2021A&A...645A..59T

Context. The detection of radio emissions from exoplanets will open up a vibrant new research field. Observing planetary auroral radio emission is the most promising method to detect exoplanetary magnetic fields, the knowledge of which will provide valuable insights into the planet's interior structure, atmospheric escape, and habitability.
Aims. We present LOFAR (LOw-Frequency ARray) Low Band Antenna (LBA: 10-90 MHz) circularly polarized beamformed observations of the exoplanetary systems 55 Cancri, υ Andromedae, and τ Bootis. All three systems are predicted to be good candidates to search for exoplanetary radio emission.
Methods. We applied the BOREALIS pipeline that we have developed to mitigate radio frequency interference and searched for both slowly varying and bursty radio emission. Our pipeline has previously been quantitatively benchmarked on attenuated Jupiter radio emission.
Results. We tentatively detect circularly polarized bursty emission from the τ Bootis system in the range 14-21 MHz with a flux density of ∼890mJy and with a statistical significance of ∼3σ. For this detection, we do not see any signal in the OFF-beams, and we do not find any potential causes which might cause false positives. We also tentatively detect slowly variable circularly polarized emission from τ Bootis in the range 21-30 MHz with a flux density of ∼400mJy and with a statistical significance of >8σ. The slow emission is structured in the time-frequency plane and shows an excess in the ON-beam with respect to the two simultaneous OFF-beams. While the bursty emission seems rather robust, close examination casts some doubts on the reality of the slowly varying signal. We discuss in detail all the arguments for and against an actual detection, and derive methodological tests that will also apply to future searches. Furthermore, a ∼2σ marginal signal is found from the υ Andromedae system in one observation of bursty emission in the range 14-38MHz and no signal is detected from the 55 Cancri system, on which we placed a 3σ upper limit of 73mJy for the flux density at the time of the observation.
Conclusions. Assuming the detected signals are real, we discuss their potential origin. Their source probably is the τ Bootis planetary system, and a possible explanation is radio emission from the exoplanet τ Bootis b via the cyclotron maser mechanism. Assuming a planetary origin, we derived limits for the planetary polar surface magnetic field strength, finding values compatible with theoretical predictions. Further observations with LOFAR-LBA and other low-frequency telescopes, such as NenuFAR or UTR-2, are required to confirm this possible first detection of an exoplanetary radio signal.