A micronova is a thermonuclear blast that happens on the surface of certain stars and lasts for just a few hours making them extremely difficult to observe.
This artist’s impression shows a binary system where micronovae may occur. Image credit: ESO / M. Kornmesser / L. Calçada.
Micronovae are extremely powerful events, but are small on astronomical scales.
They are much less energetic than the stellar explosions known as novae. Both types of explosions occur on white dwarfs.
A white dwarf in a binary system can steal material, mostly hydrogen, from its companion star if they are close enough together.
As this gas falls onto the very hot surface of the white dwarf star, it triggers the hydrogen atoms to fuse into helium explosively. In novae, these thermonuclear explosions occur over the entire stellar surface.
Micronovae are similar explosions that are smaller in scale and faster, lasting just several hours.
They occur on some white dwarfs with strong magnetic fields, which funnel material towards the star’s magnetic poles.
Durham University astronomer Simone Scaringi and colleagues first came across unusual micronova explosions when they noticed a bright flash of light lasting for a short time while analyzing data from NASA’s Transiting Exoplanet Survey Satellite (TESS).
In total, they observed micronovae in three binary systems: TV Columbae, EI Ursae Majoris and ASASSN-19bh.
Two micronovae were from already known white dwarfs, TV Columbae and EI Ursae Majoris, but the third, ASASSN-19bh, needed more observations with the X-Shooter instrument on ESO’s Very Large Telescope (VLT) for its white dwarf status to be confirmed.
“We have discovered and identified for the first time what we are calling a micronova,” Dr. Scaringi said.
“The phenomenon challenges our understanding of how thermonuclear explosions in stars occur.”
“We thought we knew this, but this discovery proposes a totally new way to achieve them.”
“Looking through astronomical data collected by TESS, we discovered something unusual: a bright flash of optical light lasting for a few hours. Searching further, we found several similar signals,” said Dr. Nathalie Degenaar, an astronomer at the University of Amsterdam.
“For the first time, we have now seen that hydrogen fusion can also happen in a localized way,” said Dr. Paul Groot, an astronomer at Radboud University.
“The hydrogen fuel can be contained at the base of the magnetic poles of some white dwarfs, so that fusion only happens at these magnetic poles.”
“This leads to micro-fusion bombs going off, which have about one millionth of the strength of a nova explosion, hence the name micronova.”
The discovery paper was published in the journal Nature.
A follow-up paper, where the authors propose a model showing that magnetic confinement of material on to an accreting white dwarf could trigger localized thermonuclear bursts, was published in the Monthly Notices of the Royal Astronomical Society Letters.
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S. Scaringi et al. 2022. Localized thermonuclear bursts from accreting magnetic white dwarfs. Nature 604, 447-450; doi: 10.1038/s41586-022-04495-6
S. Scaringi et al. Triggering micronovae through magnetically confined accretion flows in accreting white dwarfs. MNRASL, published online April 20, 2022; doi: 10.1093/mnrasl/slac042
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