Studies of Cepheus X-4 during 2018 outburst observed with AstroSat
Abstract
We present timing and spectral results of 2018 outburst of Cepheus X-4, observed twice by AstroSat at luminosity of 2.04 × 1037 erg s-1 and 1.02 × 1037 erg s-1 respectively. The light curves showed strong pulsation and co-related X-ray intensity variation in SXT (0.5--8.0 keV) and LAXPC (3--60 keV) energy bands. Spin-period and spin-down rate of the pulsar were determined from two observations as 65.350800.00014 s , (-2.100.8)×10-12 Hz s-1 at an epoch MJD 58301.61850 and 65.352900.00017 s, (-1.60.8)×10-12 Hz s-1 for an 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 years at an average-rate of (-2.4550.004)×10-14 Hz s-1, attributed to propeller-effect in the subsonic-regime of the pulsar, in addition to variations during its outburst activities. Spectra between 0.7--55 keV energy band 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 black-body. 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+0.33-0.34 keV and 30.68+0.45-0.44 keV for FD-cutoff-model and 30.46+0.32-0.28 keV and 30.30+0.36-0.34 keV for compTT-model were detected during two AstroSat observations. These when compared with earlier results, showed long term stability of its average value of 30.23 0.22 keV. The pulsar showed pulse-phase as well as luminosity dependent variations in cyclotron-line energy and width and in plasma optical-depth of its spectral continuum.