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 Prot<9.3 days. Additional analysis of TESS light-curves allowed for the rotational period estimation of 3.2±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±0.4)×1029ergs‑1 (0.3‑4.5 keV). The spectrum of the flare is characterized by temperature of ∼40 MK and luminosity (5.5±0.6)×1030ergs‑1. The total energy emitted during the flare ∼1.7×1034 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.