Using data from NASA’s Fermi Space Telescope and the enormous computing power of the citizen science project Einstein@Home, astronomers have found that PSR J2039-5617, a gamma-ray pulsar rotating 377 times a second, is in orbit with a stellar companion with a mass of about 0.15 solar masses.
An artist’s impression of PSR J2039-5617 and its companion. Image credit: B. Knispel / C.J. Clark / Max Planck Institute for Gravitational Physics / NASA’s Goddard Space Flight Center.
Millisecond pulsars are ancient neutron stars that have been spun-up to millisecond rotation periods by the accretion of matter from an orbiting companion star.
The most compelling evidence for this ‘recycling’ scenario comes from the discovery of three transitional millisecond pulsars, which have been seen to switch between rotationally powered millisecond pulsar and accretion-powered low-mass X-ray binary states.
In their rotationally powered states, these systems all belong to a class of interacting binary millisecond pulsars known as ‘redbacks,’ which are systems containing an millisecond pulsar in orbit with a low-mass companion star.
Redbacks, and the closely related ‘black widows,’ are named after species of spiders in which the heavy females have been observed to consume the smaller males after mating, reflecting the fact that the lighter companion stars are being destroyed by the pulsar’s particle wind and/or intense high-energy radiation.
Until recently, only a handful of these ‘spider’ systems had been found in surveys of the Milky Way. This is most likely due to the ablation phenomenon, which gives redbacks and black widows their nicknames: plasma from the companion can eclipse, scatter, and disperse the pulsar’s radio pulsations for large fractions of an orbit, causing these objects to be easily missed in radio pulsar surveys.
“It had been suspected for years that there is a pulsar, a rapidly rotating neutron star, at the heart of the source we now know as PSR J2039-5617,” said Lars Nieder, a Ph.D. student at the Max Planck Institute for Gravitational Physics.
“But it was only possible to lift the veil and discover the gamma-ray pulsations with the computing power donated by tens of thousands of volunteers to Einstein@Home.”
PSR J2039-5617 has been known since 2014 as the bright, pulsar-like gamma-ray source 3FGL J2039.6-5618.
All evidence obtained so far pointed at a rapidly rotating neutron star in orbit with a low-mass star being at the heart of the source. But clear proof was missing.
The first step to solving this riddle was to observe the stellar companion using optical telescopes.
The observations provided precise knowledge about the binary system without which a gamma-ray pulsar search would be unfeasible.
The system’s brightness varies during an orbital period depending on which side of the neutron star’s companion is facing the Earth.
“For PSR J2039-5617, there are two main processes at work,” said Dr. Colin Clark, an astronomer at the Jodrell Bank Centre for Astrophysics.
“The pulsar heats up one side of the light-weight companion, which appears brighter and more bluish.”
“Additionally, the companion is distorted by the pulsar’s gravitational pull causing the apparent size of the star to vary over the orbit.”
“These observations allowed our team to get the most precise measurement possible of the binary star’s 5.5-hour orbital period, as well as other properties of the system.”
The findings were published in the Monthly Notices of the Royal Astronomical Society.
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C.J. Clark et al. 2021. Einstein@Home discovery of the gamma-ray millisecond pulsar PSR J2039–5617 confirms its predicted redback nature. MNRAS 502 (1): 915-934; doi: 10.1093/mnras/staa3484
