A Radio Signal From a Nearby Red Dwarf, and Three Possible Sources

A radio survey that was not aimed at any particular star has picked up a strongly polarized signal at 45 megahertz from GJ 725A, a planet-hosting red dwarf about 11.5 light-years away. The authors report that the data cannot say whether the emission came from a planet, from the star, or from an interaction between the two.

Radio waves at these low frequencies carry information about stellar and planetary magnetic fields that is hard to get at other wavelengths, the authors write in a paper accepted for publication in Astronomy & Astrophysics. No emission of this kind had been clearly detected in this band before.

The signal turned up in V-LoLSS, a search for circularly polarized sources with LOFAR's low-band antennas. It appeared in one archival observation, on Dec. 11, 2020, and in none of the others. Its peak brightness was 135 millijanskys per beam, and 74% of it was circularly polarized, meaning the waves twisted one way. Nothing was detected above 50 megahertz, a sharp cutoff.

From the brightness and polarization, the team favors the electron cyclotron maser, the process behind Jupiter's radio aurorae. The cutoff puts the field in the emitting region at about 18 gauss, close to Jupiter's surface field.

The paper sets out three origins and settles on none. An aurora on a planet would need an undiscovered giant world on a wide orbit: the one known planet, a super-Earth on an 11.2-day orbit, is far too light to generate a field that strong. The others are an interaction between the star and that known planet, or a rare stellar flare.

GJ 725A rotates slowly and flares rarely compared with the active stars LOFAR usually finds. The authors say that makes a stellar origin less likely without excluding it, and they describe it as the lowest-frequency confirmed detection of a stellar system.

The team plans repeat monitoring with LOFAR: a signal returning every 11.2 days would indicate the known planet, while one repeating over hours would point to a giant planet's aurora.