2023A&A...669A..57T

The multi-wavelength outburst activity in the recently discovered X-ray binary transient MAXI J1348-630 has sparked a great deal of controversy about the characteristics of this binary and questions around whether the source contains a black hole (BH). Here, we present the results of our analysis of the outburst of MAXI J1348-630 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, Γ, is correlated with the mass accretion rate, Ṁ. We find that Γ increases monotonically with Ṁ from the low-hard state to the high-soft state, and then becomes saturated at Γ∼ 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 Γ is correlated with the quasi periodic oscillation frequency, ν_L. Based on this correlation, we applied a scaling method to estimate a BH mass of 14.8 ± 0.9 M_⊙, using the well-studied 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)° using the scaling technique for the correlation between Γ and normalization proportional to Ṁ. We detected a specific behavior of the disk seed photon temperature, kT_s, immediately before the outburst: kT_s initially decreases from 0.4 to 0.2 keV and increases only after the source transits to the outburst rise-maximum phase. An initial decrease in kT_s occurred simultaneously with an increase in the illumination fraction, f. We interpreted this effect in terms of the bulk motion Comptonization model. 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 Ṁ increases (or luminosity increases), the corona contracts, thus increasing the seed photon temperature, kT_s. It is possible that such a decrease in kT_s occurring simultaneously with an increase in the illumination fraction, f, can be considered a signature of the readiness of a BH object to go into an outburst phase.