Pure gamma-ray pulsars are difficult to identify because their characteristics, such as sky position, the period of rotation, and 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. This is the only way of finding a hidden periodicity in the arrival times of the gamma-ray photons.
Even high-performance computers quickly reach their limit in this process. Therefore, the researchers 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 Bruce Allen from the Albert Einstein Institute (AEI) in Hannover, Germany. Back in November 2011, Allen’s team announced the discovery of nine new Fermi gamma-ray pulsars that had escaped all previous searches. Now the scientists have made a new extraordinary find with the same methods.
The name of the newly discovered pulsar — J1838-0537 — 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 Holger Pletsch, a scientist in Allen’s group. “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 an international team led by Pletsch to solve the mystery of pulsar J1838-0537: It did not disappear, but experienced a sudden glitch after which it rotated 38-millionths 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,” said Allen. 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,” said Pletsch. 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 Lucas Guillemot from MPIfR in Bonn, Germany. “This is a good example of the collaboration of two Max Planck institutes with complementary research foci,” said Michael Kramer from the Fundamental Physics in Radio Astronomy research group.
After the discovery in data from the Fermi satellite, the researchers pointed the radio telescope in Green Bank, West Virginia, at the celestial position of the gamma-ray pulsar. In an observation of almost two hours, and by analyzing an even 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.
There were, however, noticeable overlays with observations of the High Energy Stereoscopic System (H.E.S.S.) in Namibia, which searches for high-energy gamma radiation from the depths of space. In a survey with H.E.S.S., astronomers found an extended source of this radiation near the now discovered pulsar, but have not yet been able to clarify its nature.
The discovery of the pulsar suggests that the H.E.S.S. source is a pulsar wind nebula. These are produced by particles moving at almost the speed of light, which the pulsar accelerates in its extremely strong magnetic field. Because the exact position of the pulsar is now known, H.E.S.S. can take this into account in the future and make more precise measurements than before in this celestial region.
The ATLAS computer cluster of the AEI has already assisted in the discovery of the tenth previously unknown gamma-ray pulsar; however, Allen’s team has meanwhile mobilized further computing capacity. “Since August 2011, our search has also been running on the distributed computing project Einstein@Home, which has computing power a factor of 10 greater than the ATLAS cluster. We are very optimistic about finding more unusual gamma-ray pulsars in the Fermi data,” said Allen. One goal of the expanded search is to discover the first gamma-ray-only pulsar with a rotation period in the millisecond range.