A ΔR ∼ 9.5 mag Superflare of an Ultracool Star Detected by the SVOM/GWAC System
Xu, Y.; Huang, L.; Wang, J.; Li, H. L.; Xin, L. P.; Li, G. W.; Deng, J. S.; Han, X. H.; Dai, Z. G.; Liang, E. W.; Wang, X. Y.; Wei, J. Y.; Wang, X. G.; Wu, C.; Meng, X. M.; Cai, H. B.; Lu, X. M.; Qiu, Y. L.; Zhang, J. B.; Turpin, D.; Feng, Q. C.; Sun, S. S.; Zheng, T. C.; Yang, Y. G.
China, France
Abstract
We report the detection and follow-up of a superstellar flare GWAC 181229A with an amplitude of ΔR ∼ 9.5 mag on an M9-type star by SVOM/GWAC and the dedicated follow-up telescopes. The estimated bolometric energy Ebol is (5.56-9.25) × 1034 erg, which makes the event one of the most powerful flares seen on ultracool stars. The magnetic strength is inferred to be 3.6-4.7 kG. Thanks to sampling with a cadence of 15 s, a new component near the peak time with a very steep decay is detected in the R-band light curve, followed by the two-component flare template given by Davenport et al. An effective temperature of 5340 ± 40 K is measured by fitting a blackbody shape to the spectrum in the shallower phase during the flare. The filling factors of the flare are estimated to be ∼30% and 19% at the peak time and at 54 minutes after the first detection. The detection of this particular event with large amplitude, huge emitted energy, and a new component demonstrates that high-cadence sky monitoring cooperation with fast follow-up observations is very important for understanding the violent magnetic activity.