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Four new gamma-ray millisecond pulsars discovered

An international team of astronomers reports the detection of four new gamma-ray millisecond pulsars using the Murriyang radio telescope at the Parkes Observatory in Australia. The discovery was detailed in a research paper March 16 on the arXiv preprint server.

Pulsars are highly magnetized, rotating neutron stars emitting a beam of electromagnetic radiation. The most rapidly rotating pulsars, with rotation periods below 30 milliseconds, are known as millisecond pulsars (MSPs). Astronomers assume that they are formed in binary systems when the initially more massive component turns into a neutron star that is then spun up due to accretion of matter from the secondary star.

Now, a group of astronomers led by Matthew Kerr of the Naval Research Laboratory (NRL) in Washington, DC, has detected four new MSPs with spin periods below four milliseconds. The discovery was made using the Murriyang radio telescope as part of the search for gamma-ray sources.

"We discovered four millisecond pulsars in searches of 80 gamma-ray sources conducted from 2015 to 2017 with the Murriyang radio telescope of the Parkes Observatory," the researchers wrote in the paper.

The observations carried out between 2015 and 2017 initially identified 15 targets that were further investigated. Four of them were classified as candidate MSPs. However, their nature could not be confirmed until now due to insufficient data to produce good timing solutions.

Kerr's team has now managed to obtain improved timing solutions and detected gamma-ray pulsations from all these four MSPs, which are designated PSR J0646−5455, PSR J1803−4719, PSR J2045−6837, and PSR J1833−3840.

PSR J1833−3840 is the only eclipsing pulsar out of the four and was classified as a "black widow" as it has a semi-degenerate companion with a mass of less than 0.1 solar masses. PSR J1833−3840 has the longest known orbital period among "black widows"—0.9 days.

The other three MSPs are binaries with white dwarfs in nearly circular orbits. The longest orbital period out of the three was observed in PSR J1803−4719—90.44 days, while PSR J2045−6837 has the shortest—5.17 days.

According to the paper, the newly found gamma-ray MSPs have spin periods between 1.86 and 3.67 milliseconds, while their dispersion measures range from 21.07 to 78.6 pc/cm³. The spin-down luminosities of the pulsars were measured to be between 4.5 and 108 decillion erg/s.

The authors of the study noted that PSR J0646−5455 has a strong, Vela-like gamma-ray pulse profile with two caustic-shaped gamma-ray peaks. They added that the gamma-ray brightness of this pulsar, along with a spin period of approximately 2.5 milliseconds and narrow pulse profile, make it suitable for high-precision timing.