2021ApJ...920..139M

We present timing and spectral results for the 2018 outburst of Cepheus X-4, observed twice by AstroSat at luminosities of 2.04 x 10³⁷ erg s^–1 and 1.02 x 10³⁷ erg s^–1. The light curves showed strong pulsation and co-related X-ray intensity variation in the SXT (0.5-8.0 keV) and LAXPC (3-60 keV) energy bands. The spin period and spin-down rate of the pulsar were determined from two observations to be 65.35080 ± 0.00014 s and (-2.10 ± 0.8) x 10^–12 Hz s^–1 at epoch MJD 58301.61850, and 65.35290 ± 0.00017 s and (-1.6 ± 0.8) x 10^–12 Hz s^–1 at epoch MJD 58307.40211. Pulse shape studies with AstroSat showed energy- and intensity-dependent variations. The pulsar showed an overall continuous spin-down over 30 yr at an average rate of (-2.455 ± 0.004) x 10^–14 Hz s^–1, attributed to the propeller effect in the subsonic regime of the pulsar, in addition to variations during its outburst activities. Spectra between the 0.7 keV and 55 keV energy bands were well fitted by two continuum models, an absorbed compTT model and an absorbed power law with a Fermi-Dirac cutoff (FD-cutoff) model with a blackbody. These were combined with an iron emission line and a cyclotron absorption line. The prominent cyclotron resonance scattering features with a peak absorption energy of 30.48_–0.34^+0.33 keV and 30.68_–0.44^+0.45 keV for the FD-cutoff model and 30.46_–0.28^+0.32 keV and 30.30_–0.34^+0.36 keV for the compTT model were detected during two AstroSat observations. When compared with earlier results, these showed long-term stability of an average value of 30.23 ± 0.22 keV with wide variation in source luminosity. The pulsar showed pulse phase as well as luminosity dependent variations in the cyclotron line energy and width and in the plasma optical depth of its spectral continuum.