Tuesday, January 10, 2017

Mysterious Radio Signals: The Sequel

Less than two months, ago we brought you the mysterious tale of fast radio bursts (FRBs), bright flashes of radio waves that last for just fractions of a second and most likely come from outside of our galaxy, but which we know little else about. Last week, the sequel to that story was released. In a press conference at the meeting of the American Astronomical Society, coordinated with a cover story in the journal Nature, astronomers announced that they had identified the origin of an FRB for the first time: a small, faint, dwarf galaxy more than 2.5 billion light years away. Companion papers have also been published in the journal Astrophysical Journal Letters (here, here, and here).

Gemini composite image of the field around FRB 121102 (indicated). The dwarf host galaxy was imaged, and spectroscopy performed, using the Gemini Multi-Object Spectrograph (GMOS) on the Gemini North telescope on Mauna Kea in Hawai'i. Data was obtained on October 24-25 and November 2, 2016.
Image Credit: Gemini Observatory/AURA/NSF/NRC.

“This detection really has broken open the gates of a new realm of science and discovery, using FRBs as tools to explore the universe,” said collaborator and astronomer Sarah Burke-Spolaor from West Virginia University (previously with the National Radio Astronomy Observatory).

The headlining FRB is “FRB 121102,” named after its detection date of November 2, 2012. We learned this year that it could have easily been named something else, though—the FRB repeats. Of the 20 or so FRBs detected, this is the first we’ve seen flash multiple times. It flashes often, varying in brightness and unpredictable in its timing. We don’t know what, why, or how, but the fact that the FRB repeats rules out some previously proposed models. In particular, it rules out causes related to massive explosions, since the source lives to burst another day.

Telescopes all around the world banded together to study FRB 121102 in 2016, once they learned it flashed repeatedly. Collections of radio telescopes joined forces and overcame significant technological challenges to sort through massive amounts of data in an effort to pinpoint the precise location where the burst originates. And they succeeded.

The FRB indeed originates from outside of our galaxy—really far outside of it. Optical observations show a faint smudge of a dwarf galaxy at the exact source of the bursts, presumed to be the host galaxy. The galaxy is 2.5 billion light years away, meaning the signals we are detecting today left the source 2.5 billion years ago; it takes that long to cover the distance between us, even at the speed of light.

Astronomers have also learned that the FRB is very close to a source of steady, faint radio waves in the same galaxy. It’s not clear if both the faint steady signal and the bright flashes come from the same object or from two different objects that are physically related. The researchers say this could indicate that the bursts are coming from a source embedded in a dense nebula or maybe an accreting black hole.

We’ve learned more. Observations also show that the radio sources are significantly offset from the center of their galaxy. Additionally, at the time the signals left the host galaxy, the galaxy was pretty small in size and mass. Evidence suggests that new stars were forming rapidly, so that could play into the formation of FRBs in some way.

Like all good mysteries, there are so many questions… How representative is FRB 121102 of the other FRBs we have seen? Of the total population of FRBs? Do all FRBs come from dwarf galaxies? What is the mechanism causing the burst? How is FRB 121102 related to the faint, steady radio source? What can we learn from FRBs about the space between galaxies?

With all the excitement surrounding this discovery and several ongoing studies, hopefully we’ll be bringing you part 3 of this saga shortly.

Kendra Redmond

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