A Cosmic Siren and Its Nebulous Afterglow express
Radio telescopes have caught a signal that repeats like a beacon in the fog. It's a fast radio burst — a brief flash of radio waves from a distant galaxy. Nearby, astronomers search for a persistent source — a faint glow that lingers after the burst, like embers after fireworks. In the new study, researchers pointed a network of radio telescopes at two such candidates.
The first source, linked to burst FRB 20190417A, was detected at 5 gigahertz. It turned out to be extremely compact — smaller than one milliarcsecond on the sky, which is like spotting a coin from a thousand kilometers away. Its brightness temperature exceeds one hundred thousand kelvins, meaning the emission isn't thermal but comes from charged particles moving in magnetic fields. The source's spectrum is nearly flat: it shines equally bright at low and high frequencies. This behavior resembles light from a nebula inflated by a young pulsar — a rapidly spinning neutron star.
The second candidate, linked to burst FRB 20181030A, did not show up at these frequencies. If its emission is truly compact, the spectrum must be very steep: it fades sharply at high frequencies. This sets it apart from the first source and hints at a different mechanism.
This approach, catching the faint persistent glow next to short flashes, helps separate the burst's "engine" from the surrounding nebula. The more such sources we can measure, the clearer it becomes what machines drive these cosmic sirens.
The source's brightness temperature exceeds one hundred thousand kelvins. For comparison, the Sun's surface is about six thousand kelvins. Such a high temperature for a radio source means its emission is produced not by hot atoms but by magnetic fields accelerating particles.
In Carl Sagan's novel "Contact," the protagonist picks up a radio signal from an alien civilization. Fast radio bursts were also initially mistaken for artificial signals, but now we know they're natural phenomena.