A multi-dimension, time-dependent Monte Carlo code is used to compute sample γ-ray spectra to explore whether unambiguous constraints could be obtained from γ-ray observations of type Ia supernovae. Both spherical and aspherical geometries are considered and it is shown that moderate departures from sphericity can produce viewing-angle effects that are at least as significant as those caused by...

We use the GEANT4 Monte Carlo framework to calculate the γ-ray albedo of the Moon due to interactions of cosmic ray (CR) nuclei with moon rock. Our calculation of the albedo spectrum agrees with the EGRET data. We show that the spectrum of γ-rays from the Moon is very steep with an effective cutoff around 3–4 GeV (600 MeV for the inner part of the Moon disk) and exhibits a narrow pion-decay lin...

We report simultaneous multi-frequency observations of the blazar PG 1553+113, that were carried out in March-April 2008. Optical, X-ray, high-energy (HE: ≥ 100 MeV) γ-ray, and very-highenergy (VHE: ≥ 100 GeV) γ-ray data were obtained with the KVA, REM, RossiXTE/ASM, AGILE and MAGIC telescopes. This is the first simultaneous broad-band (i.e., HE+VHE) γ-ray observation of a blazar. The source sp...

We suggest a short-living supermassive magnetar model to account for the X-ray flares following short γ−ray bursts. In this model, the central engine of the short γ−ray bursts is a supermassive millisecond magnetar, that is formed in double neutron stars coalescence. The X-ray flares are powered by the dipole radiation of the magnetar. When the magnetar has lost a significant part of its angula...

For almost one year the Large Area Telescope on board the Fermi observatory has been surveying high-energy phenomena in our Universe. We will present an overview of the status of the mission and of some results from the first year of observations, focusing on the topics of particular interest for the high-energy Physics community: detection of high-energy γ-ray bursts, the discovery of new popu...

A remarkable number of pulsar wind nebulae (PWN) are coincident with EGRET γ-ray sources. X-ray and radio imaging studies of unidentified EGRET sources have resulted in the discovery of at least 6 new pulsar wind nebulae (PWN). Stationary PWN (SPWN) appear to be associated with steady EGRET sources with hard spectra, typical for γ-ray pulsars. Their toroidal morphologies can help determine the ...

We have compared the radio emission from a sample of parsec-scale AGN jets as measured by the VLBA at 15 GHz, with their associated γ-ray properties that are reported in the Fermi LAT 3-month bright source list. We find in our radio-selected sample that the γ-ray photon flux correlates well with the quasi-simultaneously measured compact radio flux density. The LAT-detected jets in our radio-sel...

We re-examine the outer gap size by taking the geometry of dipole magnetic field into account. Furthermore, we also consider that instead of taking the gap size at half of the light cylinder radius to represent the entire outer gap it is more appropriate to average the entire outer gap size over the distance. When these two factors are considered, the derived outer gap size f(P,B,< r > (α)) is ...

Supernova remnants have long been suggested as a class of potential counterparts to unidentified γ-ray sources. The mechanisms by which such γ-rays can arise may include emission from a pulsar associated with a remnant, or a variety of processes associated with energetic particles accelerated by the SNR shock. Imaging and spectral observations in the X-ray band can be used to identify propertie...