Long-term evolution of Cyclotron Line energy in an eclipsing pulsar 4U 1538-522

Abstract

We present the timing and spectral analysis of the HMXB source 4U 1538-522 using NuSTAR observations. One of the observations partially covers the X-ray eclipse of the source along with eclipse ingress. The source is found to spin down at the rate of 0.163 $\pm$ 0.002 s $\text{yr}^{-1}$ between $\sim$ (54973-58603) MJD. It is evident that at time $\sim$ 58620 MJD, a torque reversal occurred, thereafter the source exhibited a spin-up trend at the rate - (0.305 $\pm$ 0.018) s $\text{yr}^{-1}$ until 59275 MJD. A recent NuSTAR observation finds the pulse period of the source: (526.2341 $\pm$ 0.0041) s. The pulse profile exhibits a transition from double-peaked to single-peaked nature above $\sim$ 30 keV. We analyzed the overall trend of the temporal evolution of fundamental Cyclotron Resonance Scattering Feature (CRSF), $\text{E}_\text{cyc}$, incorporating recent NuSTAR measurements. Initially, during the time span $\sim$ (50452.16-55270.8) MJD, the cyclotron line energy is found to increase at a rate of 0.11 $\pm$ 0.03 keV $\text{yr}^{-1}$ which is further followed by a decrease at a rate - 0.14 $\pm$ 0.01 keV $\text{yr}^{-1}$ between (55270.8-59267) MJD. The combined measurements in the time span (50452.16-59267) MJD reveal that the cyclotron line energy is increasing linearly at a rate of 0.08 $\pm$ 0.02 keV $\text{yr}^{-1}$.