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By analyzing the data from NASA's Fermi Gamma-ray Space Telescope, Russian astronomers have detected anti-glitches in a gamma-ray pulsar designated PSR J1522-5735. The discovery, September 28 on the pre-print server arXiv, makes PSR J1522-5735 one of the only few known anti-glitching gamma-ray pulsars.

Pulsars are highly magnetized rotating neutron stars emitting beams of electromagnetic radiation. Observations show that many pulsars exhibit sudden step-like increase of rotation frequency, known as glitches. However, in four pulsars, astronomers have detected anti-glitches—sudden spin-downs in the rotation frequency.

Discovered in 2013, PSR 1522-5735 is a radio-quiet gamma-ray rotation-powered pulsar in the Galactic plane, with a spin period of approximately 204 milliseconds. It has a surface magnetic field at a level of about 3.61 TG, spin-down power of 289 decillion erg/s, and its characteristic age is estimated to be 51,800 years.

Now, a team of astronomers led by Alexander Panin of the Institute for Nuclear Research of the Russian Academy of Sciences in Moscow, Russia, reports the detection of anti-glitches in PSR 1522-5735. The discovery was made based on the data from Fermi's Large Area Telescope (LAT) collected between August 2008 and April 2024.

"In this paper, we report on the timing analysis of over 15 years of Fermi-LAT data for the pulsar PSR J1522-5735. We detected eight glitch events, six of which were categorized as anti-glitches," the researchers wrote.

According to the study, the six anti-glitches in PSR J1522-5735 manifest as step-like changes in frequency and frequency time derivative with no post-glitch recovery terms. However, the astronomers added that a rapid recovery following anti-glitch within a few days cannot be completely ruled out due to the lack of comprehensive gamma-ray data.

All the detected anti-glitches turned out to be radiatively quiet, exhibiting no significant variations in the shape of the pulse profile or energy flux. Therefore, the authors of the paper assume that an internal mechanism is responsible for these phenomena in PSR J1522-5735.

The identification of radiation-quiet anti-glitches in a rotation-powered pulsar like PSR J1522-5735 supports the hypothesis that their potential origin may be in the neutron star's interior. Further observations, focused on the search for more anti-glitching pulsars, are required in order to confirm this assumption.

Summing up the results, the astronomers noted that PSR J1522-5735 is the newest addition to the short list of anti-glitching pulsar and that the two glitches detected besides the anti-glitch events appear to be a regular spin-up glitch and a spin-up glitch over-recovered to a spin-down.

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