Quiescent and flaring X-ray emission from the nearby M/T dwarf binary SCR 1845-6357

TL;DR

This study reports on X-ray observations of the nearby ultracool dwarf binary SCR 1845-6357, revealing persistent low-level emission and a significant flare, indicating an active corona despite the star's advanced age.

Contribution

First detection of quasi-quiescent and flaring X-ray emission from SCR 1845-6357, demonstrating coronal activity in an ultracool M/T dwarf binary.

Findings

Quasi-quiescent X-ray emission detected at log L_x = 26.2 erg/s

A strong flare increased count rate by a factor of 30 in 10 minutes

Coronal temperatures reached up to 30 MK during the flare

Abstract

We investigate an XMM-Newton observation of SCR 1845-6357, a nearby, ultracool M8.5/T5.5 dwarf binary. The binary is unresolved in the XMM detectors, however the X-ray emission is very likely from the M8.5 dwarf. We compare its flaring emission to those of similar very low mass stars and additionally present an XMM observation of the M8 dwarf VB 10. We detect quasi-quiescent X-ray emission from SCR 1845-6357 at soft X-ray energies in the 0.2-2.0 keV band, as well as a strong flare with a count rate increase of a factor of 30 and a duration of only 10 minutes. The quasi-quiescent X-ray luminosity of log L_x = 26.2 erg/s and the corresponding activity level of log L_x/L_bol = -3.8 point to a fairly active star. Coronal temperatures of up to 5 MK and frequent minor variability support this picture. During the flare, that is accompanied by a significant brightening in the near-UV, plasma temperatures of 25-30 MK are observed and an X-ray luminosity of L_x= 8 x 10^27 erg/s is reached. SCR 1845-6357 is a nearby, very low mass star that emits X-rays at detectable levels in quasi-quiescence, implying the existence of a corona. The high activity level, coronal temperatures and the observed large flare point to a rather active star, despite its estimated age of a few Gyr.