AstroSat observations of the Be/X-ray binary XTE J1946+274 during 2018 and 2021 outbursts

TL;DR

This study analyzes the timing and spectral properties of the Be/X-ray binary XTE J1946+274 during its 2018 and 2021 outbursts using AstroSat data, revealing pulsar spin evolution, energy-dependent pulse profiles, and cyclotron features.

Contribution

First detailed timing and spectral analysis of XTE J1946+274 during 2018 and 2021 outbursts with AstroSat, including spin evolution and cyclotron line detection.

Findings

Pulsar spins-up during outbursts and spins-down during quiescence.

Detection of cyclotron resonant scattering feature at ~43 keV.

Energy-dependent pulse profile variations.

Abstract

We present the timing and spectral studies of the Be/X-ray binary XTE J1946+274 during its 2018 and 2021 giant outbursts using observations with the SXT and the LAXPC instruments on the AstroSat satellite. Unlike the 1998 and 2010 outbursts, where a giant outburst was followed by several low intensity periodic outbursts, the 2018 and 2021 outbursts were single outbursts. The X-ray pulsations are detected over a broad energy band covering 0.5-80 keV from the compact object. We construct the spin evolution history of the pulsar over two decades and find that the pulsar spins-up during the outbursts but switches to spin-down state in the quiescent periods between the outbursts. Energy resolved pulse profiles generated in several bands in 0.5-80 keV show that the pulse shape varies with the energy. The energy spectrum of the pulsar is determined for the 2018 and 2021 outbursts. The best fit spectral models require presence of Cyclotron Resonant Scattering Feature (CRSF) at about 43 keV in the energy spectra of both the outbursts. We find indication of possible reversal in the correlation between the cyclotron line energy and luminosity which needs to be ascertained from future observations. Using the best fit spectra the X-ray luminosity of XTE J1946+274 is inferred to be $2.7 \times 10^{37}$ erg s$^{-1}$ for the 2018 observations and $2.3 \times 10^{37}$ erg s$^{-1}$ for the 2021 observations. We discuss possible mechanisms which can drive outbursts in this transient Be X-ray binary.