Indian astronomers have carried out long-term radio observations of a “black widow” millisecond pulsar known as PSR J1544+4937. The results of the observation campaign, published November 25 on the pre-print server arXiv, shed more light on the properties of this pulsar.
The fastest rotating pulsars, those with rotation periods of less than 30 milliseconds, are called millisecond pulsars (MSP). The researchers speculate that they form in binary systems when the initially most massive component transforms into a neutron star which is then rotated due to the accretion of material from the secondary star.
A class of extreme binary pulsars with semi-degenerate companion stars are called “spider pulsars”. These objects are further categorized as “black widows” if the companion star has extremely low mass (less than 0.1 solar masses), while they are referred to as “red backs” if the secondary star is heavier. .
Discovered by the Giant Meterwave Radio Telescope (GMRT) in 1991, PSR J1544+4937 is a black widow MSP with a rotation period of 2.16 milliseconds. The pulsar is in a binary system with an orbital period of 2.9 hours orbiting a low-mass companion star (with a mass of at least 0.017 solar masses).
In a bid to unveil more information about the properties of PSR J1544+4937, a team of astronomers led by Sangita Kumari of the Tata Institute for Basic Research in India conducted an 11-year monitoring campaign of this source in using GMRT and the Green Bank Telescope (GBT).
“The majority of the observations reported in this article were made with the GMRT, which is an interferometric radio array consisting of 30 dishes, each 45 meters in diameter… In 2012, we used the Green Bank Telescope (GBT) to make multiple observations of PSR J1544+4937, to help determine the system’s orbit,” the researchers wrote.
First, the team measured the total proper motion of PSR J1544+4937, which was found to be around 10.14 mas per year. This allowed astronomers to calculate the 2−D transverse velocity of this pulsar using two galactic electron density models, namely NE2001 and YMW16. This speed was found to be at a level of 140 and 58 km/s for NE2001 and YMW16, respectively. The distance to PSR J1544+4937 is estimated to be 3,900 (NE2001) or 9,450 light-years (YMW16).
The dispersion measurement of PSR J1544+4937 was measured at approximately 23.22 pc/cm3 and its characteristic age has been estimated at some 42 billion years. The study also revealed that the pulsar has a surface magnetic field at a level of 42 million Gauss and the rate of energy loss from this source is about 12 decillion erg/s. The mass of the companion star has been calculated to be 0.0196 solar masses.
The search detected temporal variations in the dispersion measurement of PSR J1544+4937. In addition, the results also indicate a frequency-dependent measure of dispersion, the variations of which, according to the authors of the paper, could arise due to spatial variations in electron density in the interstellar medium.
Sangita Kumari et al, Decade-long study of the Black Widow millisecond pulsar J1544+4937, arXiv (2022). DOI: 10.48550/arxiv.2211.14107
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