Detection of CRSF in Cep X-4 at a low luminosity level

Abstract

The BeXRP Cep X-4 underwent a type II outburst during July 2023, during which NuSTAR made two observations of the source. In this paper, we present the spectral and timing analysis of Cep X-4 during this outburst. Pulsations were detected from the source at the neutron star spin period of ∼ 66 s from both observations. We detected a CRSF feature at an energy of 27.3+1.5 −1.0 keV in the spectrum of Cep X-4 during the first observation. In the second observation conducted a day later when the source luminosity dropped by a factor of five, no CRSF could be detected significantly. A positive correlation between CRSF energy and luminosity was observed using data from multiple outbursts since 1998. The collisionless shock model provides a good fit to the observed variations of cyclotron line energy with luminosity, giving a surface magnetic field strength of ∼ 2.6× 1012 G.