Black hole remnants: soft X-ray flares from tidally disrupted stars

Abstract

Massive black holes may lie at the centre of many normal galaxies as remnants from earlier active phases. There is some evidence for the presence of black holes of masses 107–109 M⊙ in the nuclei of a few nearby galaxies and one of mass ~ 106 M⊙ at the centre of our own Galaxy. It has recently been suggested that the tidal disruption of a star falling into a central black hole should produce a short-lived flare lasting some months with a peak luminosity within the extreme ultraviolet (EUV) or soft X-ray band. For a given black hole mass, the event rate of such flares can be predicted, and hence may be used to set limits on the distribution of black hole masses in the galactic population. This information would provide important constraints on evolutionary models of active galactic nuclei (AGN). Recently, the ROSAT satellite performed an all-sky survey over the energy ranges 62–185 eV with the Wide Field Camera (WFC) and 0.1–2.4 keV with the Position Sensitive Proportional Counter (PSPC). We calculate the number of flares that could be detected by each instrument as a function of the mass of the central black hole. Our results indicate that at least several hundred and perhaps several thousand flare events would be expected to have been detected during the PSPC survey if remnant holes within the range a few times 107–108 M⊙ reside in a sizeable fraction of normal galaxies. If such events are not detected, then the theory that quasars are long-lived and leave remnant holes of greater than ~ 109 M⊙ in only a few per cent of the current galactic population is strongly supported.