Detection of a giant white-light flare on an L2.5 dwarf with the Next Generation Transit Survey

We present the detection of a V ∼ −10 flare from the ultracool L2.5 dwarf ULAS J224940.13−011236.9 with the Next Generation Transit Survey (NGTS). The flare was detected in a targeted search of late-type stars in NGTS full-frame images and represents one of the largest flares ever observed from an ultracool dwarf. This flare also extends the detection of white-light flares to stars with temperatures below 2000 K. We calculate the energy of the flare to be 3.4+0.9 −0.7 × 1033 erg, making it an order of magnitude more energetic than the Carrington event on the Sun. Our data show how the high-cadence NGTS full-frame images can be used to probe white-light flaring behaviour in the latest spectral types

Statistical Signatures of Nanoflare Activity. III. Evidence of Enhanced Nanoflaring Rates in Fully Convective stars as Observed by the NGTS

Abstract Previous examinations of fully convective M-dwarf stars have highlighted enhanced rates of nanoflare activity on these distant stellar sources. However, the specific role the convective boundary, which is believed to be present for spectral types earlier than M2.5V, plays on the observed nanoflare rates is not yet known. Here, we utilize a combination of statistical and Fourier techniques to examine M-dwarf stellar lightcurves that lie on either side of the convective boundary. We find that fully convective M2.5V (and later subtypes) stars have greatly enhanced nanoflare rates compared with their pre-dynamo mode-transition counterparts. Specifically, we derive a flaring power-law index in the region of 3.00 ± 0.20, alongside a decay timescale of 200 ± 100 s for M2.5V and M3V stars, matching those seen in prior observations of similar stellar subtypes. Interestingly, M4V stars exhibit longer decay timescales of 450 ± 50 s, along with an increased power-law index of 3.10 ± 0.18, suggesting an interplay between the rate of nanoflare occurrence and the intrinsic plasma parameters, e.g., the underlying Lundquist number. In contrast, partially convective (i.e., earlier subtypes from M0V to M2V) M-dwarf stars exhibit very weak nanoflare activity, which is not easily identifiable using statistical or Fourier techniques. This suggests that fully convective stellar atmospheres favor small-scale magnetic reconnection, leading to implications for the flare-energy budgets of these stars. Understanding why small-scale reconnection is enhanced in fully convective atmospheres may help solve questions relating to the dynamo behavior of these stellar sources

Detection of a giant flare displaying quasi-periodic pulsations from a pre-main sequence M star with NGTS

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 LX/LBol = −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