Is the spinning black hole energizing the disk?

To explore the implications of the MCG-6-30-15 XMM-Newton data further, we employ the standard assumption that there is little or no X-ray reflection from within the plunging region. To make a connection with theoretical accretion disk models, we must relate the distribution of observed X-ray reflection to the distribution of primary energy dissipation in the accretion disk (since it is the latter that is predicted by the models). The simplest assumption is that the local intensity of X-ray reflection from the disk surface is proportional to the local dissipation in the underlying disk. This would be the case if the ionization of the disk surface was reasonably uniform with radius, and a fixed fraction of the locally dissipated energy was transported into a geometrically-thin X-ray emitting corona. In reasonable disk models, any appreciable ionization changes would be in the sense of the innermost regions being more highly ionized, therefore producing weaker X-ray reflection signatures and requiring an even steeper dissipation profile than inferred from the X-ray reflection studies. We also note that, in MCG-6-30-15, a large fraction (25-50%) of the electromagnetic luminosity is emitted in the X-ray band by the Comptonizing corona. Thus, it cannot be grossly wrong to assume that the X-ray activity traces the underlying energetics of the disk.

Figure 14: Logarithmic derivatives (i.e. local emissivity indices) for the dissipation profiles shown in Fig. 5. As in Fig. 5, the thick line shows the dissipation profile for a standard non-torqued disk (i.e., one in which the ZTBC applies) around a near-extremal Kerr black hole with spin parameter $a=0.998$. The thin solid lines show the torque-induced dissipation component $D_{\rm tor}$ for spin parameters of (from left to right) $a=0.998,0.99,0.9,0.5,0.3$, and $0$. The dashed lines show the torque-induced component of the emitted flux, including the effects of returning radiation. To explain the MCG-6-30-15 result, one requires a torqued accretion disk around a rapidly rotating black hole.

Taking the X-ray reflection as a proxy for the underlying disk dissipation, we see that these data are clearly discrepant with the standard models for black hole accretion disks for any spin parameter. As discussed in §3.2.2, the flux emitted per unit proper area of a standard disk around a near-extremal Kerr black hole, $D_{\rm PT}(r)$, is zero at $r=r_{\rm ms}$ by virtue of the ZTBC, peaks at $r\sim 1.6r_{\rm g}$ and then gradually steepens to approach $D(r)\propto r^{-3}$. At no point does $D_{\rm PT}(r)$ become as steep as $r^{-4.5}$, as required by the XMM-Newton observations. This is true for any assumed BH spin. However, as discussed in §3.2.4, magnetically-induced torques at the radius of marginal stability can significantly enhance the amount of dissipation in the innermost regions of the accretion disk. The `best' possible case (in terms of producing steep dissipation profiles) is where the magnetic torque is applied entirely at the radius of marginal stability. In Fig. 14, we plot the logarithmic derivative of the dissipation profiles shown in Fig. 5. One finds that it is still impossible to achieve the required steep dissipation profiles in the case of non-rotating or slowly rotating black holes. Only in the case of a rapidly-rotating black hole does the dissipation profile become sufficiently steep. In fact, one finds that sufficiently steep dissipation profiles can only be obtained in the regions of parameter space where an appreciable fraction of the extra energy is derived directly from the spin of the black hole. As discussed in §3.2.4, the physical origin of this torque is likely a magnetic connection between the accretion disk and either the plunging region or the rotating event horizon itself.

Relativistic effects may also reveal themselves through the strength of the reflection features. These XMM-Newton data show that, during the DM state, the relative strength of the reflection is about twice that expected from a plane-parallel coronal geometry. While there may be geometrical explanations, such an enhancement is in fact a natural consequence of returning radiation [196,218].

It must be reiterated that, while the case for a rapidly-rotating hole in MCG-6-30-15 is solid, the argument for a torqued accretion disk is vulnerable to the possibility of powerful fluorescence/X-ray reflection from within the plunging region. More theoretical work on the plunging region is required to assess the impact it might have on these arguments. There is also another way of alleviating the need for a torqued accretion disk. Martocchia & Matt have recently suggested that the DM spectrum of MCG-6-30-15 could be explained via the gravitational focusing of X-rays emitted by a source that is $r\sim 3M$ from the black hole at high latitudes (i.e. close to the spin axis of the black hole) [219]. One problem suffered by this scenario is that it overpredicts the relative strength of the X-ray reflection by a factor of two. Thus, for this picture to be correct, only half of the disk surface area can be in a state capable of producing X-ray reflection signatures. Even if this picture is correct, the presence of such a powerful X-ray source on the spin axis of the black hole (presumably corresponding to the base of an MHD jet) is itself a strong circumstantial indicator of black hole spin-energy extraction.

Figure 15: Continuum subtracted iron line profile from the second XMM-Newton observation of MCG-6-30-15 [220]. This is the highest signal to noise relativistic line profile yet measured.

The first results from a later and very long (400ks) XMM-Newton observation of MCG-6-30-15 were reported recently by Andy Fabian and collaborators [220]. This observation has produced the best iron line profile yet obtained from any object (Fig. 15). There is strong evidence for line profile variability between the two XMM-Newton observations, with the latter observation showing a prominent blue horn to the line profile. This horn indicates the presence of emission from larger radii in the disk. However, the extreme red-wing to the line profile, requiring a very steep central emissivity profile, was also present in these data. We await further analysis of this tremendously important dataset.