1302.2907 (Hiroki Akamatsu et al.)

Suzaku X-Ray Observations of the Accreting NGC 4839 Group of Galaxies and the Radio Relic in the Coma Cluster    [PDF]

Hiroki Akamatsu, Susumu Inoue, Takuya Sato, Kyoko Matsushita, Yoshitaka Ishisaki, Craig L. Sarazin

Based on Suzaku X-ray observations, we study the hot gas in regions around the NGC 4839 group of galaxies and the radio relic in the outskirts of the Coma cluster. From spectral analysis, the temperature of the gas shows a gradual decline from 5 keV around NGC4839 to about 3.6 keV at the radio relic. Across the relic, the temperature drops steeply by approximately a factor of 2 from 3.6 to 1.5 keV. This temperature drop can be interpreted as a shock with Mach number M = 2.2 pm 0.5. The existence of a shock front suggests that it may be responsible for accelerating the non-thermal electrons. However, if they are accelerated according to the simplest theory of diffusive shock acceleration (DSA) in which test particles are injected from a thermal distribution, the electron spectrum expected from the measured Mach number and shock compression would be steeper than that inferred from the observed, spatially integrated spectrum of the radio relic with index alpha = 1.18, taking into account radiative loss effects. The expected electron acceleration efficiency may also be too low to be compatible with the observed radio flux. The radio relic may nevertheless result from DSA if it is initiated by the injection of a pre- existing population of non-thermal electrons. Based on the correspondence between the temperature jump and the radio relic as well as the metal distribution, these facts can be interpreted in terms of a picture in which NGC 4839 is falling into the Coma cluster and the accompanying, metal-enriched gas is being stripped because of ram pressure. We suggest that the shock front associated with the radio relic is generated by the combined interaction of the pre-existing intracluster medium with gas conveyed by the infalling NGC 4839 group, together with the inflow of cooler gas from the large-scale structure filament that connects Coma with Abell 1367.

View original: http://arxiv.org/abs/1302.2907