2019MNRAS.482.5553J

We present the detection of an energetic flare on the pre-main-sequence M3 star NGTS J121939.5-355557, which we estimate to be only 2 Myr old. The flare had an energy of 3.2±^0.4_0.3×10³⁶ erg and a fractional amplitude of 7.2 ± 0.8, making it one of the most energetic flares seen on an M star. The star is also X-ray active, in the saturated regime with log L_X/L_Bol = -3.1. In the flare's peak, we have identified multimode quasi-periodic pulsations formed of two statistically significant periods of approximately 320 and 660 s. This flare is one of the largest amplitude events to exhibit such pulsations. The shorter period mode is observed to start after a short-lived spike in flux lasting around 30 s, which would not have been resolved in Kepler or TESS short-cadence modes. Our data show how the high cadence of the Next Generation Transient Survey (NGTS) can be used to apply solar techniques to stellar flares and to identify potential causes of the observed oscillations. We also discuss the implications of this flare for the habitability of planets around M star hosts and how NGTS can help our understanding of this.