2003ApJS..144..213S

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 D₂₅ isophote to a limiting X-ray luminosity of ∼3x10³⁶ 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 ∼86%, or 5x10⁴⁰ 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 ∼95 µJy at λ=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 ∼18 M_☉ 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 8.1x10³⁹ ergs.s^–1. The remaining (unresolved) X-ray emission is confined within ∼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 Hα or excess UV emission seen in the central ≲1.0 kpc of the galaxy. The shape of the luminosity function of the bulge sources is a power law with a break at ∼4x10³⁷ ergs.s^–1; suggesting the presence of an aging (∼400 Myr) population of low-mass X-ray binaries. Extrapolating this luminosity function to lower luminosities accounts for only ∼10% of the unresolved X-ray emission. Spectroscopically, the unresolved emission can be represented as a combination of soft, kT∼0.3 keV, optically thin plasma emission and of a Γ=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.