Statistical Signatures of Nanoflare Activity. II. A Nanoflare Explanation for Periodic Brightenings in Flare Stars observed by NGTS

TL;DR

This study provides statistical evidence that nanoflare activity can explain periodic brightenings in flare stars, challenging the traditional wave-based interpretation and offering a new diagnostic approach for stellar activity analysis.

Contribution

It introduces a novel interpretation that stellar periodic signals can originate from nanoflare activity, supported by statistical analysis of NGTS data, expanding understanding of stellar flare phenomena.

Findings

Evidence for stellar nanoflare signals with a power-law index of 3.25

Decay timescale of nanoflares around 200 seconds

Synthetic nanoflare time series can mimic p-mode oscillations

Abstract

Several studies have documented periodic and quasi-periodic signals from the time series of dMe flare stars and other stellar sources. Such periodic signals, observed within quiescent phases (i.e., devoid of larger-scale microflare or flare activity), range in period from $1-1000$ seconds and hence have been tentatively linked to ubiquitous $p$-mode oscillations generated in the convective layers of the star. As such, most interpretations for the observed periodicities have been framed in terms of magneto-hydrodynamic wave behavior. However, we propose that a series of continuous nanoflares, based upon a power-law distribution, can provide a similar periodic signal in the associated time series. Adapting previous statistical analyses of solar nanoflare signals, we find the first statistical evidence for stellar nanoflare signals embedded within the noise envelope of M-type stellar lightcurves. Employing data collected by the Next Generation Transit Survey (NGTS), we find evidence for stellar nanoflare activity demonstrating a flaring power-law index of $3.25 \pm 0.20 $, alongside a decay timescale of $200 \pm 100$ s. We also find that synthetic time series, consistent with the observations of dMe flare star lightcurves, are capable of producing quasi-periodic signals in the same frequency range as $p$-mode signals, despite being purely comprised of impulsive signatures. Phenomena traditionally considered a consequence of wave behaviour may be described by a number of high frequency but discrete nanoflare energy events. This new physical interpretation presents a novel diagnostic capability, by linking observed periodic signals to given nanoflare model conditions.