A crack team of South African astronomers in collaboration with their international counterparts have discovered a rare and unique type of white dwarf “pulsar”. The South African team comprised researchers from the South African Astronomical Observatory (SAAO), the University of Cape Town (UCT) and the South African Radio Astronomy Observatory (SARAO). Some members of the research team were also part of the discovery of the nature of the very first such system known as AR Scorpii and found in 2016.
Using highly advanced technology
To achieve this pioneering feat, the team relied on a number of different cutting-edge telescopes which made it easy for them to observe this new object. Some of the telescopes used included the optical telescopes hosted at the Sutherland site of the SAAO, including the 10-m diameter Southern African Large Telescope (SALT).
Considered the only second one known pulsar, the discovery of this rare spinning white star in a binary star, provides a new understanding of the role of magnetic fields in stellar evolution, explained SARAO in its website. White dwarfs are explained as small dense stars, typically the size of Earth but with a mass about 200,000 times greater, nearer to the mass of the Sun. In other words, a teaspoon of white dwarf material would weigh around 15 tons.
According to SARAO, white dwarfs form when a low-mass star, like the Sun, or even less massive, has burnt all its fuel, losing its outer layers. They offer insight into different aspects of star formation and evolution. White dwarfs begin their lives at extremely hot temperatures before cooling down over billions of years. They rotate rapidly and have strong magnetic – about a billion times the Earth’s magnetic field – and lash their stellar companion with powerful beams of charged particles and radiation. This causes the entire system to brighten and fade dramatically over the minutes-long rotation period of the white dwarf. The bulk of the energy of these systems is powered by the slowing down of the spinning white dwarf, due to the drag exerted by its strong magnetic field, added the observatory.
The dynamo model
The research team says the newly detected white dwarf pulsar, referred to (J1912-4410 for short), has an orbital period of just over 4 hours. However, its rotation period is just a little over 5 minutes long, which means it’s spinning about 270 times faster than the Earth does. Said Dr Ingrid Pelisoli, research fellow at the University of Warwick’s department of physics, and lead author: “The origin of magnetic fields is a big open question in many fields of astronomy, and this is particularly true for white dwarf stars. The magnetic fields in white dwarfs can be more than a million times stronger than the magnetic field of the Sun, and the dynamo model helps to explain why. The discovery of J1912−4410 provided a critical step forward in this field”.
Dr Pelisoli said they used data from a few different survey telescopes to find potential white dwarf pulsar candidates, focusing on those that had similar characteristics to AR Sco. After observing a couple of dozen candidates, she added, they found one that showed very similar light variations to AR Sco. They then followed these up with a number of other telescopes, both from the ground and space, said Dr Pelisoli.
MeerKAT radio telescope array
Professor David Buckley of the SAAO, UCT and the University of the Free State, including his SAAO and University of Johannesburg colleague, Professor Stephen Potter, led the South African optical observations. Sharing some of their observations Professor Buckley said: “Like for the case of AR Sco, we were able to show that J1912-4410 also shows pulsed polarization and pulsed spectral lines, confirming its white dwarf pulsar nature”. Their observations were done with the help of the MeerKAT radio telescope array, undertaken following a request from professors Patrick Woudt (UCT) and Buckley.
The radio light curve
Commented Professor Woudt: “These beautiful MeerKAT observations show how short repeat snapshot radio observations can reveal the strongly pulsed radio emission, as is demonstrated in these observations of J1912-4410”. Buckley added: “The radio light curve of J1912-4410 is remarkably different to AR Sco, with only a very brief period when the very narrow pulses are seen. In fact, in the first ~40 min observation we were lucky to catch them at all and if we’d not, then possibly the second longer 8-hour observation may not have happened.”
According to Dr Pelisoli, this new discovery is important as it supports the predictions made by the dynamo model. Due to their old age, the white dwarfs in the pulsar system should be cool. Their companions also have to be close enough that the gravitational pull of the white dwarf was, in the past, strong enough to steal mass from the companion, causing them to be fast spinning. All of those assumptions hold for the new pulsar found: the white dwarf is cooler than 13,000K, has a spin of around five minutes, and the gravitational pull and radiation from the white dwarf have a strong effect on the companion star, observed Dr Pelisoli in conclusion.