The first detection of a relativistic accretion disk

MCG-6-30-15 was the first AGN where X-ray reflection from fairly neutral material was clearly detected using the EXOSAT [201] and Ginga [204] observatories. This demonstrated that there was cold and optically-thick material in the vicinity of the SMBH that was being irradiated by the power-law X-ray continuum. It was suspected by many that this material was indeed the accretion disk. However, direct confirmation of this had to await the superior spectral capabilities of ASCA.

As has already been described, ASCA was the first observatory capable of obtaining medium resolution 0.5-10keV X-ray spectra of cosmic sources. Thus, it must be appreciated that the demands for observing time on ASCA were intense, with many different sub-communities of the astrophysical world wishing to utilize this new resource to explore the universe (indeed, ASCA has made major contributions in fields as diverse as stellar coronae, the diffuse interstellar and intracluster medium, and gamma-ray bursts). Furthermore, at the time when ASCA started operating, there was a certain level of skepticism within the astrophysical community concerning the possibility of detecting strong relativistic effects via spectroscopic studies of AGN -- thus, since the majority of observing time on ASCA was allocated in open peer-reviewed competitions, it was not a foregone conclusion that ASCA would spend significant time taking high-quality spectra of AGN. The fact such spectra were obtained, thereby allowing the direct detection of relativistic effects around supermassive black holes, is largely due to the perseverance of Yasuo Tanaka (the Project Scientist) and Andy Fabian, for which they were awarded the Rossi Prize of the American Astronomical Society in 2001.

Returning to the history of MCG-6-30-15, a rather short (half day) ASCA observation of MCG-6-30-15 showed the iron line to be significantly broadened in energy space, being well modeled as a Gaussian profile with centroid energy $E_0=6.2{\rm\thinspace keV}$ and a standard-deviation of $\sigma=0.7{\rm\thinspace keV}$[205,206]. The improved signal-to-noise from a much longer (4.5 days¹²) ASCA observation was required to determine that the iron line did, indeed, possess the profile expected from the surface of a relativistic accretion disk[20]. As shown in Fig. 1, this line profile is well fit with line emission from a disk around a Schwarzschild black hole with a disk inclination of $\theta\approx 27^\circ$ (measured away from the normal to the disk plane), with line flux distributed across the disk proportional to $r^{-3}$, extending down to $r=6M$.

Figure 12: Time-averaged (upper panels) and peculiar line profiles (lower panels) of the iron K emission from MCG-6-30-15 seen in the two long ASCA observations in 1994 (left) and 1997 (right). In the 1994 observation, a very broad profile with a pronounced red-wing is seen during a period of Deep Minimum of the light curve (lower left), compared to the time-averaged line profile shown in the upper panel. In contrast, during a sharp flare in the 1997 observation, whole line emission is shifted to energies below 6 keV and there is no significant emission at the rest line-energy of 6.4 keV (lower right). Both peculiar line shapes can be explained by large gravitational redshift in small radii on the accretion disk.

In fact, these ASCA data were a great deal richer than just described. As is common with type-1 AGN, the X-ray flux of this AGN varied rapidly as a function of time. Via a detailed analysis of this and subsequent ASCA observations, Kazushi Iwasawa found that the iron line flux and profile also undergoes dramatic changes with time[207,208]. The detailed iron line changes are complex and defy simple characterization (see Fig. 12). In the 1994-July ASCA observation, it was found that, during a large flaring event, the iron line became quite narrow with an energy close to the intrinsic value. This suggests that the observed fluorescence was dominated by fairly large radii in the disk during the flare. On the other hand, during another flare observed in a 1997 ASCA observation, the line emission redshifted entirely to energies below 6keV suggesting dominance by small radii in which gravitational redshifts are large.

Of particular interest was the identification of the so-called ``deep minimum'' (DM) state. During the DM state, the continuum flux falls below its average value by a factor of 2-3, and the fluorescent iron line appears to become much broader and stronger. In fact, the redshifts needed to explain the observed line profile are so great as to require line emission from within $r=6M$. As we shall see, we believe that the DM state gives us a window into the more exotic astrophysics of rapidly rotating black holes.

Of course, the claim that iron line studies are probing the region within a few gravitational radii of the black hole is a bold one, and should be tested against other models at every opportunity. Given the quality of data, the July-1994 MCG$-$6-30-15 line profile has become a traditional test bed for such comparisons. The first issue to assess is the correctness of the continuum subtraction that underlies the determination of the iron line profile. Andy Fabian and collaborators systematically examined the effect of unmodeled spectral components (e.g., absorption edges) and found that the line profile was robust to all physically reasonable possibilities [209]. Given the reality of the spectral features, one must then critically assess its interpretation as an emission line arising from a relativistic accretion disk. The most serious challenger is a model in which a source of intrinsically narrow iron line emission is surrounded by a rather optically-thick ($\tau\sim 5$), cool ($kT\sim 0.2{\rm\thinspace keV}$) and yet highly-ionized plasma. Repeated Compton recoil suffered when the photons scatter off the electrons can then broaden and redshift the observed line[210]. However, such a scenario predicts that the continuum spectrum passes through the same Compton cloud, filtering out high-frequency temporal variability and imposing a continuum spectral break at $E\sim m_{\rm e}c^2/\tau^2$. The presence of high-frequency temporal variability and the absence of a continuum spectral break strongly argues against such a scenario [211,212].

In another alternative model, it has been has proposed that energetic protons transform iron in the surface of the disk into chromium and lower $Z$ metals via spallation which then enhances their fluorescent emission [213]. With limited spectral resolution, such a line blend might appear as a broad skewed iron line. This model suffers both theoretical and observational difficulties. On the theoretical side, high-energy protons have to be produced and slam into the inner accretion disk with a very high efficiency (one requires 10% of the binding energy of the accreted material to be channeled into this process alone). On the observational side, it should be noted that the broad line in MCG-6-30-15 [214] is well resolved by the ASCA SIS (the instrumental resolution is about 150 eV at these energies) and it would be obvious if it were due to several separate and well-spaced lines spread over 2 keV. There can of course be Doppler-blurring of all the lines, but it will still be considerable and require that the redward tails be at least 1 keV long.