A group of astronomers at the University of Toronto in Canada has performed one of the highest resolution observations in the history of astronomy by observing two intense regions of radiation around PSR B1957+20, a so-called ‘black widow’ pulsar located in the northern constellation of Sagitta, approximately 6,500 light-years from Earth. Their results appear in the journal Nature.
PSR B1957+20 is seen in the background through the cloud of gas enveloping its brown dwarf companion. Image credit: Mark A. Garlick / Dunlap Institute for Astronomy & Astrophysics, University of Toronto.
First discovered in 1988 by Princeton University astronomers, PSR B1957+20 is an eclipsing millisecond pulsar.
It orbits with its low-mass companion — a sub-stellar object called a brown dwarf — with a period of 9.2 hours.
“PSR B1957+20 is a neutron star that rotates rapidly — over 600 times a second,” said lead author Robert Main of the University of Toronto’s Dunlap Institute for Astronomy and Astrophysics and his colleagues.
“As the star spins, it emits beams of radiation from the two hotspots on its surface. The intense regions of radiation being observed are associated with the beams.”
“The brown dwarf companion is about a third the diameter of the Sun and is roughly 1.2 million miles (2 million km) from the pulsar. It is tidally locked to the pulsar so that one side always faces its pulsating companion, the way the Moon is tidally locked to the Earth.”
“Because it is so close to the pulsar, the brown dwarf is blasted by the strong radiation coming from its smaller companion. The intense radiation from the pulsar heats one side of the relatively cool dwarf star to the temperature of our Sun, or some 10,800 degrees Fahrenheit (6,000 degrees Celsius).”
“The blast from the pulsar could ultimately spell its companion’s demise. Pulsars in these types of binary systems are called ‘black widow’ pulsars. Just as a black widow spider eats its mate, it is thought that the pulsar, given the right conditions, could gradually erode gas from the dwarf star until the latter is consumed.”
The gas is acting as like a magnifying glass right in front of the pulsar, the team found.
“We are essentially looking at the pulsar through a naturally occurring magnifier which periodically allows us to see the two regions separately,” Main said.
In addition to being an observation of incredibly high resolution, the team’s result could be a clue to the nature of mysterious phenomena known as fast radio bursts (FRBs).
“Many observed properties of FRBs could be explained if they are being amplified by plasma lenses,” Main said.
“The properties of the amplified pulses we detected in our study show a remarkable similarity to the bursts from the repeating FRB, suggesting that the repeating FRB may be lensed by plasma in its host galaxy.”
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Robert Main et al. 2018. Pulsar emission amplified and resolved by plasma lensing in an eclipsing binary. Nature 557: 522-525; doi: 10.1038/s41586-018-0133-z
