Constraining the magnetic field structure of the Be/X-ray binary XTE J1946+274

Ashwin Devaraj

Neutron stars (NS) are known to host some of the highest magnetic fields in the universe. They are the ideal astrophysical laboratories for studying the behaviour of matter under the influence of extreme gravity and electromagnetism. Detection of cyclotron line features in the hard X-ray spectrum of the NS is one of the most direct ways of studying magnetic fields of NS systems. XTE J1946+274 houses one such neutron star locked in a binary orbit with a Be-type star. Being a transient, it was observed only a handful of times over the last three decades. It exhibits a cyclotron line at 39 keV, indicating a magnetic field strength of around 3.4 × 1012 G. In this work, we analyzed the hard X-ray spectrum of this source using data from various observatories and studied the cyclotron line’s luminosity and pulse phase dependence. We found that the cyclotron line feature is present in only half the phases at higher luminosities, and the cyclotron line’s luminosity dependence is different for different poles. We explain the interesting behavior of the cyclotron line feature using a multipolar magnetic field configuration of the NS.