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Using the Five-hundred-meter Aperture Spherical radio Telescope (FAST) in China, astronomers have discovered a new pulsar with a spin period of about two seconds. The newly detected pulsar, designated PSR J1922+37, was found in the direction of open cluster NGC 6791. The finding was reported in a paper Dec. 11 on the arXiv preprint server.

Pulsars are highly magnetized, rotating neutron stars emitting a beam of electromagnetic radiation. They are usually detected in the form of short bursts of radio emission; however, some of them are also observed via optical, X-ray and gamma-ray telescopes.

To date, more than 300 pulsars have been identified in globular clusters (GCs) in the Milky Way. However, no pulsar has been found in an open cluster (OC). This is most likely due to the lower stellar density and thus a lower encounter rate in OCs.

This may change with the discovery of PSR J1922+37 made by a team of astronomers led by Xioa-Jin Liu of the Beijing Normal University in China. The new pulsar was found in the line-of-sight of the old open cluster NGC 6791.

"Our first discovery is a 1.9-second pulsar (J1922+37) found in the direction of the old open cluster NGC 6791. (...) The small angular separation and the close distance predicted by the NE2001 model make PSR J1922+37 a likely member of NGC 6791," the researchers wrote in the paper.

PSR J1922+37 has a spin period of 1.92 seconds and a dispersion measure of about 85 pc/cm³. The flux density of this pulsar was calculated to be approximately 7.0 µJy.

By analyzing the position of PSR J1922+37, Liu's team found that the pulsar is in the same direction as NGC 6791 with an offset of about 14 arcminutes from the cluster center. The distance to PSR J1922+37 was measured to be about 15,600 light years, which is consistent with the cluster distance, estimated to be between 13,100 and 16,000 light years.

Based on these results, the astronomers concluded that PSR J1922+37 is likely a member of NGC 6791, which—if confirmed—will make it the first pulsar detected in an open cluster. However, more studies, including exact measurements of pulsar distance and proper motion, should be conducted in order to validate this assumption.

"There are several ways to check if PSR J1922+37 is actually associated with NGC 6791. First, a precise measurement of the distance to the pulsar and the open cluster will give a straightforward answer to the problem. (...) Second, if PSR J1922+37 is associated with NGC 6791, then both objects should have similar proper motion," the authors of the paper wrote.

Summing up the results, the researchers noted that NGC 6791 has a relatively high surface brightness and compactness, indicating a crowded stellar environment. This gives hopes that NGC 6791 harbors more pulsars. Liu's team estimates that even nine such objects are expected to be detected in this cluster.

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