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

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

Corresponding author: A. N. Parmar, aparmar@astro.estec.esa.nl

Received: 16 October 2000
Accepted: 18 December 2000

Abstract

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, α, of and a disk-blackbody with a temperature, kT, of keV and a projected inner-disk radius of 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 keV and a projected inner-disk radius of km. The spectrum of M 33 X-9 is rather hard with . 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.