Evidence for non-thermal X-ray emission from the double Wolf-Rayet colliding-wind binary Apep
Journal article, 2023

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.

Outflows

X-rays: stars

Radiation mechanisms: non-thermal

Stars: Wolf-Rayet

Acceleration of particles

Stars: winds

Author

Santiago Del Palacio

Chalmers, Space, Earth and Environment, Astronomy and Plasmaphysics

F. García

Instituto Argentino de Radioastronomia

M. De Becker

University of Liège

D. Altamirano

University of Southampton

V. Bosch-Ramon

University of Barcelona

P. Benaglia

Instituto Argentino de Radioastronomia

B. Marcote

Joint Institute for Very Long Baseline Interferometry European Research Infrastructure Consortium (JIVE)

G. E. Romero

Instituto Argentino de Radioastronomia

Astronomy and Astrophysics

0004-6361 (ISSN) 1432-0746 (eISSN)

Vol. 672 A109

Subject Categories

Astronomy, Astrophysics and Cosmology

Fusion, Plasma and Space Physics

DOI

10.1051/0004-6361/202245505

More information

Latest update

5/22/2023