2.2. M81

As discussed in Fabbiano (1995), M81 (NGC 3031) is a nearby (3.6 Mpc, Freedman et al. 1994) Sb galaxy optically similar to M31; however, in X-rays it displays a significantly more luminous population of individual sources (even discounting the nuclear AGN). To get a feel of the progress in sensitivity of X-ray telescopes in the last ~ 20 years, it is interesting to compare the Einstein observations of M81, where 9 extra-nuclear sources with L_X 2 × 10³⁸ ergs s^-1 were detected (Fabbiano 1988; total ~ 35 ks exposure time), with the ROSAT results that led to detection of 26 extra-nuclear sources with L_X > 10³⁷ ergs s^-1 (Immler & Wang 2001; 177 ks - HRI, 101 ks - PSPC), and finally with the Chandra results: 124 sources detected within the optical D₂₅ isophote to a limiting luminosity of ~ 3 × 10³⁶ ergs s^-1 in ~ 50 ks (Swartz et al. 2003).

The Chandra results show that 88% of the non-nuclear emission is resolved into individual sources. The brightest of these sources have luminosities exceeding the Eddington luminosity for a spherically accreting neutron star (see Fabbiano 1995), i.e. they are among the sources dubbed `Ultraluminous X-ray Sources' (ULX; see Section 3). Of the 66 sources that lie within Hubble Space Telescope (HST) fields, 34 have potential counterparts (but 20 ± 4 chance coincidences are expected). Five sources are coincident with supernova remnants in the spiral arms (including the well studied SN 1993J), but one of them (the ULX X-6) is identified with a XRB, based on it X-ray spectrum. Only four potential GC identifications are found. For one of the M81 sources, Ghosh et al. (2001) report a 10-year ROSAT-Chandra X-ray transient light curve.

Nine of the sources found in the Chandra observation of M81 are supersoft (SSS; Swartz et al. 2002), with L_X(0.2 - 2.0 keV) in the range of > 2 × 10³⁶ -3 × 10³⁸ ergs s^-1, and a blackbody emission temperature of 40-80 eV. The fraction of SSS is consistent with the expected values, based on the Galaxy and M31. Four sources are in the bulge and five in the disk; of the latter, four are on the spiral arms. With the exception of the most luminous of these systems, which has a bolometric luminosity L_bol ~ 1.5 × 10³⁹ ergs s^-1, and will be discussed in Section 3, all these sources are consistent with the nuclear-burning accreting white dwarf picture of SSS (van den Heuvel et al. 1992; see the Chapter by Kahabka in this book). The SSS associated with the spiral arms tend to have higher emission temperatures, suggesting more massive white dwarf counterparts, which would result from relatively massive stars in a relatively younger stellar population.

The first report of XLF studies in M81 (Tennant et al. 2001; Fig. 3) showed dramatic differences in the XLFs of bulge and disk sources. While the XLF of the bulge is reminiscent of the bulge of M31, with a relatively steep power-law flattening at L_X(0.2 - 8.0 keV) < 4 × 10³⁷ ergs s^-1, the XLF of the disk follows a uninterrupted shallow power law (cumulative slope -0.50).

Figure 3. Bulge and Disk XLFs for M81. The straight line is the best-fit power-law to the disk XLF (Tennant et al. 2001).

The subsequent more complete study of Swartz et al. (2003) confirms the break in the bulge XLF and suggests that it may be due to an aging ~ 400 Myr old population of LMXBs. The extrapolation of this XLF to lower luminosities can only explain 10% of the unresolved bulge emission, which, however, has the same spatial distribution as the detected bulge sources: besides some gaseous emission, this may suggest an undetected steepening of the XLF due to a yet fainter older population of sources in the central regions. The disk population has different XLFs, depending on the source distance from the spiral arms (Fig. 4): in particular, the very luminous (> 10³⁸ ergs s^-1) sources responsible for the flat power law are all concentrated on the arms; a break at high luminosities appears when spiral arm sources are excluded. Swartz et al. (2003) suggest that these most luminous sources are likely to be very young XRBs resulting from the star formation stimulated by the spiral density waves.

Figure 4. Steepening XLFs of disk sources of M81, at increasing distance from the spiral arms (solid line; Swartz et al. 2003).