Neutrinos from transient central engine shocks in AGN

Abstract

If shock acceleration of protons is responsible for an appreciable fraction of the non-thermal emission from AGN central engines, they are probably also powerful sources of high energy neutrinos. The neutrino background intensity at the Earth has been predicted by several authors on that basis. This paper describes a calculation of the neutrino spectrum, starting from assumptions about the state of accreting plasma near the central black hole. Our assumptions are closest to those of Stecker and Salamon. where the protons cool by photo-pion production, and neutrino emission is normalised to X-ray emission. However, rather than using the non-relativistic test-particle analytic results for proton acceleration rate and spectral index, we use the results of a numerical code developed at Imperial College, which takes into account aspects of the non-linear shock regime, and shows that relativistic shocks produce a harder spectrum with a faster acceleration time. Also, because of the narrow ranges of plasma properties which permit steady shocks, and doubts about shock stability, we believe that particle acceleration in AGN central engines occurs at transient shocks, small compared with the overall source size, and at a wide range of radii, rather than a single steady shock at a fixed distance from the central black hole, as is usually assumed. Observations of rapid X-ray variability in Seyferts are also suggestive of such a picture. We calculate neutrino spectra for 3C 273, and more importantly, overall background spectra from unresolved AGN, which are compared with other model predictions. We show that with a realistic correction to the Bohm diffusion limit for protons, parallel shocks are unlikely to produce an observable neutrino flux, and quasi-perpendicular relativistic shocks are necessary.