Einstein@Home discovers first millisecond pulsar visible only in gamma rays
(28 February 2018 - Albert Einstein Institute Hannover) The distributed computing project Einstein@Home aggregates the computing power donated by tens of thousands of volunteers from across the globe. In a survey of the gamma-ray sky, this computer network has now discovered two previously unknown rapidly rotating neutron stars in data from the Fermi gamma-ray space telescope.
While all other such millisecond pulsars have also been observed with radio telescopes, one of the two discoveries is the first millisecond pulsar detectable solely through its pulsed gamma-ray emission. The findings raise hopes of detecting other new millisecond pulsars, e.g., from a predicted large population of such objects towards the center of our Galaxy. Scientists from the Max Planck Institute for Gravitational Physics in Hannover and the Max Planck Institute for Radio Astronomy in Bonn closely collaborated to enable the discoveries.
The entire sky as seen by the Fermi Gamma-ray Space Telescope and the two pulsars discovered by Einstein@Home that were now published. The field below each inset shows the pulsar name and some of its measured characteristics, as well as the measured gamma-ray pulsations and radio pulsations (if detected). The flags in the insets show the nationalities of the volunteers whose computers found the pulsars. (courtesy: Knispel/Clark/Max Planck Institute for Gravitational Physics/NASA/DOE/Fermi LAT Collaboration)
“We made these two new discoveries in our large-scale Einstein@Home gamma-ray pulsar survey. This feat was only possible by using novel and more efficient search methods, improved Fermi Large Area Telescope (LAT) data, and the huge computing power provided by Einstein@Home,” says Dr. Colin Clark from Jodrell Bank Centre for Astrophysics, lead author of the paper published in Science Advances, who was a doctoral student at the Max Planck Institute for Gravitational Physics when he made the discoveries. “After we found the two millisecond pulsars with Einstein@Home we pointed a large radio telescope at them and expected to find their pulsed radio emission. That was the case for every other millisecond pulsar known until then. To our surprise one of our discoveries remained entirely radio-quiet.”
This shows that these “blind” gamma-ray pulsar searches have the potential to discover a hitherto unknown population of radio-quiet millisecond pulsars. These might be behind other unidentified Fermi-LAT sources, or the gamma-ray glow seen towards the center of our Galaxy.
Born in supernova explosions
Neutron stars are compact remnants from supernova explosions and consist of exotic, extremely dense matter. They measure about 20 kilometers across and weigh more than our Sun. Because of their strong magnetic fields and fast rotation they emit beamed radio waves and energetic gamma rays similar to a cosmic lighthouse. If these beams point towards Earth during the neutron star's rotation, it becomes visible as a pulsating radio or gamma-ray source – a so-called pulsar.
Millisecond pulsars form when a pulsar is spun up by accreting matter from a companion star. The inflow of material from the partner star can accelerate the pulsar up to hundreds of rotations in a single second. Once the accretion ends, the rapidly rotating neutron star can be observed as a millisecond pulsar.
Two new millisecond pulsars
The new publication describes the discovery of two previously unknown gamma-ray pulsars, which are called PSR J1035−6720 and PSR J1744−7619 after their respective sky positions. The first of the two city-sized neutron stars spins a dazzling 348 times each second, and the latter 213 times each second. After the initial discoveries, their astrophysical parameters were refined by a re-analysis of the Fermi-LAT data.
These improved parameters were used to search for the radio pulsations of the two sources in archival radio telescope data and in new observations with the Parkes Radio Telescope. While PSR J1035−6720 was discovered as an unusually faint radio millisecond pulsar, no radio waves at all were detected from PSR J1744−7619. This makes it the first radio-quiet millisecond pulsar ever discovered.
A hidden pulsar population
It is possible that the lighthouse-like radio beams of PSR J1744−7619 do not point towards Earth, while the gamma-ray beams do. The researchers addressed this question by comparing the observed gamma-ray emission with theoretical models. They showed that the models that describe the gamma-ray emission well, predict a detectable radio signal. Its absence means that PSR J1744−7619 must either be extremely radio-faint, or that the models must be incomplete.
According to some predictions, the observed excess of high-energy gamma-radiation from the central region of the Milky Way could be due to a hidden population of thousands of millisecond pulsars. While only a handful might be detectable with current large radio telescopes, gamma-ray searches might have a better chance of finding more of these sources.
Einstein@Home searches for gamma-ray pulsars hidden in binary systems
“With the help of our volunteers we searched through 152 unidentified pulsar-like sources from the Fermi-LAT Catalog,” says Prof. Dr. Bruce Allen, director of Einstein@Home and director at the Max Planck Institute for Gravitational Physics in Hanover. “We have shown that 19 of these do not only look like pulsars, they in fact are pulsars and in some cases quite unusual to boot. Personally, I would bet that many of the remaining 133 are also pulsars, but hidden in binary systems, where they are more difficult to find. At the moment Einstein@Home is chasing after those binary pulsars and I hope we will soon find some.”
“This is a marvelous example of modern-day astrophysics: we use expertise from gravitational wave astronomy to cleverly analyze gamma-ray data in order to reveal sources that complement our knowledge from radio observations. Brilliant,” concludes Michael Kramer, director at Max Planck Institute for Radio Astronomy, head of its “Fundamental Physics in Radio Astronomy” research department, and co-author of the paper.