Discovery of Extreme, Roughly-Daily Superflares on the Recurrent Nova V2487 Oph

Abstract

V2487 Oph is a recurrent nova with detected eruptions in 1900 and 1998. Startlingly, V2487 Oph shows flares, called `Superflares', with up to 1.10 mag amplitude, fast rises of under one-minute, always with an initial impulsive spike followed by a roughly-exponential tail, typically one-hour durations, and with random event times averaging once-per-day. The typical flare energy $E$ is over 10${}^{38}$ ergs, while the yearly energy budget is 10${}^{41}$ ergs. V2487 Oph Superflares obey three relations; the number distribution of flare energies scales as $E^{-2.34\pm 0.35}$, the waiting time from one flare to the next is proportional to $E$ of the first event, and flare durations scale as $E^{0.44\pm 0.03}$. Scenarios involving gravitational energy and nuclear energy fail to satisfy the three relations. The magnetic energy scenario, however, can explain all three relations. This scenario has magnetic field lines above the disc being twisted and amplified by the motions of their footprints, with magnetic reconnection releasing energy that comes out as Superflare light. This exact mechanism is already well known to occur in white light solar flares, in ordinary M-type flare stars, and in the many Superflare stars observed all across the H-R diagram. Superflares on Superflare stars have rise times, light curve shapes and durations that are very similar to those on V2487 Oph. So we conclude that the V2487 Oph Superflares are caused by large-scale magnetic reconnection. V2487 Oph is now the most extreme Superflare star, exhibiting the largest known flare energy (1.6$\times$10${}^{39}$ ergs) and the fastest occurrence rate.

MNRAS submitted, 20 pages

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