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Astron. Astrophys. 330, 97-107 (1998)

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COMPTEL observations of Centaurus A at MeV energies
in the years 1991 to 1995

H. Steinle1, K. Bennett4, H. Bloemen2, W. Collmar1, R. Diehl1, W. Hermsen2, G.G. Lichti1, D. Morris3, V. Schönfelder1, A.W. Strong1 and O.R. Williams4

1 Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse, Postfach 1603, D-85740 Garching, Germany
2 SRON-Utrecht, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
3 University of New Hampshire, Institute for Studies of Earth, Oceans and Space, Durham, NH 03824, USA
4 Astrophysics Division, Space Science Departement of ESA/ESTEC, 2200 AG Noordwijk, The Netherlands

Received 24 July 1997 / Accepted 17 September 1997

Abstract

During the Compton Gamma-Ray Observatory (CGRO) observations made in Phase I, II, III, and IV/Cycle 4 in the years 1991 to 1995, the region on the sky including the radio galaxy Centaurus A was in the wide field-of-view of COMPTEL in 15 pointings of various durations.

The analysis of the COMPTEL data obtained during these 15 pointings shows emission consistent with the position of Cen A and we detected Cen A in two emission states (intermediate and low). Our data do not require, but also cannot rule out, that emission from a nearby X-ray source (MS1312.1-4221) contributes to the measured flux. But it is unlikely, that the high flux observed in the energy band 3 - 10 MeV during Phase II of the COMPTEL observations, is due to a significant contribution of this source.

The spectra in the COMPTEL energy range 0.75 - 30 MeV are consistent with the Centaurus A spectra in the energy range 0.05 - 4 MeV from OSSE and the 0.1 - 1 GeV data points from EGRET, both instruments also on board CGRO. The spectral form differs between the two observed emission states and we derive a [FORMULA] -ray luminosity in the total energy band from 50 keV to 1 GeV of [FORMULA] erg [FORMULA] and [FORMULA] erg [FORMULA] for isotropical emission in the the intermediate and low emission state, respectively.

The inclusion of the contemporaneous OSSE and EGRET spectra stretches the covered energy range in this observations over almost five decades and puts hard constraints on models for the [FORMULA] -ray emission of this AGN.

Key words: extragalactic astronomy – galaxies: active – BL Lacertae objects: individual: MS1312.1-4221 – galaxies: individual: Cen A – gamma-rays: observations

Send offprint requests to: H. Steinle (hcs@mpe-garching.mpg.de)

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© European Southern Observatory (ESO) 1998

Online publication: January 8, 1998
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