A Chandra X-Ray Observatory ACIS-S imaging observation is used to study the population of X-ray sources in the nearby Sab galaxy M81 ( NGC 3031 ) .
A total of 177 sources are detected with 124 located within the D _ { 25 } isophote to a limiting X-ray luminosity of \sim 3 \times 10 ^ { 36 } ergs s ^ { -1 } .
Source positions , count rates , luminosities in the 0.3 – 8.0 keV band , limiting optical magnitudes , and potential counterpart identifications are tabulated .
Spectral and timing analysis of the 36 brightest sources are reported including the low-luminosity active galactic nucleus , SN 1993J , and the Einstein -discovered ultra-luminous X-ray source X6 .
The nucleus accounts for \sim 86 % , or 5 \times 10 ^ { 40 } ergs s ^ { -1 } , of the total X-ray emission from M81 .
Its spectrum is well-fit by an absorbed power law with photon index 1.98 \pm 0.08 consistent with previous observations ( average index 1.9 ) .
SN 1993J has softened and faded since its discovery .
At an age of 2594 days , SN 1993J displayed a complex thermal spectrum from a reverse shock rich in Fe L and highly-ionized Mg , Si , and S but lacking O .
A hard X-ray component , emitted by a forward shock , is also present .
X6 is spatially-coincident with a stellar object with optical brightness and colors consistent with an O9 – B1 main sequence star .
It is also coincident with a weak radio source with a flux density of \sim 95 \mu Jy at \lambda = 3.6 cm .
The continuum-dominated X-ray spectrum of X6 is most closely reproduced by a blackbody disk model suggesting the X-ray source is an \sim 18 M _ { \odot } object accreting at nearly its Eddington limit .

The non-nuclear point source population of M81 accounts for 88 % of the non-nuclear X-ray luminosity of 8.1 \times 10 ^ { 39 } ergs s ^ { -1 } .
The remaining ( unresolved ) X-ray emission is confined within \sim 2 kpc of the galactic center .
The spatial distribution of this emission and of the resolved X-ray bulge sources closely follows that of the bulge optical light .
In particular , there is no evidence for an X-ray signature accompanying the filamentary H \alpha or excess UV emission seen in the central \stackrel { \textstyle < } { \sim } 1.0 kpc of the galaxy .
The shape of the luminosity function of the bulge sources is a power law with a break at \sim 4 \times 10 ^ { 37 } ergs s ^ { -1 } ; suggesting the presence of an aging ( \sim 400 Myr ) population of low-mass X-ray binaries .
Extrapolating this luminosity function to lower luminosities accounts for only \sim 10 % of the unresolved X-ray emission .
Spectroscopically , the unresolved emission can be represented as a combination of soft , kT \sim 0.3 keV , optically-thin plasma emission and of a \Gamma = 1.6 power law .
The unresolved bulge X-ray emission is therefore most likely a combination of hot gas and of one or more large and distinct populations of low-luminosity X-ray sources confined in the gravitational potential and tracing the old population of bulge stars .
The distribution of disk sources shows a remarkably strong correlation with spiral arms with the brightest disk sources located closest to spiral arms .
The luminosity function of sources near the spiral arms is a pure power law ( slope -0.48 \pm 0.03 ) while that of sources further away exhibits a break or cut-off in the power law distribution with no high-luminosity members .
This is interpreted as a natural consequence of the passage of spiral density waves that leave the brightest ( when averaged over their lifetimes ) and shortest-lived X-ray sources immediately downstream of the spiral arms .
Consistent with model predictions , we conclude that the shapes of the X-ray luminosity functions of the different galactic components of M81 are most likely governed by the birth rates and lifespans of their constituent X-ray source populations and that the luminosity functions can be used as a measure of the star formation histories of their environments .