Coherent radio emission at 45 MHz from the nearby planet-hosting star GJ 725A
Ultra-low-frequency (<100 MHz) coherent radio emission from exoplanet-hosting stellar systems carries information about planetary and stellar magnetic fields, as well as the space weather environment - information that is difficult to obtain at other wavelengths. We carried out the circularly polarized LOFAR LBA Sky Survey, the most sensitive circularly polarized survey in this frequency range, using the LOFAR low-band antenna system. As part of this untargeted search, we detected a signal in a 1h observation that is coincident with the quiescent, planet-hosting, solar-neighborhood M dwarf GJ 725A at a distance of 3.5 pc. The signal has a high circular polarization fraction of $74\pm16$% and a peak brightness of $B=135\pm17$ mJy/beam between 42 MHz and 50 MHz. It was undetected in the 50-66 MHz range, with a 3$σ$ upper limit of $B<48$ mJy/beam, corresponding to a drop of at least a factor 3 across 12 MHz, and indicating a strong spectral feature or cutoff around 50 MHz. Our detection represents the lowest-frequency confirmed detection of a stellar system. Based on the high estimated brightness temperature ($\sim10^{14}$ K), transient character and polarization fraction, the emission is most plausibly attributed to the electron cyclotron maser instability. The available observations are not sufficient to identify the astrophysical origin of the emission. We consider three plausible scenarios: auroral emission of a yet undetected wide-orbit giant planet with a magnetic field $B\geq18$ G, star-planet interaction with the known 11 d orbit terrestrial planet GJ 725Ab, or an origin due to stellar magnetic activity. Ultra-low-frequency monitoring, combined with new, preferably simultaneous observations in the X-rays, UV, and H$α$, and a search for a wide-orbit (thousands of days) giant planet across the full allowed parameter space will allow us to distinguish between these scenarios.