Non-magnetic dwarfs turn out to be magnetic

NASA’s Kepler satellite provides detailed data to overturn conventional thinking on magnetism and accretion discs. Andrew Masterson reports.

MV Lyrae is a binary system consisting of a white dwarf accreting material from the envelope of a nearby star.
MV Lyrae is a binary system consisting of a white dwarf accreting material from the envelope of a nearby star.
Helena Uthas

Until today, roughly 85% of white dwarfs – the compact, hyper-dense corpses of burnt-out stars – were thought to lack magnetic fields.

Now, research led by New Zealand astrophysicist Simone Scaringi and published in the journal Nature looks set to prompt a major reappraisal of the idea.

Scaringi, from the University of Canterbury, with colleagues, report a previously unknown behaviour in a system called MV Lyrae, which comprises a white dwarf in a binary relationship with a red dwarf. The behaviour has been likened to binge-eating and can only be explained by the presence of a hitherto unsuspected magnetic field.

The team’s findings arose from data gathered over a four year period by NASA’s Kepler satellite.

White dwarfs are typically surrounded by an accretion disc – a massive circle comprising dust, gas and other materials. MV Lyrae’s disc normally appears hot and bright to observers, but has been known to suddenly dim and fade for months at a time. This is referred to as the disc’s “low state”.

Although Kepler is designed specifically to detect exoplanets, the white dwarf was included in its mission list, with the result that it took shots of the system once a minute for just shy of 48 months.

The period included a transition into low state. The Kepler data revealed that during this period the star was far from inactive. On the contrary, as UK astrophysicist Thomas Marsh writes in an editorial accompanying the report, “MV Lyrae showed spectacular quasi-regular flaring — brightening by as much as a factor of 6 in 30-minute-long bursts that recurred approximately once every 2 hours. This behaviour lasted for days”.

Scaringi and colleagues explain the bright bursts as the release of gravitational potential energy. The Kepler data suggests that in the binary system, matter builds up at the “magnetospheric boundary” – the co-rotation radius arising between the red and white dwarfs. The dust and gas is then sporadically pulled into the white star, the result of an unstable magnetic field.

The finding provides strong evidence for magnetic fields being present in far more than the 15% of white dwarfs previously thought.

The discovery might never have been made were it not for the attentions of Kepler. The scientists note that the field, when present, is substantial but remains too weak to be detected by any other instruments.

Scaringi has a novel way of describing its strength.

“We have seen episodes of strong flares of accretion interrupted by periods with no evidence of accretion,” he says.

“This sporadic activity is best explained by the presence of a strong magnetic field comparable to that of 1000 fridge magnets.

“This magnetic field ‘gates’ the accretion, causing the matter to pile up until it has a gravitational attraction stronger than the magnetic forces holding it back, indicating for the first time that even “non-magnetic” white dwarfs can have very strong magnetic fields.”

Accretion discs have been observed around other celestial objects, including black holes and neutron stars. The scientists say their observations constitute strong evidence that the physics that governs their behaviour is consistent regardless of the object to which they are anchored.

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Andrew Masterson is news editor of Cosmos.
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