# Probing the Cyclotron line characteristics of 4U 1538-522 using AstroSat-LAXPC

## TL;DR

This study uses AstroSat-LAXPC to analyze the cyclotron line features of the high mass X-ray binary pulsar 4U 1538-52, revealing phase-dependent variations and confirming long-term changes in the cyclotron line energy.

## Contribution

First detailed phase-resolved cyclotron line study of 4U 1538-52 using AstroSat-LAXPC, highlighting phase dependence and long-term evolution of the CRSF parameters.

## Key findings

- CRSF detected at ~22 keV with high significance
- CRSF energy varies by ~13% over pulse phase
- Long-term increase in CRSF centroid energy confirmed

## Abstract

We report the first report on cyclotron line studies with the LAXPC instrument onboard AstroSat of the High mass X-ray Binary pulsar 4U 1538-52. During the observation of source which spanned about one day with a net exposure of 50 ks,the source X-ray flux remained constant. Pulse profile is double peaked in low energy range and has a single peak in high energy range, the transition taking place around the cyclotron line energy of the source. Cyclotron Scattering Feature (CRSF) is detected at $\sim$ 22 keV with a very high significance in phase averaged spectrum. It is one of the highest signal to noise ratio detection of CRSF for this source. We performed detailed pulse phase resolved spectral analysis with 10 independent phase bins. We report the results of pulse phase resolved spectroscopy of the continuum and CRSF parameters. The cyclotron line parameters show pulse phase dependence over the entire phase with a CRSF energy variation of $\sim$ 13% which is in agreement with previous studies. We also confirm the increase in the centroid energy of the CRSF observed between the 1996-2004 (RXTE) and the 2012 (Suzaku) observations, reinforcing that the increase was a long-term change.