HESS J1640-465 and HESS J1641-463: two intriguing TeV sources in the light of new Fermi LAT observations

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 reported 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.

Constraints on cosmic-ray efficiency in the supernova remnant RCW 86 using multi-wavelength observations

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 efficient 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.

Acceleration of cosmic rays at supernova remnant shocks: constraints from gamma-ray observations

In the past few years, gamma-ray astronomy has entered a golden age. At TeV energies, only a handful of sources were known a decade ago, but the current generation of ground-based imaging atmospheric Cherenkov telescopes has increased this number to more than one hundred. At GeV energies, the Fermi Gamma-ray Space Telescope has increased the number of known sources by nearly an order of magnitude in its first 2 years of operation. The recent detection and unprecedented morphological studies of gamma-ray emission from shell-type supernova remnants is of great interest, as these analyses are directly linked to the long standing issue of the origin of the cosmic-rays. However, these detections still do not constitute a conclusive proof that supernova remnants accelerate the bulk of Galactic cosmic-rays, mainly due to the difficulty of disentangling the hadronic and leptonic contributions to the observed gamma-ray emission. In this talk, I will review the most relevant cosmic ray related results of gamma ray astronomy concerning supernova remnants.

Fermi-LAT detection of gamma-ray emission in the vicinity of the star forming regions W43 and Westerlund 2

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), collective 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.

Fermi-LAT Search for Pulsar Wind Nebulae around gamma-ray Pulsars

The high sensitivity of the Fermi-LAT (Large Area Telescope) offers the first opportunity to study faint and extended GeV sources such as pulsar wind nebulae (PWNe). After one year of observation the LAT detected and identified three pulsar wind nebulae: the Crab Nebula, Vela-X and the PWN inside MSH 15-52. In the meantime, the list of LAT detected pulsars increased steadily. These pulsars are characterized by high energy loss rates from ~3 times 10^{33} erg s$^{-1}$ to 5 times 10$^{38}$ erg s$^{-1}$ and are therefore likely to power a PWN. This paper summarizes the search for PWNe in the off-pulse windows of 54 LAT-detected pulsars using 16 months of survey observations. Ten sources show significant emission, seven of these likely being of magnetospheric origin. The detection of significant emission in the off-pulse interval offers new constraints on the gamma-ray emitting regions in pulsar magnetospheres. The three other sources with significant emission are the Crab Nebula, Vela-X and a new pulsar wind nebula candidate associated with the LAT pulsar PSR J1023-5746, coincident with the TeV source HESS J1023-575. We further explore the association between the H.E.S.S. and the Fermi source by modeling its spectral energy distribution. Flux upper limits derived for the 44 remaining sources are used to provide new constraints on famous PWNe that have been detected at keV and/or TeV energies.

Fermi Large Area Telescope observations of the Vela-X Pulsar Wind Nebula

We report on gamma-ray observations in the off-pulse window of the Vela pulsar PSR B0833-45, using 11 months of survey data from the Fermi Large Area Telescope (LAT). This pulsar is located in the 8 degree diameter Vela supernova remnant, which contains several regions of non-thermal emission detected in the radio, X-ray and gamma-ray bands. The gamma-ray emission detected by the LAT lies within one of these regions, the 2*3 degrees area south of the pulsar known as Vela-X. The LAT flux is signicantly spatially extended with a best-fit radius of 0.88 +/- 0.12 degrees for an assumed radially symmetric uniform disk. The 200 MeV to 20 GeV LAT spectrum of this source is well described by a power-law with a spectral index of 2.41 +/- 0.09 +/- 0.15 and integral flux above 100 MeV of (4.73 +/- 0.63 +/- 1.32) * 10^{-7} cm^{-2} s^{-1}. The first errors represent the statistical error on the fit parameters, while the second ones are the systematic uncertainties. Detailed morphological and spectral analyses give strong constraints on the energetics and magnetic field of the pulsar wind nebula (PWN) system and favor a scenario with two distinct electron populations.

H.E.S.S. observations of the supernova remnant RX J0852.0-4622

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 dedicated 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.