Chandra X-ray observations of the spiral galaxy M81

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 similar to3x10(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 ultraluminous X-ray source X6. The nucleus accounts for similar to86%, or 5x10(40) ergs s(-1), of the total X-ray emission from M81. Its spectrum is well fitted by an absorbed power law with photon index 1.98+/-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 similar to95 muJy 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 similar to18 M(circle dot) 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 8.1x10(39) ergs s(-1). The remaining (unresolved) X-ray emission is confined within similar to2 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 Halpha or excess UV emission seen in the central less than or similar to1.0 kpc of the galaxy. The shape of the luminosity function of the bulge sources is a power law with a break at similar to4x10(37) ergs s(-1); suggesting the presence of an aging (similar to400 Myr) population of low-mass X-ray binaries. Extrapolating this luminosity function to lower luminosities accounts for only similar to10% of the unresolved X-ray emission. Spectroscopically, the unresolved emission can be represented as a combination of soft, kTsimilar to0.3 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+/-0.03), while that of sources further away exhibits a break or cutoff 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.