X Persei: A study on the origin of its high-energy emission

Abstract

Aims. The origin of the hard X-ray emission in the Be/X-ray binary system X Persei has long been debated as its atypical ‘two-hump’ spectrum can be modelled in multiple ways. The main debate focuses on the the high-energy hump, which is fit as either a cyclotron resonance scatter frequency (CRSF) or inverse Comptonization due to bulk Comptonization. Methods. Using INTEGRAL/JEM-X and ISGRI data, we studied the temporal and spectral variability in the 3 − 250 keV energy range during observations over ∼15 years. A NuSTAR observation was also included in a joint spectral fit with the INTEGRAL spectrum. Results. We find that the joint spectrum can be described well by a low-energy component due to thermal Comptonization and a high-energy component due to bulk Comptonization, a CRSF, or a cyclotron emission line. The three models begin to diverge above ∼120 keV, where statistics are low. Conclusions. We compare our results with observations of other Be/X-ray binaries that show similar ‘two-hump’ spectra while in a low-luminosity state. As the sources are in a low accretion state, the bulk Comptonization process is likely inefficient, and thus not an explanation for the high-energy component. The broad CRSF (27 ± 2 keV) in X Persei suggests that the high-energy emission is not due to a CRSF. Thus, the high-energy component is potentially due to cyclotron emission, though other scenarios are not definitively excluded.