Gamma-ray pulsar searches

Gamma-ray pulsar searches

This group's main research aspects are computing-intense searches for and studies of pulsars - rapidly spinning neutron stars - through gamma-rays in previously unaccessible parameter spaces using efficient data analysis and powerful computing resources.

A particularly exciting focus is on extending searches to parameter spaces that in fact have been inaccessible before on computational grounds. Achieving this requires the development of efficient data analysis methodologies and the exploitation of powerful computing resources, such as the Einstein@Home volunteer supercomputer.

Pulsars are some of the most extreme objects in our Universe and important key probes for a wide range of fundamental physics. Yet many aspects are still poorly understood after decades of observations, primarily at radio wavelengths. Now gamma-ray observations with NASA’s Fermi Large Area Telescope provide complementary windows of unprecedented opportunities to discover and study new pulsars.

Gamma-ray Pulsars

The entire sky as seen by the Fermi Gamma-ray Space Telescope and the 13 pulsars discovered by Einstein@Home that were published in January 2017. The field below each inset shows the pulsar name and its rotation frequency. The flags in the insets show the nationalities of the volunteers whose computers found the pulsars.

With the Large Area Telescope (LAT) on board NASA's Fermi satellite, for the first time neutron stars were detected via their periodic gamma-ray pulsations alone. Many of these stars are invisible in the radio band. Continuously surveying the gamma-ray sky, the LAT cataloged several hundred unidentified sources, among which many are thought to be pulsars. However, extracting new pulsars and important science from LAT data is computationally limited and almost impossible with conventional methods.

Blind searches for gamma-ray pulsars are a computational challenge. Long integration times are needed to detect regular pulsations in the photon arrival times. In blind searches for gamma-ray pulsars, the relevant pulsar parameters are unknown a-priori and must be explicitly searched. For observations spanning multiple years, this requires a dense grid covering a multi-dimensional parameter space, with a tremendous number of points to be individually tested.

The same computational problem issue affects blind searches for continuous waves in data from laser-interferometric gravitational-wave detectors. The signals have the same phase model and the same search parameter space, and in both cases the data span several years. This has motivated the transfer and adaption of gravitational-wave methods to the analysis of LAT data for new gamma-ray pulsars.

The unusual PSR J1311-3430 pulsar system with the first millisecond pulsar discovered solely by its lighthouse-like gamma-ray emissions (magenta). The record-breaking pulsar system is so compact that it would fit completely inside our Sun. This schematic representation shows the Sun, the companion's orbit, and the companion at its maximum possible size true to scale; the pulsar has been greatly enlarged in contrast.

With these novel search methods developed at the AEI in Hannover, the following exciting findings were made so far:

  • Discovery of 23 previously unknown gamma-ray pulsars, among which were the
  • discovery of gamma-ray pulsations from the second-fastest rotating radio pulsar,
  • discovery of the first radio-quiet millisecond pulsar,
  • discovery of PSR J1838-0537, which suffered the largest glitch yet seen in any gamma-ray pulsar. The discovery of the pulsars also explains a previously known TeV source, which appears to be the pulsar wind nebula powered by the pulsar,
  • discovery of the first millisecond pulsar J1311-3430 via its gamma-ray pulsations. This pulsar is also part of a record-breaking binary pulsar with an orbital period of only 94 minutes, the shortest of any rotation-powered pulsar.

Today, hundreds of LAT-cataloged sources remain unidentified, with a still growing number: some of these could be new types of gamma-ray pulsars. Their detection might require further extension and advancement of the search methods following approaches used in GW astronomy. In addition to the improved search techniques, the computational resources for these searches are significantly enhanced by using the volunteer computing system Einstein@Home.

Overall, the unprecedented search sensitivity from the combination of these advances warrants optimism for further gamma-ray pulsar discoveries.

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