The Soft X-ray Lightcurves of Partially Eclipsed Stellar Flares

TL;DR

This study investigates how stellar eclipses by stars, disks, or prominences affect the observed X-ray lightcurves of stellar flares, suggesting eclipses can explain atypical flare shapes but are not the sole cause.

Contribution

It introduces a simple model considering eclipses to explain non-typical flare morphologies, highlighting eclipses' modest impact on flare measurements.

Findings

Eclipses can reduce flare emission measures and decay times.

Many atypical flare shapes can be reproduced by eclipse models.

Eclipses are often undetected but have limited influence on derived loop lengths.

Abstract

Most stellar flares' soft X-ray lightcurves possess a `typical' morphology, which consists of a rapid rise followed by a slow exponential decay. However, a study of 216 of the brightest flares on 161 pre-main sequence stars, observed during the Chandra Orion-Ultradeep Project (COUP), showed that many flare lightcurves depart from this typical morphology. While this can be attributed to the superposition of multiple typical flares, we explore the possibility that the time-variable eclipsing of flares by their host stars may also be an important factor. We assume each flare is contained within a single, uniform plasma density magnetic loop and specify the intrinsic variation of the flare's emission measure with time. We consider rotational eclipse by the star itself, but also by circumstellar discs and flare-associated prominences. Based on this simple model, we generate a set of flares similar to those observed in the COUP database. Many eclipses simply reduce the flare's maximum emission measure or decay time. We conclude therefore that eclipses often pass undetected, but usually have only a modest influence on the flare emission measure profile and hence the derived loop lengths. We show that eclipsing can easily reproduce the observed atypical flare morphologies. The number of atypical modelled flare morphologies is however much less than that found in the COUP sample. The large number of observed atypical flare morphologies, therefore, must be attributed to other processes such as multiple flaring loops.