Strange slow-spinning neutron star discovered in ‘stellar graveyard’

BY DR HILARY KAY AND BEN ROBINSON (THE UNIVERSITY OF MANCHESTER)

An international team of scientists, led by members of the European Research Councilfunded MeerTRAP group at The University of Manchester in the UK, has discovered an unusual, very slow-spinning neutron star using South Africa’s MeerKAT telescope.

The star, named PSR J0901-4046, is rotating once every 76 seconds and is unexpectedly emitting radio pulses.

Neutron stars (also known as pulsars) are extremely dense remnants of the supernova explosion of a massive star. They produce beams of radio waves that sweep across the sky as they spin, producing regular flashes like cosmic lighthouses. However, slow rotation along with a strong magnetic field, like that seen with PSR J0901-4046, is thought to inhibit radio emission, casting uncertainty on the exact nature of the object.

Whilst the radio energy produced by PSR J0901-4046 is characteristic of a pulsar, the chaotic structure within the pulses and their polarisation is similar to that seen in magnetars, and the spin rate is more consistent with that of a white dwarf. One possibility, the team suggests, is that it may belong to a new class of ultra-long period neutron stars.

PSR J0901-4046 was first discovered serendipitously when a single pulse was detected by the MeerTRAP instrument, which was piggybacking on observations by the ThunderKAT project. Combining data from both projects enabled the teams to accurately locate the position of the neutron star, allowing for more detailed and sensitive follow-up observations.

The high sensitivity of MeerKAT observations, along with MeerTRAP’s ability to detect transients in real time and the simultaneous imaging from the ThunderKAT team, combined to make the discovery possible. In the case of PSR J0901-4046, radio emission was only detected for a tiny fraction (0.5%) of its rotation period and therefore detecting similar sources will be observationally challenging.

Dr Manisha Caleb, formerly from The University of Manchester and now at the University of Sydney, Australia, who led the research said: “It is likely there are many more of these very slowly spinning sources in the galaxy which has important implications for how neutron stars are born and age. The majority of pulsar surveys do not search for periods this long and so we have no idea how many of these sources there might be.”

For now, the exact mechanisms behind the radio emission from PSR J0901-4046 remain a mystery. But with the unprecedented sensitivity of the next generation of radio telescopes like the SKA, coupled with the innovative techniques like those employed by scientists in the MeerTRAP team, previously unseen new classes of radio transients will be uncovered, advancing our understanding of the relationship between neutron stars, ultra-long period magnetars and fast radio bursts.

The paper, “Discovery of a radio-emitting neutron star with an ultra-long spin period of 76s”, is published in Nature Astronomy: https://www.nature.com/articles/s41550-022-01688-x

BELOW: Artist impression of the 76s pulsar (in magenta) compared to other more rapidly spinning sources.

Credit: Danielle Futselaar (artsource.nl)