A Chandra ACIS-S imaging observation of the nearby galaxy M81 ( NGC 3031 ) reveals 9 luminous soft X-ray sources .
The local environments , X-ray spectral properties , and X-ray light curves of the sources are presented and discussed in the context of prevailing physical models for supersoft sources .
It is shown that the sample falls within expectations based on population synthesis models taken from the literature though the high observed luminosities ( L _ { obs } \sim 2 \times 10 ^ { 36 } to \sim 3 \times 10 ^ { 38 } erg-s ^ { -1 } in the 0.2–2.0 keV band ) and equivalent blackbody temperatures ( T _ { eff } \sim 40 to 80 eV ) place the brightest detected M81 objects at the high luminosity end of the class of supersoft sources defined by previous ROSAT and Einstein studies of nearby galaxies .
This is interpreted as a natural consequence of the higher sensitivity of Chandra to hotter and more luminous systems .
Most of the sources can be explained as canonical supersoft sources , accreting white dwarfs powered by steady surface nuclear burning , with X-ray spectra well-fit by hot white dwarf local thermodynamic equilibrium atmosphere models .
An exceptionally bright source is scrutinized in greater detail as its estimated bolometric luminosity , L _ { bol } \sim 1.5 \times 10 ^ { 39 } erg s ^ { -1 } , greatly exceeds theoretical estimates for supersoft sources .
This source may be beyond the stability limit and undergoing a phase of mass outflow under extreme conditions .
Alternatively , a model in which the observed X-ray spectrum arises from an accretion disk around a blackhole of mass \sim 1200/ ( cos i ) ^ { 1 / 2 } M _ { \odot } ( viewed at an inclination angle i ) can not be excluded .