The search for radio emission from the exoplanetary systems 55 Cancri, Andromedae, and Boötis using LOFAR beam-formed observations

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 to the planet’s interior structure, atmospheric escape, and habitability. Aims. Here, we present LOFAR-LBA low-frequency circularly polarized beamformed observations of the three exoplanetary systems 55 Cancri, υ Andromedae, and τ Boötis. All three systems are predicted to be good candidates to search for exoplanet radio emission. Methods. We apply the BOREALIS pipeline that we have developed to mitigate radio frequency interference and search for both slowly varying and bursty radio emission. Our pipeline has been quantitatively benchmarked on attenuated Jupiter radio emission. Results. We detect circularly polarized bursty emission from τ Boötis b in the range 14-21 MHz with a flux density of ∼430 mJy with a statistical significance ∼3σ. For this detection, obtained with the method that we benchmarked on Jupiter bursts, we identified no potential false positives. Also, we tentatively find slowly variable emission from τ Boötis in the range 21-30 MHz with a flux density of ∼400 mJy with a statistical significance of ∼8.6σ. This emission is circularly polarized, structured in the time-frequency plane, and in excess in the ON-beam with respect to the two simultaneous OFF beams. However, close examination of the observed signature casts some doubts about the reality of the signal. We discuss in detail all the arguments for and against an actual detection, and derive from that methodological tests and conclusions that will apply to future searches. Conclusions. Assuming that these circularly polarized emissions are real and generated through the cyclotron maser mechanism, we can derive limits on the range of the polar surface magnetic field strengths for the planet τ Boötis b. No signal is detected from the 55 Cancri system, on which we place a 3σ upper limit of 73 mJy for the flux density at the time of the observation. Also, no signal is found from the υ Andromedae system with the exception of a ∼2σ marginal signal on one observation of bursty emission in the range 14-38 MHz. Further observations with LOFAR and other low-frequency telescopes such as NenuFAR or UTR-2 are required to confirm these possible first detections of exoplanetary radio signals. Key words. Planets and satellites: magnetic fields – Radio continuum: planetary systems – Magnetic fields – Astronomical instrumentation, methods and techniques – Planet-star interactions – Planets and satellites: aurorae – planets and satellites: gaseous planets