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,” says Lucas Guillemot from the Max Planck Institute for Radio Astronomy in Bonn, the second author of the study. “This is a good example of the collaboration of two Max Planck institutes with complementary research foci,” says Michael Kramer, Director and Head of 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/USA at the celestial position of the gamma-ray pulsar. In an observation of almost two hours and by analysing 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.
There were, however, noticeable overlays with observations of the High Energy Stereoscopic System (H.E.S.S.) in Namibia, which searches for very-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. Since the exact position of the pulsar is now known, H.E.S.S. can take this into account in the future and to make more precise measurements than before in this celestial region.
The ATLAS computer cluster of the Albert Einstein Institute has thus already assisted in the discovery of the tenth previously unknown gamma-ray pulsar; however, Allen’s team has meanwhile mobilised 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 ten greater than the ATLAS cluster. We are very optimistic about finding more unusual gamma-ray pulsars in the Fermi data,” says Bruce Allen. One goal of the expanded search is to discover the first gamma-ray-only pulsar with a rotation period in the millisecond range.
These cosmic beacons are compact neutron stars, born in supernova explosions, which rotate rapidly and steadily about their axis. Their intense magnetic field causes them to emit radio waves and gamma radiation. Their rotation sweeps the emission regions through space like the beam from a lighthouse. If the neutron star points towards Earth, it is visible as a pulsar. Not all pulsars show up in multiple spectral ranges. In some cases, the scientists measure only the flashing as a radio pulsar; in other cases, only the periodic arrival times of gamma photons can be registered. The latter type of pulsar is called a gamma-ray-only pulsar. The most likely cause for the different pulsar types are the different orientations of the emission regions in the extremely strong magnetic field of the neutron star.
But pulsars are even more mysterious: when they are young, their steady rotation is irregular, and disturbed by sudden, jerky accelerations known as glitches. Only about five percent of the pulsars exhibit this behaviour. In such a glitch, the neutron star suddenly rotates faster, then slowly decelerates again and returns to the old rotation period a few weeks later. Astronomers do not yet know why this happens, but accurate measurements of these glitches provide insights into the structure of the compact celestial bodies.
To date, astronomers have found most pulsars in the radio wave range, but thanks to NASA’s Fermi satellite they are finding more and more of these celestial bodies via their high-energy gamma radiation. Fermi has been observing the universe with its Large Area Telescope (LAT) in the gamma-rays since 2008. It has discovered hundreds of new sources, many of which are probably undiscovered pulsars.