  The recent discovery by Bachetti et al. (2014) of a pulsar in M82 that can
reach luminosities of up to 10^40 ergs s^-1, a factor of ~100 the Eddington
luminosity for a 1.4 Msol compact object, poses a challenge for accretion
physics. In order to better understand the nature of this source and its duty
cycle, and in the light of several physical models that have been subsequently
published, we conduct a spectral and temporal analysis of the 0.5-8 keV X-ray
emission from this source from 15 years of Chandra observations. We fit the
Chandra spectra of the pulsar with a power-law model and a disk black body
model, subjected to interstellar absorption in M82. We carefully assess for the
effect of pile-up in our observations, where 4/19 observations have a pile-up
fraction >10%, which we account for during spectral modeling with a convolution
model. When fitted with a power-law model, the average photon index when the
source is at high luminosity (L_X>10^39 ergs s^-1) is Gamma=1.33+/-0.15. For
the disk black body model, the average temperature is T=3.24+/-0.65 keV,
consistent with other luminous X-ray pulsars. We also investigated the
inclusion of a soft excess component and spectral break, finding that the
spectra are also consistent with these features common to luminous X-ray
pulsars. In addition, we present spectral analysis from NuSTAR over the 3-50
keV range where we have isolated the pulsed component. We find that the pulsed
emission 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 previously
been identified as a transient, we find from our longer-baseline study that it
has been remarkably active over the 15-year period, where for 9/19 (47%)
observations that we analyzed, the pulsar appears to be emitting at a
luminosity in excess of 10^39 ergs s^-1, greater than 10 times its Eddington
limit.
