The Properties of X‐Ray and Optical Light Curves of X‐Ray Novae

We have collected the available data from the literature and from public dataarchives covering the past two decades for the long-term X-ray and opticallight curves of X-ray nova (XN) outbursts, and carry out for the first time asystematic, statistical study of XN light curves which are classified into 5morphological types. Basic light curve parameters, e.g., the outburst peakflux, amplitude, luminosity, rise and decay timescales, the observed andexpected outburst durations, and total energy radiated, are tabulated anddiscussed. The rise timescales are found to have a flat distribution while thedecay timescales have a much narrower and near-Gaussian distribution, centeredaround 30 days and dominated by the strongest outbursts. The peak luminosity isalso distributed like a Gaussian, centered around 0.2 in Eddington units, whilethe total energy released has a much broader distribution around 10E44 ergs. We identify and discuss additional light curve features, such as precursors,plateaus, and secondary maxima. The plateaus exhibited in the light curves ofblack hole sources are found to have, on average, longer durations and they arefollowed by longer decays. The identified secondary maxima seem to occur mostlyin black hole systems. For the frequency of outbursts, we find that the averageXN outburst rate is about 2.6 per year for events >0.3 Crab, and that the meanrecurrence time between outbursts from a single source is 6 years. The spatialand logN-logS distribution of the XN sources, with limited statistics, agreeswith a source population in the Galactic disk, as observed from a point at adistance of 8.5 kpc from the Galactic center. Finally, we point out that theobserved XN light curve properties can in general be explained by a diskthermal instability model, although some important problems still remain.Comment: 68 pages including 27 Postscript figures and 12 tables. To be published in the Astrophysical Journal, Part