Studies of Cepheus X-4 during 2018 outburst observed with AstroSat

TL;DR

This study analyzes the 2018 outburst of Cepheus X-4 using AstroSat, revealing detailed timing and spectral properties, including stable cyclotron line energies and pulse-phase dependent spectral variations, enhancing understanding of accretion physics in X-ray pulsars.

Contribution

First detailed AstroSat timing and spectral analysis of Cepheus X-4 during 2018 outburst, including cyclotron line stability and pulse-phase spectral variations.

Findings

Detected stable cyclotron resonance scattering features around 30.2 keV.

Observed energy and intensity dependent pulse-shape variations.

Documented long-term spin-down trend over 30 years.

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 $65.35080\pm0.00014$ s , $(-2.10\pm0.8)\times10^{-12}$ Hz s$^{-1}$ at an epoch MJD 58301.61850 and $65.35290\pm0.00017$ s, $(-1.6\pm0.8)\times10^{-12}$ 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 $(-2.455\pm0.004)\times10^{-14}$ 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 $30.23 \pm 0.22$ 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.