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Astron. Astrophys. 348, 43-62 (1999)


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Broad band X-ray spectra of M31 sources with BeppoSAX

G. Trinchieri 1, G.L. Israel 2, L. Chiappetti 3, T. Belloni 1,4, L. Stella 2, F. Primini 5, P. Fabbiano 5 and W. Pietsch 6

1 Osservatorio Astronomico di Brera, via Brera 28, I-20121 Milano, Italy
2 Osservatorio Astronomico di Roma, via Frascati 33, I-00044 Roma, Italy
3 Istituto di Fisica Cosmica "G. Occhialini" (CNR), via Bassini 15, Milano, Italy
4 Astronomical Institute "A. Pannekoek" and Center for High Energy Astrophysics, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
5 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
6 Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstrasse, D-85748 Garching, Germany

Received 16 April 1999 / Accepted 4 June 1999

Abstract

We present the first spectral study of the X-ray emitting stellar sources in M31 in the energy band from [FORMULA] 0.1 to 10 keV. We find that the globular cluster sources have spectral characteristics consistent with those of the Milky Way object, namely that the spectrum can be described by a thermal model with [FORMULA] keV from [FORMULA] 2 to 10 keV. Evidence of high absorption in some of these sources is most likely an indication that they lie in or behind the HI ring in the disk of the galaxy. We also find one peculiar globular cluster source, with spectral characteristics more typically associated with either High Mass X-ray Binaries or black hole candidates. We therefore suggest that either the source has been wrongly identified with a globular cluster or that the system contains a Black Hole.

We confirm earlier report that the spectrum of the bulge of M31 is consistent with the superposition of many LMXB spectra. It is likely that a large fraction of the [FORMULA] keV detection obtained from the PDS instrument is associated with the bulge, thus extending the spectral data for this complex of sources up to [FORMULA] keV. The high energy part of the spectrum can be parameterized with typical LMXB spectra, while at low energies an additional component is required.

No significant variability is observed within the BeppoSAX observation, while a few sources appear to have varied (brightened) since ROSAT and Einstein observations.

Key words: galaxies: individual: M31 – galaxies: spiral – X-rays: galaxies

Send offprint requests to: G. Trinchieri

Correspondence to: ginevra@brera.mi.astro.it

© European Southern Observatory (ESO) 1999

Online publication: July 16, 1999

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