A R 9.5 mag Super Flare of An Ultracool Star Detected by SVOM/GWAC System
Abstract
In this paper, we report the detection and follow-ups of a super stellar flare GWAC\,181229A with an amplitude of R9.5 mag on a M9 type star by SVOM/GWAC and the dedicated follow-up telescopes. The estimated bolometric energy Ebol is (5.56-9.25)×1034 ergs, which places the event to be one of the most powerful flares on ultracool stars. The magnetic strength is inferred to be (3.6-4.7) kG. Thanks to the sampling with a cadence of 15 seconds, 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. (2014). An effective temperature of 534040 K is measured by a blackbody shape fitting to the spectrum at 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 min after the first detection. The detection of the particular event with large amplitude, huge-emitted energy and a new component demonstrates that a high cadence sky monitoring cooperating with fast follow-up observations is very essential for understanding the violent magnetic activity.