Accretion states of ultraluminous X-ray sources

Details Accretion states of ultraluminous X-ray sources Accretion states of ultraluminous X-ray sources

Sep 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009cfdd.confe..59s&link_type=abstract

Chandra's First Decade of Discovery, Proceedings of the conference held 22-25 September, 2009 in Boston, MA. Edited by Scott Wo

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Accretion, Winds, And Outflows

Scientific paper

Ultraluminous X-ray sources (ULXs) have extended our knowledge of accretion onto black holes, and in particular of their different ``states'' as a function of accretion rate. At moderate luminosities (˜ 1E39-1E40 erg/s), the X-ray spectra of most ULXs are either fitted by non-standard accretion disks (eg, slim disks) or by a power-law, consistent with inverse-Compton emission (probably an extension of the ``steep-power-law'' state of Galactic black holes). At the highest luminosities (>˜ 1E40 erg/s), most ULXs have a power-law dominated spectrum; in particular, about half of them have hard photon indices (high/hard state, Gamma <˜ 1.7). In addition, two more elements are often found: a thermal ``soft excess'' is the signature of the standard thin disk at large radii, which constrains the radial size of the inner Comptonizing/outflow region; and a break or downturn of the power-law at ˜ 5 keV provides a characteristic temperature of the electrons in the inner region. Thus, the physics of super-Eddington accretion states correlates with that of the low states, with different systems dominated either by energy advection, or mechanical output, or Comptonizing coronae. Another intriguing issue we will discuss is whether there is a cutoff in the luminosity distribution at ˜ a few E40 erg/s, which would still be consistent with stellar black holes formed from direct collapse in metal-poor environments (maximum mass ˜ 70 Msun). If the power-law distribution extends to higher luminosities, it requires more massive black holes, perhaps formed from mergers in dense stellar/protostellar cluster environments

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