SIMBAD references

TheChandra X-Ray Observatory obtained a 50 ks observation of the central region of M81 using the ACIS-S in imaging mode. The global properties of the 97 X-ray sources detected in the inner 8'.3x8'.3 field of M81 are examined. Roughly half the sources are concentrated within the central bulge. The remainder are distributed throughout the disk, with the brightest disk sources lying preferentially along spiral arms. The average hardness ratios of both bulge and disk sources are consistent with power-law spectra of index Γ∼1.6, indicative of a population of X-ray binaries. A group of much softer sources is also present. The background-source-subtracted logN-logS distribution of the disk follows a power law of index ~-0.5 with no change in slope over three decades in flux. The logN-logS distribution of the bulge follows a similar shape but with a steeper slope above ∼4x10³⁷ ergs.s^–1. There is unresolved X-ray flux from the bulge with a radial profile similar to that of the bulge sources. This unresolved flux is softer than the average of the bulge sources, and extrapolating the bulge logN-logS distribution toward weaker sources can account for only 20% of the unresolved flux. No strong time variability was observed for any source with the exception of one bright, soft source.