4.1. ULXs in Early-Type Galaxies
As can be seen from Table 1, in early-type galaxies the occurrence of sources with LX = 1 - 2 × 1039 ergs s-1 is common, although generally limited to a few sources per galaxy. These sources could easily be explained with normal black hole binaries or moderately beamed neutron star binaries (King 2002). In their mini-survey of 14 galaxies observed with Chandra (which include some of the ones listed in Table 1), Irwin, Athey & Bregman (2003) find that of the four sources with X-ray luminosities in the 1 - 2 × 1039 ergs s-1 range for which they can derive spectra, three have soft spectra, similar to those of black hole binaries in high state (see also Finoguenov & Jones 2002).
Not much can be said about the variability of ULXs in early-type galaxies, because repeated Chandra observations of a given galaxy are not generally available. In the case of NGC 5128, comparison with previous ROSAT images (see Colbert and Ptak 2002) shows considerable flux variability in these very luminous sources: two ULXs were detected in ROSAT observations, both have considerable lower luminosities in the Chandra data (Kraft et al. 2001), and one of them may have disappeared.
While, in general, sources with LX > 2 × 1039 ergs s-1 are relatively rare in early-type galaxies as compared to actively star-forming galaxies (see Section 3), and may be preferentially associated with GCs (e.g. Angelini, Loewenstein & Mushotzky 2001; see Irwin, Athey & Bregman 2003), this is not always the case, as exemplified by NGC 720. This galaxy (Jeltema et al. 2003) is peculiar in possessing nine ULXs (this number is of course dependendent on the assumed distance, 35 Mpc), a population as rich as that of the actively starforming merger galaxies The Antennae (Fabbiano, Zezas & Murray 2001, Zezas & Fabbiano 2002). Only three of these ULXs can be associated with GCs. The sources in NGC 720 are also peculiar in their spatial distribution, which does not follow the distribution of the optical light, as it would be expected from LMXBs evolving from low-mass bulge binaries: these sources are distributed in arcs. Their large number and their spatial distribution may suggest that they are younger systems, perhaps the remnants of a recent merger event.
The associations of some ULXs in early-type galaxies with GCs may support the possibility that a subset of these sources may be associated with IMBH (> 10M) (see Fabbiano 1989 and refs. therein; Irwin, Athey & Bregman 2003). However, most of the ULXs in early-type galaxies are likely to be lower mass binaries, given the stellar population of the parent galaxy. King (2002; see also Piro & Bildsten 2002) suggests that they may be a class of ULXs associated with outbursts of soft X-ray transients, resulting in moderately beamed emission from the inner regions of a thick accretion disk. In the case of NGC 720 they may be related to a `hidden' younger stellar population (Jeltema et al. 2003).