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 data archives covering the past two decades for the long-term X-ray and optical light curves of X-ray nova (XN) outbursts. XN outbursts are due to episodic accretion events, primarily in low-mass X-ray binaries normally characterized by low mass transfer rates. Dynamical studies indicate that most XNs contain a black hole. The soft X-ray emission during outburst traces the accretion rate through the inner edge of the accretion disk, while the optical light curve traces the physical conditions at the outer disk--thus, collectively, they contain information on the time-dependent behavior of accretion processes through the disks.

In this paper, we carry out for the first time a systematic, statistical study of XN light curves that are classified into five morphological types. Basic light-curve parameters, such as the outburst peak flux, amplitude, luminosity, rise and decay timescales, the observed and expected outburst durations, and the total energy radiated, are tabulated and discussed. We find that the rise timescales have a flat distribution while the decay timescales have a much narrower and near-Gaussian distribution, centered around 30 days and dominated by the strongest outbursts. The peak luminosity is also distributed like a Gaussian, centered around 0.2 in Eddington units, while the total energy released has a much broader distribution around 10⁴⁴ ergs. We find no intrinsic difference between black hole and neutron star systems in their distribution of peak amplitudes.

We identify and discuss additional light-curve features, such as precursors, plateaus, and secondary maxima. The plateaus exhibited in the light curves of black hole sources are found to have, on average, longer durations, and they are followed by longer decays. The identified secondary maxima seem to occur mostly in black hole systems. For the frequency of outbursts, we find that the average XN outburst rate is about 2.6 yr^-1 for events greater than 0.3 crab and that the mean recurrence time between outbursts from a single source is 6 yr. The spatial and log N-log S distribution of the XN sources, with limited statistics, agrees with a source population in the Galactic disk, as observed from a point at a distance of 8.5 kpc from the Galactic center. Finally, we point out that the observed XN light-curve properties can be explained in general by a disk thermal instability model, although some important problems still remain.