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 similar to 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 similar to 1.7 and an electron temperature of similar to 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 +/- 0.4 (cos i/ cos 70 degrees)(-1/2) (D/8 kpc) M-circle dot (where i and D are the inclination angle and the distance, respectively) from the inner disk radius obtained in the high/soft state.