Mr. Ashwin Devaraj

Detection of Cyclotron Resonant Scattering Features (CRSF) in the hard X-ray spectrum of HMXBs is one of the most direct ways of estimating the magnetic field strengths of Neutron Stars (NS) in these systems. The variation of the cyclotron line parameters showing a correlation or an anti-correlation with the X-ray luminosity has been attributed to the transitions in accretion regimes in several of these sources. Studying the pulse phase dependence of these parameters also helps constrain the accretion and emission geometry since these systems' complex magnetic field structures largely dictate the flow of matter onto the NS and the outflow of radiation. In this context, I will present the results of cyclotron line studies involving three HMXBs: GRO J1750-27, XTE J1946+274, and A0535+26. The Be/X-ray binary GRO J1750-27, during an outburst in 2021, exhibited the presence of a rather deep cyclotron line feature at 43 keV. We found the source to be in a supercritical accretion regime while almost reaching the Eddington limit at the peak of its outburst. XTE J1946+274 is another Be/X-ray binary that shows strongly energy-dependent pulse profiles with a double-peaked structure evolving to a single-peaked structure near the cyclotron resonance energy of ~39 keV. Using data from NuSTAR, Insight HXMT, and Astrosat, we found cyclotron line energy to vary with luminosity for the first time. Another interesting feature observed was that the cyclotron line was significantly detected in only one of the two peaks when observed at higher fluxes but in all phases when observed at lower fluxes. The cyclotron line at the first peak showed luminosity dependence, while the line energy was nearly constant, with luminosity at the second peak. We explain this peculiar behavior of the cyclotron line using the phenomena of photon spawning and a non-dipolar magnetic field structure in this NS. The Be/X-ray binary, A0535+26, is a well-studied X-ray pulsar exhibiting a cyclotron line at 44 keV and has shown clear luminosity dependence, indicating accretion regime transitions. In 2020, it underwent a major outburst, reaching up to a flux of around 12 Crab. I will present the results of the spectral and timing analysis of this source, offering new and interesting insights into the accretion geometry of this system.