Recurrent Nova V2487 Oph Had Superflares in 1941 and 1942 With Radiant Energies 10$^{42.5\pm1.6}$ Ergs

TL;DR

This paper reports the discovery and analysis of superflares from the recurrent nova V2487 Oph, recorded in archival photographs and recent observations, revealing energies up to 10^{42.5} ergs and consistent superflare activity over 82 years.

Contribution

The study provides the first archival evidence of superflares in 1941 and 1942 from V2487 Oph and confirms its long-term superflare activity from 1941 to 2023.

Findings

Superflares recorded in 1941 and 1942 with energies ~10^{42.4} and 10^{42.5} ergs.

V2487 Oph has been superflaring continuously from 1941 to 2023.

Total superflare energy emitted from 1941 to 2023 is approximately 10^{44.1} ergs.

Abstract

V2487 Ophiuchi (V2487 Oph) is a recurrent nova with classical nova eruptions in 1900 and 1998, and it is also the most extreme known superflare star. These superflares are roughly-hour-long flares with amplitudes and optical energies reaching up to 1.10 mag and $10^{39.21}$ ergs, with the superflares recurring once-a-day. The V2487 Oph superflares are certainly operating with the same mechanism as all the other types of superflare stars, where magnetic loops are twisted and stretched until reconnection occurs, whereupon ambient electrons are accelerated to relativistic energies and then emitted bremsstrahlung radiation from X-ray to radio. V2487 Oph is unique among known superflare stars in that one of the loop footprints is in an accretion disk. This exact mechanism was theoretically predicted by M. R. Hayashi and colleagues in 1996. Now, I have found two superflares recorded on Harvard archival photographs from the years 1941 and 1942. These two superflares have $B$ magnitude amplitudes of $>$1.83 and $>$2.00 mag and total radiated energies of $10^{42.4}$ and $10^{42.5}$ ergs with bolometric corrections. Each has emitted energies of $\sim$30-billion Carringtons, in units of the most energetic solar flare. Further, I find superflares in the Zwicky Transient Factory light curves, so V2487 Oph has been superflaring from 1941 to 2023. For the observed number distribution of $dN/dE$=$4E^{-2}$ superflares per year, for $E$ in units of $10^{41}$ ergs, the emitted energy in superflare light is $10^{42.1}$ erg in each year, or $10^{44.1}$ ergs from 1941 to 2023.