Finding a fragile fridge magnet, 170,000 light years away

Astronomers have measured the magnetic field of a recent supernova. Andrew Masterson reports.

Rings of debris expand from supernova SN1978.
Rings of debris expand from supernova SN1978.

A magnetic field 50,000 times weaker than that of a fridge magnet has been detected across a distance of 1.6 million trillion kilometres by a team of astronomers in Toronto, Canada, and Western Australia.

The field surrounds the remains of a supernova dubbed SN 1987A, and its confirmation constitutes the earliest possible detection of the magnetic field that forms after the explosion of massive star.

SN 1987A was discovered by University of Toronto astronomer Ian Shelton 31 years ago. It was the first supernova to be detected using the naked eye since Johannes Kepler saw one 400 years earlier.

It immediately became an object of intense interest to astronomers around the world, presented with a unique opportunity to observe the progress of the blast shockwave, and the ever-expanding rings of debris expelled by the titanic explosion.

A team of researchers including Bryan Gaensler from Toronto University’s Dunlap Institute for Astronomy and Astrophysics used the Telescope Compact Array at the Paul Wild Observatory near the Australian town of Narrabri to study the radiation emitted by the supernova.

“The picture shows what it would look like if you could sprinkle iron filings over the expanding cloud of debris, 170 thousand light years away,” says Gaensler.

By taking careful measurements of the radiation, the researchers were able to deduce the strength and structure of the nascent magnetic field.

Research into other supernovas has shown that as the remnants grow older, the magnetic field becomes stronger, stretches and forms into ordered patterns.

Gaensler and colleagues were surprised to find that even just three decades after the initial explosion, that process was already well underway.

“At such a young age,” says lead author Giovanna Zanardo, “everything in the stellar remnant is moving incredibly fast and changing rapidly, but the magnetic field looks nicely combed out all the way to the edge of the shell.”

Adds Gaensler: “As it continues to expand and evolve, we will be watching the shape of the magnetic field to see how it changes as the shock wave and debris cloud run into new material.”

The research is published in the journal Astrophysical Journal Letters, with a full access copy lodged on the preprint server Arxiv.

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