AstroSat view of the newly discovered X-ray transient MAXI~J1803--298 in the Hard-intermediate state

Abstract

We perform comprehensive temporal and spectral analysis of the newly discovered X-ray transient MAXI J1803-298 using an AstroSat target of opportunity observation on May 11, 2021 during its outburst. The source was found to be in the hard intermediate state. We detect type C quasi-periodic oscillations (QPOs) at the frequencies of ∼ 5.4 Hz and ∼ 6.3 Hz along with a sub-harmonic at ∼ 2.8 Hz in the 3 − 15 keV band. The frequency and fractional rms amplitude of the QPO in the 15 − 30 keV band are found to be higher than those in the 3 − 15 keV band. We find soft lags of ∼ 3.8 ms and ∼ 6.8 ms for the respective QPOs at ∼ 5.4 Hz and ∼ 6.3 Hz, whereas soft lag of ∼ 4.7 ms is found at the sub-harmonic frequency. The increase in the soft lags at the QPO frequencies with energy is also observed in other black hole transients and is attributed to the inclination dependence of the lags. The rms-energy spectra indicate the power-law component to be more variable than the disk and the reflection components. We find a broad iron line with an equivalent width of ∼ 174 − 193 eV and a reflection hump above ∼ 12 keV in the energy spectrum. The estimated mass of the black hole (∼ 11 − 14 M ⊙ ) and the spin parameter suggest the source likely to be a stellar mass Kerr black hole X-ray binary.

AstroSat observation of X-ray dips and state transition in the black hole candidate MAXI J1803–298

“…NICER, NuSTAR and AstroSat observations revealed the presence of periodic absorption dips with a periodicity of ∼ 7 hrs in the light curves of MAXI J1803-298 (Homan et al 2021;Xu & Harrison 2021;Jana et al 2021b). QPOs were also detected in the PDS of MAXI J1803-298 (Chand et al 2021;Ubach et al 2021;Wang et al 2021). During MAXI/GSC observations, state transition of the source from the LHS to HSS was observed (Shidatsu et al 2021).…”