There are only few very-high-energy sources in our Galaxy which might accelerate particles up to the knee of the cosmic-ray spectrum. To understand the mechanisms of particle acceleration in these PeVatron candidates, textit{Fermi}-LAT and H.E.S.S. o
bservations are essential to characterize their $gamma$-ray emission. HESS J1640$-$465 and the PeVatron candidate HESS J1641$-$463 are two neighboring (ang[astroang]{0.25}) $gamma$-ray sources, spatially coincident with the radio supernova remnants (SNRs) G338.3$-$0.0 and G338.5+0.1. Detected both by H.E.S.S. and textit{Fermi}-LAT, we present here a morphological and spectral analysis of these two sources using 8 years of textit{Fermi}-LAT data between 200 si{megaelectronvolt} and 1 si{teraelectronvolt} with multi-wavelength observations to assess their nature. The morphology of HESS J1640$-$465 is described by a 2D Gaussian ($sigma=$ ang[astroang]{0.053} $pm$ ang[astroang]{0.011}$_{stat}$ $ pm$ ang[astroang]{0.03}$_{syst}$) and its spectrum is modeled by a power-law with a spectral index $Gamma = 1.8pm0.1_{rm stat}pm0.2_{rm syst}$. HESS J1641$-$463 is detected as a point-like source and its GeV emission is described by a logarithmic-parabola spectrum with $alpha = 2.7 pm 0.1_ {rm stat} pm 0.2_ {rm syst} $ and significant curvature of $beta = 0.11 pm 0.03_ {rm stat} pm 0.05_ {rm syst} $. Radio and X-ray flux upper limits were derived. We investigated scenarios to explain their emission, namely the emission from accelerated particles within the SNRs spatially coincident with each source, molecular clouds illuminated by cosmic rays from the close-by SNRs, and a pulsar/PWN origin. Our new emph{Fermi}-LAT results and the radio and flux X-ray upper limits pose severe constraints on some of these models.
The breakthrough developments of Cherenkov telescopes in the last decade have led to angular resolution of 0.1{deg} and an unprecedented sensitivity. This has allowed the current generation of Cherenkov telescopes to discover a population of supernov
a remnants (SNRs) radiating in very-high-energy (VHE, E>100 GeV) gamma-rays. A number of those VHE SNRs exhibit a shell-type morphology spatially coincident with the shock front of the SNR. The members of this VHE shell SNR club are RX J1713.7-3946, Vela Jr, RCW 86, SN 1006, and HESS J1731-347. The latter two objects have been poorly studied in high-energy (HE, 0.1<E<100 GeV) gamma-rays and need to be investigated in order to draw the global picture of this class of SNRs and constrain the characteristics of the underlying population of accelerated particles. Using 6 years of Fermi P7 reprocessed data, we studied the HE counterpart of the SNRs HESS J1731-347 and SN 1006. The two SNRs are not detected in the data and given that there is no hint of detection, we do not expect any detection in the coming years from the SNRs. However in both cases, we derived upper limits that significantly constrain the gamma-ray emission mechanism and can rule out a standard hadronic scenario with a confidence level > 5 sigma. With this Fermi analysis, we now have a complete view of the HE to VHE gamma-ray emission of TeV shell SNRs. All five sources have a hard HE photon index (<1.8) suggesting a common scenario where the bulk of the emission is produced by accelerated electrons radiating from radio to VHE gamma-rays through synchrotron and inverse Compton processes. In addition when correcting for the distance, all SNRs show a surprisingly similar gamma-ray luminosity supporting the idea of a common emission mechanism. While the gamma-ray emission is likely to be leptonic dominated, this does not rule out efficient hadron acceleration in those SNRs.
We report on gamma-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 rep
orted in this region and was associated with HESS J1640-465. With an increased dataset 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 Gamma = 2.47 +/- 0.05 +/- 0.06 and presents no significant gamma-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 Gamma = 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 gamma-ray sources.
The Vela supernova remnant is the closest supernova remnant to Earth containing an active pulsar, the Vela pulsar (PSR B0833-45). This pulsar is the archetype of the middle-aged pulsar class and powers a bright pulsar wind nebula (PWN), Vela-X, spann
ing a region of 2{deg} x 3{deg} south of the pulsar and observed in the radio, X-ray and very high energy gamma-ray domains. The detection of the Vela-X PWN by the Fermi Large Area Telescope (LAT) was reported in the first year of the mission. Subsequently, we have re-investigated this complex region and performed a detailed morphological and spectral analysis of this source using 4 years of Fermi-LAT observations. This study lowers the threshold for morphological analysis of the nebula from 0.8 GeV to 0.3 GeV, allowing inspection of distinct energy bands by the LAT for the first time. We describe the recent results obtained on this PWN and discuss the origin of the newly detected spatial features.
Several young supernova remnants (SNRs) have recently been detected in the high-energy and very-high-energy gamma-ray domains. As exemplified by RX J1713.7-3946, the nature of this emission has been hotly debated, and direct evidence for the efficien
t acceleration of cosmic-ray protons at the SNR shocks still remains elusive. We 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. In the hadronic scenario, the derived HE upper limits together with the HESS measurements in the VHE domain can only be accommodated by a spectral index Gamma <= 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. 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 muG. For these two scenarios, we assessed the level of acceleration efficiency. 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.
Forty six gamma-ray pulsars were reported in the First Fermi Large Area Telescope (LAT) Catalog of Gamma-ray Pulsars. Over forty more have been seen since then. A simple but effective figure-of-merit for gamma-detectability is sqrt(Edot)/d^2, where E
dot is the pulsar spindown power and d the distance. We are tracking down the best gamma-ray candidates not yet seen. We present the timing and spectral analysis results of some new high spindown power, nearby gamma-ray pulsars. We also update some population distribution plots in preparation for the 2nd Fermi LAT gamma-ray Pulsar Catalog.
Particle acceleration in massive star forming regions can proceed via a large variety of possible emission scenarios, including high-energy gamma-ray production in the colliding wind zone of the massive Wolf-Rayet binary (here WR 20a and WR 121a), co
llective wind scenarios, diffusive shock acceleration at the boundaries of wind-blown bubbles in the stellar cluster, and outbreak phenomena from hot stellar winds into the interstellar medium. In view of the recent Fermi-LAT detection of HESS J1023-575 (in the vicinity of Westerlund 2), we examine another very high energy (VHE) gamma-ray source, HESS J1848-0145 (in the vicinity of W43), possibly associated with a massive star cluster. Considering multi-wavelength data, in particular TeV gamma-rays, we examine the available evidence that the gamma-ray emission coincident with Westerlund 2 and W43 could originate in particles accelerated by the above-mentioned mechanisms in massive star clusters.
The shell-type supernova remnant RX J0852.0-4622 was detected in 2004 and re-observed between December 2004 and May 2005 with the High Energy Stereoscopic System (H.E.S.S.), an array of four Imaging Cherenkov Telescopes located in Namibia and dedicat
ed to the observations of gamma-rays above 100 GeV. The angular resolution of <0.1 degree and the large field of view of H.E.S.S. (5 degrees diameter) are well adapted to studying the morphology of the object in very high energy gamma-rays, which exhibits a remarkably thin shell very similar to the features observed in the radio range and in X-rays. The spectral analysis of the source from 300 GeV to 20 TeV will be presented. Finally, the possible origins of the very high energy gamma-ray emission (Inverse Compton scattering by electrons or the decay of neutral pions produced by proton interactions) will be discussed, on the basis of morphological and spectral features obtained at different wavelengths.
