XMM-Newton observations of the brightest Ultraluminous X-ray sources

Abstract

We present an analysis of 13 of the best quality EPIC ULX datasets. We utilise the high signal-to-noise in these ULX spectra to investigate the best descriptions of their spectral shape in the 0.3-10 keV range. Simple models of an absorbed power-law or multicolour disc blackbody prove inadequate at describing the spectra. Better fits are found using a combination of these two components, with both variants of this model - a cool (0.2 keV) disc blackbody plus hard power-law continuum, and a soft power-law continuum plus a warm (1.7 keV) disc blackbody - providing good fits to 8/13 ULX spectra. However, by examining the data above 2 keV, we find evidence for curvature in the majority of datasets, inconsistent with the dominance of a power-law in this regime. The most successful empirical description of the spectra proved to be a combination of a cool (0.2 keV) classic blackbody spectrum, plus a warm disc blackbody (good fits to 10/13 ULX spectra). The best overall fits are provided by a physically self-consistent accretion disc plus Comptonised corona model (diskpn + eqpair), which fits acceptably to 11/13 ULXs. This model provides a physical explanation for the spectral curvature, namely that it originates in an optically-thick corona, though the accretion disc photons seeding this corona still originate in an apparently cool disc. We note similarities between this fit and models of Galactic black hole binaries at high accretion rates. We conclude that this analysis of the best spectral data for ULXs shows it to be plausible that the majority of the population are high accretion rate stellar-mass (perhaps up to 80-Msun) black holes, though we cannot categorically rule out the presence of larger, 1000-Msun intermediate-mass black holes (IMBHs) in individual sources with the current X-ray data.

17 pages, 3 figures, accepted for publication in MNRAS

Cited by in corpus (25)

• Super-critically accreting stellar-mass black holes as ultraluminous X-ray sources
• X-ray observations of ultraluminous X-ray sources
• Spectral State Transitions of the Ultraluminous X-ray Sources X-1 and X-2 in NGC 1313
• On the weakness of disc models in bright ULXs
• Optical emission from massive donors in ULX binary systems
• Testing the Paradigm that Ultraluminous X-ray Sources as a Class Represent Accreting Intermediate-Mass Black Holes
• Bridging the gap between stellar-mass black holes and ultraluminous X-ray sources
• Irradiation models for ULXs and fits to optical data
• Weighing the black holes in ultraluminous X-ray sources through timing
• New Hubble Space Telescope imaging of the counterparts to six ultraluminous X-ray sources
• Low-Level Nuclear Activity in Nearby Spiral Galaxies
• Chandra monitoring observations of the ultraluminous X-ray source NGC 5204 X-1
• Intermediate-mass black holes and ultraluminous X-ray sources in the Cartwheel ring galaxy
• Accretion Disk Spectra of the Brightest Ultra-luminous X-ray Source in M82
• Radio and X-ray Properties of Relativistic Beaming Models for Ultra-Luminous X-ray Sources
• Elemental Abundances of Nearby Galaxies through High Signal-to-Noise XMM-Newton Observations of ULXs
• New flaring of an ultraluminous X-ray source in NGC 1365
• The dependence of the estimated luminosities of ULX on spectral models
• Multi-band study of NGC 7424 and its two newly-discovered ULXs
• The X-ray properties of the dwarf Magellanic-type galaxy NGC 55
• A ULX associated with a cloud collision in M99
• Multi-epoch spectroscopy of the globular cluster black hole in NGC 4472
• Multiwavelength study of the bright X-ray source population in the interacting galaxies NGC5774/NGC5775
• Black hole mass estimates from soft X-ray spectra
• Detection of X-ray elongated emission from a ultraluminous X-ray source in the interacting pair of galaxies NGC 5953/5954