The only known repeating Fast Radio Bursts (FRBs) have only become more interesting by giving astronomers more clues about their home.
Fast Radio Bursts are flash-like bursts of radio energy with an extragalactic origin that only last a few milliseconds. Astronomers only know of about 30 FRBs since the discovery of the first one in 2007. They originally thought these intense bright bursts were caused by explosions, collisions, or collapses, until the first repeating FRB, FRB121102, was discovered by the Canadian Paul Scholz, while he was a PhD student at McGill University.
Last year, researchers discovered that FRB121102 is in a star-forming region of a dwarf galaxy, over 3 billion light years away from Earth. The extremely far distance of this source means that each burst contains a lot of energy, and perhaps each millisecond of these flash-like bursts contains as much energy as the Sun releases in an entire day. However, the physical nature of these FRBs remains unknown and mysterious, rendering them a hot topic in astronomy.
In a new study published in Nature, Daniele Michilli, a PhD Candidate from the University of Amsterdam and ASTRON, the Netherlands Institute for Radio Astronomy, and his international team of colleagues, present more clues about the source of the repeating FRB121102. This international team includes McGill University’s Victoria Kaspi and Shriharsh Tendulkar. At the recent American Astronomical Society meeting, the team revealed evidence for why they think FRB121102 comes from an extremely magnetic environment, suggesting that the source of this FRB is most likely located near a massive black hole.
Data from Arecibo Observatory in Puerto Rico and Green Bank Observatory in West Virginia reveal that these repeating radio bursts are highly polarized. When radio waves, or any light waves, are transmitted, they have an electric and a magnetic field. In polaridiscovering more FRBszed light, the electric field only points in one direction. “We use polarization every day, such as polarized glasses to remove annoying glare of the opposite polarization,” said Dr. Kaspi, a professor of physics at McGill University.
The team of astronomers have observed that these highly polarized radio waves are twisted, meaning that the angle of polarization changes depending on what radio frequency they are observing at. Twisted polarization is explained by the Faraday rotation, discovered by a British physicist named Michael Faraday. Faraday rotation is a magneto-optical effect caused as light travels through a magnetic environment.
“Sometimes you can have a little bit of Faraday rotation, but we saw this huge number in the units of 100,000, which you never hear of! It’s usually 10, 20, so when you say 100,000, it’s a lot,” Dr. Kaspi told SpaceQ in an interview. The huge amount of Faraday rotation is suggestive of an extreme magnetic field in a dense plasma. Previously, such highly magnetic plasmas had only been observed around a massive black hole near the center of the Milky Way.
The high Faraday rotation values of FRB121102, which show up as a twist, suggests that the source of these FRBs is located close to a massive black hole. Black holes themselves aren’t the cause of high magnetization. The region around black holes where material falls into them and gets heated to really high temperatures forms currents and jets. “A black hole alone wouldn’t have a magnetic field, but if you put stuff around it, magnetic field gets generated in them because of different plasma instabilities as they get heated so high when they fall in and towards the black hole,” says Dr. Kaspi.
Some astronomers believe that the cause of the bursts themselves are rapidly spinning neutron stars. Neutron stars are the small, dense, collapsed cores of large stars that die in supernovas. First, the neutron stars send out radio bursts, then when they propagate through magnetized regions, they get twisted. “You first need the source to make the bright radio waves, and then understand why they get twisted. You need two crazy things: a crazy neutron star to make the bursts in the first place, then the crazy black hole to cause all the rotation,” Dr. Kaspi told SpaceQ. “Neither of those crazy ideas are secure. These are hypotheses.”
Michilli and his team also speculate that the source of FRB121102 could be located in a powerful nebula or amongst the remains of a dead star, instead of close to a black hole, which could also explain the twist of these radio bursts. “It doesn’t mean that nature isn’t creative and doesn’t make magnetic fields in other areas. These FRBs could be coming from something that we don’t understand yet,” says Dr. Kaspi. What is known is that the source of these FRBs is in an extremely magnetic field, one that is at least 200 times stronger than the average magnetic field in our galaxy.
The source of FRB121102 emits a wide variety of bursts. “Every burst comes out looking different,” says Dr. Kaspi. This variation could be caused by the object that creates them, meaning the source emits each burst differently, or it could be that all bursts are emitted the same, but the signals gets distorted as they travel from the source to us.
Since FRB121102 is the only known repeating FRB, astronomers are still unsure if it shares a similar origin to the non-repeating FRBs. Although astronomers have caught a glimpse of FRB121102’s environment, the true physical nature of all discovered FRBs still remains a mystery. New telescopes could hold the answer to all of these questions. The CHIME Telescope (Canadian Hydrogen Intensity Mapping Experiment) in Penticton, British Columbia, is promising for discovering more FRBs.
Currently, the naming system for FRBs is the term “FRB” followed by the discovery date. For example, FRB121102 was discovered on November 02, 2012. Astronomers might need to change their naming system soon, since CHIME is capable of detecting multiple FRBs every day!
One of FRB 121102’s radio bursts, as detected with the Arecibo telescope. The colour panel shows the brightness of the burst as a function of radio frequency and time, whereas the curve above shows the brightness of the burst summed across all observed radio frequencies. This movie illustrates how detecting the bursts at the highest possible time resolution has been critical in resolving their complex structures. Credit: Andrew Seymour, NAIC, Arecibo.