Ultra Luminous X-ray sources: disc models and alternative scenarios

Details Ultra Luminous X-ray sources: disc models and alternative scenarios Ultra Luminous X-ray sources: disc models and alternative scenarios

Jun 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006sf2a.conf..153g&link_type=abstract

SF2A-2006: Proceedings of the Annual meeting of the French Society of Astronomy and Astrophysics Eds.: D. Barret, F. Casoli, G.

Astronomy and Astrophysics

Astrophysics

Scientific paper

It was suggested that phenomenological power-law plus cool disc-blackbody models represent the simplest, most robust interpretation of the X-ray spectra of bright ultraluminous X-ray sources (ULXs); this has been taken as evidence for the presence of intermediate-mass black holes in those sources. We assess this claim by comparing the cool disc-blackbody model with a range of other models. Using the XMM-Newton/EPIC spectra of NGC 4559 as an example, we show that ULX spectra can be fitted equally well by subtracting a disc-blackbody component from a dominant power-law component, thus turning a soft excess into a soft deficit. We also propose a more complex physical model, based on a power-law component slightly modified at various energies by smeared emission and absorption lines from highly-ionized, fast-moving gas. Our main conclusion is that the presence of a soft excess or a soft deficit depends on the energy range over which we choose to fit the ``true" power-law continuum, and that the observed deviations from such a continuum, which are usually modelled by disc-blackbody components, should not be taken as evidence for accretion disc emission, nor used to infer black hole masses.

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