High-energy emission from off-axis relativistic jets.pdf

a r X i v : a s t r o - p h / 0 5 0 1 1 9 7 v 1 1 1 J a n 2 0 0 5 High-energy emission from off-axis relativistic jets E.V. Derishev ? , F.A. Aharonian ? and Vl.V. Kocharovsky ? ? Institute of Applied Physics, 46 Ulyanov st., 603950 Nizhny Novgorod, Russia ? Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, D-69117 Heidelberg, Germany Abstract. We analyze how the spectrum of synchrotron and inverse Compton radiation from a narrow rela- tivistic jet changes with the observation angle. It is shown that diversity of acceleration mechanisms (in particular, taking the converter mechanism (Derishev et al. 2003) into account) allows for numer- ous modifications of the observed spectrum. In general, the off-axis emission in GeV-TeV energy range appears to be brighter, has a much harder spectrum and a much higher cut-off frequency com- pared to the values derived from Doppler boosting considerations alone. The magnitude of these effects depends on the details of particle acceleration mechanisms, what can be used to discriminate between different models. One of the implications is the possibility to explain high-latitude unidentified EGRET sources as off-axis but otherwise typical relativistic-jet sources, such as blazars. We also discuss the broadening of beam pattern in application to bright transient jet sources, such as Gamma-Ray Bursts. INTRODUCTION Relativistic flows are attractive as a solution of the compactness problem for bright and rapidly variable sources, such as Active Galactic Nuclei (AGNs) and Gamma-Ray Bursts (GRBs). Lorentz boosting lowers both the required comoving number density and the required energy of individual photons, so that the pair-production opacity can be made smaller than one for a source of given size. Often (if not always), the relativistic outflows are in the form of narrow jets, as seen in many AGNs (e.g., Urry Padovani 1995) and deduced theoretically for GRBs (e.g., Mészáros 2002). The central dogma for narrow and highly relati