Eclipsing Stellar Flare on the Demon Star Algol Binary System Observed during the MAXI–NICER Follow-up Campaign in 2018

Abstract Algol is a well-known eclipsing binary hosting an active and variable star that exhibits frequent stellar flares. Here, we report our preplanned and coordinated rapid X-ray follow-up observations of an eclipsing flare on Algol. The Monitor of All-sky X-ray Image detected a flare on Algol at 05:52 UT on 2018 July 4. Subsequently, we carried out a prompt X-ray monitoring with the Neutron star Interior Composition ExploreR starting at 19:45 UT on the same day, and the observation ended at 06:02 UT on 2018 July 6. During the decaying phase of the flare, we successfully detected a 5.8 hr long eclipse, corresponding to the secondary eclipse in which Algol A blocks the line of sight to Algol B. During the eclipse, the 2–10 keV X-ray flux is decreased to the 20% level from 1.9 × 10 −10 to 4.5 × 10 −11 erg cm −2 s −1 . We found a configuration of the flare size and location to explain the X-ray observations; e.g., the flare occurred at the latitude 45°S of the Algol B surface with a flare height of 1.9 × 10 11 cm, corresponding to 0.8 times the stellar radius of Algol B, giving 80% obscuration of the flare loop by Algol A. The apparent absorption increase before the eclipse might originate from coronal mass ejection in the line of sight ejected during the flare.