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. Furthermore, the internal consistencies of the accretion disk hypothesis must be critically examined. Given the quality of data, the July-1994 MCG-6-30-15 line profile has become a testbed for such comparisons.
Fabian et al. (1995)
examined many alternative models including lines
from mildly relativistic outflows, the effect of absorption edges on the
observed spectrum, and broadening of the line via comptonization.
Fabian et al. found that none of these models were viable alternatives
for the MCG-6-30-15 line profile. The idea of producing the broad
line via Comptonization has been revived recently by
Misra & Kembhavi
(1998) and
Misra & Sutaria (1999).
They argue that the spectrum
initially consists of a narrow iron line superposed on a power-law
continuum and that Comptonization in a surrounding cloud with optical
depth 
~ 4 produces the broad
line. The Comptonizing cloud
must be both cold (kT < 0.5 keV in order to predominately
downscatter rather than upscatter the line photons), and fully-ionized
(since no strong iron absorption edges are seen in the continuum
spectrum). The cloud is kept fully ionized and yet cool by postulating
that the continuum source has a very luminous optical/UV component.
There are strong arguments against such a model. Since the power-law
continuum emission also passes through any such Comptonizing cloud, one
would observe a break in the continuum spectrum at Ebr ~
me c2 /
2 ~ 30 keV. Such a break
is not observed in the BeppoSAX
(Guainazzi et al. 1999)
or RXTE data
(Lee et al. 1999)
for MCG-6-30-15 (see
Misra 1999).
Also, both continuum
variability (which is seen on timescales as short as 100 s) and
ionization arguments limit the size of the Comptonizing cloud in
MCG-6-30-15 to R < 1012 cm. The
essence of this ionization argument
is that the ionization parameter at the outer edge of the cloud (which,
for a fixed cloud optical depth, scales with cloud size as
1/R) must be sufficiently high that all abundant metals, including
iron, are fully ionized
(Fabian et al. 1995;
Reynolds & Wilms 2000).
In the case of MCG-6-30-15, these constraints on the cloud size turn
out to so tight that the postulated optical/UV component required to
Compton cool the cloud would violate the black body limit
(Reynolds & Wilms 2000).
Comptonization moreover provides a poor fit
(Ruszkowski & Fabian
2000).
Hence, we consider the Comptonization model for the broad
iron line not to be viable.
In another alternative model,
Skibo (1997)
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 (see Fig. 1). 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 (Skibo assumes
= 0.1 for
this process alone). On the observational side, it should be noted that
the broad line in MCG-6-30-15
(Tanaka et al 1995)
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, as suggested by
Skibo (1997),
but it
will still be considerable and require that the redward tails be at
least 1 keV long.
Finally it is worth noting that the line profile indicates that most of the Doppler shifts are due to matter orbiting at about 30 degrees to the line of sight. The lack of any large blue shifted component rules out most models in which the broad line results from iron line emission from bipolar outflows or jets. What we cannot determine at present is the geometry in more detail. For example, we cannot rule out a `blobby' disk (Nandra & George 1994). We do, however, require that any corona be either optically-thin or localized, in order that passage of the reflection component back through the corona does not smear out the sharp features. (Note though that an optically-thick corona over the inner regions of a disc would explain the lack of an iron line from that region.)