Ground-based detection of G star superflares with NGTS

James A G Jackman ,David J Armstrong ,Simón R Walker ,Chloe E Pugh ,S Udry ,A Thompson ,Maximilian N Günther ,Andrew Grange ,James S Jenkins ,A Erikson ,Anne-Marie Broomhall ,Alexander Chaushev ,M R Goad ,Edward Gillen ,Liam Raynard ,C A Watson ,B T Gänsicke ,M R Burleigh ,J Mccormac ,Philipp Eigmüller ,P J Wheatley ,R G West

doi:10.1093/mnras/sty897

Abstract

We present high cadence detections of two superflares from a bright G8 star (V = 11.56) with the Next Generation Transit Survey (NGTS). We improve upon previous superflare detections by resolving the flare rise and peak, allowing us to fit a solar flare inspired model without the need for arbitrary break points between rise and decay. Our data also enables us to identify substructure in the flares. From changing starspot modulation in the NGTS data we detect a stellar rotation period of 59 hours, along with evidence for differential rotation. We combine this rotation period with the observed \textit{ROSAT} X-ray flux to determine that the star's X-ray activity is saturated. We calculate the flare bolometric energies as $5.4^{+0.8}_{-0.7}\times10^{34}$ and $2.6^{+0.4}_{-0.3}\times10^{34}$ erg and compare our detections with G star superflares detected in the \textit{Kepler} survey. We find our main flare to be one of the largest amplitude superflares detected from a bright G star. With energies more than 100 times greater than the Carrington event, our flare detections demonstrate the role that ground-based instruments such as NGTS can have in assessing the habitability of Earth-like exoplanets, particularly in the era of \textit{PLATO}.

Highlights

Stellar flares are explosive phenomena caused by reconnection events in a star’s magnetic field (e.g. Benz & Gudel 2010)
We present high cadence detections of two superflares from a bright G8 star (V = 11.56) with the Generation Transit Survey (NGTS)
When searching for flares, we started from the raw Next Generation Transit Survey (NGTS) lightcurves and detrended them using a custom version of the sysrem algorithm (Mazeh et al 2007)

Summary

Introduction

Stellar flares are explosive phenomena caused by reconnection events in a star’s magnetic field (e.g. Benz & Gudel 2010). When previously observed from the ground, they have been synonymous with active M stars, which flare regularly and brightly compared to their quiescent flux Estrela & Valio 2017) These effects are relatively well studied for M dwarf hosts, it is expected that in future PLATO (Rauer et al 2014) will reveal habitable zone planets around K and G stars. Compared to flares from later-type counterparts, detections of superflares from G stars are relatively rare. No G star superflares have been detected with a CCD detector from the ground, several have been seen either visually, in photography or with vidicon detectors (Schaefer 1989; Schaefer et al 2000)

Results

Discussion

Conclusion