Ground- and space-based observation: Quiescent
accretion discs are hosted by white dwarf accretors in cataclysmic
variables and black hole accretors in X-ray transients. We will
acquire spectroscopic data of accretion discs with high time- and
velocity-resolution over several energy bands, employing HST to detect
the inner disc regions in the UV, and ground-based optical and IR
instruments to detect the outer disc; e.g. the 4.2-m William
Herschell Telescope on La Palma, the 3.9-m Anglo Australian Telescope
in New South Wales and the 3.8-m UK IR Telescope on Hawaii. Several
useful data sets from each of these instruments already exist.
Indirect accretion disc imaging: The accretion disc
targets are too small to resolve directly, therefore we intend to use
maximum entropy tomography techniques to spatially resolve disc
spectra. We will construct maps of discs in velocity coordinates from
orbital line profile variations and transform these to spatial
coordinates by assuming a velocity field (Marsh & Horne 1988). Disc
eclipses by the companion star will provide spectrally-resolved
spatial maps more directly (Horne 1985). The advantage of the
resolution provided by these techniques is that we can discriminate
between contributions from the disc, stellar components and localized
shocked material forming spots or waves. This fulfills one of the
original scientific goals of disc tomography.
Disc atmosphere models: The discipline of disc
atmosphere building has now reached a sophistication with which the
physical state of discs can be realistically modelled. Our models can
include the effects of external irradiation, vertical gravity
gradients, finite optical depths, energy dissipation, turbulence and
supersonic velocity gradients. Our current code has been used with
notable success in probing the structure of absorption curtains in
accretion discs and to investigate optically thick, steady-state
atmospheres (Horne et al. 1994). A successful fit to synthetic data is
provided in Fig. 3.
Figure 3: A synthetic optical
disc spectrum in green constructed using atmosphere models and treated
with noise. The black line is a fit. The three panels on the left
present the run of 
(= 8000
K), 
(= 0.1
g cm
) and 
(=
100
) as a function of
R in green, and power-law fits in black. Consequently, even
face-on accretion disc spectra yield potentially enough information to
determine the radial runs of fundamental physical properties. From
Still, Horne & Hubeny (1998).
References:
Horne K., 1985, MNRAS, 213, 129
Horne K., Marsh T.R., Cheng F.H., Hubeny I., Lanz T., 1994, ApJ, 426, 294
Marsh T.R., Horne K., 1988, MNRAS, 235, 269
Shakura N.I., Sunyaev R.A., 1973, A&A, 24, 337
Still M.D., Horne K., Hubeny I., 1998, in S.S. Holt, T. Kallman, eds.,
Proc. of the 8th Astrophysics Conference in Maryland, AIP
Wijers R., Pringle, J., 1998, MNRAS, in press.