Torque reversal and cyclotron absorption feature in HMXB 4U 1538−522.

We present a comprehensive timing and spectral analysis of the HMXB 4U 1538−522 by using the Nuclear Spectroscopic Telescope Array (NuSTAR) observatory data. Using three archived observations made between 2019 and 2021, we have detected ∼ 526 s coherent pulsations up to 60 keV. We have found an instantaneous spin-down rate of |$\dot{P} = 6.6_{-6.0}^{+2.4} \times 10^{-6}$| s s−1 during the first observation. The pulse profiles had a double peaked structure consisting of a broad primary peak and an energy dependent, weak secondary peak. We have also analysed the long-term spin period evolution of 4U 1538−522 from data spanning more than four decades, including the data from Fermi /GBM. Based on the recent spin trends, we have found that the third torque reversal in 4U 1538−522 happened around MJD 58800. The source is currently spinning up with |$\dot{P} = -1.9(1) \times 10^{-9}$| s s−1. We also report a periodic fluctuation in the spin period of 4U 1538−522. The broad-band persistent spectra can be described with a blackbody component and either power law or Comptonization component along with a Fe Kα line at 6.4 keV and a cyclotron absorption feature around 22 keV. We have also found a relatively weak absorption feature around 27 keV in the persistent spectra of 4U 1538−522 in all three observations. We have estimated a magnetic field strength of |$1.84_{-0.06}^{+0.04} (1+z) \times 10^{12}$| and |$2.33_{-0.24}^{+0.15} (1+z) \times 10^{12}$|  G for the two features, respectively. [ABSTRACT FROM AUTHOR]