Second Discovery of a White Dwarf Pulsar Reinforces Prior Models
Pulsars are a class of celestial bodies, typically associated with Neutron stars. These are characterised by the so-called lighthouse effect. The poles of a pulsar are subject to two intense jets of high energy electromagnetic radiation. If the star has a high enough obliquity, as seen by an observer, these jets will will sweep across the observer, resulting in the measurement of periodic pulses of intensity. A commonly accepted model describes charged particles trapped within the magnetic field being accelerated as they approach the magnetic poles of the star. This results in the seen emission, via synchrotron radiation.
2016 marked the first discovery of a white dwarf exhibiting the properties of a pulsar, with a period of 1.97 mins and temperature of ~20,000K. This star became known as AR Scorpii. The mechanism by which this occurs is yet to be concretely determined, as white dwarves should not have magnetic fields intense enough to allow for the same pulsar mechanism.
The leading description that aims to explain this was published in 2021 by in Schreiber et al. It postulates that the star is initially without a magnetic field, gravitationally accreting material from its companion, and raising its angular momentum, and causing high rates of rotation. As the star cools, its core crystallises, resulting in a solid inner core surrounded by a liquid mantle, undergoing convection. This is known as the dynamo model, and accounts for the presence of a magnetic field. Strong enough fields lead to disruption of the binary system, separating the two, and temporarily ceasing the mass transfer. This results in the binary pulsar systems we see today.
Last year, a team of astronomers, led by Dr Ingrid Pelisoli, identified a further white dwarf pulsar, J191213.72-441045.1, found within a binary system with a red dwarf. The average temperature of a white dwarf has been found to lie in the order of 106 K and the typically rotation period is in the order of days. J191213.72-441045.1 was found to only have a temperature of ~13,000K, well below the norm. Additionally, the period with which pulses were detected lay at around 5.3 mins, highlighting a far shorter rotational period than is standard.
The team aimed to identify systems of stars with properties similar to those observed within AR Scorpii. Candidates were directly measured photometrically with a system referred to as ULTRACAM on the 3.58 m New Technology Telescope at the la Silla observatory in Chile. Using this, they first identified the short pulses emitted by J191213.72-441045.1, leading the team to probe the star further.
The discovery provides further support for the descriptions outlined by the dynamo model, as the newly discovered system shares a striking similarity to that of AR Scorpii, providing robust security to the descriptions outlined by Schreiber et al. The discovery sheds light not only on the behaviour and nature of white dwarves as pulsars but exhibits implications for our understanding of stellar evolution in binary systems, particularly towards the end of a star’s lifecycle.
--
Journal Source: I. Pelisoli et al, A 5.3-min-period pulsing white dwarf in a binary detected from radio to X-rays, Nature Astronomy, https://doi.org/10.1038/s41550-023-01995-x, 2023
M. Schreiber et al, The origin and evolution of magnetic white dwarfs in close binary stars, Nature Astronomy, https://doi.org/10.1038/s41550-021-01346-8, 2021
Cover Image: Dr Mark Garlick//University of Warwick