Chandra and XMM–Newton observations of A2256: cold fronts, merger shocks, and constraint on the IC emission

Abstract

We present the results of deep Chandra and XMM-Newton observations of a complex merging galaxy cluster Abell 2256 (A2256) that hosts a spectacular radio relic. The temperature and metallicity maps show clear evidence of a merger between the western subcluster and the primary cluster. We detect five X-ray surface brightness edges. Three of them near the cluster center are cold fronts (CFs): CF1 is associated with the infalling subcluster; CF2 is located in the east of the primary cluster and CF3 is to the west of the primary cluster core. The other two edges at cluster outskirts are shock fronts (SFs): SF1 near the radio relic in the NW has Mach numbers derived from the temperature and the density jumps respectively of MT = 1.62 ± 0.12 and Mρ = 1.23 ± 0.06; SF2 in the SE has MT = 1.54 ± 0.05 and Mρ = 1.16 ± 0.13. In the region of the radio relic there is no evidence for the correlation between X-ray and radio substructures, from which we estimate an upper limit for the inverse Compton emission, and therefore set a lower limit on the magnetic field (∼ 450 kpc from primary cluster center) of B > 1.0 μG for a single power-law electron spectrum or B > 0.4 μG for a broken power-law electron spectrum. We propose a merger scenario including a primary cluster, a subcluster, and a group. Our merger scenario accounts for the X-ray edges, diffuse radio features, and galaxy kinematics, as well as projection effects.

Deep low-frequency radio observations of Abell 2256 I: The filamentary radio relic

“…The emission is mainly visible in low resolution images. No feature is detected in the Chandra and XMM-Newton X-ray images (Ge et al 2020) at that location. The arc-shaped feature is apparently connected to the large relic, the central halo emission, and the source F. Based only on the visual appearance, whether the connection between these features is real or just in projection is not obvious.…”

“…At X-ray wavelengths, the cluster is luminous (L X,0.1−2.4 keV = 3.7 × 10 44 erg s −1 ; Ebeling et al 1998). Recent X-ray studies provide evidence for five X-ray surface brightness discontinuities: three cold fronts and two shock fronts (Ge et al 2020;Breuer et al 2020). One of the shock fronts, which has a Mach number M = 1.26 ± 0.06, is detected at the northwest of the large relic (Ge et al 2020).…”

“…Recent X-ray studies provide evidence for five X-ray surface brightness discontinuities: three cold fronts and two shock fronts (Ge et al 2020;Breuer et al 2020). One of the shock fronts, which has a Mach number M = 1.26 ± 0.06, is detected at the northwest of the large relic (Ge et al 2020). However, there is an offset of about 150 kpc between the relic emission at 1.5 GHz and the detected X-ray density/temperature jump.…”

“…Optical studies of the galaxy distribution reveal that the cluster consists of three separate components (Berrington et al 2002;Miller et al 2003). Both optical and X-ray observations provide strong evidence that Abell 2256 is undergoing a merger event between a main cluster component, a major sub-component and a third infalling group (Briel et al 1991;Briel & Henry 1994;Sun et al 2002;Berrington et al 2002;Miller et al 2003;Ge et al 2020).…”

“…where ξ e is the fraction of the kinetic energy dissipated at the shock front channeled into the acceleration of supra-thermal electrons, A is the surface area of the relic, n e, d is the downstream electron density, T d is the downstream electron temperature, B is the magnetic field strength, B CMB is the field strength equivalent to the Cosmic Microwave Background energy density, B CMB = 3.24 (1 + z) 2 µG, and Ψ(M, T d ) comprises all Mach number dependencies. For the relic, we adopt n e, d = 1 × 10 −3 cm −3 and T d = 8 keV (Trasatti et al 2015;Ge et al 2020). The other measured values are A = 0.8 Mpc 2 , α = −1.07, and Ψ(M) = 0.76 (assuming a shock of M = 5.4).…”