3.2. Spiral Galaxies
Plenty of spirals have been observed with Chandra, but there is very little information on the optical identifications. Globular clusters are hard to find because of the patchy extinction. There is also the difficulty of subtracting the diffuse light of the galaxy. These problems are exacerbated for nearly face-on spirals. In addition to the low-mass X-ray binaries, a spiral galaxy also hosts high-mass X-ray binaries and supernova remnants among the luminous X-ray sources.
M31 (Andromeda Nebula). The apparent size of M31 is so big that only ROSAT has studied the whole (Magnier et al. 1992; Supper et al. 1997). Di Stefano et al. (2002) have conducted Chandra observations of ~ 2560 arcmin2 in four different areas so as to be representative of the whole. About one third of the 90 Chandra sources have luminosities (0.5-7 keV) in excess of 1037 erg s-1; the most luminous source is probably associated with the globular cluster Bo 375. Its luminosity (0.5-2.4 keV) varied between ~ 2 × 1038 and ~ 5 × 1038 erg s-1. Supper et al. (1997) reported regular variations of ~ 50% on a timescale of ~ 16 hours. A similar percentage variability was found in the 500 day X-ray light curves of two other highly luminous globular clusters in M31, Bo 82 and Bo 86 (Di Stefano et al. 2002). Some of the more luminous globular cluster X-ray sources could be multiple sources.
It has been stated on the basis of different data sets that the X-ray luminosity function of globular cluster X-ray sources is different in M31 than in the Milky Way (Van Speybroeck et al. 1979 on the basis of Einstein data; Di Stefano et al. 2002), and that it is the same (Supper et al. 1997). In Figure 6 we show the normalized cumulative distributions for clusters in the Milky Way and in M31. The distributions look different, but a Kolmogorov-Smirnov test shows that there is a non-negligible probability, 0.03, that the difference is due to chance. It is therefore possible that the extent to higher luminosities in M31 is due to the larger number of X-ray sources (and of globular clusters).
Figure 6. Left: Comparison of the cumulative (from high luminosities downward) X-ray luminosity distributions of globular clusters in the Milky Way (dashed curve) and M31 (solid curve). After Di Stefano et al. (2002). Right: Normalized cumulative (from low-luminosities upward) X-ray luminosity function for sources with Lx > 1035.5erg s-1 in the Milky Way (dashed curve) and M31 (solid curve). The Chandra luminosities given by Di Stefano et al. (2002) were multiplied by 0.46 to convert them to the energy range of the ROSAT data from Verbunt et al. (1995). The probability that the normalized distributions are the same is 0.03.
M104 (NGC4594, Sombrero galaxy) is an Sa galaxy at a distance of ~ 8.9 Mpc. Only optically bright globular clusters house the luminous LMXBs detected with Chandra (Di Stefano et al. 2003). The majority of the sources with luminosities in excess of 1038 erg s-1 are located in globular clusters. The luminosity function of X-ray sources in the globular clusters has a cut-off near the Eddington limit for a 1.4 M neutron star. One globular cluster houses a super-soft source (see Chapter 11). There is a connection between metal-rich, red globular clusters and the X-ray sources. However, the most luminous X-ray sources are equally likely to be located in metal-poor globular clusters with lower optical luminosities. The optically brightest blue globular clusters do not seem to house very luminous X-ray sources.