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20th of March 2015
Another shock for the Bullet cluster
by Tim Shimwell (Leiden University, The Netherlands)
With Australia Telescope Compact Array (ATCA) observations we (Shimwell et al. 2015) detect a highly elongated Mpc-scale diffuse radio source on the eastern periphery of the Bullet cluster 1E 0657-55.8, which we argue has the positional, spectral and polarimetric characteristics of a radio relic. This powerful relic consists of a bright northern bulb and a faint linear tail. Exactly coincident with the linear tail we find a sharp X-ray surface brightness edge in the deep Chandra image of the cluster - a signature of a shock front in the hot intracluster medium (ICM), located on the opposite side of the cluster to the famous bow shock. This new example of an X-ray shock coincident with a relic further supports the hypothesis that shocks in the outer regions of clusters can form relics via diffusive shock (re-)acceleration. Intriguingly, our new relic suggests that seed electrons for reacceleration are coming from a local remnant of a radio galaxy, which we are lucky to catch before its complete disruption. If this scenario, in which a relic forms when a shock crosses a well-defined region of the ICM polluted with aged relativistic plasma - as opposed to the usual assumption that seeds are uniformly mixed in the ICM - is also the case for other relics, this may explain a number of peculiar properties of peripheral relics.

The image to the left shows the X-ray emission of the Bullet cluster (colour) overlaid with the ATCA radio continuum emission (green contours). The extended emission on the eastern periphery of the X-ray emission is the newly discovered radio relic. The brightness profile (right image) shows the radio (dashed) and X-ray brightness (red line) through the radio relic and reveals an X-ray shock at the location of the radio relic.

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