NEUTRINOS FROM TRANSIENT CENTRAL ENGINE SHOCKS IN AGN

If shock acceleration of protons is responsible for an appreciable fra ction of the non-thermal emission from AGN central engines, they are p robably also powerful sources of high energy neutrinos. The neutrino b ackground intensity at the Earth has been predicted by several authors on that basis. This paper describes a calculation of the neutrino spe ctrum, starting from assumptions about the state of accreting plasma n ear the central black hole. Our assumptions are closest to those of St ecker and Salamon, where the protons cool by photo-pion production, an d neutrino emission is normalised to X-ray emission. However, rather t han using the non-relativistic test-particle analytic results for prot on acceleration rate and spectral index, we use the results of a numer ical code developed at Imperial College, which takes into account aspe cts of the non-linear shock regime, and shows that relativistic shocks produce a harder spectrum with a faster acceleration time. Also, beca use of the narrow ranges of plasma properties which permit steady shoc ks, and doubts about shock stability, we believe that particle acceler ation in AGN central engines occurs at transient shocks, small compare d with the overall source size, and at a wide range of radii, rather t han a single steady shock at a fixed distance from the central black h ole, 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 f or protons, parallel shocks are unlikely to produce an observable neut rino flux, and quasi-perpendicular relativistic shocks are necessary.