NuSTAR and XMM-Newton observations of the binary 4FGL J1405.1–6119: A γ -ray-emitting microquasar?

Context. 4FGL J1405.1−6119 is a high-mass γ -ray-emitting binary that has been studied at several wavelengths. The nature of this type of binary is still under debate, with three possible scenarios usually invoked to explain the origin of the γ -ray emission: collisions between the winds of a rapidly rotating neutron star and its companion, collisions between the winds of two massive stars, and nonthermal emission from the jet of a microquasar. Aims. We analyzed two pairs of simultaneous NuSTAR and XMM-Newton observations to investigate the origin of the radio, X-ray, and γ -ray emissions. Methods. We extracted light curves between 0.5 and 78 keV from two different epochs, which we call Epoch 1 and Epoch 2. We then extracted and analyzed the associated spectra to gain insight into the characteristics of the emission in each epoch. To explain these observations, along with the overall spectral energy distribution, we developed a model of a microquasar jet. This allowed us to make some inferences about the origin of the observed emission and to discuss the nature of the system. Results. A power-law model combined with the inclusion of a blackbody accurately characterizes the X-ray spectrum. The power-law index ( E −Γ ) was found to be ∼1.7 for Epoch 1 and ∼1.4 for Epoch 2. Furthermore, the associated blackbody temperature was ∼1 keV and with a modeled emitting region of size ≲16 km. The scenario we propose to explain the observations involves a parabolic, mildly relativistic, lepto-hadronic jet. This jet has a compact acceleration region that injects a hard spectrum of relativistic particles. The dominant nonthermal emission processes include synchrotron radiation of electrons, inverse Compton scattering of photons from the stellar radiation field, and the decay of neutral pions resulting from inelastic proton-proton collisions within the bulk matter of the jet. These estimates are in accordance with the values of a super-Eddington lepto-hadronic jet scenario. The compact object could be either a black hole or a neutron star with a weak magnetic field. Most of the X-ray emission from the disk could be absorbed by the dense wind that is ejected from the same disk. Conclusions. We conclude that the binary 4FGL J1405.1−6119 could be a supercritical microquasar similar to SS 433.