Kavli Affiliate: Gregory Herczeg
| Summary:
Multiwavelength observations of stellar flares trace the activity of different components of the stars’ outer atmosphere, providing insight into their interactions. In the present paper, we report a superflare from BP Tau, simultaneously observed with the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) satellite and TESS. While we attribute the X-ray flux increase to a magnetically powered flare, the optical light curve likely results from the superposition of the flare and an accretion burst. The X-ray flare has a mean flux of $(1.5^+0.3_-0.4)times10^-11$ erg cm$^-2$ s$^-1$ in the WXT energy band (0.5-4.0 keV), with e-folding times of $1.7pm1.0$ ks and $14pm5$ ks for the rise and decay phase, respectively. The corresponding time-integrated flare energy is $(1.0pm 0.2)times 10^36$ erg. The optical flare has an e-folding time of $0.33pm0.04$ ks for the rise phase, but the data do not constrain the decay timescale. Assuming a decay phase equal to the rise phase, the resulting optical flare energy is $(2.8pm0.4)times10^34$ erg in the TESS band ($sim6,000$-$sim10,000$ Å), corresponding to a bolometric energy of $(1.9pm0.3)times10^35$ erg (assuming a blackbody at 11000 K). The Follow-up X-ray Telescope (FXT) on EP triggered an observation $sim1.5$ day after the flare, with a flux of $(4.6^+0.2_-0.5)times10^-13$ erg cm$^-2$ s$^-1$ (0.5-10.0 keV), indicating that BP Tau had returned to quiescence. This work demonstrates the potential of jointly analyzing EP and TESS data for superflares. WXT is expected to detect $sim800$ superflares per year, with FXT capable of slewing to the flaring star within $sim3$-5 minutes. The large field of view of both missions offers us the opportunity to study multiwavelength variability during energetic flares.
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