3.2. Intermediate Mass Black Holes or Beamed XRBs?
Although there is clear evidence pointing towards an XRB nature for ULXs, the presence of IMBHs in these systems is by no means universally accepted, and it may be quite possible that ULXs are indeed a heterogeneous population. As discussed above, the ASCA spectra were interpreted by Makishima et al. (2000) as evidence for rotating Kerr IMBH, to reconcile the high accretion disk temperature suggested by the model fitting of these spectra with the large black hole masses implied by the bolometric luminosity of the ULXs, which would require much cooler disks for a non-rotating IMBH. Colbert & Mushotzky (1999) suggested that these cooler accretion disk components may be present in their ASCA survey of ULXs, but the statistical significance of these early claims is not very high. The Chandra detections of super-soft ULXs (e.g., Swartz et al. 2002, in M81; Di Stefano et al. 2003 in M104; see also later in this Section) could be interpreted as evidence for IMBHs. More important, low-temperature components vere discovered in the XMM-Newton spectra of `normal' ULXs: in the NGC 1313 ULXs, which do not require a Kerr black hole, and are entirely consistent with emission from an IMBH accretion disk (Miller et al. 2003a; Fig. 8); and in at least one of the ULXs in the Antennae galaxies (kT ~ 0.13 keV) (Miller et al. 2003b).
Considerable attention has been devoted to an extremely luminous variable 1040 ergs s-1 ULX detected with Chandra near the dynamical center of M82. In the picture of spherical accretion onto an IMBH, the luminosity of this source would imply masses in excess of 100 M for the accretor. This ULX appears to be at the center of an expanding molecular superbubble with 200 pc diameter (Matsushita et al. 2000). Based on its accurate Chandra position, which is not at the nucleus, Kaaret et al. (2001) set an upper limit of 105 - 106 M to its mass. Strohmayer & Mushotzky (2003) report quasi periodic oscillations (QPOs) in the XMM-Newton data of this source. They argue that their discovery suggests emission from an accretion disk and is incompatible with the radiation being beamed, and therefore implying a less extreme emitted luminosity, as in King et al. (2001; see below). On the assumption that the highest QPO frequency is associated with the Kepler frequency at the innermost circular orbit around a Schwarzschild black hole, these authors set an upper limit of 1.87 × 104 M to the black hole mass: this source could therefore be an IMBH, with masses in the 100-10,100 M range. However, as noted by Strohmayer & Mushotzky (2003), the crowded M82 field cannot be spatially resolved with XMM-Newton, making the association of the QPO with the most luminous ULX in the field not entirely proven. Moreover, the spectral fit of these data suggests a temperature kT ~ 3 keV, much higher than the one expected from an IMBH accretion disk.
As we will discuss below, some results are hard to explain in the IMBH scenario. Two other models have been advanced, which do not require IMBH masses. The large number of ULXs found in The Antennae led to the suggestion that they may represent a normal stage of XRB evolution (King et al. 2001). In the King et al. (2001) model, the apparent (spherical) accretion luminosity is boosted because of geometrical collimation of the emitting area in thick accretion disks, resulting from the large thermal-timescale mass transfer characterizing the later stages of a massive XRB (see Chapter by King in this book). Exploiting the similarity with Galactic microquasars, the jet emission model of Körding et al. (2002) produces enhanced luminosity via relativistic beaming. In at least one case, the variable luminous ULX 2E1400.2-4108 in NGC 5408, there is observational evidence pointing to this relativistic jet model: Kaaret et al. (2003) find weak radio emission associated with the X-ray source, and argue that the both the multi-wavelength spectral energy distribution, and the X-ray spectrum are consistent with the Körding et al. (2002) scenario.
In some cases at least the IMBH hypothesis is supported by the association of the ULX with diffuse H nebulae, suggesting isotropic illumination of the interstellar medium by the ULX, and therefore absence of beaming (e.g. Pakull & Mirioni 2002 in the case of the NGC 1313 sources, see Miller et al. 2003). M81 X-9 is also associated with an optical nebula, which also contains hot gas (La Parola et al. 2001; Wang 2002). Wang (2002) considers the possibility that this nebula may be powered by the ULX and also speculates that it may be the remnant of the formation of the ULX. Weaver et al. (2002) discuss a heavily absorbed ULX in the nuclear starburst of NGC 253; this source appears to photoionize the surrounding gas. Weaver et al. speculate that it may be an IMBH, perhaps connected with either the beginning or the end of AGN activity. However, in at least one case (IC 342 X-1, Roberts et al. 2003), there is a suggestion of anisotropic photoionization, that may indicate beamed emission from the ULX.
In The Antennae, comparison with HST data shows that the ULXs are offset from starforming stellar clusters. While coincidence with a stellar cluster may be due to happenstance because of the crowded fields, the absence of an optical counterpart is a solid result and suggests that the ULXs may have received kicks at their formation (Zezas & Fabbiano 2002), which would be highly unlikely in the case of a massive IMBH forming in a dense stallar cluster (e.g. Miller & Hamilton 2002). An alternate IMBH scenario, discussed by Zezas & Fabbiano, is that of primordial IMBHs drifting through stellar clusters after capturing a companion (Madau & Rees 2001).
Other optical studies find counterparts to ULXs, and set indirect constraints on the nature of the accretor. A blue optical continuum counterpart to the variable ULX NGC 5204 X-1 was found by Roberts et al. (2001), and subsequently resolved by Goad et al. (2002) with HST. These authors conclude that the stellar counterpart points to an early-type binary. Similarly, Liu, Bregman & Seizer (2002) find an 08V star conterpart for M81 X-1, a ULX with average LX ~ 2 × 1039 ergs s-1. These counterparts may be consistent with the picture of King et al. (2001), of ULXs as XRBs experiencing thermal timescale mass transfer.
Recent results on supersoft variable ULXs suggest that the emitting region may not be associated with the inner regions of IMBH accretion disks in these sources, but may be due to Eddington-driven outflows from a stellar mass black hole. The spectral variability (at constant bolometric luminosity) of the soft ULX P098 in M101 (detected with Chandra; Mukai et al. 2003) led to the suggestion of an optically thick outflow from a 15-25 M black hole, regulated by the Eddington limit. Chandra time monitoring observations of The Antennae have led to the discovery of a variable super-soft source (kT = 90 - 100 eV for a blackbody spectrum), reaching ULX luminosities of 2.4 × 1040 ergs s-1 (Fabbiano et al. 2003b). The assumption of unbeamed emission would suggest a black hole of 100 M. However the radiating area would have to vary by a factor ~ 1000 in this case, inconsistent with gravitational energy release from within a few Schwarzschild radii of a black hole. As discussed in (Fabbiano et al. 2003b), a surprising possible solution is a white dwarf with M ~ 1 M, at the Eddington limit, with a variable beaming factor (up to a beaming factor b ~ 10-2). A second possible solution involves outflows from a stellar-mass black hole, accreting near the Eddington limit (as in Mukai et al. 2003) but with mildly anisotropic radiation patterns (b ~ 0.1, as in King et al. 2001). Similar sources are reported in M81 (Swartz et al. 2002), NGC 300 (Kong & Di Stefano 2003), and other nearby spiral galaxies (Di Stefano & Kong 2003; see also Di Stefano et al. (2003) for SSSs in M31).
Transient behavior has been shown to be an important observational diagnostic that could allow us to distinguish between beamed models and IMBH accretion for the origin of ULXs in young, star-forming regions (Henninger et al. 2003). Accretion onto IMBH black holes can lead to unstable disks and hence transient behavior whereas beamed binary systems have transfer rates that are high enough for the disks to be stable and X-ray emission to be persistent. Therefore long-term monitoring can prove a valuable and possibly unique tool in unraveling the nature of ULXs.