Magnetically gated accretion in an accreting 'non-magnetic' white dwarf

Localized thermonuclear bursts from accreting magnetic white dwarfs

Nova explosions are caused by global thermonuclear runaways triggered in the surface layers of accreting white dwarfs^1-3. It has been predicted^4-6 that localized 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 magnetized white dwarf from a low-mass companion, have been observed on several occasions in the past 40 years^7-11. During these bursts, the optical/ultraviolet luminosity increases by a factor of more than  three in less than an hour and fades in around ten hours. Fast outflows have been observed in ultraviolet spectral lines⁷, with velocities of more than 3,500 kilometres per second, comparable to the escape velocity from the white dwarf surface. Here we report on optical bursts observed in TV Columbae and in two additional accreting systems, EI Ursae Majoris and ASASSN-19bh. The bursts have a total energy of approximately 10^-6  times than those of classical nova explosions (micronovae) and bear a strong resemblance to type-I X-ray bursts^12-14. 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.

Decay of the coronal magnetic field can release sufficient energy to power a solar flare

Solar flares are powered by a rapid release of energy in the solar corona, thought to be produced by the decay of the coronal magnetic field strength. Direct quantitative measurements of the evolving magnetic field strength are required to test this. We report microwave observations of a solar flare, showing spatial and temporal changes in the coronal magnetic field. The field decays at a rate of ~5 Gauss per second for 2 minutes, as measured within a flare subvolume of ~10²⁸ cubic centimeters. This fast rate of decay implies a sufficiently strong electric field to account for the particle acceleration that produces the microwave emission. The decrease in stored magnetic energy is enough to power the solar flare, including the associated eruption, particle acceleration, and plasma heating.