XMM‐Newton Spectra of Intermediate‐Mass Black Hole Candidates: Application of a Monte Carlo Simulated Model

Abstract

We present a systematic spectral analysis of six ultraluminous X-raysources (NGC 1313 X-1 and X-2, IC 342 X-1, Ho IX X-1, NGC 5408 X-1, andNGC 3628 X-1) observed with the XMM-Newton observatory. Theseextranuclear X-ray sources in nearby late-type galaxies have beenconsidered as intermediate-mass black hole candidates. We have performedMonte Carlo simulations of Comptonized multicolor blackbody accretiondisks. This unified and self-consistent spectral model assumes aspherically symmetric, thermal corona around each disk and accounts forthe radiation transfer in the Comptonization. We find that the modelprovides satisfactory fits to the XMM-Newton spectra of the sources. Thecharacteristic temperatures of the accretion disks (T_{in}), forexample, are in the range of ~0.05-0.3 keV, consistent with theintermediate-mass black hole interpretation. We find that the black holemass is typically about a few times 10^{3} M_{solar} andhas an accretion rate of ~10^{-6} to 10^{-5} M_{solar} yr^{-1}. For the spectra considered here, we find thatthe commonly used multicolor blackbody accretion disk model with anadditive power-law component, although not physical, provides a goodmathematical approximation to the Monte Carlo simulated model. However,the latter model provides additional constraints on the properties ofthe accretion systems, such as the disk inclination angles and coronaoptical depths.