Hard X-ray Structures in the Jet Hotspots of Radio Galaxies

1. Hard X-ray Structures in the Jet Hotspots of Radio Galaxies D. Reed1, M. Bałucińska-Church1,2, M.J. Church1,2, M. Ostrowski2,3 and Ł. Stawarz2 1School of Physics and Astronomy, University of Birmingham, UK 2Obserwarium Astronomiczne, Uniwersytet Jagielloński, Poland 3Institut für Theoretische Physik IV, Ruhr-Universität Bochum, Germany In the most powerful radio galaxies, hotspots which radiate from radio to X-rays are often found at the end of the relativistic radio jets. These hotspots are thought to form where the jet pummels into the ambient intergalactic medium (IGM) (Blandford & Rees 1974; Scheuer 1974). The jet material decelerates abruptly forming a double shock structure. The forward ‘bow’ shock compresses and heats the IGM. The reverse ‘Mach’ shock propagates within the jet, converting jet bulk kinetic energy into cosmic rays and magnetic field energy. Cygnus A – VLA 6cm Radio Cygnus A – Chandra 0.3-8.0 keV X-ray A B We have analysed several archival Chandra observations of a number of FRII radio galaxies which exhibit X-ray hotspots. Given the superb spatial resolution of Chandra, the hotspots of a few of these galaxies are mapped in detail for the first time. Although a lack of counts prohibits a thorough spectral analysis, we have simply extracted hardness ratio (HR) maps to search for any spectral variation, and thus structure, across the hotspots. Images were extracted in two energy bands: 2.0-8.0 keV and 0.3-2.0 keV, which were divided to give the hardness ratio maps. D Eastern Hotspot – Hardness Ratio image Western Hotspots – Hardness Ratio image (2.0-8.0)/(0.3-2.0) keV (2.0-8.0)/(0.3-2.0) keV A smoothed hardness map of the hotspots of Cygnus A (right) reveals previously unknown structure around the hotspots in the form of outer and inner arcs of enhanced hardness (Bałucińska-Church et al. 2004). The contours represent the smoothed 0.3-8.0 keV X-ray intensity. The outer hard arcs may represent the first detection of the bow shock; while the inner hard arcs may reveal where the jets impact on the hotspots. Below are the results of a follow-up study of the galaxy 3C 123 with smoothed 0.3-8.0 keV intensity contours overlaid. The eastern Hotspot clearly shows an outer hard arc similar to those seen in Cygnus A. Understanding the origin of the hotspot emission is a major problem in astrophysics. For some time, it has been clear that the X-ray emission from hotspots cannot be explained by a single model. Some objects show hotspot X-ray emission consistent with the predictions of a synchrotron self-Compton (SSC) model, whereby the synchrotron emitting electrons inverse-Compton (IC) scatter synchrotron photons into the X-ray band. However, other objects show hotspot X-ray emission which is substantially stronger than the SSC model would predict and indicative of simple synchrotron radiation rather than IC scattering. That is to say, can be modelled (in most cases) by a simple power law. The reasons for these differences are at present not understood. 3C123 – Chandra 0.3-8.0 keV image The hard arcs discovered in these data cannot be explained within the framework of the SSC model. Work by Carilli et al. (1991) suggests that at the edges of the hotspots the shock-generated electron energy distribution will adopt a steep power law shape or sharp cut-off in the high energy range. Thus SSC X-ray emission is expected to be softer at the outer edges of the hotspot, not harder. Given that detailed spectral modelling of the hard arcs is not possible due to a lack of counts we qualitatively consider two other possible origins for the hard regions: 1) A Non-thermal origin Both inner and outer hard arcs may be due to synchrotron radiation of electrons accelerated in turbulent regions highly perturbed by shocks and shear flows. 2) A Thermal origin The outer hard arcs may be explained by the thermal radiation of ambient intracluster gas compressed and heated by the bow shock. This thermal explanation does not account for the inner hard arcs. Hardness Ratio Image : (2.0-8.0)/(0.3-2.0) keV In the above figures, we have simulated spectral data to assess the implications of the HR values obtained from the Chandra data. We have simulated a simple absorbed power law (non-thermal case) for a range of photon power law index , and an absorbed mekal model (thermal case) for a range of kT. In Cygnus A, the column density was fixed at the standard 0.3x1022 cm-2 as determined from radio observations (Dickey & Lockman 1990). In 3C123, two column densities were used, the standard radio derived column density (lower curves in figures) and a column density derived from a spectral fit to the total hotspot emission. These simulated spectra were divided into the same two energy bands as in our analysis and the HR found. The dashed lines represent the simulated variation in HR. From the Chandra data, HR values were calculated simply by taking the mean of a region enclosing the hard emission/central intensity in the HR map. The calculated HR values were placed on the curve at the same simulated HR value from which it is possible to then read off  or kT (note in 3C123 the points are placed on the lower curve). In Cygnus A it is clear to see that the differences in hardness between regions is significant. However, in the thermal case, a temperature ~90 – 200 keV is suggested. Such high temperatures are unrealistic and thus we favour the non-thermal case. In the case of 3C123, due to poor Poisson statistics, we have introduced a soft region which is the whole hotspot not including the hard and central intensity region. The difference in HR between the hard arc and this soft region is statistically significant. The point labelled ‘Hotspot – Total’ represents an actual spectral fit to the total hotspot emission and as expected lies between the hard and soft limits. As in Cygnus A, the hard arc and central intensity HR in 3C123 suggests an unrealistically high thermal temperature >400 keV, in addition, these points do not intersect the simulated HR curve. For these reasons we argue in favour of a non-thermal explanation. Hotspot close-up hardness ratio Cygnus A properties Redshift = 0.0565 Distance = 240 Mpc Chandra exposure = 35 ks 3C 123 properties Redshift = 0.2177 Distance = 930 Mpc Chandra exposure = 43.66 ks