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The recent discovery by Bachetti et al. (2014) of a pulsar in M82 that canreach luminosities of up to 10^40 ergs s^-1, a factor of ~100 the Eddingtonluminosity for a 1.4 Msol compact object, poses a challenge for accretionphysics. In order to better understand the nature of this source and its dutycycle, and in the light of several physical models that have been subsequentlypublished, we conduct a spectral and temporal analysis of the 0.5-8 keV X-rayemission from this source from 15 years of Chandra observations. We fit theChandra spectra of the pulsar with a power-law model and a disk black bodymodel, subjected to interstellar absorption in M82. We carefully assess for theeffect of pile-up in our observations, where 4/19 observations have a pile-upfraction >10%, which we account for during spectral modeling with a convolutionmodel. When fitted with a power-law model, the average photon index when thesource is at high luminosity (L_X>10^39 ergs s^-1) is Gamma=1.33+/-0.15. Forthe disk black body model, the average temperature is T=3.24+/-0.65 keV,consistent with other luminous X-ray pulsars. We also investigated theinclusion of a soft excess component and spectral break, finding that thespectra are also consistent with these features common to luminous X-raypulsars. In addition, we present spectral analysis from NuSTAR over the 3-50keV range where we have isolated the pulsed component. We find that the pulsedemission in this band is best fit by a power-law with a high-energy cut-off,where Gamma=0.6+/-0.3 and E_C=14^{+5}_{-3} keV. While the pulsar has previouslybeen identified as a transient, we find from our longer-baseline study that ithas been remarkably active over the 15-year period, where for 9/19 (47%)observations that we analyzed, the pulsar appears to be emitting at aluminosity in excess of 10^39 ergs s^-1, greater than 10 times its Eddingtonlimit.

astrophysical journal/the astrophysical journal

Spectral and temporal properties of the ultra-luminous X-ray pulsar in  M82 from 15 years of Chandra observations and analysis of the pulsed emission  using NuSTAR