Superflare on a rapidly-rotating solar-type star captured in X-rays

In this work, we studied X-ray source SRGe J021932.4−040154 (SRGe J021932), which we associated with a single X-ray active star of spectral class G2V-G4V and the rotational period $P_{\mathrm{rot}}<9.3$ days. Additional analysis of TESS light-curves allowed for the rotational period estimation of $3.2\pm 0.5$ days. SRGe J021932 was observed with the SRG/eROSITA during eUDS survey in 2019 in a much dimmer state compared to the XMM-Newton catalogue 4XMM-DR12. Detailed analysis revealed that the archival XMM-Newton observations captured the source during a flaring event in 2017. The XMM-Newton light curve demonstrates a strong flare described with the Gaussian rise and exponential decay, typical for stellar flares, characterized by timescales of ∼400 s and ∼1300 s, respectively. The spectral analysis of the quiescent state reveals ∼10 MK plasma at luminosity of $(1.4\pm 0.4)\times {10}^{29}$ $\mathrm{erg}{s}^{-1}$ (0.3−4.5 keV). The spectrum of the flare is characterized by temperature of ∼40 MK and luminosity $(5.5\pm 0.6)\times {10}^{30}$ $\mathrm{erg}{s}^{-1}$. The total energy emitted during the flare $\sim 1.7\times {10}^{34}$ erg exceeds the canonical threshold of 10³³ erg, allowing us to classify the observed event as a superflare on a rapidly-rotating solar-type star. Additionally, we present the upper limit on the surface starspot area based on the brightness variations and consider the hypothesis of the object being a binary system with G-type and M-type stars, suggested by two independent estimations of radial velocity variations from APOGEE-2 and Gaia.