A black widow pulsar shows a possible connection to fast radio bursts

Illustration of PSR B1957+20 and its companion
Illustration ofIllustration of PSR B1957+20 and its companion

An article published in the journal “Nature” describes a high resolution observation of a pulsar cataloged as PSR B1957+20. A team of astronomers used data collected using the Arecibo radio telescope, obtaining one of the best results in the history of astronomy thanks to the presence of a trail of plasma left by a brown dwarf, a companion of the pulsar in a binary system. According to the astronomers, the lens effect generated suggests that it’s also the cause of fast radio bursts.

Approximately 6,500 light years from Earth, the pulsar PSR B1957+20 is one of the most massive known and has a rotation speed of more than 600 times per second. A pulsar is a neutron star, one of the possible remains of a star that exploded in a supernova, that’s rotating quickly.

This pulsar’s companion is a brown dwarf, an object at the boundary between star and planet with a diameter about a third of the Sun’s about 2 million kilometers (1.2 million miles) away from the pulsar. It has an orbital period around the pulsar of 9 hours and it’s tidally locked to it, which means that it always shows the same face to its companion, like the Moon to the Earth.

The brown dwarf has a surface temperature estimated around 6,000° Celsius (10,800° Fahrenheit) on the side facing its companion due to the strong radiation that hits it. The consequence is that the gas that forms the brown dwarf becomes plasma that expands considerably forming a sort of trail similar to a comet’s tails. That gas is slowly getting lost in space and this means that one day it will run out. That’s the reason why these pulsars that steal gas from their companions are nicknamed black widows.

A curious but very useful characteristics of the plasma emitted by the brown dwarf is that it acts like a magnifying glass and allows to see images of the pulsar 70-80 times larger. The image (courtesy Dr. Mark A. Garlick, Dunlap Institute for Astronomy & Astrophysics, University of Toronto. All rights reserved) shows an artistic representation of that lens with the pulsar in the background seen through the plasma cloud surrounding the brown dwarf, seen in the foreground.

This situation is useful to study the pulsar PSR B1957+20 but suggested the researchers an interesting possibility. Robert Main of the University of Toronto, the article’s lead author, explained that many properties observed in fast radio bursts (FRBs) could be explained if they were amplifications caused by plasma lenses, noting the similarities with the pulses coming from the pulsar PSR B1957+20 observed thanks to that type of amplification.

So far, astronomers cataloged a couple of dozens of fast radio bursts, radio pulses that last no more than a few thousandths of a second. Their nature has remained mysterious so far because it’s difficult to study so few events of such short duration. If Robert Main team’s hypothesis is right, things could change.

This study was conducted thanks to data collected using the Arecibo radio telescope before it was damaged by Hurricane Maria in September 2017. The hope is to be able to conduct follow-up observations of the pulsar PSR B1957+20 and obtain more data to test the theory on the connection with fast radio bursts.

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