Volume 374, Number 3, August II 2001
|Page(s)||895 - 906|
|Section||Cosmology (including clusters of galaxies)|
|Published online||15 August 2001|
Temporal and spectral gamma-ray properties of Mkn 421 above 250 GeV from CAT observations between 1996 and 2000
Laboratoire de Physique Nucléaire des Hautes Énergies, École Polytechnique, Palaiseau, France (IN2P3/CNRS)
2 Physique Corpuscul aire et Cosmologie, Collège de France et Université Paris VII, France (IN2P3/CNRS)
3 Laboratoire de Physique Nucléaire et de Hautes Energies, Universités Paris VI/VII, France (IN2P3/CNRS)
4 Centre d'Études Spatiales des Rayonnements, Université Paul Sabatier, Toulouse, France (INSU/CNRS)
5 Groupe de Physique Fondamentale, Université de Perpignan, France
6 Service d'Astrophysique, Centre d'Études de Saclay, France (CEA/DSM/DAPNIA)
7 Nuclear Center, Charles University, Prague, Czech Republic
Corresponding author: F. Piron, email@example.com
Accepted: 30 May 2001
The γ-ray emission above from the BL Lac object Markarian 421 was observed by the CAT Cherenkov imaging telescope between December, 1996, and June, 2000. In 1998, the source produced a series of small flares, making it the second extragalactic source detected by CAT. The time-averaged differential spectrum has been measured from 0.3 to , which is well fitted with a power law: . In 2000, the source showed an unprecedented activity, with variability time-scales as short as one hour, as for instance observed during the night between 4 and 5 February. The 2000 time-averaged spectrum measured is compatible with that of 1998, but some indication of a spectral curvature is found between 0.3 and . The possibility of spectral hardening during flares is also discussed, and the results are compared to those obtained on the other BL Lac, Markarian 501.
Key words: galaxies: active / galaxies: nuclei / BL Lacertæobjects: individual: Mkn 421 / gamma-rays: observations
© ESO, 2001
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