Context :

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 - 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 in detail the physical and dynamical state of the cluster ICM .
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 3 \sigma upper limit on the IC flux in the 20-80 keV band is in the range \left [ 2.2 - 4.0 \right ] \times 10 ^ { -13 } \mathrm { erg } \mathrm { s } ^ { -1 } \mathrm { cm% } ^ { -2 } , implying a lower limit on the magnetic field strength in the range B > [ 0.23 - 0.31 ] \mu G .
Locally , we looked for IC emission in the central region of the cluster radio halo finding a 3 \sigma upper limit on the 20-80 keV non-thermal flux of 3.17 \times 10 ^ { -14 } \mathrm { erg } \mathrm { s } ^ { -1 } \mathrm { cm } ^ { -2 } , corresponding to a lower limit on the magnetic field strength of B \gtrsim 0.81 \mu G .

Conclusions :