Jets and gamma-ray emission from isolated accreting black holes

Abstract

Isolated black holes (IBHs) are not usually considered to be important astrophysical sources, since, even in the case of a high accretion rate, an accretion disc rarely can be formed due to the small angular momentum of the infalling matter. Thus, such systems are not expected to feature thermal disc emission which makes the dominant contribution to the radiative output of binary systems harbouring a BH. Moreover, due to their relatively modest accretion rates, these objects are not conventionally treated as feasible jet sources. However, the large number of IBHs in the Galaxy, estimated to be ~108, implies a very high density of 10-4pc-3 and an average distance between IBHs of ~10pc. Our study shows that the magnetic flux, accumulated on the horizon of an IBH because of accretion of interstellar matter, allows the Blandford-Znajeck mechanism to be activated. Thus, electron-positron jets can be launched. We have performed 2D numerical modelling which allowed the jet power to be estimated. Their inferred properties make such jets a feasible electron accelerator which, in molecular clouds (MCs), allows electron energy to be boosted up to ~1 PeV. For the conditions expected in MCs, the radiative cooling time should be comparable to the escape time. Thus, these sources can contribute both to the population of unidentified point-like sources and to the local cosmic-ray (CR) electron spectrum. The impact of the generated electron CRs depends on the diffusion rate inside MCs. If the diffusion regime in a MC is similar to Galactic diffusion, the produced electrons should rapidly escape the cloud and contribute to the Galactic CR population at very high energies, >100 TeV. However, due to the modest jet luminosity (at the level of ~1035ergs-1) and low filling factor of MCs, these sources cannot make a significant contribution to the spectrum of CR electrons at lower energies. On the other hand, if the diffusion within MCs operates at a rate close to the Bohm limit, the CR electrons escaping from the source should be confined in the cloud, significantly contributing to the local density of CRs. The inverse Compton emission of these locally generated CRs may explain the variety of gamma-ray spectra detected from nearby MCs. © 2012 The Authors Monthly Notices of the Royal Astronomical Society. © 2012 RAS.