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

Abstract

ABSTRACT We report on the analysis of the AstroSat data set of the accreting millisecond X-ray pulsar SAX J1808.4–3658, obtained during its 2019 outburst. We found coherent pulsations at ∼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 amplitudes of $\sim 3.5 {{\ \rm per\ cent}}$, $\sim 1.2 {{\ \rm per\ cent}}$ and $\sim 0.37 {{\ \rm per\ cent}}$ for the fundamental, second and third harmonics, respectively. Our energy-resolved pulse profile evolution study indicates a strong energy dependence. We also observed a soft lag in the fundamental and hard lags during its harmonic. The broad-band spectrum of SAX J1808.4–3658 can be described well using a combination of the thermal emission component with kT ∼ 1 keV, a thermal Comptonization (Γ ∼ 1.67) from the hot corona and broad emission lines due to Fe.