Discovery and Long-term Broadband X-ray monitoring of Galactic Black Hole Candidate MAXI J1803-298
Abstract
We report the results from the broad-band X-ray monitoring of the new Galactic black hole candidate MAXI J1803-298 with the MAXI/GSC and Swift/BAT during its outburst. After the discovery on 2021 May 1, the soft X-ray flux below 10 keV rapidly increased for 10 days and then have been gradually decreasing over 5 months. At the brightest phase, the source exhibited the state transition from the low/hard state to the high/soft state via the intermediate state. The broad-band X-ray spectrum during the outburst was well described with a disk blackbody plus its thermal or non-thermal Comptonization. Before the transition the source spectrum was described by a thermal Comptonization component with a photon index of 1.7 and an electron temperature of 30 keV, whereas a strong disk blackbody component was observed after the transition. The spectral properties in these periods are consistent with the low/hard state and the high/soft state, respectively. A sudden flux drop with a few days duration, unassociated with a significant change in the hardness ratio, was found in the intermediate state. A possible cause of this variation is that the mass accretion rate rapidly increased at the disk transition, which induced a strong Compton-thick outflow and scattered out the X-ray flux. Assuming a non-spinning black hole, we estimated a black hole mass of MAXI J1803-298 as 5.8 0.4~( i/ 70)-1/2 (D/8~kpc)~M (where i and D are the inclination angle and the distance) from the inner disk radius obtained in the high/soft state.