X-ray spectral scaling of black hole mass in quasi-periodic eruptive source RX J1301.9+2747
We present a spectral and timing analysis of X-ray emission from the actively accreting supermassive black hole (SMBH) in RX~J1301.9+2747 with prominent quasi-periodic eruptions (QPEs) using {\it ASCA, ROSAT, XMM-Newton}, and {\it NICER} observations. {\it NICER} detected renewed QPE activity from this source in 2024. We examined in detail the X-ray spectrum formation in this source during QPE outbursts applying the first-principal model, Bulk Motion Comptonization. We find an increase of the photon index $Γ$ with a mass accretion rate $\dot M$, the index saturation at $Γ_{sat}=2.7\pm 0.1$ at high $\dot M$ value during outburst peaks. Based on the derived $Γ-\dot M$ correlation, we apply the scaling technique for the first time to measure the black hole mass in RX~J1301.9+2747. The SMBH mass in this source was estimated to be between $4\times 10^5$ M$_ {\odot}$ and $7\times 10^6$ M$_ {\odot}$ for the inclination range from 0$^{\circ}$ to 70$^{\circ}$, which is adopted as a plausible range based on the AGN unified model assumption, applying HLX--1~ESO 243--49, OJ~287, and 1H~0707--495 as reference scaling sources. We also estimate a BH mass in RX~J1301.9+2747 applying a timing analysis, in which the Compton cloud size to be $\sim 10^{11}$ cm, estimated from the quasi-periodic oscillation frequency, $ν\sim 10^{-4}$ Hz, indicates SMBH mass in RX~J1301.9+2747 is of order of $10^6$ M$_{\odot}$. Overall, the BH mass obtained by the scaling method in this source is consistent with previous measurements using other methods.