The soft X-ray light curves of partially eclipsed stellar flares

Abstract

Most stellar flares' soft X-ray light curves 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 light curves 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 not only 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 data base. 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.

Detection of a giant flare displaying quasi-periodic pulsations from a pre-main-sequence M star by the Next Generation Transit Survey

“…This ratio of loop length to stellar radius, while too large for the Sun and similar spectral types, is consistent with previous inferred measurements of loop lengths on mainsequence M dwarfs (which can go up to 2R * in length, Mullan et al 2006) and is well within loop lengths for pre-main sequence stars (e.g. Favata et al 2005;Johnstone et al 2012). Indeed, for stars with a measured NIR excess Favata et al (2005) measured loop lengths up to tens of times the stellar radii, with the flaring loops connecting the star and disc.…”

HD 314884: a slowly pulsating B star in a close binary

“…We know from Solar observations that magnetic phenomena are solely responsible for coronal activity and, in turn, the production and detection of X-rays. The same holds true for Solar-like G stars except that stellar flare peak temperatures and emission measures can be orders of magnitude greater than that of the Sun (Johnstone et al 2012). The range of Lx for G stars spans almost 3 orders of magnitude from a few times 10 26 erg s −1 to (2-4)×10 30 erg s −1 (Maggio et al 1987).…”

Giant white-light flares on fully convective stars occur at high latitudes

“…The convex shape of a flare light curve in X-rays on V773 Tau (Skinner et al 1997) could be well-described by rotational modulation. Later, effects of rotation and eclipses have been put forward as an explanation for unusual flare light curves multiple times (Stelzer et al 1999;Montmerle et al 2000;Johnstone et al 2012). However, only few examples of unambiguously located stellar flares exist to date (Schmitt & Favata 1999;Wolter et al 2008;Peterson et al 2010).…”