Ten years of intermediate polar X-ray lightcurve interpretation

Norton, A. J. (1995). Ten years of intermediate polar X-ray lightcurve interpretation. In: Cape Workshop on Magnetic Cataclysmic Variables, Astronomical Society of the Pacific Conference Series, Astronomical Society of the Pacific, pp. 285–291.

URL: http://www.aspbooks.org/a/volumes/article_details/...


X-ray light curves provide some of the best clues to understanding the accretion and emission processes in intermediate polars (IPs). In particular, the spin-folded data contain a wealth of information about the location and shape of the accretion and emission regions, the method by which the accretion occurs, and other physical parameters of the system. The problem is one of interpreting the light curves in order to discover these parameters.

About 10 years ago, the first attempts were made to do this, assuming the modulation to be caused entirely by self occulation of the emission region by the white dwarf. With the advent of energy resolved pulse profiles obtained using EXOSAT, it became clear that photoelectric absorption played a major part in shaping the profiles too. Interpretations based around the idea of 'accretion curtains' were then proposed. Higher quality Ginga and ROSAT data have more recently thrown up further complications. Firstly, not all the pulse profiles are roughly sinusoidal. This indicates that there must be some asymmetry in the system, either between the properties of the polar emitting regions, or between their shape, size or location. Secondly, some systems show evidence for modulation at the 'beat period' between the spin and orbit. This indicates that some of the accretion flow does not pass through the accretion disc, but attaches to the field lines directly.

Any attempt to determine system parameters from the X-ray pulse profiles must take these factors into account. Therefore, 'fitting' or 'modelling' the light curves is a complicated procedure, but nevertheless offers some of the best hope for resolving many of the unanswered questions about IPs.

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