Astronomy and Astrophysics – Astronomy
Scientific paper
Jul 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010aipc.1248..123r&link_type=abstract
X-RAY ASTRONOMY 2009; PRESENT STATUS, MULTI-WAVELENGTH APPROACH AND FUTURE PERSPECTIVES: Proceedings of the International Confer
Astronomy and Astrophysics
Astronomy
1
X-Ray Binary Stars, Brightness, Galactic Nuclei, Stellar Mass, Accretion Disks, X-Ray Binaries, Magnitudes And Colors, Luminosities, Galactic Center, Bar, Circumnuclear Matter, And Bulge, Masses, Infall, Accretion, And Accretion Disks
Scientific paper
Although a great deal of progress has been made towards understanding the nature of ultraluminous X-ray sources (ULXs) over the past decade, many basic issues remain to be settled. Chief amongst these is the mass of the accretor in these systems: what proportion of ULXs are powered by accretion onto an intermediate-mass black hole (IMBH), and how big are these objects? Or can we explain their extraordinary X-ray luminosities by exotic, extreme accretion rate processes onto more prosaic stellar-mass black holes? Here we report the results of two new studies using the best available XMM-Newton observations to probe the astrophysics of ULXs. A systematic study of ULX power spectra shows a significant proportion of luminous objects with unusually suppressed variability compared to standard black hole binaries and AGN. Furthermore, detailed spectral studies confirm that ULX spectra appear unlike any standard accretion state in the XMM-Newton band pass. Hence we infer that many ULXs contain small black holes operating in a new, super-Eddington ``ultraluminous'' accretion state. We show that ULX spectra appear to vary with accretion rate, being closest to standard states at around Eddington, with an optically thick corona becoming the characteristic feature as we reach super-Eddington rates. At the highest rates we see evidence for the emergence of a cool photosphere, likely related to a massive outflowing wind.
Done Christine
Gladstone Jeanette C.
Hetf L. M.
Roberts Timothy P.
Vaughan S. A.
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