A team of international astronomers reported that the RS CVn-type star GT Mus (HR4492, HD 101379+HD 101380) was the most active star in the X-ray sky in the last decade in terms of the scale of recurrent energetic flares. Their study recently appeared in Astrophysical Journal.
MAXI is an all-sky X-ray monitor that has been operating on the Japanese Experiment Module (JEM; Kibo) on the International Space Station (ISS) since 2009 August 15. It observes a large area of the sky once per 92 minute orbital cycle and makes it possible to search for transients effectively.
Among the MAXI-detected stellar flare sources, the RS CVn-type star GT Mus showed remarkably energies flared with energies up to ~ 1038 erg repeatedly. So far, MAXI has detected flare candidates with the MAXI “nova-alert system” designed to detect transients from MAXI all-sky images in real time.
The quadruple system GT Mus (HR4492) consists of two binary systems named HD 101379 and HD 101380, located at (R.A., Dec.)(J2000) = (11h39m29s.497, −65°23’52”.0135) at a distance of 109.594 pc. The two binaries (HD 101379 and HD 101380) are separated by 0.23 arcsec, which is spatially resolved by speckle methods.
The RS CVn-type single-lined spectroscopic binary HD 101379 has a G5/8 giant primary with a radius of 16.56 R. This binary shows strong CaII H, CaII K, and variable Hα emissions. Moreover, it shows a periodic photometric variation of 61.4 days, which dominates any other variations of GT Mus. This 61.4 day variation may be attributed to a rotational modulation of one or more starspots on HD 101379. These features indicate high magnetic activity, which implies that the flare observed by MAXI may have originated on HD 101379.
The other system, HD 101380, is a binary consisting of an A0 and an A2 main-sequence star. In the folded V-band GT Mus light curve, a small dip is detected. It is interpreted to be due to an eclipse of this binary with a period of 2.75 day. No variations by spots have ever been observed. Thus, it is feasible to speculate that HD 101379 has higher chromospheric activity than HD 101380.
All of the reported MAXI flares from GT Mus so far have been detected by the MAXI “nova-alert system”. However, there is a real potential that some flares have been missed by this automated system. Given the current small number (23) in the MAXI stellar flare sample and the highly active nature of GT Mus, GT Mus provides a good opportunity to study the physical characteristics of stellar flares and their mechanism.
Now, Sasaki and colleagues carried out a detailed analysis of the MAXI data of GT Mus to search for Xray flares. They successfully detected 11 flares (including the three that have been already reported) in 8 yr of observations with Monitor of All-sky X-ray Image (MAXI) from 2009 August to 2017 August. All flared showed a total released energy of 1038 erg or higher. They also performed a unified analysis for all of them and found that the detected flare peak luminosities were 1–4 × 1033 erg s¯1 in the 2.0–20.0 keV band for its distance of 109.6 pc.
Their timing analysis showed long durations (τr +τd) of 2–6 days with long decay times (τd) of 1–4 days. The released energies during the decay phases of the flares in the 0.1–100 keV band ranged 1–11 × 1038 erg, which are at the upper end of the observed stellar flare. The released energies during whole duration time ranged 2–13 × 1038 erg in the same band.
They also carried out X-ray follow-up observations for one of the 11 flares with Neutron star Interior Composition Explorer (NICER) on 2017 July 18 and found that the flare cooled quasi-statically. On the basis of a quasi-static cooling model, the flare loop length is derived to be 4 × 1012 cm (or 60 R). This size is a 2–3 orders of magnitude larger than that of the typical solar flare loop of 109–1010. While, the electron density is derived to be 1 × 1010 cm¯3, which is consistent with the typical value of solar and stellar flares (1010¯13 cm¯3). The ratio of the cooling timescales between radiative cooling (τrad) and conductive cooling (τcond) is estimated to be τrad ∼ 0.1 τcond from the temperature; thus radiative cooling was dominant in this flare.
Furthermore, for the first time, they plotted the G and K giant binary samples in the diagram of X-ray to bolometric luminosity ratio versus Rossby number (shown in fig 1) and obtained a consistent distribution with those for the low-mass stars. The Rossby number and log(LX/Lbol) of GT Mus are 0.614 and −3.5, respectively, which puts GT Mus in line with the relation derived from low-mass and giant binary stars in the diagram. It shows a considerably higher LX/Lbol than other giant binaries. This high X-ray fraction suggests that GT Mus is at a high magnetic activity level, which is consistent with what is inferred from its recurring large flares.
Featured image: R/B-band color composite image of GT Mus from the Second Digitized Sky Survey (DSS2), measuring 30 arcminutes across. © In-the-Sky
Reference: Ryo Sasaki, Yohko Tsuboi, Wataru Iwakiri, Satoshi Nakahira, Yoshitomo Maeda, Keith C. Gendreau, Michael F. Corcoran, Kenji Hamaguchi, Zaven Arzoumanian, Craig Markwardt, Teruaki Enoto, Tatsuki Sato, Hiroki Kawai, Tatehiro Mihara, Megumi Shidatsu, Hitoshi Negoro, Motoko Serino, “The RS CVn type star GT Mus shows most energetic X-ray flares throughout the 2010s”, the Astrophysical Journal, 910(1), 23 Mar 2021. https://iopscience.iop.org/article/10.3847/1538-4357/abde38
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