TeV blazars as seen by the CAT telescope

To date, only two extragalactic objects have been firmly established as very high-energy gamma-ray sources in the Northern sky: these are the two blazars Markarian 501 and Markarian 421. This paper reviews the most striking results obtained from these sources by the CAT atmospheric Cherenkov imaging telescope, with a particular emphasis on the 1999-2000 and 2000-2001 observation campaigns of Markarian 421.Comment: 4 pages, 5 figures, to appear in Proc. of the SF2A conference "Semaine de l'Astrophysique Francaise", May 28 - June 1st, 2001 (Lyon, France

Observations of the gamma-Ray Emission Above 250 Gev from the Blazars Markarian 501 and Markarian 421 by the Cat Cherenkov Atmospheric Imaging Telescope

The Very High Energy (VHE) gamma-ray emission of the closest BL Lacertae objects Markarian 501 and Markarian 421 has been observed by the CAT telescope in 1997 and 1998. In 1997 Mrk 501 exhibited a remarkable series of flares, with a VHE emission peaking above 250 GeV. The source showed correlated emissions in the X-ray and VHE gamma-ray bands, together with intensity-spectral hardness correlation in the latter energy range. During small flares in 1998, Mrk 421 became the second extragalactic source detected by CAT. Its spectral properties are compared to those of Mrk 501. Theoretical implications for jet astrophysics are briefly discussed.Comment: 4 pages, 3 figures. To appear in the proceedings of the XIth rencontres de Blois, June 27 - July 3, 199

Very high-energy gamma-ray sources as seen by the CAT imaging telescope

To date, only three objects have been firmly established as very high-energy gamma-ray sources in the Northern sky: the Crab nebula, which is a plerion, and the two blazars Markarian 501 and Markarian 421. This paper reviews the most striking results obtained for these sources by the CAT atmospheric Cherenkov imaging telescope.Comment: 4 pages, 7 figures, to appear in Proc. of the 36th Rencontres de Moriond on "Very High-Energy Phenomena in the Universe'' (Les Arcs, France

Gamma-Ray Bursts at high and very high energies

Gamma-Ray Bursts (GRBs) are extra-galactic and extremely energetic transient emissions of gamma rays, which are thought to be associated with the death of massive stars or the merger of compact objects in binary systems. Their huge luminosities involve the presence a newborn stellar-mass black hole emitting a relativistic collimated outflow, which accelerates particles and produces non-thermal emissions from the radio domain to the highest energies. In this article, I review recent progresses in the understanding of GRB jet physics above 100 MeV, based on Fermi observations of bright GRBs. I discuss the physical implications of these observations and their impact on GRB modeling, and I present some prospects for GRB observation at very high energies in the near future.Comment: 18 pages, 13 figures -- To be published in a topical review of Comptes Rendus Physique (French Academy of Sciences) on "Gamma-ray Astronomy", Eds. B. Degrange & G. Fontain

Calibration of the CAT Telescope

Due to the lack of test-beams in ground-based gamma-ray astronomy, detector calibration has been a major challenge in this field. However, with the use of Cherenkov ring-images due to cosmic-ray muons and of strong gamma-ray signals, the CAT telescope could be rather well monitored and understood. Here we present a few oustanding aspects of this work.Comment: 5 pages, 5 figures. To appear in the proceedings of the GeV-TeV Gamma Ray Astrophysics Workshop "Towards a Major Atmospheric Cherenkov Detector VI", Snowbird, Utah, August 13-16, 199

VHE gamma-ray spectral properties of the blazars Mrk 501 and Mrk 421 from CAT observations in 1997 and 1998

The Very High Energy (VHE) gamma-ray emission of the BL Lacertae objects Markarian 501 and Markarian 421 has been observed by the CAT Imaging Atmospheric Cherenkov Telescope in 1997 and 1998. The spectrum extraction method is presented, and the spectral properties of both sources are compared in different activity states. Theoretical implications for jet astrophysics are discussed.Comment: 4 figures, 6 pages. To appear in the proceedings of the GeV-TeV Gamma Ray Astrophysics Workshop "Towards a Major Atmospheric Cherenkov Detector VI", Snowbird, Utah, August 13-16, 199