A pulsar in SNR 1987A?

Supernova 1987A, imaged with the Anglo-Australian Telescope. Photo: David Malin, AAO.

For astronomers, the highlight of 1987 was supernova 1987A, the brightest exploding star seen since telescopes were invented. The star that exploded had about 20 times the mass of the Sun, and its demise could have created a pulsar—a spinning neutron star that emits a beam of radio waves, which we detect as a train of radio pulses. There is evidence that a compact object of some kind—a neutron star or even a black hole—formed when the star’s core collapsed, as neutrinos (subatomic particles) were detected on Earth a few hours before the light from the explosion was seen.

Astronomers have spent the past quarter century looking for proof of a pulsar lurking within the supernova remnant. Now Giovanna Zanardo (UWA) and her colleagues have found the strongest evidence yet of one’s existence.

Zanardo’s research team imaged the emission from the supernova remnant at wavelengths from radio to far infrared, using the Atacama Large Millimeter/submillimeter Array (ALMA) and the Australia Telescope Compact Array (ATCA). Observations with the ATCA reveal the radio emission—‘synchroton emission’—generated by the shock wave from the explosion (which is still travelling outwards) as this collides with the material left behind by the defunct star. The highest-frequency ALMA image, on the other hand, shows the emission from cold dust within the debris from the explosion.

Subtracting these two emission components from the other images leaves some residual emission. This could be additional synchrotron emission; it could be emission from even colder dust. But it also has the characteristics of the radiation you’d expect from a ‘wind’ of magnetised particles streaming from a pulsar.

Clinching evidence for a pulsar would be, of course, its pulses. These might now have become evident at radio wavelengths. “We are trying with Parkes [CSIRO’s Parkes radio telescope],” said CAASTRO Deputy Director Lister Staveley-Smith (UWA), one of the paper’s authors. “But there’s a lot of ionised material to look through.” The hunt continues.

Images of the remnant of supernova 1987A, made at wavelengths from radio to far infrared. (Figure 1 from Zanardo et al. 2014)

Left column, top to bottom: images of the remnant of supernova 1987A, made at different frequencies (94, 102, 213, 345 and 672 GHz). Middle column: the images after subtraction of the emission from cold dust. Right column: the images after subtraction of the main synchrotron emission. The white spot in each image shows the angular resolution. The green cross indicates the position of the supernova. (Figure 1 from Zanardo et al. 2014)

Giovanna Zanardo et al. “Spectral and morphological analysis of the remnant of supernova 1987A with ALMA and ATCA.” ApJ 796, 82 (2014) doi:10.1088/0004-637X/796/2/82