The spin the tidal disruption models left out may explain fading black hole flares

Brightness in these models tracks a ratio rather than a mass. The flare peaks with the rate at which stripped debris falls back onto the black hole, which is roughly the material removed divided by the time it takes to return ^[14]. Earlier hydrodynamical runs had the numerator shrinking pass after pass, which should have produced fading and did not ^[5]. The reason sat in the group's own earlier work: the tidal field that strips material also torques the survivor, so the star leaves each encounter spinning faster and the smaller amount of debris comes back over a shorter window ^[6]. Both terms fall together, and the predicted light curve barely moves.

The new result closes that loop by capping the spin-up. A star already rotating quickly on arrival is not spun up significantly by each passage, the fallback time stays roughly fixed, and declining mass loss finally shows up as a declining peak fallback rate and a dimmer flare ^[7]. Bandopadhyay calls the fast initial rotation "a new ingredient" ^[9], and says the problem had puzzled the group for two years ^[8].

It is worth being clear about what kind of ingredient it is. Initial spin is not new physics; it is an initial condition, set before the first encounter and not something an observer can go and measure. Nothing in this system is seen directly. The black hole emits no light, and the flare, debris radiating over days to months as it accretes, is the whole of the evidence ^[12]. So the model gains a parameter it can fit, and the fading light curve is the only handle on it.

That matters because the dimmers are a minority. Four of roughly ten known repeating systems fade ^[3], about 40 percent ^[4], and the other six have to emerge from the same physics with a different setting. Stellar structure already supplies part of that spread: Bandopadhyay likens a low-mass star to a fluffy meringue that grows more vulnerable to the tidal force, while a heavier star with an onion-like interior sheds outer layers and keeps a dense core relatively untouched, losing decreasing amounts each time ^[11]. Spin now sits alongside mass and structure as something a system's history has to deliver.

History is where the paper ends up. It asks why the star would already be rotating so rapidly ^[15], and that question lands next to one Coughlin states plainly: it is "extremely difficult" to bind a star to a supermassive black hole tightly enough that it orbits in a matter of months, "and yet they seem to do so in rpTDEs" ^[10]. Whatever channel produces these systems now has to produce the tight orbit and the fast rotation together. Until it does, the dimming is accounted for by an assumption that is itself unaccounted for.