I present a review of Fermi observations of Gamma-Ray Burst during the first year of data survey.

The LAT instrument on the Fermi mission will reveal the rich spectral and temporal gamma-ray burst phenomena in the > 100 MeV band. The synergy with Fermi's GBM detectors will link these observations to those in the well explored 10-1000 keV range; the addition of the > 100 MeV band observations will resolve theoretical uncertainties about burst emission in both the prompt and afterglow phases. Trigger algorithms will be applied to the LAT data both onboard the spacecraft and on the ground. The sensitivity of these triggers will differ because of the available computing resources onboard and on the ground. Here we present the LAT's burst detection methodologies and the instrument's GRB capabilities.

We seek to understand the propagation mechanisms of positrons in theinterstellar medium (ISM). This understanding is a key to determine whether thespatial distribution of the annihilation emission observed in our Galaxyreflects the spatial distribution of positron sources and, therefore, makes itpossible to place constraints on the origin of positrons. We review thedifferent processes that are likely to affect the transport of positrons in theISM. These processes fall into three broad categories: scattering offmagnetohydrodynamic waves, collisions with particles of the interstellar gasand advection with large-scale fluid motions. We assess the efficiency of eachprocess and describe its impact on the propagation of positrons. We alsodevelop a model of positron propagation, based on Monte-Carlo simulations,which enable us to estimate the distances traveled by positrons in thedifferent phases of the ISM. We find that low-energy (< 10 MeV) positronsgenerally have negligible interactions with magnetohydrodynamic waves, insofaras these waves are heavily damped. Positron propagation is mainly controlled bycollisions with gas particles. Under these circumstances, positrons can travelhuge distances (up to 30 kpc/n_H for 1 MeV positrons) along magnetic fieldlines before annihilating.

The High Energy Stereoscopic System (H.E.S.S.) consists of four ImagingAtmospheric Cherenkov Telescopes (IACTs) in Namibia for the detection of cosmicvery-high-energy (VHE) gamma-rays. Gamma-ray bursts (GRBs) are among thepotential VHE gamma-ray sources. VHE gamma-emission from GRBs is predicted bymany GRB models. Because of its generally fast-fading nature in many wavebands,the time evolution of any VHE gamma-radiation is still unknown. In order toprobe the largely unexplored VHE gamma-ray spectra of GRBs, a GRB observingprogram has been set up by the H.E.S.S. collaboration. With the highsensitivity of the H.E.S.S. array, VHE gamma-ray flux levels predicted by GRBmodels are well within reach. We report the H.E.S.S. observations of andresults from some of the reported GRB positions during March 2003 - May 2006.

We present an upper limit of galactic diffuse gamma-ray flux, measured withthe GAMMA experiment at energy about 175 TeV. The results were obtained usingselection of muon poor extensive air showers at mountain level (700 g/cm^2, Mt.Aragats, Armenia) for 5 GeV muon energy threshold. An upper limit for thedifferential flux at energy E=175+/-25 TeV is equal to (5.8-7.0)x10^-12 [ergm^2 s sr]^-1 for 95% confidence level.

In the survey of the Galactic plane conducted with H.E.S.S., many VHEgamma-ray sources were discovered for which no clear counterpart at otherwavelengths could be identified. HESS J1731-347 initially belonged to thissource class. Recently however, the new shell-type supernova remnant (SNR)G353.6-0.7 was discovered in radio data, positionally coinciding with the VHEsource. We will present new X-ray observations that cover a fraction of the VHEsource, revealing nonthermal emission that most likely can be interpreted assynchrotron emission from high-energy electrons. This, along with a largerH.E.S.S. data set which comprises more than twice the observation time used inthe discovery paper, allows us to test whether the VHE source may indeed beattributed to shell-type emission from that new SNR. If true, this would makeHESS J1731-347 a new object in the small but growing class of non-thermalshell-type supernova remnants with VHE emission.

Observations by the Large Area Telescope (LAT) on the \textit{Fermi} mission of diffuse $\gamma$-rays in a mid-latitude region in the third quadrant (Galactic longitude $l$ from $200\arcdeg$ to $260\arcdeg$ and latitude $| b |$ from $22\arcdeg$ to $60\arcdeg$) are reported. The region contains no known large molecular cloud and most of the atomic hydrogen is within 1 kpc of the solar system. The contributions of $\gamma$-ray point sources and inverse Compton scattering are estimated and subtracted. The residual $\gamma$-ray intensity exhibits a linear correlation with the atomic gas column density in energy from 100 MeV to 10 GeV. The measured integrated $\gamma$-ray emissivity is $(1.63 \pm 0.05) \times 10^{-26} {\rm photons s^{-1} sr^{-1} H\mathchar`-atom^{-1}}$ and $(0.66 \pm 0.02) \times 10^{-26} {\rm photons s^{-1} sr^{-1} H\mathchar`-atom^{-1}}$ above 100 MeV and above 300 MeV, respectively, with additional systematic error of $\sim 10%$. The differential emissivity in 100 MeV--10 GeV agrees with calculations based on cosmic ray spectra consistent with those directly measured, at the 10% level. The results obtained indicate that cosmic ray nuclei spectra within 1 kpc from the solar system in regions studied are close to the local interstellar spectra inferred from direct measurements at the Earth within $\sim 10%$.