Suzaku observations of the merging galaxy cluster Abell 2255: The northeast radio relic

¹ SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
e-mail: h.akamatsu@sron.nl
² Department of Physics, Nara Women’s University, Kitauoyanishimachi, Nara, 630-8506 Nara, Japan
³ Argelander Institute for Astronomy, Bonn University, Auf dem Hügel 71, 53121 Bonn, Germany
⁴ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
⁵ Department of Earth and Planetary Science, The University of Tokyo, 113-0033 Tokyo, Japan
⁶ Research Center for the Early Universe, School of Science, The University of Tokyo, 113-0033 Tokyo, Japan
⁷ Department of Physical Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, 739-8526 Hiroshima, Japan
⁸ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁹ Department of High Energy Astrophysics, Institute of Space and Astronautical Science, 229 Kamagawa, Japan
¹⁰ Department of Physics, Graduate School of Science, University of Tokyo, 7-3-1 Hongo, Bunkyo, 113-0033 Tokyo, Japan
¹¹ Department of Physics, Tokyo Metropolitan University, 1-1 Minami-Osawa, Hachioji, 192-0397 Tokyo, Japan
¹² Department of Physics, Yamagata University, Kojirakawa-machi 1-4-12, 990-8560 Yamagata, Japan
¹³ Astronomical Institute Anton Pannekoek, University of Amsterdam, Science Park 904, 1098XH Amsterdam, The Netherlands
¹⁴ GRAPPA Institute, University of Amsterdam, 1098 XH Amsterdam, The Netherlands

Received: 28 February 2016
Accepted: 24 November 2016

Abstract

We present the results of deep 140 ks Suzaku X-ray observations of the north-east (NE) radio relic of the merging galaxy cluster Abell 2255. The temperature structure of Abell 2255 is measured out to 0.9 times the virial radius (1.9 Mpc) in the NE direction for the first time. The Suzaku temperature map of the central region suggests a complex temperature distribution, which agrees with previous work. Additionally, on a larger-scale, we confirm that the temperature drops from 6 keV around the cluster center to 3 keV at the outskirts, with two discontinuities at r ~ 5′ (450 kpc) and ~12′ (1100 kpc) from the cluster center. Their locations coincide with surface brightness discontinuities marginally detected in the XMM-Newton image, which indicates the presence of shock structures. From the temperature drop, we estimate the Mach numbers to be ℳ_inner ~ 1.2 and, ℳ_outer ~ 1.4. The first structure is most likely related to the large cluster core region (~350–430 kpc), and its Mach number is consistent with the XMM-Newton observation (ℳ ~ 1.24: Sakelliou & Ponman 2006, MNRAS, 367, 1409). Our detection of the second temperature jump, based on the Suzaku key project observation, shows the presence of a shock structure across the NE radio relic. This indicates a connection between the shock structure and the relativistic electrons that generate radio emission. Across the NE radio relic, however, we find a significantly lower temperature ratio (T₁/T₂ ~ 1.44 ± 0.16 corresponds to ℳ_X−ray ~ 1.4) than the value expected from radio wavelengths, based on the standard diffusive shock acceleration mechanism (T₁/T₂> 3.2 or ℳ_Radio> 2.8). This may suggest that under some conditions, in particular the NE relic of A2255 case, the simple diffusive shock acceleration mechanism is unlikely to be valid, and therefore, more a sophisticated mechanism is required.