How to measure a magnetic field that's very long way away, and is very, very weak. An international group of boffins have announced that they figured out how.
The magnetic field in question belongs to a distant supernova remnant, Supernova 1987A, 168,000 light years from Earth. While the supernova exploded in 1987, its remnants remains a favourite observation target.
The magnetism was inferred from the direction in which radio waves leaving 1987A were polarised, which as the team wrote in its paper “points to a primarily radial magnetic field across the inner ring, encompassing both the reverse and forward shocks”.
Ho-hum, right? Not according to team member Brian Gaensler of the University of Toronto's Dunlap Institute:
Presenting our discovery of magnetism in the remnant of Supernova 1987A! 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. Great work led by @StellarEngineer. https://t.co/xCvd3PtBBD pic.twitter.com/ndG1Pw7PUf— Bryan Gaensler (@SciBry) June 14, 2018
The magnetism isn't oriented the way you might expect, either:
Like other somewhat older supernovae, we find that the magnetism points outward. Weird.— Bryan Gaensler (@SciBry) June 14, 2018
This means that if you were standing on the surface of the debris cloud holding a compass, north would be straight up, above your head!
The magnetism we’ve detected is around 50,000 weaker than a fridge magnet. And we’ve been able to measure this from a distance of a million trillion kilometres. Not too shabby, eh?— Bryan Gaensler (@SciBry) June 14, 2018
You could think of this as a “right hand rule”, but on a cosmic scale: the direction of the particles accelerating away from 1987A dictates the orientation of the magnetic field, that orientation dictates the polarisation of the light, and that (rather than magnetism) is what you can measure from such a distance.
The magnetic field was discovered using polarimetric observations carried out in the 20 GHz to 50 GHz radio spectrum, observed using the Australia Telescope Compact Array between October 2015 and May 2016.
The study was led by Giovanna Zanardo of Western Australia's International Centre for Radio Astronomy Research (ICRAR), with collaboration from Gaensler, Lister Staveley-Smith (also of ICRAR), Remy Indebetouw of America's National Radio Astronomy Observatory and the University of Virginia, C-Y Ng of the University of Hong Kong, Mikako Matsuura of University College London and Cardiff University, and A K Tzioumis of CSIRO's Australia Telescope National Facility. ®