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tokens: et al. 1993 ; Steiner et al. 2010 ), which is determined by black hole mass and spin. In the case of non-rotating black holes, ISCO is equal to three times the Schwarzschild radius. Thus, from the average innermost radius of the disk, km, the black hole mass M BH is estimated to beThe expected distance–mass relation in i = 0° and 60° are shown in Figure 5 . Figure 5. Observational constrains on the distance–mass diagram. The two solid colored lines represent Equation ( 3 ) when i = 0° (blue) and 60° (cyan). The dashed lines with the same colors denote the error region due to systematic error. The gray curves give the empirical relation that the luminosity after the soft-to-hard transition ( T = 96) is 0.01–0.04 L Edd . The shaded region represents the plausible mass and distance range. In addition, BHBs are known to show similar luminosity dependence of the spectral states. For instance, in the current case of MAXI J1348–630, the flux at the soft-to-hard transition ( T = 91) is ∼10% of the peak flux in the high/soft state ( T = 14); this value is consistent with those of other BHBs discovered by MAXI, such as MAXI J1820+070 (∼12%; Shidatsu et al. 2019 ) and MAXI J1910−057 (∼10%–15%; Nakahira et al. 2014 ). Furthermore, it was pointed out that the soft-to-hard transition typically occurs at 1%–4% of the Eddington luminosity ( L Edd ; Maccarone 2003 ). Namely, the bolometric luminosity soon after the soft-to-hard transition is expected to be 0.01–0.04 L Edd . The gray curves in Figure 5 give the distance–mass relation of L trans / L Edd = 0.01 and 0.04, where we employed the bolometric flux at the soft-to-hard transition ∼1.7 × 10 −8 erg s −1 cm −2 on T = 96 and L Edd = 1.3 × 10 38 ( M/M ⊙ ) erg s −1 . Next, we try to constrain the source distance. The galactic coordinate of the source is ( l, b ) = (309.3, −1.1), which is tangential to the Galactic Scutum–Centaurus arm. If MAXI J1348−630 locates in the Scutum–Centaurus arm, the distance is estimated to be ∼4–8 kpc (e.g., Xu et al. 2018 ). However, the value of N H = 8.6 × 10 21 cm −2 implies that the source is located in front of the arm at ∼3–4 kpc. If we adopt the most likely index L trans / L Edd ≈ 0.02 (Vahdat Motlagh et al. 2019 ), the distance becomes ∼3.8 kpc for the face-on disk. It is consistent with the estimates from optical observations (Charles et al. 2019 ; Russell et al. 2019b ). In order to estimate the black hole mass more precisely, taking account of the relativistic effects, we tried the kerrbb model instead of the diskbb model (Li et al. 2005 ). The kerrbb model gives relations among the spinning parameter a , inclination angle i , and black hole mass M under a given source distance, mass accretion rate, and spectral hardening factor ( κ in Equation ( 2 )). We carried out a joint fit to the MAXI/GSC and Swift/XRT spectra at T = 85 fixing D = 4 kpc and κ = 1.7 for several discrete a and i values, only allowing the black hole mass and mass accretion rate to be free.