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

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 similar to 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 similar to 3 . 5 per cent , similar to 1 . 2 per cent and similar to 0 . 37 per cent for the fundamental, second and third harmonics, respectiv ely. Our energy-resolv ed pulse profile evolution study indicates a strong energy dependence. We also observ ed 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 similar to 1 keV, a thermal Comptonization ( F similar to 1.67) from the hot corona and broad emission lines due to Fe.

Automated summary

We analyzed the data set of the accreting millisecond X-ray pulsar SAX J1808.4-3658 obtained during its 2019 outburst using AstroSat. The pulsar showed coherent pulsations at around 401 Hz and had a consistent orbital solution with previous studies. The pulse profile in the 3-20 keV energy range could be well fitted with three harmonically related sinusoidal components, and the energy-resolved pulse profile evolution study indicated a strong energy dependence. The broad-band spectrum of SAX J1808.4-3658 could be described by a combination of thermal emission, thermal Comptonization from the hot corona, and broad emission lines due to Fe.