Gotcha: Fast radio burst's home nabbed

GRAPEVINE, TEXAS — A mysterious, recurring blast of cosmic radio waves finally has a home address. For the first time, astronomers have definitively traced a fast radio burst back to its source: a faint galaxy about 2.5 billion light-years away. The finding confirms a decade-long suspicion that these outbursts originate well outside our galaxy, although the mystery as to what’s causing them remains unsolved.

“Now with the first proven distance, we can see how remote and how bright the source must be,” Sarah Burke-Spolaor, an astrophysicist at West Virginia University, said January 4 at a meeting of the American Astronomical Society. For roughly five milliseconds, the burst outshined all the stars in its own galaxy and rivaled the luminosity of blazing disks of gas that swirl around supermassive black holes, said Burke-Spolaor, one of the researchers involved with the project.

Fast radio bursts have stumped astronomers since the first one was reported in 2007 (SN: 8/9/14, p. 22). Since then, 17 more bursts have been detected by several radio telescopes around the world. In nearly every case, the outburst lasted just a few milliseconds and was never seen again. Only one, first detected at the Arecibo Observatory in Puerto Rico in 2012, has been seen multiple times (SN Online: 12/21/16).

Most radio telescopes can provide only a fuzzy idea of where on the sky a burst comes from. But the repetitive nature of this burst, dubbed FRB 121102, gave astronomers a heads up of where to point the Very Large Array, a network of radio dishes near Socorro, N.M., which could provide a sharper image.

“We have imaged the burst itself with the VLA and pinpointed where it is on the sky,” said Shami Chatterjee, an astrophysicist at Cornell University. Over the span of six months, the VLA detected nine outbursts coming from the same direction as previous repetitions. A persistent glow of radio waves also comes from the same spot. Further observations with the Gemini telescope in Hawaii revealed that the radio outbursts coincide with a faint galaxy. By measuring how much the expansion of the universe has stretched the light coming from the galaxy, the researchers were able to measure the distance to the source of the burst.

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The findings appear in a paper in the Jan. 5 Nature and two papers in the Jan. 10 Astrophysical Journal Letters.

“Without a doubt, this is a landmark event,” said Duncan Lorimer, an astrophysicist at West Virginia University who was not involved with these studies but did discover the first radio burst roughly a decade ago. “There’s no question about the validity of the result.”

The host galaxy is tiny. “We’re barely able to distinguish it from a star,” said project member Shriharsh Tendulkar, an astrophysicist at McGill University in Montreal. It has roughly one one-thousandth of the stars as the Milky Way and is less than one-tenth as wide. “That’s weird,” he said. One favored explanation for fast radio bursts is that they come from neutron stars, the dense cores left behind after a massive star explodes. But if neutron stars are responsible, then astronomers expect to find bursts in places with lots of stars, Tendulkar said.

Tracing FRB 121102 back to a dwarf galaxy doesn’t rule out neutron stars as a source. The gas in dwarf galaxies is more pristine than in other locales such as the Milky Way — with relatively low amounts of elements heavier than helium. Such gas makes it easier for massive stars to form. More heavyweight stars lead to more neutron stars, which could lead to more radio bursts.

Some of the new data, however, also suggest that the source sits near a supermassive black hole, indicating that perhaps the radio blast is somehow connected to gas and dust swirling down the black hole’s gravitational throat.

“We’ve made this huge breakthrough in getting the distance, and it still doesn’t want to let its identity be known,” Lorimer said.

With a host galaxy in hand, astronomers can now point telescopes covering a broad range of the electromagnetic spectrum — from radio waves to gamma rays — at the galaxy to learn more about the burst’s home. One thing that researchers will look for is whether or not the bursts have a steady beat; all the detections so far have appeared randomly. If the signal has a regular period, then something that is spinning (like a neutron star) might be the culprit. Pinpointing more radio bursts and seeing if they originate in dwarf galaxies could also help researchers figure out if this object is unusual or typical of all radio bursts.