Plasma-Lensing Effect Enables Observation of Pulsar 6500 Light-Years from Earth

Astronomers from the Dunlap Institute for Astronomy & Astrophysics have performed one of the highest-resolution observations in astronomical history by observing two intense regions of radiation, 20 km apart, around a star that is 6500 light-years away. The observation is equivalent to using a telescope on Earth to see a flea on the surface of Pluto.

The pulsar PSR B1957+20 (formerly known as the "Black Widow") is seen in the background through the cloud of gas enveloping its brown dwarf star companion. Courtesy of Mark A. Garlick, Dunlap Institute for Astronomy & Astrophysics, University of Toronto.

Astronomers used the 305-m-diameter William E. Gordon Telescope at the Arecibo Observatory in Puerto Rico to study the pulsar, informally known as the "Black Widow." The team mapped how the observed brightness of the pulsar changes on timescales as short as microseconds and found that the pulsar seems to be much brighter at certain points of its orbit — in particular, at the edges of the plasma cloud, just before and after the pulsar is eclipsed.

The astronomers hypothesize that the edges of the plasma cloud act as a lens, boosting the observed brightness of the pulsar for short periods.

In a plasma lens, radio waves are bent, and the waves arriving at an observer from different angles can overlap to produce a bright spot (known as a caustic). A similar effect with light can be seen at the bottom of a swimming pool on a sunny day.

“The gas is acting like a magnifying glass right in front of the pulsar,” said astronomer Robert Main. “We are essentially looking at the pulsar through a naturally occurring magnifier [that] periodically allows us to see the two regions separately.”

Main and colleagues also demonstrated that the plasma-lensing effect can be used to zoom in on the pulsar, by studying how the effect changes with time and with observed radio frequency.

The observation could also hold clues to the nature of phenomena known as fast radio bursts (FRBs).

“Many observed properties of FRBs could be explained if they are being amplified by plasma lenses,” said Main. “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.”

The research was published in Nature (doi:10.1038/s41586-018-0133-z).