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Issue A&A
Volume 374, Number 3, August II 2001
Page(s) 895 - 906
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20010798



A&A 374, 895-906 (2001)
DOI: 10.1051/0004-6361:20010798

Temporal and spectral gamma-ray properties of Mkn 421 above 250 GeV from CAT observations between 1996 and 2000

F. Piron1, A. Djannati-Ataï2, M. Punch2, J.-P. Tavernet3, A. Barrau3, R. Bazer-Bachi4, L.-M. Chounet1, G. Debiais5, B. Degrange1, J.-P. Dezalay4, P. Espigat2, B. Fabre5, P. Fleury1, G. Fontaine1, P. Goret6, C. Gouiffes6, B. Khelifi2, I. Malet4, C. Masterson2, G. Mohanty1, E. Nuss5, C. Renault3, M. Rivoal3, L. Rob7 and S. Vorobiov1

1  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

(Received 10 April 2001 / Accepted 30 May 2001 )

Abstract
The $\gamma$-ray emission above 250 GeV 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 5 TeV, which is well fitted with a power law: $\frac{\mathrm{d}\phi}{\mathrm{d}E}\varpropto E_{\mathrm{TeV}}^{-2.88\pm0.12^\mathrm{stat}\pm0.06^\mathrm{syst}}$. 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 5 TeV. The possibility of TeV spectral hardening during flares is also discussed, and the results are compared to those obtained on the other TeV BL Lac, Markarian 501.


Key words: galaxies: active -- galaxies: nuclei -- BL Lacertæobjects: individual: Mkn 421 -- gamma-rays: observations

Offprint request: F. Piron, piron@in2p3.fr

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© ESO 2001


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