NICER / ISS Science Nugget
for January 2, 2025

Tracking the aftermath of a superflare

Magnetic activity in stars produces flares of varying strength and duration. One of the strongest X-ray flares ever observed - about 100,000 times stronger than any recorded from our Sun and an order of magnitude stronger than any seen in long-duration surveys - was detected on 7 November, 2022, from a star cataloged as HD 251108; follow-up observations with NICER were initiated within two days and continued for nearly a month as the flare emission decayed back to quiescent levels previously measured by the European eROSITA survey instrument. The star's basic properties (e.g., type "K", abundant in lithium, rotation period of 21.3 days, mass of approximately 1 Sun but with a radius 6-8 times larger) and analysis of the X-ray flare's long decay are described in a peer-reviewed paper by H. Günther (MIT) and collaborators recently published in The Astrophysical Journal.

Each NICER observation provides a sensitive spectrum from which elemental abundances, plasma densities and temperatures, and their evolution with time can be inferred. These in turn are related to the geometry of the flare's magnetic loop above the star's surface. Günther et al. find that the data are best fit with thermal emission models from four plasma components with distinct temperatures, where the hottest decays the fastest in its contribution to the flare emission in the early stages of its decay, while the lowest-temperature component is more stable. The length of the flare's magnetic loop is estimated to be 2-4 times the radius of the star. Günther et al. find that, despite the exceptional strength of the flare, its observed properties in X-rays relative to those at other wavelengths are similar to much smaller flares from a variety of stars, suggesting that they all share common underlying physical processes that can be reliably described by existing and ever-improving models.