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

We present the results obtained from broad-band X-ray timing and spectral analysis of black hole (BH) candidate MAXI J1803-298 using an AstroSat observation on 2021 May 11-12. Four periodic absorption dips with a periodicity of 7.02 ± 0.18 h are detected in the light curve. AstroSat observe the source when it was undergoing a transition from hard-intermediate state to soft-intermediate state. Our timing analysis reveals the presence of a sharp Type-C quasi-periodic oscillation (QPO) in the power-density spectra (PDS) with an evolving QPO frequency ranging from 5.31 ± 0.02 to 7.61 ± 0.09 Hz. We investigate the energy dependence of the QPO and do not find this feature in the PDS above 30 keV. The combined 0.7-80 keV SXT and LAXPC spectra are fitted with a model consisting of thermal multi-colour blackbody emission and Comptonized emission components. We perform time-resolved spectroscopy by extracting spectra during the dip and non-dip phases of the observation. A neutral absorber is detected during the dip and non-dip phases though a signature of an ionized absorber is also present in the dip phases. The spectral and temporal parameters are found to evolve during our observation. We estimate the mass function of the system as f(M) = 2.1-7.2 M_⊙ and the mass of the BH candidate in the range of M_BH ~ 3.5-12.5 M_⊙.