Homing in on the Source of a Mysterious Fast Radio Burst

Ground- and space-based observations have now shed intriguing new light on a mysterious radio source more than 3 billion light-years away.

Fast radio burst FRB 121102

This artist's impression shows the radio dishes of the Very Large Array receiving the signal of FRB 121102.
Danielle Futselaar

A puzzling source of ultra-brief radio bursts is slowly giving up some of its secrets. Three new papers, published on the arXiv preprint server in late May, have presented surprising new results on the only known repeating fast radio burst, dubbed FRB 121102. But despite the concerted efforts of some of the largest observatories, both on the ground and in space, the true nature of these enigmatic events remains a mystery.

Fast radio bursts are very brief explosions of radio waves, first discovered at the 64-meter Parkes radio telescope in Australia. Because of their unpredictability, their short duration (on the order of milliseconds), and the small field of view of a typical radio telescope, such bursts are hard to spot. But from the two dozen or so found so far, it follows that there must be thousands of these bursts occurring all over the sky every day.

Of these, FRB121102, first detected by the 305-meter Arecibo radio telescope in Puerto Rico, is special. After its discovery on November 2, 2012 (hence its name), it became the first of its kind to repeat, producing many additional bursts of radio waves. Whatever FRBs might be, they’re apparently not some one-off cataclysm.

Fast radio burst FRB 121102

Gemini composite image of the field around FRB 121102. The dwarf host galaxy was imaged, and spectroscopy performed, using the Gemini Multi-Object Spectrograph (GMOS) on the Gemini North telescope on Maunakea in Hawai'i.
Gemini Observatory / AURA / NSF / NRC

In 2016 observations by the Very Large Array in New Mexico and the European VLBI Network succeeded in precisely pinpointing the sky location of the repeating radio source. It turned out to coincide with a remote, dim dwarf galaxy that also hosts a faint persistent source of radio waves. A paper published earlier this year already revealed that the FRB and the persistent radio source are separated by less than 12 milliarcseconds.

Ground- and space-based observations by a large international team of astronomers have now shed intriguing new light on this mysterious FRB. On February 23rd, the dwarf galaxy — 3.2 billion light-years away in southern Auriga — was studied in detail by the Hubble Space Telescope, following earlier observations by the 8.1-meter Gemini-North telescope at Mauna Kea, Hawaii, and by NASA’s Spitzer Space Telescope. Meanwhile, a Japanese team also observed the puny galaxy with the 8.2-meter Subaru telescope, also at Mauna Kea.

The new observations reveal that the bursts originate in an active, compact star-forming region some 4,500 light-years across in the outskirts of the tiny galaxy. According to team member Jason Hessels (ASTRON, the Netherlands Institute for Radio Astronomy), this strongly suggests that the source of the bursts is a relatively young object — probably a recently formed neutron star. “FRB121102 might be considered a pulsar on steroids,” says Hessels. But it’s not yet clear whether the intermittent bursts are part of a regularly repeating signal, characteristic of pulsars.

The Crab Nebula

This photo shows a three-color composite of the Crab Nebula. Sitting in the center is a pulsar — a burnt-out stellar core that might be similar to the source FRB 121102. However, while the Crab pulsar emits X-rays, FRB 121102 does not.
ESO

The burst location has also been monitored by ESA’s XMM Newton observatory (in September 2016) and by NASA’s Chandra X-ray Observatory (in November 2016 and January 2017). But although Arecibo and the 110-meter Green Bank Telescope in West Virginia recorded a total of 12 new bursts during these periods (two of which were detected by both instruments), nothing was seen in X-rays. This indicates that the bursts’ source is not just a scaled-up version of the young pulsar at the heart of the Crab Nebula. The Crab pulsar also emits giant radio flares (albeit half a million times less powerful than FRB121102!), but it’s a conspicuous X-ray source too.

Still, Hessels believes that the culprit must be a young stellar remnant — maybe a rapidly spinning magnetar: a strongly magnetized neutron star. The faint persistent radio source could be the shock wave from the original supernova explosion, or a so-called pulsar wind nebula. Then again, known magnetars also produce powerful bursts of X-rays and gamma rays, which have not (yet) been observed in the case of FRB121102.

One of the other nagging questions is whether or not the repeating source is representative of all FRBs. No other fast radio bursts have been seen to repeat, despite various monitoring programs. Therefore, some researchers believe the repeater to be the odd one out. One of them is radio astronomer Duncan Lorimer of West Virginia University, who discovered the first FRB in Parkes data from 2001. “I suspect there may be multiple classes,” he says.

References:
C. G. Bassa et al. "FRB 121102 is coincident with a star forming region in its host galaxy." arXiv preprint server.

M. Kokubo et al. "H-alpha intensity map of the repeating fast radio burst FRB 121102 host galaxy from Subaru/Kyoto 3DII AO-assisted optical integral-field spectroscopy." arXiv preprint server.

C. J. Law et al. "A multi-telescope campaign on FRB 121102: Implications for the FRB population." arXiv preprint server.

B. Marcote et al. "The repeating fast radio burst FRB 121102 as seen on milliarcsecond angular scales." arXiv preprint server.

P. Scholz et al. "Simultaneous X-ray, gamma-ray, and radio observations of the repeating fast radio burst FRB 121102." arXiv preprint server.

 

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