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 $2.04\times 10^{37}$ erg s${}^{-1}$ and $1.02\times 10^{37}$ 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 $65.35080\pm 0.00014$ s , $(-2.10\pm 0.8)\times 10^{-12}$ Hz s${}^{-1}$ at an epoch MJD 58301.61850 and $65.35290\pm 0.00017$ s, $(-1.6\pm 0.8)\times 10^{-12}$ 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 $(-2.455\pm 0.004)\times 10^{-14}$ 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 $30.48_{-0.34}^{+0.33}$ keV and $30.68_{-0.44}^{+0.45}$ keV for FD-cutoff-model and $30.46_{-0.28}^{+0.32}$ keV and $30.30_{-0.34}^{+0.36}$ 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 $30.23\pm 0.22$ 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.

25 pages, 18 figures, 3 tables, Accepted for Publication in The Astrophysical Journal

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