Discovery and Long-term Broadband X-Ray Monitoring of Galactic Black Hole Candidate MAXI J1803-298

We report the results from the broadband X-ray monitoring of the new Galactic black hole candidate MAXI J1803-298 with 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, then gradually decreased over five months. In the brightest phase, the source exhibited the state transition from the low/hard state to the high/soft state via the intermediate state. The broadband X-ray spectrum during the outburst is well described with a disk blackbody plus its thermal or nonthermal Comptonization. Before the transition, the source spectrum is described by a thermal Comptonization component with a photon index of ~1.7 and an electron temperature of ~30 keV, while a strong disk blackbody component is 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 duration of a few days, 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 nonspinning black hole, we estimate the black hole mass of MAXI J1803-298 to be $5.8\pm 0.4\,{(\cos i/\cos 70^\circ )}^{-1/2}(D/8\,\mathrm{kpc})\,{M}_{\odot }$ (where i and D are the inclination angle and the distance, respectively) from the inner disk radius obtained in the high/soft state.