60 3.4. Mass Estimates from the Radio/X-Ray Correlation
0 2 4 6 8 10 12 Lg MMsol
0 2 4 6 8 10 12
LgMMsolCorr
GX339-4 Sgr A LLAGN
FRIXBL RBL
F3.8—Black hole mass derived form the radio/X-ray correlation vrs. measured black hole mass. The solid line denotes that both methods yield the same result.
picture the radiative output of sub-Eddington black holes is non-thermally dominated, while near-Eddington black holes are thermally dominated. This scheme allows one to unify the radiative properties of black holes over a large range of accretion powers. At sub-Eddington accretion rates, the scaling be-tween radio and optical or X-ray cores is then predicted to follow the scaling laws outlined in Falcke & Biermann (1995) and Markoffet al. (2003). This requires taking the black hole mass into account.
Near-Eddington black holes are presumably found in quasars, luminous Seyfert galaxies, and soft-state X-ray binaries which are considered to be in the high state. As pointed out elsewhere (Pounds et al. 1995; Maccarone et al.
2003) Narrow-Line Seyfert 1s may also be related to the very high state of X-Ray binaries.
On the other hand, candidates for sub-Eddington black holes are XRBs in the low-hard state, Sgr A*, LINERs, FR I radio galaxies, and BL Lac objects.
In terms of beaming and inclination-based unified schemes, which we do not explicitly discuss but consider valid. It may be worth pointing out that ultra-luminous X-ray sources might be low-mass analogs to BL Lacs and blazars as discussed in the next section or (K¨ording et al. 2002).
Using various samples of sub-Eddington black holes, we are able to show that all these different types of sources seem to fall near the predicted
radio/X-62 3.5. Conclusion and Discussion ray correlation, if the scaling with black hole mass is taken into account.
The crucial underlying assumption is that all these latter sources are in-trinsically jet-dominated and have essentially the same SED in common: a flat, optically thick radio spectrum and an optically thin power law beyond a turn-over frequency. Shape and scaling of the SED needed to explain the radio/X-ray correlation is just what one expects in a pure jet model and sup-ports the notion of jet-dominated accretion flows (“JDAF”). On the other hand, some form of radiative inefficient accretion flows/corona is also clearly needed for this picture to work, since there is always a need for a power and matter source for the outflow. It may be possible to adapt the scheme for a situation where the X-ray emission is dominated by emission from optically thin accre-tion flows, if their X-ray flux follows a similar non-linear scaling as predicted in the jet case.
An interesting corollary for jets is that, in order to obtain the scaling with mass, one has to assume that the region of the onset of particle acceleration in the jet – producing the optically thin power law – is always around a fixed location in mass-scaled units (∼100−1000Rg).
With the large range of black hole powers and masses discovered the pro-posed picture may warrant further investigation and detailed tests. If solidified and further evolved it may help to predict the luminosity evolution of black holes at various wavebands over many orders of magnitude.
4
Ultra-Luminous X-ray Sources
4.1 Introduction
Ultra-luminous X-ray sources are among the most widely discussed objects, but their true nature is yet unknown. These objects have X-ray luminosities around LX ∼ 1039−42 erg/s which seems too bright for normal black hole X-Ray binaries (BHXRBs) but far dimmer than normal Active galactic nuclei (AGN). The first hints to these intermediate-luminosity X-ray point sources have been found in the 1980s (Fabbiano 1989; Colbert et al. 1995). ROSAT and subsequent high resolution X-ray satellites confirmed these findings and showed that these intriguing sources are often not in the centers of the galaxies (Colbert & Mushotzky 1999; Roberts & Warwick 2000). More recent surveys (see e.g., Colbert & Ptak 2002) find approximately one ULX in every five galaxies confirming that ULXs are a common phenomenom.
The Eddington limit for an accreting object with mass MisLEdd≈1.25× 1038MMerg s−1, which implies that these sources are super-Eddington for stellar mass objects. Some ULXs show spectral transitions from a soft spectrum to a hard power law and luminosity variability (e.g. Mizuno et al. 2001; Kubota et al. 2001), ruling out supernova remnants and supporting the idea that ULXs can be attributed to accreting black holes.
To achieve the observed X-ray luminosities with isotropically radiating ac-cretion disks should require a population of intermediate-mass black holes (IMBHs) of 50−500M. The existence of IMBHs would be extremely ex-citing, as they could be the ’missing link’ between stellar mass black holes and the supermassive black holes in the center of the galaxies (see e.g., Ebisuzaki
63
64 4.1. Introduction et al. 2001). Such objects would have important implications for Cosmology and arise in many theories on the collapse of primordial stars. There is also an ongoing discussion whether the inner disk temperatures of ULXs are too high for IMBHs, for high temperatures similar to the values found in XRBs (see e.g., Colbert & Mushotzky 1999; Mizuno et al. 1999; Makishima et al. 2000).
However, recent XMM observations often find lower inner disk temperatures, compatible with IMBHs, than earlier ASCA observations (see e.g. Miller et al.
2003).
While there is some evidence that a few of these objects might indeed be intermediate mass black holes (IMBH, e.g., Strohmayer & Mushotzky 2003), the creation and feeding mechanisms for black holes of this size are totally unknown. The phenomenom seems to be connected to star formation, see e.g., the cartwheel galaxy (Gao et al. 2003). This galaxy would need hundreds to thousands of IMBHs feeding from a yet unknown non-stellar mass reservoir or the ULX phenomenom is connected to high mass XRBs (King 2004). Grimm et al. (2002); Gilfanov (2004) show the existence of a universal X-ray lumi-nosity function that may extend up to ULXs, if one takes the star forming rate into account. There seems to be a dependence of the ULX abundance on the galaxy type, especially dwarf galaxies host many nearby ULXs.
These problems with isotropic emission models have already been dis-cussed by King et al. (2001), where the authors propose some form of anisotropic emission as an alternative. A beaming factor of ten already re-duces the required mass of the black holes to ”normal” stellar values, but this is difficult to achieve with pure disk models. In this thesis we suggest that the spectrum of some XRBs could be explained by a coupled disk/jet model (see also Markoff et al. 2001a), where some of the X-ray emission is pro-duced by synchrotron and inverse-Compton radiation in a jet. This emission would naturally be relativistically beamed. Mirabel & Rodr´ıguez (1999) (see also Reynolds et al. 1997) have pointed out that a number of nearby galax-ies should host microblazars - microquasars with relativistically beamed jets pointed towards the observer. In this chapter we will investigate whether such populations of microblazars or intermediate mass black holes can indeed ex-plain current data on ULXs and constrain the basic parameters required for these models. This chapter is based on K¨ording et al. (2002). In the next chap-ter we will explore if these potential microblazars also show compact radio emission like Blazars.
4.2 Simple Models