A joint XMM-NuSTAR observation of the galaxy cluster Abell 523: Constraints on inverse Compton emission

Aims: We present the results of a joint XMM-Newton and NuSTAR observation (200 ks) of the galaxy cluster Abell 523 at z = 0.104. The peculiar morphology of the cluster radio halo and its outlier position in the radio power P(1.4 GHz) - X-ray luminosity plane make it an ideal candidate for the study of radio and X-ray correlations and for the search of inverse Compton (IC) emission.
Methods: We constructed bi-dimensional maps for the main thermodynamic quantities (i.e., temperature, pressure and entropy) derived from the XMM observations to describe the physical and dynamical state of the cluster's intracluster medium (ICM) in detail. We performed a point-to-point comparison in terms of surface brightness between the X-ray and radio emissions to quantify their morphological discrepancies. Making use of NuSTAR's unprecedented hard X-ray focusing capability, we looked for IC emission both globally and locally after properly modeling the purely thermal component with a multi-temperature description.
Results: The thermodynamic maps obtained from the XMM observation suggest the presence of a secondary merging process that could be responsible for the peculiar radio halo morphology. This hypothesis is supported by the comparison between the X-ray and radio surface brightnesses, which shows a broad intrinsic scatter and a series of outliers from the best-fit relation, corresponding to those regions that could be influenced by a secondary merger. The global NuSTAR spectrum can be explained by purely thermal gas emission, and there is no convincing evidence that an IC component is needed. The 3σ upper limit on the IC flux in the 20-80 keV band is in the [2.2-4.0] × 10^-13 erg s^-1 cm^-2 range, implying a lower limit on the magnetic field strength in the B > [0.23 - 0.31] μG range. Locally, we looked for IC emission in the central region of the cluster radio halo finding a 3σ upper limit on the 20-80 keV nonthermal flux of 3.17 × 10^-14 erg s^-1 cm^-2, corresponding to a lower limit on the magnetic field strength of B ≳ 0.81 μG.