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 $\sim5.4$ Hz and $\sim6.3$ Hz along with a sub-harmonic at $\sim2.8$ 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 $\sim3.8$ ms and $\sim6.8$ ms for the respective QPOs at $\sim5.4$ Hz and $\sim6.3$ Hz, whereas soft lag of $\sim4.7$ 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 $\sim0.17-0.19$ keV and a reflection hump above $\sim12$ keV in the energy spectrum. Based on the X-ray spectroscopy and considering the distance to the source as 8 kpc, the estimated mass ($\sim8.5-16$ M$_\odot$) and spin ($a\gtrsim0.7$) of the black hole suggest that the source is likely to be a stellar mass Kerr black hole X-ray binary.