New Hubble observations have pinned five enigmatic radio bursts to the arms of spiral galaxies, narrowing down possible explanations.
Fast radio bursts (FRBs) are radio signals that can emit as much energy in a fraction of a second as the Sun does in an entire year. The radio flashes are so brief they’re hard to pin down, but scientists using the Hubble Space Telescope have now traced five FRBs to the arms of spiral galaxies.
These birthplaces are consistent with supernovae or the exotic magnetars they produce as the source of the radio bursts. The study, led by Alexandra Mannings (University of California, Santa Cruz) will appear in the Astrophysical Journal (preprint available here).
Since their discovery in 2007, more than 100 FRBs have been detected, but scientists have traced only about 15 of them to specific sources. Some of these are repeating sources, which makes it easier to locate the radio flashes within their host galaxies, which are observed at infrared, visible-light, or ultraviolet wavelengths.
But single flashes are harder to pin down. For example, the CHIME radio telescope in Canada can scan vast regions of the sky and capture many FRBs but cannot identify their exact location. Other telescopes, such as the ASKAP radio telescope in Australia, do a better job of pinpointing their locations but capture fewer signals.
In the latest study, scientists used the Hubble Space Telescope to follow up on six FRBs that ASKAP had detected and localized, as well as two repeating sources seen by other radio telescopes.
“The Hubble Space Telescope is so sensitive that it uncovers features that we can’t see in our ground-based imaging,” explains team member Wen-fai Fong (Northwestern University).
The Hubble images showed that the sources of five of the FRBs were in the spiral arms of different galaxies at vastly different distances. “The farthest localized FRB is nearly 8 billion light years away,” explains Sunil Simha (University of California, Santa Cruz), also a team member. “Most localized FRBs are a few billion light years away and there are some really close ones, including one from our own galaxy.”
The ASKAP-obtained positions were so precise, and the Hubble images sharp enough, that scientists were able to trace the FRBs to specific regions in the galaxy arms. Surprisingly, they were not in the brightest regions of intense star formation, which allowed scientists to rule out some causes of the radio bursts.
Fast Radio Burst Origins
“We know that long gamma-ray bursts and super-luminous supernovae likely originate from young, very massive stars,” says Fong. “The fact that we do not find FRBs to be highly correlated to the brightest regions of the spiral arms, and of their hosts overall, means we can rule out a strong connection to long gamma-ray bursts and certain types of supernovae.”
Short gamma-ray bursts — high-energy flashes from neutron star mergers that last as short as a few milliseconds — are also unlikely to be the source of FRBs. Mergers of neutron stars usually happen on the outskirts of galaxies, far away from the spiral arms where FRBs are mostly found.
On the other hand, the core-collapse supernovae of more average-sized stars might be a more likely FRB source. Such stars would be more likely to be in a fainter region within spiral arms — exactly the type of region where FRBs have been found.
Another possible FRB source is magnetars, neutron stars with extremely strong magnetic fields. In fact, this suspicion was recently confirmed for an FRB detected in our own galaxy last year, which was traced back to a magnetar. However, it’s possible different types of sources might make similar FRB signals, so new sources for the radio bursts might yet be discovered.
Shami Chatterjee (Cornell University), who was not involved with the study, is excited about the results. He says that now astronomers can start looking for trends: “Are all FRBs associated with star formation? Just some of them? So that’s a big step forward,” he adds, “although we are still dealing with small numbers of sources so far, and many more such detections are needed before we have a definitive picture.”
Duncan Lorimer (West Virginia University), the discoverer of the first FRB who was also not involved with the study, agrees. “The sample is admittedly very small, but the signs from this work point in favor of a neutron star origin, consistent with the FRBs coming from magnetars in these galaxies,” he says. “This study is just so impressive and gives a clear roadmap as to what will be possible with a larger number of objects that is undoubtedly forthcoming in the next few years.”