Long-term cycles in cosmic X-ray sources

Abstract

Most of what we know about galactic X-ray binaries comes from their time variation, particularly periodic variations corresponding to neutron star rotation, and binary motion. Longer cycles or quasi-cycles are much harder to observe because of the shortage of instrumentation suitable for long-term monitoring. Nonetheless, cycle with periods up to a few years have been seen in several galactic binaries.Cycles of ~ 30-300 days have been confirmed for four high-mass systems, LMC X-4, Her X-l, SS433, and Cyg X-l, and are suspected in several others. These cycles are observed in both the X-ray and optical bands, and represent cyclic variations in both the inner and outer parts of the accretion disk. Some component of these systems is precessing, but we are not certain which. It could be a misaligned companion star; the outer rim of the accretion disk, driven by radiative feedback; or the neutron star.Several low-mass X-ray binaries have quasi-periodic cycles, with periods ranging from 89 to 2 years. The amplitude of modulation ranges between ~ 50 and 100 ~o, i.e., both 'persistent' and 'transient' objects fall into this class. This activity is reminiscent of the superoutburst cycles of the SU UMa cataclysmic variables, and may be caused by similar mass-transfer instabilities.Periodic outbursts in the Be/neutron star systems seem to result from variable mass transfer in a wide, eccentric orbit. The relationship between the orbital cycle and the flux outbursts, however, is not well understood, and even the equivalence of the outburst and binary cycles remains hypothetical for most objects. Most likely, the periodic outbursts result from enhanced mass transfer at periastron.Compared to other aspects of X-ray astronomy, long-term activity has been much less intensively studied by both observers and theoreticians. A simple all-sky monitor in permanent operation could provide for the X-ray sky the same kind of data base provided to optical observers by the Harvard plates.

Anomalous Low States and Long-Term Variability in the Black Hole Binary LMC X-3

“…The canonical example is the eclipsing X-ray pulsar Her X-1, in which the long-term X-ray light curve is dominated by a high amplitude, double-peaked 35 day variation. The main features of this periodicity can be successfully explained by the presence of a warped or inclined accretion disk precessing with respect to the binary orbital plane (Priedhorsky & Holt 1987). Other systems that show nearly periodic variations at least an order of magnitude longer than the orbital period include the accreting X-ray pulsars SMC X-1 (50-60 days; Gruber & Rothschild 1984;Wojdowski et al 2000, but see Clarkson et al 2003) and LMC X-4 (30.5 days; Lang et al 1981, but see Trowbridge et al 2007), as well as the paradigmatic disk jet source SS 433 (Fabian & Rees 1979;Abell & Margon 1979).…”

Long-term X-ray variability and state transition of GX 339–4

“…X-ray variability on long time-scales (from days to years) has been found in many low-mass and high-mass X-ray binaries, but its origin is still an open question (e.g. Priedhorsky & Holt 1987;Schwarzenberg-Czerny 1992;Wijers & Pringle 1999;Ogilvie & Dubus 2001). Long-term X-ray variability has also been seen in the persistent black hole candidates Cyg X -1 (e.g.…”

Searching for periodicities in the MACHO light curve of LMC X-2

“…In high-mass X-ray binaries (HMXB) superorbital periods are known for several sources. Theories to explain these long-term variations include precession of a tilted accretion disc, precession of the neutron star, mass transfer feedback and triple systems (see Priedhorsky &Holt 1987 andSchwarzenberg-Czerny 1992). In LMXBs such superorbital periods are much less common.…”

“…It is thought that they may be owing to radiation-driven warped accretion discs (e.g. Wijers & Pringle 1999) or a disc instability in the system (Priedhorsky & Holt 1987).…”