Galaxies and the Universe - Jets, Superluminal Motion, and Gamma-Ray Bursts

Galaxies and the Universe - Jets, Superluminal Motion, and Gamma-Ray Bursts

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1/15/12 Galaxies and the Universe - Jets, Superluminal Motion, and Gamma-Ray Bu« 1/6 www.astr.ua.edu/keel/galaxies/jets.html Jets, Superluminal Motion, and Gamma-Ray Bursts Radio synthesis maps have shown jets in hundreds of AGN, on scales from subparsec to megaparsecs. The continuity of jets in direction indicates that the central generator has a memory over millions of years, and disk structures provide a natural way to control the direction of the jets. There is a vast literature on the collimation and production of jets; I will mention only a few points here. How fast are they? Structures of jets can indicate their Mach numbers (with respect to the external medium), but not immediately their absolute velocities. Some sources look as if the jets are rather slow and flexible, while others look like highly relativistic blowtorches. We do not even know for sure whether we are seeing a phase or group velocity when motions can be measured. Strong evidence for relativistic bulk motions comes from superluminal sources, in which the projected speed of motion (always outward from the core) of distinct blobs is 1-10 c . A natural explanation is (backwards) time dilation in material approaching us at ~0.9 c ; the Doppler boosting of this material would make these objects bright, so that the boosted sources were observed first. The emitter stays only slightly behind its earlier radiated wavefronts, so the projected motion is quite rapid (see Superluminal Radio Sources , ed. Zensus and Pearson, Cambridge 1987). The governing equations reflect the relativistic Doppler dilation and boost effects. If we consider the projected separation between a stationary core and a blob moving away from it at a rate c β at an angle θ to our line of sight, the apparent transverse velocity will be which has a maximum value v ma[ ~ γ c . The apparent jet/counterjet ratio R (for physically identical jets) becomes where ȟ is related to the spectral index α by 3-α for a confined blob and 2-α for a continuous jet. The relativistic γ factor does not appear in the ratio because it is identical for both components, so that the geometric factors alone are left. The data show v = 1-10 c for superluminal sources (and subluminals also exist, mostly for nearby and fairly low-power objects like M87 and Cen A). This material is not always the conventional jet in an early stage; Barthel has shown that radio galaxies of large projected size (i.e. presumably viewed 90° to the jet axes) can have superluminal motions, and proposed an intermediate model in which material is initially ejected over a broad cone angle. Only the tiny fraction coming near our line of sight is boosted enough to see at high angular resolution, and it is this fraction that would exhibit superluminal motion. On larger scales, structures in the M87 jet a kiloparsec from the core have been found to show transverse velocities of 0.3 - 0.5 c (Reid et al 1989 ApJ 336, 112; Biretta et al. 1989 ApJ 342, 128) - so far the only direct evidence that something in large-scale jets is moving at high speeds. In M87, HST imaging
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Galaxies and the Universe - Jets, Superluminal Motion, and Gamma-Ray Bursts

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