Using the Arecibo Observatory in Puerto Rico and the Green Bank Telescope in West Virginia, astronomers have gained new insight into the extreme home of a mysterious source of so-called fast radio bursts (FRB). According to the team, radio waves from the recently-discovered repeating FRB, named FRB 121102, are 100% linearly polarized, indicating that they come from an extragalactic source embedded in a strong magnetic field, like that around a massive black hole at the center of a galaxy.
The 305-m Arecibo telescope and its suspended support platform of radio receivers is shown amid a starry night. A flash from the FRB 121102 source is seen traveling toward the telescope. Image credit: Danielle Futselaar.
Fast radio bursts are brief, bright pulses of radio emission from distant but so far unknown sources, and FRB 121102 — discovered by Arecibo Observatory astronomers in November 2012 — is the only one known to repeat: more than 200 high-energy bursts have been observed coming from this source.
In 2017, researchers pinpointed the location of the FRB 121102 source and reported that it lies in a star-forming region of a dwarf galaxy more than 3 billion light years from Earth.
The vast distance to the source implies that it releases an enormous amount of energy in each burst — roughly as much energy in a single millisecond as the Sun releases in an entire day.
Now, using Arecibo and Green Bank data, Dr. Jason Hessels from the Netherlands Institute for Radio Astronomy (ASTRON) and the University of Amsterdam and his colleagues have shown that FRB 121102’s radio bursts have a property known as polarization.
The behavior of this polarized light allows them to probe the source’s environment in a new way and to ‘peer into the lair’ of the mysterious burster.
“Polarized light is likely familiar to anyone who has used polarized sunglasses to cut down on the glare of sunlight reflected off water,” the astronomers said.
“If polarized radio waves travel through a region with a magnetic field, the polarization gets ‘twisted’ by an effect known as Faraday rotation: the stronger the magnetic field, the greater the twisting.”
The radio bursts from FRB 121102 twist to such an extreme — more than 500 times greater any other fast radio burst observed to date — that Dr. Hessels and co-authors concluded the bursts must pass through a high magnetic field in dense plasma.
“The nearly 100% polarization of the radio bursts is unusual, and has only been seen in radio emissions from the extreme magnetic environments around massive black holes, such as those at the centers of galaxies,” they said.
The short bursts, which range from 30 microseconds to 9 milliseconds in duration, indicate that the source could be as small as 6 miles (10 km) across — the typical size of a neutron star.
Other possible sources are a magnetar interacting with the nebula of material shed when the original star exploded to produce the magnetar; or interactions with the highly magnetized wind from a rotating neutron star, or pulsar.
“At this point, we don’t really know the mechanism. There are many questions, such as, how can a rotating neutron star produce the high amount of energy typical of an FRB?” said team member Dr. Vishal Gajjar, a postdoctoral fellow at the University of California, Berkeley
“We estimate the magnetic field and gas density surrounding the blast source, and we can link them, for example, with a model involving a young magnetar — a neutron star with an especially large magnetic field – to the central engine that produces the bursts,” said team member Professor James Cordes, of Cornell University.
“The radio source and its environment are unique, which indicates a new type of object not seen previously.”
“The only known sources in the Milky Way that are twisted as much as FRB 121102 are in the Galactic center, which is a dynamic region near a massive black hole. Maybe FRB 121102 is in a similar environment in its host galaxy,” added Daniele Michilli, PhD candidate at the University of Amsterdam and ASTRON.
“However, the twisting of the radio bursts could also be explained if the source is located in a powerful nebula or supernova remnant.”
Details of the research are published in the journal Nature.
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D. Michilli et al. 2018. An extreme magneto-ionic environment associated with the fast radio burst source FRB 121102. Nature 553, 182-185; doi: 10.1038/nature25149
