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 \times 10 ^ { 40 } ~ { } { erg } ~ { } { s } ^ { 1 } ( 0.3–10.0 keV ) .
The variability previously observed in this source signals that it is an accreting source which 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 which combined spectra from different observatories , and the strongest evidence of a cool disk in an ultraluminous X-ray source apart from NGC 1313 X-1 .
Like 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 \simeq 1 keV ) may imply that M81 X-9 harbors a black hole with a mass on the order of 10 ^ { 3 } ~ { } M _ { \odot } ; the measured disk component normalization and broad-band luminosity imply black hole masses on the order of 10 ^ { 2 } ~ { } M _ { \odot } .
It is therefore possible that these sources harbor 10 ^ { 3 } ~ { } M _ { \odot } black holes accreting at L _ { X } \simeq 0.1 \times 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For comparison to M81 X-9 , we have also analyzed the previously unpublished EPIC-pn spectrum of NGC 1313 X-1 ; cool disk emission is again observed and refined spectral fit parameters and mass estimates are reported .