EDP Sciences
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Volume 368, Number 2, March III 2001
Page(s) 420 - 430
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20010027

A&A 368, 420-430 (2001)
DOI: 10.1051/0004-6361:20010027

BeppoSAX spectroscopy of the luminous X-ray sources in M 33

A. N. Parmar1, L. Sidoli1, T. Oosterbroek1, P. A. Charles2, G. Dubus3, M. Guainazzi4, P. Hakala5, W. Pietsch6 and G. Trinchieri7

1  Astrophysics Division, Space Science Department of ESA, ESTEC, Postbus 299, 2200 AG Noordwijk, The Netherlands
2  Dept of Physics & Astronomy, University of Southampton, Southampton, Hants, SO17 1BJ, UK
3  Astronomical Institute "Anton Pannekoek", Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
4  XMM-Newton SOC, ESA Villafranca, Apartado 50727, 28080, Madrid Spain
5  Observatory & Astrophysics Lab., University of Helsinki, Helsinki, Finland
6  Max-Planck-Institut für Extraterrestrische Physik, 85740 Garching bei München, Germany
7  Osservatorio Astronomico di Brera, Via Bianchi 46, Merate 22055, Italy

(Received 16 October 2000 / Accepted 18 December 2000 )

The nearby galaxy M 33 was observed by the imaging X-ray instruments on-board BeppoSAX. Two observations at different phases of the 105.9 day intensity cycle of the luminous central source X-8 failed to reveal the expected modulation, suggesting that it is probably transitory. Similar behavior has been observed from several X-ray binary sources. This strengthens somewhat the idea that M 33 X-8 is a black hole accreting from a binary companion. The 0.2-10 keV spectrum of M 33 X-8 can best be modeled by an absorbed power-law with a photon index, $\alpha$, of $1.89 \pm ^{0.40} _{0.79}$ and a disk-blackbody with a temperature, kT, of $1.10 \pm 0.05$ keV and a projected inner-disk radius of $55.4 \pm ^{6.0} _{7.7}$ km. This spectral shape is in good agreement with earlier ASCA results. The 2-10 keV spectra of M 33 X-4, X-5, X-7, X-9 and X-10 are all consistent with power-law or bremsstrahlung models, whilst that of X-6 appears to be significantly more complex and may be reasonably well modeled with a disk-blackbody with $kT = 1.7 \pm 0.2$ keV and a projected inner-disk radius of $7 \pm 2$ km. The spectrum of M 33 X-9 is rather hard with $\alpha = 1.3$. Compared to earlier Einstein and ROSAT observations, M 33 X-7 and X-9 varied in intensity, M 33 X-4, X-6, and X-10 may have varied and M 33 X-5 remained constant.

Key words: galaxies: individual (M 33) -- galaxies: nuclei -- X-ray: galaxies

Offprint request: A. N. Parmar, aparmar@astro.estec.esa.nl

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© ESO 2001