Recent X-ray observations of many active galactic nuclei (AGN) have found
the K 
emission line of cold iron (at 6.40keV), and show this line
to be broadened and skewed towards low-energies (Mushotzky et al. 1995;
Tanaka et al. 1995; Nandra et al. 1997). The line profile is usually found
to be in good agreement with that expected if it were to originate from the
inner regions of an accretion disk around a black hole, with gravitational
redshifts and (relativistic) Doppler shifts being the processes relevant to
shaping this profile. In the cases with the best data, many alternative
mechanisms for forming a such a line can be examined and rejected (Fabian
et al. 1995).
The iron line is believed to be a fluorescent line which results when
Thomson-thick material, with a relatively low ionization state, is
externally illuminated by a hard X-ray source (Basko 1978; George & Fabian
1991; Matt, Perola & Piro 1991). In the case of accreting black hole
systems, the optically-thick material can be identified with a thin
accretion disk whereas the external X-ray source is probably a disk-corona.
The equivalent width (EW), W, of the iron line can be predicted given the
source geometry, illuminating X-ray spectrum, and chemical abundances. In
the `standard' case where the cold material possesses Morrison & McCammon
(1983) cosmic abundances and subtends 
sr at the X-ray source (which
is assumed to have an AGN-like spectrum), the EW is 
.
For a more recent set of cosmic abundances (Anders & Grevesse 1993), this
EW increases slightly to 
(Reynolds, Fabian & Inoue
1995; Matt, Fabian & Reynolds 1997), primarily due to the increased
abundance of iron.
Observationally, many iron lines are significantly stronger than the
predictions of the previous paragraph. Although the mean EW in the sample
of Nandra et al. (1997) is only slightly in excess of 
,
, there are many objects in this sample with
very strong lines, 
. Several possible explanations for
the strength of these lines have been previously discussed. First, the
iron may be over-abundant (George & Fabian 1991; Reynolds, Fabian & Inoue
1995). However, the EW grows only logarithmically with iron abundance due
to the fact that iron itself contributes to its own K line opacity
through L-shell photoelectric absorption. Thus, extreme iron
overabundances ( 
or more) are required to explain
the strongest lines. Secondly, the geometry might be such that the cold
material subtends more than sr at the X-ray source. This geometry
is difficult to reconcile with a disk/corona model. Thirdly, gravitational
focusing of X-ray flux from a source which is at some height above the disk
plane can enhance the EW of the line (Martocchia & Matt 1996; Reynolds &
Begelman 1997). However, these General Relativistic (GR) effects are only
important for enhancing the emission from the innermost regions of the disk
(radii 
, where 
is the
Schwarzschild radius of the central black hole), which produce very
redshifted line emission (with observed photon energies of 
or
less). Thus, whilst GR enhancement effects might be important for
understanding the broadest line known, they cannot be relevant to strong
iron lines from typical objects.
In this letter, we note that any relative motion between the X-ray source and the accretion disk will also affect (and usually enhance) the EW of the iron line through the effects of special relativistic (SR) aberration and Doppler shifts. Such relative disk/corona motion will naturally occur if the accretion disk and the corona are not rigidly coupled together. For example, some authors treat the disk-corona as an independent, slim accretion disk. Due to the subsequent sub-Keplerian motion of the corona, the disk and corona will be in relative motion at any given radius. However, the corona may well be tightly coupled to the accretion disk by magnetic fields (which will force the disk and corona to accrete together). Even in this circumstance, the SR effects discussed here may be important. Field & Rogers (1993; hereafter FR93) have argued that magnetic instabilities and reconnection events in a disk corona could produce shock waves and/or the streaming of relativistic particles along the magnetic field lines. Thus, the plasma which is instantaneously responsible for the X-ray emission might well be in bulk motion relative to the disk. Such arguments gain qualitative support by drawing an analogy with the solar corona and solar flares.
For convenience, we set the speed of light to unity, c=1, throughout this work.