A detailed study of the rise phase of a long duration X-ray flare in the young star TWA 11B

TL;DR

This study analyzes the rise phase of a long-duration X-ray flare in the young star TWA 11B, revealing large magnetic loops and iron fluorescence, providing insights into stellar magnetic activity in slightly evolved young stars.

Contribution

First detailed analysis of a long-duration flare's rise phase in TWA 11B, inferring large magnetic loops and detecting iron fluorescence in a star without a disk.

Findings

Large magnetic loops (~4 R*) inferred from flare analysis.

Detection of Fe 6.4 keV fluorescence line during the flare.

First evidence of such large loops in a star slightly older than very young stellar objects.

Abstract

We analyzed a long duration flare observed in a serendipitous XMM-Newton detection of the M star CD-39 7717B (TWA 11B), member of the young stellar association TW Hya (~ 8 Myr). Only the rise phase (with a duration of ~ 35 ks) and possibly the flare peak were observed. We took advantage of the high count-rate of the X-ray source to carry out a detailed analysis of its spectrum during the whole exposure. After a careful analysis, we interpreted the rise phase as resulting from the ignition of a first group of loops (event A) which triggered a subsequent two-ribbon flare (event B). Event A was analyzed using a single-loop model, while a two-ribbon model was applied for event B. Loop semi-lengths of ~ 4 R* were obtained. Such large structures had been previously observed in very young stellar objects (~ 1 - 4 Myr). This is the first time that they have been inferred in a slightly more evolved star. The fluorescent iron emission line at 6.4 keV was detected during event B. Since TWA 11B seems to have no disk, the most plausible explanation found for its presence in the X-ray spectrum of this star is collisional - or photo- ionization. As far as we are concerned, this is only the third clear detection of Fe photospheric fluorescence in stars other than the Sun.