Team finds companion, starspots and dynamo power Kepler field superflares

Astronomers led by Zsolt Kővári of HUN-REN CSFK have identified the engine behind the most powerful stellar flares in the Kepler field, Phys.org reported. KIC 2852961 is not the biggest or brightest star Kepler ever observed, yet it is by a wide margin the most violent red giant in the field. Its eruptions are superflares, energy outbursts that dwarf even the strongest flares the sun can produce.

The researchers say the star’s violent outbursts stem from a mix of a nearby stellar companion, enormous dark starspots, and a powerful internal magnetic dynamo. Zsolt Kővári, the lead author from HUN-REN CSFK, said the team wanted to know what makes the star so extreme, and that the answer proved subtler than a companion or internal forces by themselves. The article frames the mechanism as the answer to an open question about what has fueled the star’s eruptions since it first attracted notice.

Ground-based spectroscopy, Kepler space photometry, and Gaia data showed that KIC 2852961 is the main star in a close binary system, with a faint dwarf as its partner. The group first made the star better known in a 2020 Astronomy & Astrophysics paper listing 59 (super)flares drawn from all Kepler observations, and that earlier work indicated flares and superflares across many different star types probably share one underlying physical mechanism. The accepted Astronomy & Astrophysics paper builds on that foundation with radial velocity measurements that confirm the system’s binary nature and give a fuller picture of the star’s extreme behavior. The paper carries the title What powers the most energetic flares in the Kepler field?

The arXiv preprint Revisiting the superflaring star KIC 2852961 appeared in 2026 under DOI 10.48550/arxiv.2610.11799.

The pair pulls on each other with tides much like the moon lifts ocean water on Earth, though the stellar version is far more dramatic. During the red giant phase, which lasts tens or hundreds of millions of years, the primary swelled up and spun more slowly. Tidal effects locked the giant’s spin to its orbit and made the orbit more circular while the star reached about 12 solar radii, and the circularization timescale scales as t_c proportional to (a/R) to the eighth power, so weak main-sequence tides became strong after expansion. The team found the star is almost, but not entirely, synchronized and circularized, and they said that condition is crucial to the observed activity.

Stars are not solid, so they do not spin as one rigid body; instead, different latitudes rotate at different rates, a phenomenon called differential rotation. Our own sun spins faster at the equator than at the poles, while tidal forces in KIC 2852961 have sharply limited but not removed the same latitude-based difference. The leftover spin difference drives an internal magnetic dynamo that constantly makes and strengthens magnetic fields, storing energy until it bursts out as a flare. Kővári said differential rotation works in two ways: it helps create and maintain enormous starspots, and it twists and shears coronal magnetic field lines until their reconnection triggers a superflare.

Brightness changes in KIC 2852961 that stand out in Kepler data come from large dark starspots covering parts of the stellar surface as the star turns. These spots are intensely magnetic areas like sunspots, but vastly larger, and they act as reservoirs of stored magnetic energy. Larger spots hold more magnetic energy and produce stronger flares, making the link between spot size and flare energy causal instead of coincidental.

The researchers checked if the companion sets off flares by linking magnetic field lines or by timing them to certain points in the orbit, but found no sign of that. The partner still matters indirectly, since its tides spun up and partially matched the giant’s rotation. That arrangement lets differential rotation function well, meaning the companion put the engine in place and the giant provides the flare trigger.

Older red giants are usually considered magnetically quiet since their activity fades as they age. A red giant spun up by a companion that keeps rotating fast can become the most magnetically active giant star in the Kepler field. The findings help separate whether strong flaring in a binary system comes from the partner or from internal processes, and for KIC 2852961 the internal processes clearly drive the activity, shaped by but not controlled from the companion. The work matters for understanding space weather around stars other than the sun, and it indicates that knowing how often flares happen and how powerful they are here demands a detailed view of internal rotation, magnetic condition, and binary evolution.