MAXI~J1348--630: Estimating the black hole mass and binary inclination using a scaling technique

Abstract

The outburst activity in the MAXI~J1348--630 has sparked a great deal of controversy, whether the source contains a black hole (BH). Here, we present the results of our analysis of the outburst of MAXI J348--630 using Swift/XRT data. We find that energy spectra in all spectral states can be modeled using a combination of Comptonization and Gaussian iron-line components. In addition, we show that the X-ray photon index, Gamma, is correlated with the mass accretion rate, Mdot. We find that Gamma increases monotonically with Mdot from the LHS to the HSS, and then saturated at Gamma~3. This index behavior is similar to that exhibited by a number of other BH candidates. This result represents observational evidence of the presence of a BH in MAXI~J1348--630. We also show that the value of Gamma is correlated with the QPO frequency, \nu_{L}. Based on this correlation, we applied a scaling method to estimate a BH mass of 14.8 +/- 0.9 M_{\odot}, using the BH binary XTE~J1550--564 as a reference source. The recent discovery of a giant dust scattering ring around MAXI~J1348--630 by SRG/eROSITA has refined distance estimates to this X-ray source. With this distance, we were able to estimate the disk inclination i = (65+/- 7)^0 using the scaling technique for the correlation between Gamma and normalization proportional to Mdot. An initial decrease in kT_s occurred simultaneously with an increase in the illumination fraction, f. At the start of the outburst, the Compton cloud (or "corona") is very extended and, thus, the seed photons injected to the corona from the relatively far-away disk region, where kT_s is about 0.2--0.4 keV. While Mdot increases (or luminosity increases), the corona contracts, thus increasing the seed photon temperature, kT_s.