Insights into the phase-dependent cyclotron line feature in XTE J1946+274: an AstroSat and Insight-HXMT view

Abstract

ABSTRACT XTE J1946+274 is a Be/X-ray binary with a 15.8 s spin period and 172 d orbital period. Using RXTE/PCA data of the 1998 outburst, a cyclotron line around 37 keV was reported. The presence of this line, its dependence on the pulse phase, and its variation with luminosity have been of some debate since. In this work, we present the re-analysis of two AstroSat observations: one made during the rising phase of the 2018 outburst and the other during the declining phase of the 2021 outburst. We also present a new analysis of the Insight-HXMT observations of the source at the peak of the 2018 outburst. We find the source to be spinning up over the course of the outburst and spinning down between the two outbursts. We report the presence of a higher cyclotron line energy using the 2018 AstroSat observation (∼45 keV) and 2018 Insight-HXMT observation (∼50 keV) and a line at ∼40 keV during the declining phase of the 2021 outburst using data from AstroSat. We also investigate the pulse phase dependence of the cyclotron line parameters and find that the line is significantly detected in all the phases of both AstroSat observations, along with showing variation with the pulse phase. This differs from the previous results reported using BeppoSAX and NuSTAR. We explain this behaviour of the cyclotron line to be due to photon spawning and different accretion column radii at the two poles of this neutron star.