The search for radio emission from the exoplanetary systems 55 Cancri, υ Andromedae, and τ Bootis 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 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, upsilon Andromedae, and tau 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 tau Bootis system in the range 14-21 MHz with a flux density of similar to 890 mJy and with a statistical significance of similar to 3 sigma. 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 tau Bootis in the range 21-30 MHz with a flux density of similar to 400 mJy and with a statistical significance of >8 sigma. 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 similar to 2 sigma marginal signal is found from the upsilon Andromedae system in one observation of bursty emission in the range 14-38 MHz and no signal is detected from the 55 Cancri system, on which we placed a 3 sigma upper limit of 73 mJy 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 tau Bootis planetary system, and a possible explanation is radio emission from the exoplanet tau 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.

Automated summary

The research presents observations of exoplanetary radio emissions using LOFAR low-frequency antennas targeting the systems of 55 Cancri, upsilon Andromedae, and tau Bootis. The study developed the BOREALIS pipeline to mitigate radio frequency interference and tentatively detected bursty and slowly variable radio emissions from the tau Bootis system. Further observations are needed to confirm these potential first detections of exoplanetary radio signals.