Accretion Disk Spectra of the Brightest Ultra-luminous X-ray Source in M82

Abstract

Emission spectra of hot accretion disks characteristic of advection dominated accretion flow (ADAF) models are investigated for comparison with the brightest ultra-luminous source, X-1, in the galaxy M82. If the spectral state of the source is similar to the low luminosity hard state of stellar mass black holes in our Galaxy, a fit to the {\it Chandra} X-ray spectrum and constraints from the radio and infrared upper limits, require a black hole mass in the range of $9 \times 10^4 - 5 \times 10^5 \msun$. Lower black hole masses ($\la 10^4 \msun$) are possible if M82 X-1 corresponds to the high luminosity hard state of Galactic black hole X-ray binary sources. Both of these spectrally degenerate hot accretion disk solutions lead to an intermediate mass black hole interpretation for M82 X-1. Since these solutions have different spectral variability with X-ray luminosity and predict different infrared emission, they can be distinguished by future off axis {\it Chandra} observations or simultaneous sensitive infrared detections.