| Abstract: Cyclotron Resonant Scattering Features (CRSFs) appear as absorption-like features in the hard X-ray spectra of accretion-powered pulsars and provide the most direct measure of neutron-star magnetic fields. Their dependence on X-ray luminosity often signals accretion regime transitions, while their pulse-phase dependence offers insights into emission geometry shaped by complex magnetic fields. I will present our comprehensive observational studies of cyclotron line variability in three Be/X-ray binaries—GRO J1750−27, XTE J1946+274, and A0535+26—using data from NuSTAR, Insight-HXMT, and AstroSat.
In GRO J1750−27, we discovered a deep cyclotron line feature at ~43 keV during its giant 2021 outburst, observed at luminosities approaching the Eddington limit. In XTE J1946+274, we carried out the first detailed investigation of luminosity- and pulse-phase-dependent cyclotron line behavior. The CRSF was detected only over a subset of rotational phases at higher fluxes, while emerging across all pulse phases at lower fluxes. This behavior, coupled with pulse profile evolution from a double-peaked structure to a single peak near the ~39 keV resonance, indicates a non-dipolar magnetic field configuration in this system. A0535+26, a well-studied pulsar with a known 44 keV cyclotron line, showed luminosity-dependent behavior during its 2020 outburst, when it brightened to nearly 12 Crab. High-resolution phase-resolved spectroscopy revealed a new phase-transient cyclotron line in addition to the known feature, detected in only ~16% of the rotational phases and varying strongly with pulse phase and luminosity. This discovery constrains the system’s accretion geometry, suggesting that the accretion column sweeps across the observer’s line of sight. Together, these results constitute a comprehensive and systematic exploration of cyclotron line variability in accretion-powered pulsars. This thesis reports new observational discoveries and reveals previously unexplored phase- and luminosity-dependent behaviors of cyclotron lines, providing evidence for complex magnetic field geometries. In this talk, I will present these results and discuss their broader implications. |
