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 thebright ultraluminous X-ray source M81 X-9 (Holmberg IX X-1), obtained insnapshot observations. Soft thermal emission is clearly revealed in spectradominated by hard power-law components. Depending on the model used, M81 X-9was observed at a luminosity of L_X = 1.0-1.6 E+40 erg/s (0.3--10.0 keV). Thevariability previously observed in this source signals that it is an accretingsource which likely harbors a black hole. Remarkably, accretion disk models forthe soft thermal emission yield very low inner disk temperatures (kT =0.17-0.29 keV, including 90% confidence errors and variations betweenobservations and disk models), and improve the fit statistic over anysingle-component continuum model at the 6 sigma level of confidence. Like NGC1313 X-1, scaling the temperatures measured in M81 X-9 to those commonly seenin stellar-mass Galactic black holes at their highest observed fluxes (kT ~ 1keV) may imply that M81 X-9 harbors a black hole with a mass on the order of10^3 Msun; the measured disk component normalization and broad-band luminosityimply black hole masses on the order of 10^2 Msun. It is therefore possiblethat these sources harbor 10^3 Msun black holes accreting such at L_X ~0.1*L_Edd. Radio and optical observations suggest that beaming and anisotropicemission from a stellar-mass black hole are unlikely to account for the impliedluminosity. We further argue that the strength of the hard emission in thesesources and well-established phenomena frequently observed in stellar-massblack holes near to the Eddington limit suggest that optically-thickphotospheres are unlikely to be the origin of the cool thermal emission inbright ULXs.Comment: 8 pages, 6 figres, 3 in color, uses emulateapj.sty and apjfonts.sty, ApJ accepte