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Astron. Astrophys. 350, 17-24 (1999)

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Very High Energy Gamma-ray spectral properties of Mkn 501 from CAT Cerenkov telescope observations in 1997 *

A. Djannati-Atai 7, F. Piron 4,**, A. Barrau 5, L. Iacoucci 4, M. Punch 7, J.-P. Tavernet 5, R. Bazer-Bachi 2, H. Cabot 3, L.-M. Chounet 4, G. Debiais 3, B. Degrange 4, J.-P. Dezalay 2, D. Dumora 1, P. Espigat 7, B. Fabre 3, P. Fleury 4, G. Fontaine 4, C. Ghesquière 7, P. Goret 8, C. Gouiffes 8, I.A. Grenier 8,9, S. Le Bohec 4,***, I. Malet 2, C. Meynadier 3, G. Mohanty 4, E. Nuss 3, E. Paré 4, J. Québert 1, K. Ragan 1,****, C. Renault 5, M. Rivoal 5, L. Rob 6, K. Schahmaneche 5 and D.A. Smith 1

1 Centre d'Etudes Nucléaire de Bordeaux-Gradignan, France (IN2P3/CNRS)
2 Centre d'Etudes Spatiales des Rayonnements, Toulouse, France (INSU/CNRS)
3 Groupe de Physique Fondamentale, Université de Perpignan, France (IN2P3/CNRS)
4 Laboratoire de Physique Nucléaire des Hautes Energies, Ecole Polytechnique, Palaiseau, France (IN2P3/CNRS)
5 Laboratoire de Physique Nucléaire et de Hautes Energies, Universités de Paris VI/VII, France (IN2P3/CNRS)
6 Nuclear Center, Charles University, Prague, Czech Republic
7 Physique Corpusculaire et Cosmologie, Collège de France, Paris, France (IN2P3/CNRS)
8 Service d'Astrophysique, Centre d'Etudes de Saclay, France (DAPNIA/CEA)
9 Université Paris VII, France

Received 28 May 1999 / Accepted 13 August 1999


The BL Lac object Mkn 501 went into a very high state of activity during 1997, both in VHE [FORMULA]-rays and X-rays. We present here results from observations at energies above [FORMULA] carried out between March and October 1997 with the CAT erenkov imaging Telescope.

The average differential spectrum between [FORMULA] and [FORMULA] shows significant curvature and is well represented by [FORMULA], with:


The TeV spectral energy distribution of Mkn 501 clearly peaks in the range [FORMULA]-[FORMULA]. Investigation of spectral variations shows a significant hardness-intensity correlation with no measurable effect on the curvature. This can be described as an increase of the peak TeV emission energy with intensity.

Simultaneous and quasi-simultaneous CAT VHE [FORMULA]-ray and BeppoSAX hard X-ray detections for the highest recorded flare on [FORMULA] April and for lower-activity states of the same period show correlated variability with a higher luminosity in X-rays than in [FORMULA]-rays.

The observed spectral energy distribution and the correlated variability between X-rays and [FORMULA]-rays, both in amplitude and in hardening of spectra, favour a two-component emission scheme where the low and high energy components are attributed to synchrotron and inverse Compton (IC) radiation, respectively.

Key words: galaxies: active – galaxies: nuclei – galaxies: BL Lacertae objects: individual: Mkn 501 – gamma rays: observations

* This article is dedicated to the memory of Eric Paré, one of the originators of CAT, who died in a road accident in July, 1998.
** Present address: Institut des Sciences Nucléaires, 53 avenue des Martyrs, F-38026 Grenoble cedex, France
*** Present address: Physics & Astronomy Department, Iowa State University, Ames, IA 50011, USA
**** Present address: Physics Department, McGill University, Montreal, H3A 2T8, Canada

Send offprint requests to: A. Djannati-Atai

Correspondence to: djannati@cdf.in2p3.fr

© European Southern Observatory (ESO) 1999

Online publication: September 24, 1999