The detection of a radial magnetic field across the inner ring of SNR 1987A, the remnant of a supernova first witnessed three decades ago, provides insight into the early stages of the evolution of supernova remnants and the cosmic magnetism within them.
SN 1987A is a stellar explosion that occurred from a star about 20 times the mass of the Sun.
This supernova was first observed on February 23, 1987 in a nearby dwarf galaxy, the Large Magellanic Cloud, some 164,000 light-years away.
It was the first naked-eye supernova to be observed since Johannes Kepler witnessed a supernova over 400 years ago.
In the three decades since SN 1987A occurred, material expelled by the blast, as well as the shockwave from the star’s death throes, have been traveling outward through the gas and dust that surrounded the star before it exploded.
Today, when we look at SNR 1987A, we see rings of material set aglow by the supernova’s expanding debris and shockwave.
Using the Australia Telescope Compact Array at the Paul Wild Observatory, Professor Bryan Gaensler and co-authors observed the magnetic field by studying the radiation coming from the object.
By analyzing the properties of this radiation, they were able to trace the magnetic field.
What they found was that the remnant’s magnetic field was not chaotic but already showed a degree of order.
“The magnetism we’ve detected is around 50,000 times weaker than a fridge magnet,” said Professor Gaensler, director of the Dunlap Institute for Astronomy Astrophysics at the University of Toronto.
Astronomers have known that as supernova remnants get older, their magnetic fields are stretched and aligned into ordered patterns.
So, the new observation showed that a supernova remnant can bring order to a magnetic field in the relatively short period of thirty years.
The magnetic field lines of the Earth run north and south, causing a compass to point to the Earth’s poles.
By comparison, SNR 1987A’s magnetic field lines are like the spokes of a bicycle wheel aligned from the centre out.
“At such a young age, 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,” said Dr. Giovanna Zanardo, from the International Centre for Radio Astronomy Research at the University of Western Australia.
“As SNR 1987A 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,” Professor Gaensler said.
The findings will be published in the Astrophysical Journal Letters.
Giovanna Zanardo et al. 2018. Detection of Linear Polarization in the Radio Remnant of Supernova 1987A. ApJL, in press; arXiv: 1806.04741