Does an orbiting star cause periodic modulation of X-rays from NGC6814?
USING data from the Ginga satellite, Done et al.1 have confirmed an Exosat observation2 of high-amplitude periodic modulation in the X-ray emission from the Seyfert galaxy NGC6814. To within observational errors of ∼1%, the period seems to have remained constant at ∼12,100 seconds between 1985 and 1989. An obvious candidate for the phenomenon underlying the periodicity is the orbital motion of a star or low-mass compact object around the central black hole3. As we show here, the presence of an orbiting star could be verified easily by looking for the effects of Lense–Thirring precession of the orbital plane, caused by the dragging of inertia! frames around a rotating black hole. Precession-induced variations in the waveform and in the phase of the observed periodicity should have a period of between a month and a year. Such variations could account for the different waveforms present in the Ginga and Exosat data sets4, and may be detectable in existing Ginga and future Rosat, OSSE/GRO, and Astro-D data.
Star–disc interactions near a massive black hole
We examine the possibility that stars in the nuclei of galaxies could be ground down into short-period orbits, by interaction with a disc around a central massive black hole. A star on a highly eccentric orbit, which passed close to the hole, would lose energy and momentum in passing through the disc. The cumulative effect of many passages would bring the star into a circular orbit corotating with the disc. The star would then either open up a gap in the disc (if the disc is thinner than the star's Roche radius and the viscosity in the disc is sufficiently low), or accrete material from the disc.
Pair production instabilities as a source of X-ray flares from accreting black holes
Several active galaxies emit most of their energy at hv > 100 keV (ref. 1). Observations suggest that their high-energy spectra are strongly variable and are cut off or broken at ∼1 MeV (see for example, refs 2–6 and references therein). A possible explanation of these properties is that the high-energy radiation is produced in the central region (radius R<10Rs, where Rs is the Schwarzshild radius) of the two-temperature plasma accreting onto a supermassive black hole7,8. There, the e+–e− pair creation can be very efficient and, according to our calculations, at a certain luminosity range around Lcr = 0.01 LEdd (where LEdd is the Eddington luminosity) it gives rise to a plasma instability. We show that for a proton temperature close to its virial value, kTpvirial≍ GMmp/R (where mp and M are masses of proton and of the central object respectively), this pair-production instability leads to cyclic variations of the accretion flow, during which high-energy flares are produced. We propose here that the large-amplitude luminosity changes noted in hard X rays and soft γ rays2–6, as well as the soft X-ray outbursts observed in several active galactic nuclei (AGNs)9–11 result from the above mechanism. The same scenario may also be responsible for the short-timescale quasiperiodic variability reported in some proposed galactic black holes12–15.