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

Abstract

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 $\rm P_{rot} < 9.3$ 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 ${\sim}400$~s and ${\sim}1300$~s, respectively. The spectral analysis of the quiescent state reveals ${\sim}10$~MK plasma at luminosity of $(1.4\pm0.4) \times 10^{29}$ erg/s (0.3-4.5 keV). The spectrum of the flare is characterized by temperature of ${\sim}40$ MK and luminosity $(5.5\pm0.6)\times 10^{30}$ erg/s. The total energy emitted during the flare ${\sim}1.7 \times 10^{34}$ erg exceeds the canonical threshold of $10^{33}$ 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.