Pair production instabilities as a source of X-ray flares from accreting black holes

Abstract

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<10R_s, where R_s is the Schwarzshild radius) of the two-temperature plasma accreting onto a supermassive black hole^7,8. There, the e₊–e₋ pair creation can be very efficient and, according to our calculations, at a certain luminosity range around L_cr = 0.01 L_Edd (where L_Edd is the Eddington luminosity) it gives rise to a plasma instability. We show that for a proton temperature close to its virial value, kT_p^virial≍ GMm_p/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 rays^2–6, 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 holes^12–15.