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Monster CME from the Demon Star

posted Oct 30, 2017, 9:10 PM by Jeremy Drake   [ updated Nov 1, 2017, 8:06 AM ]
The Arabic name "Algol", of the eclipsing close binary star sometimes known as the Demon Star, derives from the "head of the ghoul"in Greek tradition the head of the venomous snake-haired Medusa severed by the demigod fellow Perseus. It comprises a B8 dwarf and a K0 subgiant in a 2.86 day orbit.  When the fainter K star passes in front of the brighter B star, Algol dims significantlybehaviour thought of as somewhat spooky long before its binary nature was known.  The stars are tidally-locked, and both rotate with the same period as their orbit, much like the Moon is tidally-locked to its orbit about the Earth.  The consequent rapid rotation of the K star, in concert with its outer convection zone, engenders vigorous and violent magnetic activity.  The hot B8 dwarf is instead bereft of an outer convection zone and appears magnetically quiescent.  This is handy because when the B star passes in front of the K star the eclipse signature in X-rays can be used to infer the spatial structure of the K star's X-ray emitting corona.

Back in late August of 1997, the joint Italian-Dutch BeppoSAX X-ray satellite caught Medusa's head having a bad hair day. An immensely energetic magnetic flare was observed that lasted  for nearly three days, and released a total X-ray energy of 1037 ergequivalent to the entire visible light output of the Sun for almost an hour and a million times more energetic than the largest solar flares. The flare was eclipsed by the B star, enabling the scientists who first studied it to estimate its size and location. Part of the X-ray signal from the flare was also observed to suddenly dim, then recover.  It was suggested that this dimming was caused by a coronal mass ejection like those that commonly accompany large solar flares.  SAO Postdoctoral Scholar Sofia Moschou took up this idea and analysed the ejection using a geometrical CME description developed for solar CMEs called the "ice cream cone" model.  She found the event had a mass of two to twenty thousand trillion tonnes, similar to that of the fifth largest asteroid in the solar system, Interamnia, and moved at a speed of about 3 million kph.  This work, lead by Dr. Moschou, is accepted for publication in the Astrophysical Journal.