NICER / ISS Science Nugget for February 7, 2019

MAXI-triggered observations of stellar flares yield rapid NICER results

NICER's coordination of giant stellar-flare observations with the MAXI all-sky X-ray monitor (a JAXA payload on the JEM-EF) continues to reap benefits this week. With its huge sky coverage, MAXI finds stars erupting in giant flares, while NICER quickly follows up with its large collecting area and tolerance for high X-ray brightness. Together, these instruments are a formidable team for solving the mysteries of stellar flares. Their communication is currently accomplished manually, on the ground, using a virtual "hotline" called the MAXI And NICER Ground Alert, or MANGA (which also means 'graphic novel' in Japanese).

Although giant flares are dazzling for NICER, their MAXI signals tend to be barely above noise. MAXI duty scientists detect flares with careful data analysis, alert the NICER team, and NICER schedulers quickly implement their observations. Time is essential, as stellar flares fade quickly, on timescales of hours.

MAXI detected a new giant flare at 06:30 UT on Feb 4th from the magnetically active (RS CVn-type) low-mass binary system called sigma Geminorum, located near the star Pollux in the sky. Thanks to swift teamwork, NICER followed up the flare within 10 hours of the MAXI detection. NICER caught approximately 900 X-ray photons per second from the flare (see figure) at the beginning of the observation, among the highest count rates ever collected from a stellar X-ray flare. The flare decayed quickly, by 50% in flux in 15 hours.

Rapid decay in an X-ray flare detected from the Geminorum binary star system

An early result of the data analysis is already of interest: NICER spectra in the early decay phase show an emission line (at 6.9 keV photon energy) from highly ionized iron atoms in a very hot plasma – at a temperature of roughly 70 million Kelvin. The subsequent NICER observations suggest cooling of this plasma over a day. This result likely requires a large magnetic flare loop on the stellar surface.

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