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M82 hosts two well-known ultraluminous X-ray sources (ULXs). X-1, anintermediate-mass black hole (IMBH) candidate, and X-2, an ultraluminous X-raypulsar (ULXP). Here we present a broadband X-ray spectral analysis of bothsources based on ten observations made simultaneously with Chandra and NuSTAR.Chandra provides the high spatial resolution to resolve the crowded field inthe 0.5--8 keV band, and NuSTAR provides the sensitive hard X-ray spectraldata, extending the bandpass of our study above 10 keV. The observations, takenin the period 2015--2016, cover a period of flaring from X-1, allowing us tostudy the spectral evolution of this source with luminosity. During four ofthese observations, X-2 was found to be at a low flux level, allowing anunambiguous view of the emission from X-1. We find that the broadband X-rayemission from X-1 is consistent with that seen in other ULXs observed in detailwith NuSTAR, with a spectrum that includes a broadened disk-like component anda high-energy tail. We find that the luminosity of the disk scales with innerdisk temperature as L~T^-3/2 contrary to expectations of a standard accretiondisk and previous results. These findings rule out a thermal state forsub-Eddington accretion and therefore do not support M82 X-1 as an IMBHcandidate. We also find evidence that the neutral column density of thematerial in the line of sight increases with L$_X$, perhaps due to an increasedmass outflow with accretion rate. For X-2, we do not find any significantspectral evolution, but we find the spectral parameters of the phase-averagedbroadband emission are consistent with the pulsed emission at the highest X-rayluminosities.

Spectral evolution of the ultraluminous X-ray sources M82 X-1 and X-2