First pulsar discovered by the GCGPS survey

An international team of astronomers reports the detection of the first pulsar by the Globular Clusters GMRT Pulsar Search (GCGPS) survey. The pulsar, designated PSR J1617−2258A, was found in the globular cluster NGC 6093. The discovery was detailed in a research paper published Feb. 13 on the arXiv pre-print server.

Pulsars are highly magnetized, rotating neutron stars emitting a beam of electromagnetic radiation from their poles, most commonly in the form of radio waves. 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.

Due to their large stellar densities, globular clusters (GCs) are perceived as excellent places for the formation of MSPs. Their stellar densities are so large that many neutron stars can acquire a companion via binary exchange encounters.

One of the surveys dedicated to pulsar search in GCs is GCGPS. Its main aim is to identify MSPs in the Galactic GCs using the upgraded Giant Metrewave Radio Telescope (uGMRT). GCGPS utilizes uGMRT’s unique seamless low-frequency coverage (300–850 MHz) to look for steep-spectral-index radio sources like MSPs.

Now, a group of astronomers led by Jyotirmoy Das of the National Center for Radio Astrophysics (NCRA) in India, reports the first promising results from GCGPS.

“In this paper, we present the discovery and follow-up study of the first pulsar from this survey, PSR J1617−2258A,” the scientists wrote.

According to the study, PSR J1617−2258A is a binary MSP with a spin period of approximately 4.32 milliseconds and a dispersion measure of 66.8 pc/cm³. The pulsar was found in NGC 6093 and is therefore the first such object identified in this GC.

The study found that PSR J1617−2258A has an orbital period of approximately 18.94 hours and an orbital eccentricity of 0.54. This makes it one of the most compact and eccentric binary MSPs known to date.

The mass of the pulsar was calculated to be not greater than 1.6 solar masses, while the minimum mass of the companion was estimated to be 0.07 solar masses, which yields the total mass of the system at a level of about 1.67 solar masses.

Based on the findings, the astronomers conclude that PSR J1617−2258A is likely a perturbed MSP-helium white dwarf system seen at a low orbital inclination. They assume that the white dwarf progenitor spun up the pulsar; however, they also take into account the possibility that the system might have formed in an exchange interaction.

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