High-energy emission from jet-cloud interactions in AGNs

arXiv:1003.4676v1 [astro-ph.HE] 24 Mar 2010 June 24, 2018 17:5 WSPC/INSTRUCTION FILE Araudo˙3 International Journal of Modern Physics D c⃝World Scientific Publishing Company High-energy emission from jet-cloud interactions in AGNs Anabella T. Araudo Instituto Argentino de Radioastronom´ıa (CCT La Plata - CONICET), C.C.5, 1894 Villa Elisa, Buenos Aires, Argentina Facultad de Ciencias Astron´omicas y Geof´ısicas, Universidad Nacional de La Plata, Paseo del Bosque, 1900 La Plata, Argentina aaraudo@fcaglp.unlp.edu.ar Valent´ı Bosch-Ramon Max Planck Institut f¨ur Kernphysik, Saupfercheckweg 1, Heidelberg 69117, Germany Valenti.Bosch-Ramon@mpi-hd.mpg.de Gustavo E. Romero Instituto Argentino de Radioastronom´ıa (CCT La Plata - CONICET), C.C.5, 1894 Villa Elisa, Buenos Aires, Argentina Facultad de Ciencias Astron´omicas y Geof´ısicas, Universidad Nacional de La Plata, Paseo del Bosque, 1900 La Plata, Argentina romero@fcaglp.unlp.edu.ar Active galactic nuclei present continuum and line emission. The emission lines are orig- inated by gas located close to the central super-massive black hole. Some of these lines are broad, and would be produced in a small region called broad-line region. This region could be formed by clouds surrounding the central black hole. In this work, we study the interaction of such clouds with the base of the jets in active galactic nuclei, and we compute the produced high-energy emission. We focus on sources with low luminosities in the inner jet regions, to avoid strong gamma-ray absorption. We find that the re- sulting high-energy radiation may be significant in Centaurus A. Also, this phenomenon might be behind the variable γ-ray emission detected in M87, if very large dark clouds are present. The detection of jet-cloud interactions in active galactic nuclei would give information on the properties of the jet base and the very central regions. Keywords: galaxies: active; galaxies: individual: Centaurus A; radiation mechanism: non- thermal 1. Introduction Active galactic nuclei (AGN) are extragalactic sources composed by a super-massive black hole (SMBH), an accretion disk and bipolar relativistic jets. Some AGNs present continuum emission in the whole electromagnetic spectrum, from radio to γ-rays. Besides the continuum radiation, AGNs also have optical and ultra-violet line emission. Some of these lines are broad, emitted by gas moving at velocities vg > 1000 km s−1 and located in a region close (d ∼1017 cm) to the SMBH. The structure of the matter in the broad line region (BLR) is not well known 1 June 24, 2018 17:5 WSPC/INSTRUCTION FILE Araudo˙3 2 Araudo, Bosch-Ramon & Romero but some models assume that the gas could be clumpy. Dense clouds, confined by the hot external medium or by magnetic fields, would be ionized by photons from the accretion disk producing the emission lines broadened by the movement of the clouds around the SMBH. In the particular case of FaranoffRiley (FR) I galaxies, where the accretion disks have low luminosities, the photoionization of the clouds will be inefficient to produce lines and the clouds might be dark. Centaurus A (Cen A) and M87 are the closest AGNs, located at distances of ∼4 and ∼16 Mpc, respectively. These AGNs are classified as FR I radio sources and in the case of Cen A the nuclear region is obscured by a dense torus of gas and dust. Although the BLRs of Cen A and M87 have not been detected 1, clouds with similar characteristic to those detected in FR II AGNs may surround the SMBH 2. We are interested in the high-energy emission produced by the interaction of these possibly dark clouds with the jets of the AGN. We focus here on Cen A and M87, since their moderate accretion rate would imply reduced photon-photon opacities in the interaction region, allowing γ-rays to escape. Assuming standard parameters for the clouds and jets, we study the main physical processes that take place as a consequence of the interaction, and calculate the expected high-energy emission. 2. Jet-cloud interaction We consider that clouds with density nc = 1010 cm−3 and size Rc = 1013 cm are surrounding the SMBH and one of these clouds penetrates into one of the relativistic jets. We assume that the jet has a Lorentz factor Γ = 10 (i.e. a bulk velocity vj ∼c), a radius Rj = 0.1z (z is the distance to the black hole), and a kinetic luminosity Lj = 1044 erg s−1. The penetration time of the cloud into the jet is determined by tc ∼2Rc/vc = 2 × 104 s, where vc = 109 cm s−1 is the cloud velocity. As a consequence of the interaction of the jet material with the cloud, two shocks form. One of these shocks propagates back in the reference frame of the jet with a velocity vbs ∼vj, producing a bow shock. This bow shock reaches the steady state configuration in a time tbs ∼ xbs/vbs, where the stagnation point is taken at a distance xbs ∼0.3Rc from the cloud (this value is obtained considering particle flux conservation and assuming an escape velocity equal to the downstream sound speed). On t

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