In 2007 a bright, milliseconds-duration burst of radio waves emanating from far outside the Milky Way captured the attention of astronomers. Since then, astrophysicists have identified the home galaxies of similar fast radio bursts, or FRBs. To explain the enigmatic signals, many theorists have proposed that stellar remnants emit high-energy bursts of radiation, but until now, observational evidence has not directly associated an FRB with a specific astronomical phenomenon. Now an international effort that includes three radio observatories has identified the first FRB known to originate within our galaxy and has determined that the signal coincides with x-ray and gamma-ray emissions from the same location. The site corresponds to a collapsed neutron star with an intense magnetic field, called a magnetar. The findings provide new constraints on FRB progenitor theories and direction for future work.
On 27 April 2020, NASA’s Neil Gehrels Swift Observatory detected increased gamma-ray activity from SGR 1935+2154, a magnetar in the constellation Vulpecula. Within a day, the Canadian Hydrogen Intensity Mapping Experiment (CHIME) and the Survey for Transient Astronomical Radio Emission 2 radio telescopes detected FRBs coming from the direction of the magnetar. At the same time, several space telescopes reported the detection of an x-ray burst that occurred concurrently with the radio bursts. On 29 April the Five-Hundred-Meter Aperture Spherical Radio Telescope (FAST) in China saw another, fainter radio burst typical of a magnetar from precisely the same location, confirming the magnetar’s association with the FRB. In the days leading up to 27 April, FAST had reported a lack of FRB-like events from SGR 1935+2154 despite 29 x-ray bursts, which indicates that FRBs don’t accompany every magnetar outburst.
The observations are the first to associate nonradio emissions with an FRB and to make a compelling link between FRBs and magnetars. One proposed mechanism is that a magnetar produces short-lived flares of electrons and other charged particles that collide with those emitted during previous flares, thus generating a shock front and huge magnetic fields. Electrons swirling around the magnetic fields emit bursts of radio waves, and the heated electrons emit x rays. Another possibility is that a starquake triggers disturbances in magnetic field lines near the magnetar surface. Those disturbances induce relativistic particles to stream from the magnetosphere and generate radio emissions.
Further study could help astronomers home in both on the specific mechanisms that drive magnetars to generate FRBs and on other possible sources. The FRB source in the Milky Way may also help answer open-ended questions, such as how frequently the FRB signals repeat over years to decades and whether similar pulses can be used to identify specific objects in other galaxies. (CHIME/FRB collaboration, Nature 587, 54, 2020; C. D. Bochenek et al., Nature 587, 59, 2020; L. Lin et al., Nature 587, 63, 2020.)
