Revealing a cool accretion disk in the ultraluminous X-ray source M81 X-9 (Holmberg IX X-1): Evidence for an intermediate-mass black hole

We report the results of an analysis of two XMM-Newton EPIC-pn spectra of the bright ultraluminous X-ray source M81 X-9 (Holmberg IX X-1) obtained in snapshot observations. Soft thermal emission is clearly revealed in spectra dominated by hard power-law components. Depending on the model used, M81 X-9 was observed at a luminosity of L-X (1.0-1.6) x 10(40) ergs s(-1) (0.3-10.0 keV). The variability previously observed in this source signals that it is an accreting source that likely harbors a black hole. Remarkably, accretion disk models for the soft thermal emission yield very low inner disk temperatures (kT 0.17-0.29 keV, including 90% confidence errors and variations between observations and disk models) and improve the fit statistic over any single-component continuum model at the 6 sigma level of confidence. This represents much stronger evidence for a cool disk than prior evidence that combined spectra from different observatories, and the strongest evidence of a cool disk in an ultraluminous X-ray source apart from that for NGC 1313 X-1. In common with NGC 1313 X-1, scaling the temperatures measured in M81 X-9 to those commonly seen in stellar-mass Galactic black holes at their highest observed fluxes (kT similar or equal to 1 keV) may imply that M81 X-9 harbors a black hole with a mass on the order of 10(3) M-circle dot; the measured disk component normalization and broadband luminosity imply black hole masses on the order of 10(2) M-circle dot. It is therefore possible that these sources harbor 10(3) M-circle dot black holes accreting at L-X similar or equal to 0.1 x L-Edd. While these results do not represent proof that M81 X-9 harbors an intermediate-mass black hole, radio and optical observations suggest that beaming and anisotropic emission from a stellar-mass black hole are unlikely to account for the implied luminosity. We further argue that the strength of the hard emission in these sources and well-established phenomena frequently observed in stellar-mass black holes near to the Eddington limit suggest that optically thick photospheres are unlikely to be the origin of the cool thermal emission in bright ultraluminous X-ray sources. For comparison to M81 X-9, we have also analyzed the previously unpublished EPIC-pn spectrum of NGC 1313 X-1; cool disk emission is again observed, and refined spectral fit parameters and mass estimates are reported.