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)×10⁴⁰ergss^-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 σ 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~=1 keV) may imply that M81 X-9 harbors a black hole with a mass on the order of 10³ M_solar the measured disk component normalization and broadband luminosity imply black hole masses on the order of 10² M_solar. It is therefore possible that these sources harbor 10³ M_solar black holes accreting at L_X~=0.1×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 For comparison to M81 X-9, we have also analyzed the previously unpublished EPIC-pn spectrum of NGC 1313 X-1 cool disk emission is again observed, and refined spectral fit parameters and mass estimates are reported.