2021ApJS..256....1T

The short-period variable star MG1-688432 has been discovered to exhibit occasional extremely high energy optical outbursts as high as 10³⁸ erg. Outbursts are typically several hours in duration. These events are often highly structured, resembling sequential associated releases of energy. Twenty years of time sequence photometry are presented, indicating a basic sinusoidal light curve of mean period 6.65 days, with some phase shifting and long-term temporal trends in amplitude and mean brightness. Spectroscopy reveals a peculiar star, best resembling a K3 subgiant that has evolved off the main sequence moderately redward of the giant branch. Spectroscopic and radial velocity analyses indicate a binary system orbiting its barycenter with an unseen companion to the K3 IV primary. This is not an eclipsing system, with the inclination of the orbit precluding eclipse by the secondary. The system is at a distance of 1.5 kpc, and analysis of Gaia observations leads to the conclusion that the H-R diagram position of MG1-688432 is established by an intrinsic feature of the system, most likely either the stellar evolutionary state of the observed star or the presence of small (nongray) dust within the system. Two mechanisms (or combinations thereof) that might give rise to characteristics of the system are (1) magnetically induced chromospheric activity and (2) impacts with tidally disrupted planetary debris.