Artist's impression of a black widow pulsar in a binary system. Credit: ESA

Magnetic Fields Play Huge Role in the Eclipsing Mechanism of Black Widow Pulsars

This phenomenon has been known since the 1980s, but its exact nature has not yet been clarified - eclipses can be created by matter ripped off by pulsar wind from a companion star, or material from the pulsar wind itself.


Astronomers using the FAST radio telescope saw a rare phenomenon in the PSR J1720-0533 binary system with a black widow pulsar – the lensing of radio emission from a neutron star by a plasma cloud in the system. This is only the fifth case of such observations. The size of the lens cloud is estimated at 2,300 kilometers. 

What is a black widow pulsar?

Black widow pulsars are an unusual subgroup of rapidly rotating neutron stars that reside in close binaries where a low-mass star acts like a companion. These compact objects acquired rotation periods of several milliseconds due to the accretion of matter from a companion star, while gradually destroying it due to the pulsar wind and high-energy radiation.


Thanks to this, they acquired their name in honor of the species of terrestrial spiders. It is possible that such systems are descendants of low-mass X-ray binaries after the termination of accretion of matter onto the pulsar.

Types of ‘spider’ pulsars

There are two types of spider pulsars, which differ in the masses of companion stars – 0.2-0.4 solar masses in the case of RB pulsars named after the redback spiders or the Australian black widow) and 0.01-0.05 solar masses in the case of BW pulsars (named after the common black widow).

Many of these objects exhibit an eclipse of radio emission when the companion star approaches the line of sight for the terrestrial observer, and the degree of eclipse of the pulsar’s radio pulses is related to its frequency.

This phenomenon has been known since the 1980s, but its exact nature has not yet been clarified – eclipses can be created by matter ripped off by a pulsar wind from a companion star, or material from the pulsar wind itself.


Black widow pulsar PSR J1720-0533

A group of astronomers led by Wang Shuangqiang of the Xinjiang Astronomical Observatory published the results of an analysis of data from observations of the black widow pulsar PSR J1720-0533 in the radio range using the ground-based 500-meter FAST radio telescope in August last year.

The pulsar’s rotation period is 3.26 milliseconds, it enters the system with an orbital period of 3.16 hours, and its companion is a star with a mass of 0.034 solar masses. The system lies 622 light-years from the Sun.

The researchers noticed that during the initial phase of the eclipse, quasi-periodic variations in the emission of radio pulses with a modulation period of 22 seconds were observed, which were not recorded during the final phase of the eclipse. These results can be explained by the phenomenon of lensing of radiation by a plasma cloud, 2300 kilometers in size.

It is only the fifth time astronomers observe this phenomenon

Similar phenomena have been observed for only four spider pulsars. In addition, observational data indicate that the eclipsing medium has a magnetic field of 8 gausses. Scientists also calculated the rate of mass loss of the companion star under the influence of the pulsar wind, which was 10-12 solar masses in a year. In this case, the star will be completely destroyed in 1010 years.

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Corongiu, A., Mignani, R. P., & Seyffert, A. S. (2020, November 23). Radio pulsations from the γ-ray millisecond pulsar PSR J2039–5617. OUP Academic.
Garner, R. (2015, February 24). ‘spidery’ pulsars consume their mates. NASA.
Kansabanik, D. (n.d.). Unraveling the Eclipse Mechanism of a Binary Millisecond Pulsar Using Broadband Radio Spectra.
Nannan, Z. (2021, December 30). Plasma lensing discovered in Black widow pulsar.
Wang, S. Q. (n.d.). Unusual Emission Variations Near the Eclipse of Black Widow Pulsar PSR J1720−0533.

Written by Vladislav Tchakarov

Hello, my name is Vladislav and I am glad to have you here on Curiosmos. As a history student, I have a strong passion for history and science, and the opportunity to research and write in this field on a daily basis is a dream come true.

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