Probing Accretion Geometries of X-ray Pulsars using Cyclotron Resonant Scattering Features

Abstract

Cyclotron Resonant Scattering Features (CRSFs) appear as absorption-like features in the hard X-ray spectra of accretion-powered pulsars and provide a direct measure of neutron star magnetic fields. Their correlation or anti-correlation with X-ray luminosity often signals accretion regime transitions, while pulse phase dependence offers insights into emission geometry shaped by complex magnetic fields. I will present cyclotron line studies of three Be/X-ray binaries: GRO J1750-27, XTE J1946+274, and A0535+26, using data from NuSTAR, Insight-HXMT, and AstroSat. We report the discovery of a deep 43 keV cyclotron line feature in the X-ray spectrum of GRO J1750-27 during its 2021 outburst, reaching luminosities near the Eddington limit. XTE J1946+274 displayed strongly energy-dependent pulse profiles, evolving from double- to single-peaked near its ~39 keV resonance. We observed the luminosity dependence of the cyclotron line for the first time. At higher fluxes, the line appeared only in one pulse peak, while at lower fluxes, it was detected through all the rotational phases. We explain this peculiar behavior of the cyclotron line using the phenomena of photon spawning and a non-dipolar magnetic field structure in this neutron star. A0535+26, a well-studied pulsar with a 44 keV cyclotron line, has shown clear luminosity dependence indicative of accretion regime transitions. During its 2020 outburst, it brightened to nearly 12 Crab and was observed multiple times. Our high-resolution phase-resolved spectral analysis revealed a new phase-transient cyclotron line feature, detected in only 16% of the rotational phases, with dramatic variability across both pulse phase and luminosity. This discovery provides new constraints on the system’s accretion geometry, suggesting that the accretion column sweeps across the observer’s line of sight.