Large X-ray flares on stars detected with MAXI/GSC: A universal correlation between the duration of a flare and its X-ray luminosity

Abstract

23 giant flares from 13 active stars (eight RS CVn systems, one Algol system, three dMe stars and one YSO) were detected during the first two years of our all-sky X-ray monitoring with the gas propotional counters (GSC) of the Monitor of All-sky X-ray Image (MAXI). The observed parameters of all of these MAXI/GSC flares are found to be at the upper ends for stellar flares with the luminosity of 10 31−34 ergs s −1 in the 2-20 keV band, the emission measure of 10 54−57 cm −3 , the e-folding time of 1 hour to 1.5 days, and the total radiative energy released during the flare of 10 34−39 ergs. Notably, the peak X-ray luminosity of 5 +4 −2 × 10 33 ergs s −1 in the 2-20 keV band was detected in one of the flares on II Peg, which is one of the, or potentially the, largest ever observed in stellar flares. X-ray flares were detected from GT Mus, V841 Cen, SZ Psc, and TWA-7 for the first time in this survey. Whereas most of our detected sources are multiple-star systems, two of them are single stars (YZ CMi and TWA-7). Among the stellar sources within 100 pc distance, the MAXI/GSC sources have larger rotation velocities than the 2 other sources. This suggests that the rapid rotation velocity may play a key role in generating large flares. Combining the X-ray flare data of nearby stars and the sun, taken from literature and ou r own data, we discovered a universal correlation of τ ∝ L 0.2 X for the flare duration τ and the intrinsic X-ray luminosity L X in the 0.1-100 keV band, which holds for 5 and 12 orders of magnitude in τ and L X , respectively. The MAXI/GSC sample is located at the highest ends on the correlation.

On the expected γ-ray emission from nearby flaring stars

“…In the model described above, and based on the properties such as fluence and duration, all superflares would have been within reach of CTA. In a systematic search for flares with the MAXI/GSC instrument aboard the ISS, Tsuboi et al (2016) found only 13 stars to be flaring out of a population of 256 active binary stars (Eker et al 2008). Some of these stars flared multiple times and experienced radiative energy releases of up to 10 39 ergs.…”

X-ray Emission from Stars ∼The Results Obtained with MAXI∼

“…MAXI detected more than one-hundred flares on twenty-eight low-mass stars; fourteen RS CVn systems, one Algol system (Algol), ten dMe stars, one dKe star, one Young Stellar Object (TWA-7) and one K-type variable star. The detection of the flare from TWA-7 and the results from the first two-year monitoring with MAXI have been already reported in Uzawa et al (2011) [2] and Tsuboi et al (2016) [3], respectively. Most of the categories are classified as active binaries, which consist of the pair of a sub-giant and a main sequence, a pair of a giants and main-sequence, and a pair of two main-sequence stars.…”

A Universal Correlation between the Duration and the X-ray Luminosity in Stellar Flares

“…A Unified Correlation between the Duration and the X-ray Luminosity in Stellar Flares Y. Tsuboi type star) [15]. The physical parameters of the flares are very large for stellar flares in all of the followings: the X-ray luminosity L X (10 31−34 ergs s −1 in the 2-20 keV band), the emission measure EM (10 54−57 cm −3 ), the e-folding time (10 3−6 s), and the total energy released during the flare (10 34−39 ergs).…”

“…The number of the detection of the large flare was increased significantly. Tsuboi et al (2016) found the correlation between X-ray bolometric luminosity and the duration time which holds from solar microflares to giant stellar flares detected with MAXI [15].…”

“…The enlarged sample of intense flares has enabled us to do systematic studies in various viewpoints. One of the studies is the discovery of a universal correlation between the flare duration and the intrinsic Xray luminosity, which holds for 5 and 12 orders of magnitude in the duration and L X , respectively (Tsuboi et al 2016). Here, we review the studies of stellar flares obtained with MAXI.…”