Probing the Energy-dependent Temporal Nature of MAXI J1803-298 with AstroSat and NICER

Abstract We perform a spectral and temporal analysis of MAXI J1803-298, using AstroSat/Large Area X-ray Proportional Counters and Neutron Star Interior Composition Explorer observations taken in 2021 May, during the initial phase of the outburst. We find that the source traverses through the hard, intermediate, and soft spectral states during the outburst. The spectrum in all states can be described using soft emissions from the thermal disk and hard emissions from the coronal regions. The variation in the inner-disk temperature and normalization of the disk indicate the motion of the truncated disk across these different spectral states. We confirm the presence of broad features, type C, and type B quasiperiodic oscillations in the power spectra of the different spectral states. We investigate the fractional rms and lags of all the variability features and discover that the lags swing between positive and negative during the outburst evolution. While modeling the features with a simple model that considers variations in accretion parameters, such as the accretion rate, heating rate, and inner-disk radius, along with the delays between them, we find a dynamic reversal in the origin of the variability between the corona and the disk. Furthermore, our results are consistent with previous works and a radio study conducted on this source during its outburst.