BLACK-HOLE REMNANTS - SOFT-X-RAY FLARES FROM TIDALLY DISRUPTED STARS

Massive black holes may lie at the centre of many normal galaxies as r emnants from earlier active phases. There is some evidence for the pre sence of black holes of masses 10(7)-10(9) M. in the nuclei of a few n earby galaxies and one of mass approximately 10(6) M. at the centre of our own Galaxy. It has recently been suggested that the tidal disrupt ion of a star falling into a central black hole should produce a short -lived flare lasting some months with a peak luminosity within the ext reme 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 use d to set limits on the distribution of black hole masses in the galact ic population. This information would provide important constraints on evolutionary models of active galactic nuclei (AGN). Recently, the RO SAT satellite performed an all-sky survey over the energy ranges 62-18 5 eV with the Wide Field Camera (WFC) and 0.1-2.4 keV with the Positio n Sensitive Proportional Counter (PSPC). We calculate the number of fl ares that could be detected by each instrument as a function of the ma ss of the central black hole. Our results indicate that at least sever al 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 10(7)-10(8) 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 approx imately 10(9) M. in only a few per cent of the current galactic popula tion is strongly supported.