| Name: Prince Sharma |
| Affiliation: UPES, Dehradun |
| Conference ID: ASI2026_1047 |
| Title: Torque reversal and cyclotron absorption feature in HMXB 4U 1538-522 |
| Abstract Type: Oral |
| Abstract Category: High Energy Phenomena, Fundamental Physics and Astronomy |
| Author(s) and Co-Author(s) with Affiliation: Prince Sharma(UPES, Dehradun - 248007, India), Chetana Jain(University of Delhi, Delhi - 110007), Anjan Dutta(University of Delhi, Delhi - 110007) |
| Abstract: This work present a detailed timing and spectral analysis of the high-mass X-ray binary 4U 1538−522 using observations from the Nuclear Spectroscopic Telescope Array (NuSTAR). The study is based on three observations obtained between 2019 and 2021. Coherent X-ray pulsations with a period of approximately 526 s are detected up to energies of 60 keV. During the first observation, we measure an instantaneous spin-down rate of P_dot = 6.6(+2.4/−6.0) × 10^(−6) s /s. The pulse profiles show a double-peaked structure, consisting of a broad primary peak and a weaker, energy-dependent secondary peak. We further examine the long-term spin period evolution of 4U 1538−522 using data spanning more than four decades, including measurements from Fermi/GBM. From recent spin-period trends, we find that the third torque reversal in the system occurred around MJD 58800. The neutron star was found to be in a spin-up phase with a rate of Pdot = −1.9(1) × 10−9 s s−1. In addition, we detect a periodic modulation in the spin period. The broadband persistent X-ray spectra are well described by a model consisting of a blackbody component combined with either a power-law or a Comptonization component. The spectra also show an Fe Kα emission line at 6.4 keV and a cyclotron absorption feature around 22 keV. In all three observations, we identify an additional weak absorption feature near 27 keV. These cyclotron features correspond to magnetic field strengths of 1.84(+0.04/−0.06)(1 + z) × 10^(12) G and 2.33(+0.15/−0.24)(1 + z) × 10^(12) G, respectively. |
