We present the results from Suzaku observations of the hottest Abell galaxy cluster A2163 at $z=0.2$. To study the physics of gas heating in cluster mergers, we investigated hard X-ray emission from the merging cluster A2163, which hosts the brightes
t synchrotron radio halo. We analyzed hard X-ray spectra accumulated from two-pointed Suzaku observations. Non-thermal hard X-ray emission should result from the inverse Compton (IC) scattering of relativistic electrons by the CMB photons. 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 broad-band 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. From the Suzaku data, we detected significant hard X-ray emission from A2163 in the 12-60 keV band at the $28sigma$ level (or at the $5.5sigma$ level if a systematic error is considered). The Suzaku HXD spectrum alone is consistent with the single-T thermal model of gas temperature $kT=14$ keV. From the XMM data, we constructed a multi-T model including a very hot ($kT=18$ keV) component in the NE region. Incorporating the multi-T and the power-law models into a two-component model with a radio-band photon index, the 12-60 keV energy flux of non-thermal emission is constrained within $5.3 pm 0.9 (pm 3.8)times 10^{-12}~{rm erg, s^{-1} cm^{-2}}$. The 90% upper limit of detected IC emission is marginal ($< 1.2times 10^{-11}~{rm erg, s^{-1} cm^{-2}}$ in the 12-60 keV). The estimated magnetic field in A2163 is $B > 0.098~{rm mu G}$. While the present results represent a three-fold increase in the accuracy of the broad band 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 IC emission.
As the nodes of the cosmic web, clusters of galaxies trace the large-scale distribution of matter in the Universe. They are thus privileged sites in which to investigate the complex physics of structure formation. However, the complete story of how t
hese structures grow, and how they dissipate the gravitational and non-thermal components of their energy budget over cosmic time, is still beyond our grasp. Fundamental questions such as How do hot diffuse baryons accrete and dynamically evolve in dark matter potentials? How and when was the energy that we observe in the ICM generated and distributed? Where and when are heavy elements produced and how are they circulated? are still unanswered. Most of the cluster baryons exists in the form of a diffuse, hot, metal-enriched plasma that radiates primarily in the X-ray band (the intracluster medium, ICM), allowing the X-ray observations of the evolving cluster population to provide a unique opportunity to address these topics. Athena+ with its large collecting area and unprecedented combination of high spectral and angular resolution offers the only way to make major advances in answering these questions. Athena+ will show how the baryonic gas evolves in the dark matter potential wells by studying the motions and turbulence in the ICM. Athena+ will be able to resolve the accreting region both spatially and spectroscopically, probing the true nature and physical state of the X-ray emitting plasma. Athena+ has the capabilities to permit a definitive understanding of the formation and evolution of large-scale cosmic structure through the study of the cluster population.
