Investigating the hard X-ray emission from the hottest Abell cluster A2163 with Suzaku

Context. We present the results from Suzaku of the hottest Abell galaxy cluster A2163 at z = 0.2.
Aims: To study the physics of gas heating in cluster mergers, we investigated hard X-ray emission from the merging cluster A2163, which hosts the brightest synchrotron radio halo.
Methods: We analyzed hard X-ray emission spectra accumulated from two-pointed Suzaku observations. Non-thermal hard X-ray emission should result from the inverse Compton scattering of relativistic electrons by photons in the cosmic microwave background. To measure this emission, the dominant thermal emission in the hard X-ray band must be modeled in detail. To this end, we analyzed the combined broadband X-ray data of A2163 collected by Suzaku and XMM-Newton, assuming single- and multi-temperature models for thermal emission and the power-law model for non-thermal emission. Comparing the non-thermal hard X-ray flux to radio synchrotron emission, we also estimated the magnetic field in the cluster.
Results: From the Suzaku data, we detected significant hard X-ray emission from A2163 in the 12-60 keV band at the 28σ level (or at the 5.5σ level if a systematic error of the non-X-ray background model is considered). The Suzaku HXD spectrum alone is consistent with the single-temperature thermal model of gas temperature kT = 14 keV. From the XMM-Newton data, we constructed a multi-temperature model including a very hot (kT = 18 keV) component in the north-east region. Incorporating the multi-temperature and the power-law models into a two-component model with a radio-band photon index, where Γ = 2.18, the 12-60 keV energy flux of non-thermal emission is constrained within 5.3 ± 0.9 (±3.8) × 10^-12 erg s^-1cm^-2 (the first and second errors refer to the 1σ statistical and systematic uncertainties, respectively). The 90% upper limit of detected inverse Compton emission is marginal (F_NT < 1.2 × 10^-11 erg s^-1cm^-2 in the 12-60 keV band). The estimated magnetic field in A2163 is B > 0.098 μG. While the present results represent a three-fold increase in the accuracy of the broadband (0.3-60 keV) spectral model of A2163, more sensitive hard X-ray observations are needed to decisively test for the presence of hard X-ray emission due to inverse Compton emission.