Localised thermonuclear bursts from accreting magnetic white dwarfs

TL;DR

This paper reports the discovery of localized thermonuclear bursts in accreting magnetic white dwarfs, resembling Type I X-ray bursts, and suggests they result from thermonuclear runaways in magnetic accretion columns, expanding understanding of white dwarf explosions.

Contribution

It provides observational evidence of localized thermonuclear bursts in magnetic white dwarfs and proposes a new explanation involving thermonuclear runaways in magnetic accretion columns.

Findings

Detected optical bursts in TV Columbae, EI Ursae Majoris, and ASASSN-19bh.

Bursts have energies about 10^-6 of classical novae.

Bursts resemble Type I X-ray bursts and are not caused by accretion or magnetic reconnection.

Abstract

Nova explosions are caused by global thermonuclear runaways triggered in the surface layers of accreting white dwarfs. It has been predicted that localised thermonuclear bursts on white dwarfs can also take place, similar to Type I X-ray bursts observed in accreting neutron stars. Unexplained rapid bursts from the binary system TV Columbae, in which mass is accreted onto a moderately-strong magnetised white dwarf from a low-mass companion, have been observed on several occasions in the past $\approx40$ years. During these bursts the optical/UV luminosity increases by a factor of $>3$ in less than an hour and fades over $\approx10$ hours. Fast outflows have been observed in UV spectral lines, with velocities $>3500$ km s$^{-1}$, comparable to the escape velocity from the white dwarf surface. Here we report on optical bursts observed in TV Columbae as well as in two additional accreting systems, EI Ursae Majoris and ASASSN-19bh. The bursts have a total energy $\approx~10^{-6}$ those of classical nova explosions ("micronovae"), and bear a strong resemblance to Type I X-ray bursts. We exclude accretion or stellar magnetic reconnection events as their origin and suggest thermonuclear runaway events in magnetically-confined accretion columns as a viable explanation.