[AGN, Quasars, Black Holes -- Oral ]
Accretion states of ultraluminous X-ray sources
Roberto Soria, University College London
Doug Swartz (NASA/MSFC) et al.
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