The AstroSat observation of accreting millisecond X-ray pulsar SAX J1808.4-3658 during its 2019 outburst

TL;DR

This study analyzes AstroSat data of SAX J1808.4-3658 during its 2019 outburst, revealing coherent pulsations, energy-dependent pulse profiles, and a broadband spectrum consistent with thermal and Comptonization components.

Contribution

First detailed AstroSat analysis of SAX J1808.4-3658 during its 2019 outburst, providing new insights into its pulsation and spectral properties.

Findings

Detected coherent pulsations at ~401 Hz.

Pulse profile shows energy dependence and harmonic structure.

Broadband spectrum fits thermal, Comptonization, and Fe line components.

Abstract

We report on the analysis of the AstroSat dataset of the accreting millisecond X-ray pulsar SAX J1808.4-3658, obtained during its 2019 outburst. We found coherent pulsations at $\sim 401$ Hz and an orbital solution consistent with previous studies. The 3-20 keV pulse profile can be well fitted with three harmonically related sinusoidal components with background-corrected fractional amplitude of $\sim 3.5 \%$, $\sim 1.2 \%$ and $\sim 0.37 \%$ for fundamental, second and third harmonic, respectively. Our energy-resolved pulse profile evolution study indicate a strong energy dependence. We also observed a soft lag in fundamental and hard lag during its harmonic. The broadband spectrum of SAX J1808.4-3658 can be well described with a combination of thermal emission component with $kT \sim 1$ keV, a thermal Comptonization ($\Gamma \sim 1.67$) from the hot corona and broad emission lines due to Fe.