Evidence for non-thermal X-ray emission from the double Wolf-Rayet colliding-wind binary Apep

[Context] Massive colliding-wind binaries (CWBs) can be non-thermal sources. The emission produced in their wind-collision region (WCR) encodes information of both the shock properties and the relativistic electrons accelerated in them. The recently discovered system Apep, a unique massive system hosting two Wolf-Rayet stars, is the most powerful synchrotron radio emitter among the known CWBs. It is an exciting candidate in which to investigate the non-thermal processes associated with stellar wind shocks.[Aims] We intend to break the degeneracy between the relativistic particle population and the magnetic field strength in the WCR of Apep by probing its hard X-ray spectrum, where inverse-Compton (IC) emission is expected to dominate.[Methods] We observed Apep with NuSTAR for 60 ks and combined this with a reanalysis of a deep archival XMM-Newton observation to better constrain the X-ray spectrum. We used a non-thermal emission model to derive physical parameters from the results.[Results] We detect hard X-ray emission consistent with a power-law component from Apep. This is compatible with IC emission produced in the WCR for a magnetic field of ≈105–190 mG, corresponding to a magnetic-to-thermal pressure ratio in the shocks of ≈0.007–0.021, and a fraction of ∼1.5 × 10−4 of the total wind kinetic power being transferred to relativistic electrons.[Conclusions] The non-thermal emission from a CWB is detected for the first time in radio and at high energies. This allows us to derive the most robust constraints so far for the particle acceleration efficiency and magnetic field intensity in a CWB, reducing the typical uncertainty of a few orders of magnitude to just within a factor of a few. This constitutes an important step forward in our characterisation of the physical properties of CWBs.This work was carried out in the framework of the PANTERA-Stars (https://www.astro.uliege.be/~debecker/pantera/) initiative. FG is CONICET Researcher and was supported by PIP 0113 (CONICET), PICT-2017-2865 (ANPCyT), and PIBAA 1275 (CONICET). FG was also supported by grant PID2019-105510GB-C32/AEI/10.13039/501100011033 from the Agencia Estatal de Investigación of the Spanish Ministerio de Ciencia, Innovación y Universidades, and by Consejería de Economía, Innovación, Ciencia y Empleo of Junta de Andalucía as research group FQM-322, as well as FEDER funds. D.A. acknowledges support from the Royal Society. V.B.-R. is Correspondent Researcher of CONICET, Argentina, at the IAR. G.E.R. was supported by grant PIP 11220200100554CO (CONICET). This work received financial support from the State Agency for Research of the Spanish Ministry of Science and Innovation under grant PID2019-105510GB-C31/AEI/10.13039/501100011033 and through the “Unit of Excellence María de Maeztu 2020-2023” award to the Institute of Cosmos Sciences (CEX2019-000918-M).Peer reviewe