Extreme stars in the Milky Way

Researchers discover hundreds of new gamma-ray pulsars

November 28, 2023

An international team of astronomers has produced a new catalog of 294 gamma-ray emitting pulsars discovered in data from NASA's Fermi Gamma-ray Space Telescope, plus 34 suspects awaiting confirmation. This is 27 times the number known before the mission's launch in 2008. Researchers at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute) in Hannover, Germany, contributed 53 pulsars to the catalog. They made their discoveries by implementing novel, highly efficient data analysis methods on the Einstein@Home volunteer distributed computing project and the Atlas computing cluster.

“The sheer number of nearly 300 gamma-ray pulsars is a fantastic achievement and highlights the important role the Fermi Gamma-ray Telescope has played since it began observing our Universe,” says Colin Clark, co-author of the study and group leader at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute, AEI) in Hannover. “I’m very proud that one in six gamma-ray pulsars in the catalog was discovered by researchers at our institute.”

Observing pulsars advances the understanding of astronomy in several areas, including the search for gravitational waves and dark matter, cosmic rays, and the evolution of stars. The catalog, now published in The Astrophysical Journal Supplement, provides comprehensive information on all 294 known gamma-ray pulsars and 34 candidates.

Cosmic Lighthouses

Pulsars are a type of neutron star, the city-sized remnant of a massive sun that has exploded as a supernova. Containing more mass than our Sun in a ball less than 27 kilometers wide, neutron stars have strong magnetic fields, produce streams of energetic particles, and spin rapidly – up to 700 times per second. Pulsars also emit narrow beams of energy that sweep through space like lighthouses as the objects rotate. When these beams periodically pass by Earth, astronomers detect pulses of energy.

The new catalog represents the work of 170 scientists across the globe. Of the 300 pulsars reported in the catalog researchers at AEI Hannover have discovered 53. Using the Einstein@Home project and the institute’s large computer cluster Atlas, they have also teased out gamma-ray pulsars that have no radio counterparts. This required millions of hours of computer calculation, a process called a blind search.

This visualization shows 294 gamma-ray pulsars, first plotted on an image of the entire starry sky as seen from Earth and then transitioning to a view from above our galaxy.

Fermi’s pulsar sky

This visualization shows 294 gamma-ray pulsars, first plotted on an image of the entire starry sky as seen from Earth and then transitioning to a view from above our galaxy.

The volunteer distributed computing project Einstein@Home provided the necessary computing power by connecting tens of thousands of citizen scientists' computers around the world into a global supercomputer. The catalog also includes the latest 14 gamma-ray pulsars found by Einstein@Home. Details of their discoveries will be published in a forthcoming paper.

The Young and the Old

Of the 3,400 known pulsars, most of which are observable in radio waves and located in our Milky Way galaxy, only about 10% pulse in gamma rays, the most energetic form of light.

Astronomers distinguish different types of pulsars. Young pulsars are usually solitary objects, formed in a supernova explosion several thousand years ago. They typically spin a few to a few dozen times per second. Their emissions gradually slow down their rotation.

Paradoxically, however, pulsars that are thousands of times older spin much faster, up to hundreds of times per second, and are therefore called millisecond pulsars. “Almost half of the objects in our new catalog are fast-spinning millisecond pulsars. Before Fermi, we had never seen a millisecond pulsar in gamma rays,” says Lars Nieder, co-author of the paper and a researcher at AEI Hannover.

Observing millisecond pulsars in binary systems with a stellar partner provides clues to understanding the age-spin paradox. On its own, a pulsar's emissions slow it down, and as this happens, its emissions dim. But if it is closely paired with a normal star, the pulsar's gravity can pull in a stream of matter from its companion, which over time can spin the pulsar up.

Along Come the Spiders

“Spider” systems offer a glimpse of what happens next. They’re named for spiders known for consuming their mates: black widows have light companions (less than about 5% of the Sun's mass) while redbacks have heavier companions. As the pulsar spins up, its emissions and particle outflows become so energetic that, through processes still poorly understood, it heats up and slowly vaporizes its companion. The most energetic spiders may completely evaporate their companions, leaving only an isolated millisecond pulsar.

J1555–2908 is a black widow with a surprise – its gravitational web may have ensnared a passing planet. An analysis of 12 years of Fermi data led by AEI researchers reveals long-term spin variations much larger than those seen in other millisecond pulsars. “We think a model incorporating the planet as a third body in a wide orbit around the pulsar and its companion describes the changes a little better than other explanations, but we need a few more years of Fermi observations to confirm it,” says Clark.

The new catalog also includes about a dozen candidate spider systems. “These objects look a lot like spider pulsars, but we have yet to identify their gamma-ray pulsations,” Nieder explains. “Some of them may be worthwhile targets for searches with Einstein@Home. We look forward to discovering more such pulsars by using observations from large optical telescopes to focus the computational power of Einstein@Home.”

With Fermi's still-growing baseline of observations, new celestial gamma-ray sources are being found, and hundreds of them remain unidentified. Some of these may be new gamma-ray pulsars that, if discovered in the future, would find their way into the next version of Fermi's gamma-ray pulsar catalog.

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