NICER / ISS Science Nugget
for July 27, 2023
NICER's speedy follow-up to a giant stellar flare from IM Peg
Two stars with masses similar to our Sun, closely orbiting each other with a period of 2-3 weeks or less, can form a type of system known as an RS Canum Venaticorum (CVn) binary. These systems occasionally light up in X-rays through the release of magnetic energy. Our Sun also blasts X-ray flares powered by magnetic fields, causing space weather alerts, but the flares from RS CVn are a million times more luminous than the biggest Solar flare ever recorded. Such vigorous activity probably originates within the enhanced internal magnetic dynamos driven by the two stars' orbital motion. The flares emerge along magnetic-field loops larger than the star itself, connecting the two stars and filling the space between them with hot plasma. We don't fully understand how these flares occur and how they may affect any planetary systems around close binary stars.
Giant flares occur rarely, once per year or less. To observe flares, NICER relies on alerts from JAXA's MAXI all-sky monitor through the OHMAN network. MAXI's "nova alert" system detected a possible X-ray flare from the RS CVn system IM Pegasi on 23 July at 10:41 UT, and confirmed the event during the next pass at 12:14 UT. NICER responded promptly to MAXI's alert and began observing IM Peg four hours after the confirmation. Thanks to this rapid follow-up, NICER observed the still-scorching-hot X-ray plasma, at approximately 93 million Kelvin, during its first snapshot (see figure), detecting two clear emission lines from highly ionized iron atoms. NICER monitored the long-lasting decay for several days.
W. Iwakiri (Chiba Univ., Japan) and collaborators reported preliminary results of MAXI and NICER analysis through Astronomer's Telegram #16152. This NICER follow-up observation was one of the earliest among flares from RS CVn stars, but data acquired even earlier, during the rise of the flare, would probe the powerful plasma heating process. MAXI and NICER continue to coordinate efforts to achieve speedy responses to understand the physical mechanisms underlying giant flares.
The right-hand panel plots the X-ray brightness evolution of the July 23 flare from the IM Pegasi system, captured with MAXI (inset) and NICER. NICER monitoring began soon after the flare's peak and tracked its decay with high-quality data that enabled spectroscopy within each observation. The left-hand panel shows the high-energy-band X-ray spectrum during the first NICER observation, clearly revealing two emission lines from highly ionized iron.
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