An international team of astronomers using NASA’s Fermi Gamma-ray Space Telescope has detected a very special gamma-ray pulsar.
A gamma-ray pulsar is a compact neutron star that accelerates charged particles to relativistic speeds in its extremely strong magnetic field. This process produces gamma radiation, violet, far above the surface of the compact remains of the star, while radio waves, green, are emitted over the magnetic poles in the form of a cone. The rotation sweeps the emission regions across the terrestrial line of sight, making the pulsar light up periodically in the sky (NASA / Fermi / Cruz de Wilde)
The newly discovered pulsar J1838-0537 is radio-quiet, very young, and, during the observation period, experienced the strongest rotation glitch ever observed for a gamma-ray-only pulsar.
Pulsars are superlative cosmic beacons. These compact neutron stars rotate about their axes many times per second, emitting radio waves and gamma radiation into space. Pure gamma-ray pulsars are difficult to identify because their characteristics, such as its sky position, the period of rotation and its change in time, are unknown. And astronomers can only determine their approximate position in the sky from the original Fermi observations. They must therefore check many combinations of these characteristics in a blind search, which requires a great deal of computing time.
The team used algorithms originally developed for the analysis of gravitational-wave data to conduct a particularly efficient hunt through the Fermi data.
“By employing new optimal algorithms on our ATLAS computer cluster, we were able to identify many previously-missed signals,” said Dr Bruce Allen, Director of the Albert Einstein Institute (AEI) in Hannover, Germany, and co-author of the study that will be published in the Astrophysical Journal Letters.
The gamma-ray pulsar J1838-0537 is located towards the Scutum constellation, which is above the southern horizon in the summer sky. This detailed map shows the position of the pulsar, yellow, which is invisible to terrestrial telescopes, in a simulated view of the sky (Stellarium / AEI / Knispel)
The name of the new pulsar comes from its celestial coordinates. “The pulsar is, at 5,000 years of age, very young. It rotates about its own axis roughly seven times per second and its position in the sky is towards the Scutum constellation,” said lead author Dr Holger Pletsch of the AEI. “After the discovery we were very surprised that the pulsar was initially only visible until September 2009. Then it seemed to suddenly disappear.”
Only a complex follow-up analysis enabled the team to solve the mystery of pulsar J1838-0537: it did not disappear, but experienced a sudden glitch after which it rotated 38 millions of a Hertz faster than before. “This difference may appear negligibly small, but it’s the largest glitch ever measured for a pure gamma-ray pulsar,” Dr Allen said. And this behavior has consequences.
“If the sudden frequency change is neglected, then after only eight hours, a complete rotation of the pulsar is lost in our counting, and we can no longer determine at which rotational phase the gamma-ray photons reach the detector aboard Fermi,” Dr Pletsch explained. The ‘flashing’ of the neutron star then disappears. If the researchers take the glitch into account and correct the change in rotation, the pulsar shows up again in the observational data.
The precise cause of the glitches observed in many young pulsars is unknown. Astronomers consider ‘star quakes’ of the neutron star crust or interactions between the superfluid stellar interior and the crust to be possible explanations.
“Detecting a large number of strong pulsar glitches makes it possible to learn more about the inner structure of these compact celestial bodies,” said Dr Lucas Guillemot of the Max Planck Institute for Radio Astronomy in Bonn, the second author of the study.
After the discovery in data from the Fermi satellite, the researchers pointed the radio telescope in Green Bank at the celestial position of the gamma-ray pulsar. In an observation of almost two hours and by analyzing a further, older, one-hour observation of the source they found no indications of pulsations in the radio range, indicating that J1838-0537 is a rare gamma-ray-only pulsar.
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Bibliographic information: Pletsch HJ et al. 2012. PSR J1838-0537: Discovery of a young, energetic gamma-ray pulsar. Accepted for publication in the Astrophysical Journal Letters; arXiv: 1207.5333v1
