Relativistic Flows at the Hotspots of Radio Galaxies and Quasars?

We review the broad band properties of X-ray detected hotspots in radio galaxies and quasars. We show that their collective spectral properties can be unified in a framework involving frequency dependent relativistic beaming and varying orientations to the observer’s line of sight. The simplest dynamic model consistent with this picture is a slowing-down relativistic flow downstream from the hotspot shock, suggesting that the jet flows remain relativistic to the hotspot distances. 1 Cygnus A vs. Pictor A Pairs of radio emitting jets with lenghts up to several hundred kpc originate from the central region of radio loud active galaxies. In the most powerful of them, the jets terminate in the hotspots, compact high brightness regions, where the jet collides with the intergalactic medium (IGM). The first hotspots to be detected in X-rays were those of the nearby powerful radio galaxy Cygnus A (1), whose X-ray flux was found in agreement with synchrotron self Compton emission in equipartition between electron and magnetic field energy densities (SSCE). These hotspots show no optical emission, suggesting a spectral cutoff at lower frequencies. While Chandra observations of Cygnus A confirmed the SSCE picture (2), observations of Pictor A, another nearby powerful galaxy (3) showed a very different picture: a.) an one-sided large scale X-ray jet on the same direction with the known VLBI jet (4); b.) Detection of X-ray emission only from the hotspot on the jet side, which was also detected in the optical. In addition, SSCE models for the Pictor A hotspots underproduce the observed X-ray flux, requiring a magnetic field ∼ 14 times below its the equipartition value in order to achieve agreement with the observed the X-ray flux. The one-sided X-ray jet of Pictor A suggests the potential importance of relativistic beaming and orientation as a discriminant of the hotspot properties of these two sources. An indicator of orientation is the ratio R of the core Preprint submitted to Elsevier Science 2 February 2008 Table 1 Sources with X-ray hotspot detection Source Type Log R Optical X-ray SSCE 3C 330 NLRG -3.5 (7) NO YES (8), 2 sides YES Cygnus A NLRG -3.3(5) NO YES(1; 2) 2 sides YES 3C 295 NLRG -2.7(5) YES YES (9; 10), 2 sides YES 3C 123 NLRG -1.9(11) NO YES(12), 1 side YES 3C 263 Q -1.0(13) YES(8), jet side YES(8), Jet side See text 3C 351 Q -1.9(14) YES(8; 15), jet side YES(8; 15), jet side NO Pictor A BLRG -1.2(5) YES(3), jet side YES(3), jet side NO 3C 303 Q -0.7(5) YES(16), jet side YES(17), jet side NO 3C 390.3 BLRG -1.1(5) YES(16; 18), jet side YES(19), jet side NO (beamed) to the extended (isotropic) radio emission. Sources with jets closer to the line of sight are expected to have higher values of R than sources with jets closer to the plane of the sky. Cygnus A has logR ≈ −3.3 (5), while Pictor A has logR ≈ −1.2 (5), suggesting that the jets of Pictor A are closer to the line of sight than those of Cygnus A. Another indicator of source orientation is the detection of broad emission lines in the optical-UV spectrum of the core of a source. According to the unification scheme for radio loud active galaxies (e.g. (6)), broad line radio galaxies (BLRG) and quasars have jets pointing close to the line of sight, while narrow line radio galaxies (NLRG) have jets closer to the plane of the sky. Cygnus A is a NLRG, and Pictor A is a BLRG, suggesting again that Pictor A is aligned closer to the line of sight. 2 Collective X-ray–detected hotspot properties; A Unified Picture We examine here the properties of the X-ray detected hotspots in relation to the orientation of their jets relative to the line of sight. In table 1 we present the sources with X-ray hotspot detections, their emission line classification, core dominance R, their optical and X-ray hotspot properties, and the possibility of modeling their spectra with SSCE. An orientation sequence emerges from these observations: as the jet aligns closer to the line of sight, measured by an increase in R, the source changes from a NLRG to a BLRG/Quasar. Sources closer to the plane of the sky show X-ray hotspots in both lobes (except for the peculiar radio galaxy 3C 123, where only one hotspot is detected (12)), while more aligned sources show X-ray hotspots only on the side of the near jet, as identified through VLBI observations. While in NLRG, SSCE models are in agreement with the observed X-ray flux, in the more aligned BLRG