Ground-based detection of G star superflares with NGTS

TL;DR

This paper reports high-cadence ground-based detections of two superflares from a bright G8 star using NGTS, revealing detailed flare structures, stellar rotation, and implications for exoplanet habitability.

Contribution

It presents the first detailed flare modeling from NGTS, including substructure detection and rotation analysis, enhancing understanding of G star superflares.

Findings

Detected two superflares with energies >10^34 erg

Resolved flare rise and peak, enabling detailed modeling

Identified stellar rotation period of 59 hours and differential rotation

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}.