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

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.

Accepted for publication in MNRAS

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