Gravitational-wave detectors picked up a strange burst — the signal GW241011. Two black holes merged, but the pair looked unusual: one hole was about three times lighter than the other, and the system as a whole was spinning rapidly. Such a combination is a rare guest in the catalogs.

Usually, fast spin and a strong mass imbalance are attributed to “hierarchical mergers”: a second-generation black hole, already born in a previous collision, meets an ordinary first-generation hole. But here two difficulties arose. First, the hierarchical scenario rarely yields such a light companion (mass less than a third of the heavy one). Second, after the merger, the newborn hole receives a powerful gravitational kick and often gets ejected from its parent cluster — no one left to merge with. Moreover, the data on GW241011 had already cast doubt on its dynamical origin.

The authors tested an alternative — the “mass-ratio reversal” (MRR) channel in an isolated binary system. The scenario resembles a cosmic role swap: the initially more massive star ends its life first and becomes a black hole. Then it begins to pull matter from its companion star, which has swelled in its late stages. As a result, the roles reverse: the former heavyweight loses mass, while its partner gains weight, turning into a more massive black hole. The outcome is a pair with an inverted mass ratio and high total spin, like GW241011.

The scientists quantitatively assessed under what conditions such a channel works and identified key factors: the rate of mass loss from stars, the efficiency of mass transfer in a close pair, and the strength of stellar winds. It turned out that with certain settings of binary physics, the MRR channel becomes a viable alternative to hierarchical mergers. This is not just an explanation of one event: it is a way to peek into the mechanisms governing the lives of massive binary stars.