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

Abstract

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. We present LOFAR-LBA circularly polarized beamformed observations of the exoplanetary systems 55 Cancri, $\\upsilon$ Andromedae, and $\\tau$ Bootis. We tentatively detect circularly polarized bursty emission from the $\\tau$ Bootis system in the range 14-21 MHz with a flux density of $\\sim$890 mJy and with a significance of $\\sim$3$\\sigma$. For this detection, no signal is seen 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 $\\sim$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. 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. Furthermore, a $\\sim$2$\\sigma$ marginal signal is found from the $\\upsilon$ Andromedae system and no signal is detected from the 55 Cancri system. 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 low-frequency observations are required to confirm this possible first detection of an exoplanetary radio signal. [Abridged]