XMM–Newtonobservations of the ultraluminous nuclear X-ray source in M 33

Abstract

Abstract. We present observations with XMM-Newton of M 33 X-8, the ultraluminous X-ray source (L 0.5−10 keV ≈ 2×10 39 erg/s) closest to the centre of the galaxy. The best-fit model is similar to the typical model of Galactic black holes in very high state. Comparison with previous observations indicates that the source is still in a very high state after about 20 years of observations. No state transition has been observed even during the present set of XMM-Newton observations. We estimate the lower limit of the mass of the black hole >6 M , but with proper parameters taking into account different effects, the best estimate becomes 12 M . Our analysis favours the hypothesis that M 33 X-8 is a stellar mass black hole candidate, in agreement with the findings of other authors. In addition, we propose a different model where the high luminosity of the source is likely to be due to orientation effects of the accretion disc and anisotropies in the Comptonized emission.

X-ray observations of ultraluminous X-ray sources

“…McClintock & Remillard Furthermore, the IMBH model is not the only model that fits ULX spectra. Several sources have been identified in which a variant of this model, where the disc component fits to the hard end of the spectrum, provides a far superior fit (Stobbart, Roberts & Warwick 2004;Foschini et al 2004;Feng & Kaaret 2005). As discussed by Roberts et al (2005) this variant of the empirical black hole spectrum model does not provide a physical model for the X-ray emission -for example there cannot be sufficient photons present in the vicinity of the black hole to produce the dominant soft power-law through Compton up-scattering.…”

XMM-Newton survey of the Local Group galaxy M 33

“…The brightest source (X-8 in the nomenclature of Long et al 1981, luminosity of 10 39−40 erg s −1 ) is the most luminous X-ray source in the Local Group of galaxies and coincides with the optical center of M 33. Its time variability (Dubus et al 1997), its point-like nature seen by ROSAT HRI (Pietsch & Haberl 2000) and Chandra (Dubus & Rutledge 2002) and its X-ray spectrum best described by an absorbed power law plus disk blackbody model (e.g., Ehle et al 2001;La Parola et al 2003;Foschini et al 2004), point towards a black hole XRB. A possible periodicity of 106 days was not confirmed in later observations (see Parmar et al 2001).…”

The nature of the ultraluminous X-ray sources inside galaxies and their relation to local QSOs

“…(a) Spectral properties -As far as the spectral properties are concerned, Foschini et al (2002a) give ULX photon indices from 1.1 to about 2.3, with an average of about 1.9. This close to the canonical value for broad line, unabsorbed AGN according to Piconcelli et al (2002).…”

“…(f) The source in NGC 4698 - Foschini et al (2002b) have reported that they have found what they call a background BL Lac object with z = 0.43 in NGC 4698 (z = 0.0033). This clearly can be interpreted as evidence for a local QSO with an intrinsic redshift of 0.43 being ejected from NGC 4698.…”

“…These discoveries, due to the high resolution properties of Chandra and XMM Newton, show that ultraluminous sources are present in NGC 1073, M 82, 3628, 4038-39, 4151, 4565, 4698, 5204 and other galaxies (Strickland et al 2001;Fabbiano et al 2001;Foschini et al 2002a,b;Kaaret et al 2001;Makishima et al 2000;Wu et al 2002;Colbert & Ptak 2002). A recent study of a sample of nearby Seyfert galaxies with XMM-Newton has shown that Send offprint requests to: G. Burbidge, e-mail: gburbidge@ucsd.edu they also contain many off-center compact X-ray sources with luminosities in the range 10 39 −10 40 erg s −1 (Foschini et al 2002b).…”