part of phase space. This method is expected to yield Monte Carlo validations complementary to those already obtained at CERN and at other particle accelerators

[en] PSR B 1951+32 has the hardest spectrum of the six pulsars seen clearly in GeV gamma rays with EGRET, with no indication of a spectral break. The diffuse gamma background in the Cygnus region where it is located is so high that it is the only EGRET pulsar that was not also listed as an unpulsed source in the third catalog. These two properties make it an especially useful case to explore the selection cuts used to enhance the signals from gamma ray emitters observed with the GLAST Large Area Telescope (LAT). The paper presents a new set of cuts using the simulated Data Challenge 2 (DC2), which increases the faint source statistics in the high diffuse gamma background, as for the radio pulsar PSR B 1951+32

[en] We report the first high-significance GeV γ-ray detections of supernova remnants HESS J1731-347 and SN 1006, both of which have been previously detected by imaging atmospheric Cherenkov Telescopes above 1 TeV. Using 8 years of Fermi-LAT Pass 8 data at energies between 1 GeV and 2 TeV, we detect emission at the position of HESS. J1731-347 with a significance of ∼ 5 σ and a spectral index of Γ= 1.66 ± 0.16(stat) ± 0.12(syst). The hardness of the index and the good connection with the TeV spectrum of HESS J1731-347 support an association between the two sources. We also confirm the detection of SN. 1006 at ∼ 6 σ with a spectral index of Γ= 1.79 ± 0.17(stat) ± 0.27(syst). The northeast (NE) and southwest (SW) limbs of SN. 1006 were also fit separately, resulting in the detection of the NE region (Γ= 1.47 ± 0.26(stat)) and the non-detection of the SW region. The significance of different spectral components for the two limbs is 3.6 σ, providing first indications of an asymmetry in the GeV γ-ray emission. (authors)

[en] Millisecond pulsars have weaker magnetic fields than normal pulsars, but their rapid rotation implies open field line voltages similar to those of young pulsars and they are therefore candidates for accelerating particles to the high energies required to emit in the GeV energy domain. PSR J0218+4232 was the only millisecond pulsar detected by EGRET. Its detection was rendered difficult by the powerful BL Lacertae 3C 66A a degree away, in spite of the low background (b= -16.8 degrees). Pulsations were detected at the 4.9 σ level, reinforcing the expectation that millisecond pulsars are a promising class for detection by the forthcoming high-energy mission GLAST. To estimate the GLAST LAT performance for both PSR J0218+4232 and 3C 66A, we have simulated LAT data using predictions for the 'polar cap' and the 'outer gap' pulsar models along with spectral energy distribution estimates for the blazar, normalizing to the EGRET observed fluxes. We demonstrate that GLAST is able to separate unambiguously the two objects in a few weeks. The spectral parameters of the pulsar derived in this context could be used to discriminate between the polar cap and outer gap models

[en] Context. Since the launch of the Fermi satellite, the number of known gamma-ray pulsars has increased tenfold. Most gamma-ray detected pulsars are young and energetic, and many are associated with TeV sources. PSR J1357-6429 is a high spin-down power pulsar (È = 3.1 × 1036 erg s"-"1), discovered during the Parkes multibeam survey of the Galactic plane, with significant timing noise typical of very young pulsars. In the very-high-energy domain (E > 100 GeV), H.E.S.S. has reported the detection of the extended source HESS J1356-645 (intrinsic Gaussian width of 12') whose centroid lies 7' from PSR J1357-6429. Aims. We search for gamma- and X-ray pulsations from this pulsar, characterize the neutron star emission and explore the environment of PSR J1357-6429. Methods. Using a rotational ephemeris obtained with 74 observations made with the Parkes telescope at 1.4 GHz, we phase-fold more than two years of gamma-ray data acquired by the Large Area Telescope on-board Fermi as well as those collected with XMM-Newton, and perform gamma-ray spectral modeling. Results. Significant gamma- and X-ray pulsations are detected from PSR J1357-6429. The light curve in both bands shows one broad peak. Gamma-ray spectral analysis of the pulsed emission suggests that it is well described by a simple power-law of index 1.5 ± 0.3_s_t_a_t ± 0.3_s_y_s_t with an exponential cut-off at 0.8 ± 0.3_s_t_a_t ± 0.3_s_y_s_t GeV and an integral photon flux above 100 MeV of (6.5 ± 1.6_s_t_a_t ± 2.3_s_y_s_t) × 10"-"8 cm"-"2 s"-"1. The X-ray spectra obtained from the new data provide results consistent with previous work. Upper limits on the gamma-ray emission from its potential pulsar wind nebula (PWN) are also reported. Conclusions. Assuming a distance of 2.4 kpc, the Fermi LAT energy flux yields a gamma-ray luminosity for PSR J1357-6429 of L_γ = (2.13 ± 0.25_s_t_a_t ± 0.83_s_y_s_t) × 1034 erg s"-"1, consistent with an relationship. The Fermi non-detection of the pulsar wind nebula associated with HESS J1356-645 provides new constraints on the electron population responsible for the extended TeV emission.

[en] Several young supernova remnants (SNRs) have recently been detected in the high-energy (HE; 0.1 < E < 100 GeV) and very-high-energy (VHE; E > 100 GeV) gamma-ray domains. As exemplified by RX J1713.7-3946, the nature of this emission has been hotly debated, and direct evidence for the efficient acceleration of cosmic-ray protons at the SNR shocks still remains elusive. Here, we study the broadband gamma-ray emission from one of these young SNRs, namely RCW 86, for which several observational lines of evidence indirectly point towards the presence of accelerated hadrons. We then attempt to detect any putative hadronic signal from this SNR in the available gamma-ray data, in order to assess the level of acceleration efficiency. We also analyzed more than 40 months of data acquired by the Large Area Telescope (LAT) on-board the Fermi Gamma-Ray Space Telescope in the HE domain, and gathered all of the relevant multi-wavelength (from radio to VHE gamma-rays) information about the broadband nonthermal emission from RCW 86. For this purpose, we re-analyzed the archival X-ray data from the ASCA/Gas Imaging Spectrometer (GIS), the XMM-Newton/EPIC-MOS, and the RXTE/Proportional Counter Array (PCA). Beyond the expected Galactic diffuse background, no significant gamma-ray emission in the direction of RCW 86 is detected in any of the 0.1–1, 1–10 and 10–100 GeV Fermi-LAT maps. The derived HE upper limits, together with the H.E.S.S. measurements in the VHE domain, are incompatible with a standard D"−"2_p hadronic emission arising from proton-proton interactions, and can only be accommodated by a spectral index Γ ≤ 1.8, i.e. a value in-between the standard (test-particle) index and the asymptotic limit of theoretical particle spectra in the case of strongly modified shocks. In such a hadronic scenario, the total energy in accelerated particles is at the level of ηCR = ECR/ESN ~0.07 d"2_2_._5kpc/ñ (with the distance d_2_._5 _k_p_c ≡ d/2.5 kpc and the effective density ñ ≡ ñ /1 cm"-"3), and the average magnetic field must be stronger than 50 μG in order to significantly suppress any leptonic contribution. On the other hand, the interpretation of the gamma-ray emission by inverse Compton scattering of high energy electrons reproduces the multi-wavelength data using a reasonable value for the average magnetic field of 15–25 μG. In this leptonic scenario, we derive a conservative upper limit to ηCR of 0.04 d"2_2_._5 kpc/ñ. Furthermore, we discuss these results in the light of existing estimates of the magnetic field strength, the effective density and the acceleration efficiency in RCW 86.

[en] Since its launch, the Fermi satellite has firmly identified 5 pulsar wind nebulae plus a large number of candidates, all powered by young and energetic pulsars. HESS J1857 + 026 is a spatially extended γ-ray source detected by H.E.S.S. and classified as a possible pulsar wind nebula candidate powered by PSR J1856 + 0245. Here, we search for γ-ray pulsations from PSR J1856+0245 and explore the characteristics of its associated pulsar wind nebula. Using a rotational ephemeris obtained from the Lovell telescope at Jodrell Bank Observatory at 1.5 GHz, we phase-fold 36 months of γ-ray data acquired by the Large Area Telescope (LAT) aboard Fermi. We also perform a complete γ-ray spectral and morphological analysis. No γ-ray pulsations were detected from PSR J1856+0245. But, significant emission is detected at a position coincident with the TeV source HESS J1857 + 026. The γ-ray spectrum is well described by a simple power-law with a spectral index of Γ = 1.53 ± 0.11_s_t_a_t ± 0.55_s_y_s_t and an energy flux of G(0.1–100 GeV) = (2.71 ± 0.52_s_t_a_t ± 1.51_s_y_s_t) × 10"-"1"1 erg cm"-"2 s"-"1. The γ-ray luminosity is L_P_W_N"γ (0.1–100 GeV)=(2.5 ± 0.5_s_t_a_t ± 1.5_s_y_s_t) × 10"3"5 (d/9 kpc)"2 erg s"-"1, assuming a distance of 9 kpc. This implies a γ-ray efficiency of ~5% for E-dot = 4.6 × 10"3"6 erg s"-1, in the range expected for pulsar wind nebulae. This detailed multi-wavelength modeling provides new constraints on its pulsar wind nebula nature.

[en] We report on γ-ray analysis of the region containing the bright TeV source HESS J1640–465 and the close-by TeV source HESS J1641–463 using 64 months of observations with the Fermi Large Area Telescope (LAT). Previously only one GeV source was reported in this region and was associated with HESS J1640–465. With an increased data set and the improved sensitivity afforded by the reprocessed data (P7REP) of the LAT, we now report the detection, morphological study, and spectral analysis of two distinct sources above 100 MeV. The softest emission in this region comes from the TeV source HESS J1641–463 which is well fitted with a power law of index Γ = 2.47 ± 0.05 ± 0.06 and presents no significant γ-ray signal above 10 GeV, which contrasts with its hard spectrum at TeV energies. The Fermi-LAT spectrum of the second TeV source, HESS J1640–465 is well described by a power-law shape of index Γ = 1.99 ± 0.04 ± 0.07 that links up naturally with the spectral data points obtained by the High Energy Stereoscopic System (H.E.S.S.). These new results provide new constraints concerning the identification of these two puzzling γ-ray sources