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Description
MAXI J1631-479 was discovered on 2018 December 21 by MAXI, and exhibited a fast rise and exponential decay type of the outburst. After the low/hard state at the beginning of the outburst, the source underwent a variety of state transitions. The source, however, had been mostly in the high/soft state after about 100 days from the discovery until the X-ray flux decreased by about four orders of magnitude from a peak flux of about 2.5 Crab in the 2-10 keV band. A hardness-intensity diagram of count ratios at the energy bands, (4-10 keV)/(2-4 keV), and 2-10 keV flux does not show a typical 'q'-shape but an inverted 'Y' just like observed in XTE J1550-564 and some other black hole X-ray nova. It is naturally explained by increase and decrease in mass accretion rates and corresponding various solutions in accretion disk theories. A dust-scattered X-ray halo was observed with the X-ray telescope of the Neil Gehrels Swift Observatory. Our dust-echo analysis to the direction of MAXI J1631-479 shows that two or three dust layers are present, and the source distance is estimated at about 8 kpc and 12 kpc, respectively. If the source distance is 12 kpc, the mass is the largest among X-ray binary discovered in the Milky Way.
