Observation of planetary radio emissions using large arrays

International audiencePlanetary radio astronomy mostly concerns plasma phenomena at low frequencies (i.e., below a few hundred MHz). The low frequency limit for ground-based observations of these phenomena is given by the Earth's ionosphere, which limits ground based radio observations to frequencies ≥10 MHz. We give an overview and update on the status of a few representative ground-based radio arrays that have been used for planetary studies within the frequency range 10–200 MHz, and we discuss their potential for the four types of planetary radio emissions that can be observed within this frequency range: (1) synchrotron emission from Jupiter's radiation belts, (2) radio bursts caused by solar system planetary lightning, (3) Jupiter's magnetospheric emission, and (4) magnetospheric radio emission from extrasolar planets, for which we also give an update to previous predictive studies. Comparing the four emission modes with the characteristics of existing ground-based radio arrays, we show that the Low Frequency Array (LOFAR) has the potential to bring considerable advances to those four fields of planetary radio science

PALANTIR: An updated prediction tool for exoplanetary radio emissions

In the past two decades, it has been convincingly argued that magnetospheric radio emissions, of cyclotron maser origin, can occur for exoplanetary systems, similarly as solar planets, with the same periodicity as the planetary orbit. These emissions are primarily expected at low frequencies (usually below 100 MHz, c.f. Farrell et al., 1999; Zarka, 2007). The radio detection of exoplanets will considerably expand the field of comparative magnetospheric physics and star-planet plasma interactions (Hess & Zarka, 2011). We have developped a prediction code for exoplanetary radio emissions, PALANTIR: "Prediction Algorithm for star-pLANeT Interactions in Radio". This code has been developed for the construction of an up-to-date and evolutive target catalog, based on observed exoplanet physical parameters, radio emission theory, and magnetospheric physics embedded in scaling laws. It is based on, and extends, previous work by Grießmeier et al. (2007b). Using PALANTIR, we prepared an updated list of targets of interest for radio emissions. Additionally, we compare our results with previous studies conducted with similar models (Griessmeier, 2017). For the next steps, we aim at improving this code by adding new models and updating those already used

Identifying transient and variable sources in radio images

With the arrival of a number of wide-field snapshot image-plane radio transient surveys, there will be a huge influx of images in the coming years making it impossible to manually analyse the datasets. Automated pipelines to process the information stored in the images are being developed, such as the LOFAR Transients Pipeline, outputting light curves and various transient parameters. These pipelines have a number of tuneable parameters that require training to meet the survey requirements. This paper utilises both observed and simulated datasets to demonstrate different machine learning strategies that can be used to train these parameters. The datasets used are from LOFAR observations and we process the data using the LOFAR Transients Pipeline; however the strategies developed are applicable to any light curve datasets at different frequencies and can be adapted to different automated pipelines. These machine learning strategies are publicly available as Python tools that can be downloaded and adapted to different datasets (https://github.com/AntoniaR/TraP_ML_tools). Comment: Astronomy & Computing Accepted, 25 pages, 20 figur

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

International audienceContext. 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 Boötis. 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 Boötis system in the range 14–21 MHz with a flux density of ~890 mJy and with a statistical significance of ~3 σ . 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 Boötis in the range 21–30 MHz with a flux density of ~400 mJy and with a statistical significance of >8 σ . 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 ~2 σ marginal signal is found from the υ 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 σ 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 τ Boötis planetary system, and a possible explanation is radio emission from the exoplanet τ Boötis 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

Pulsar scintillation studies with LOFAR. I. The census

Wu Z, Verbiest J, Main RA, et al. Pulsar scintillation studies with LOFAR. I. The census. arXiv:2203.10409. 2022.Context. Interstellar scintillation (ISS) of pulsar emission can be used both as a probe of the ionised interstellar medium (IISM) and cause corruptions in pulsar timing experiments. Of particular interest are so-called scintillation arcs which can be used to measure time-variable interstellar scattering delays directly, potentially allowing high-precision improvements to timing precision. Aims. The primary aim of this study is to carry out the first sizeable and self-consistent census of diffractive pulsar scintillation and scintillation-arc detectability at low frequencies, as a primer for larger-scale IISM studies and pulsar-timing related propagation studies with the LOw-Frequency ARray (LOFAR) High Band Antennae (HBA). Results. In this initial set of 31 sources, 15 allow full determination of the scintillation properties; nine of these show detectable scintillation arcs at 120-180 MHz. Eight of the observed sources show unresolved scintillation; and the final eight don't display diffractive scintillation. Some correlation between scintillation detectability and pulsar brightness and dispersion measure is apparent, although no clear cut-off values can be determined. Our measurements across a large fractional bandwidth allow a meaningful test of the frequency scaling of scintillation parameters, uncorrupted by influences from refractive scintillation variations. Conclusions. Our results indicate the powerful advantage and great potential of ISS studies at low frequencies and the complex dependence of scintillation detectability on parameters like pulsar brightness and interstellar dispersion. This work provides the first installment of a larger-scale census and longer-term monitoring of interstellar scintillation effects at low frequencies

Pulsar scintillation studies with LOFAR, I. The census

Wu Z, Verbiest J, Main RA, et al. Pulsar scintillation studies with LOFAR, I. The census. Astronomy & Astrophysics. 2022;663: A116.Context.Interstellar scintillation (ISS) of pulsar emission can be used both as a probe of the ionized interstellar medium (IISM) and cause corruptions in pulsar timing experiments. Of particular interest are so-called scintillation arcs which can be used to measure time-variable interstellar scattering delays directly, potentially allowing high-precision improvements to timing precision.Aims.The primary aim of this study is to carry out the first sizeable and self-consistent census of diffractive pulsar scintillation and scintillation-arc detectability at low frequencies, as a primer for larger-scale IISM studies and pulsar-timing related propagation studies with the LOw-Frequency ARray (LOFAR) High Band Antennae (HBA).Methods.We use observations from five international LOFAR stations and the LOFAR core in the Netherlands. We analyze the 2D auto-covariance function of the dynamic spectra of these observations to determine the characteristic bandwidth and timescale of the ISS toward the pulsars in our sample and investigate the 2D power spectra of the dynamic spectra to determine the presence of scintillation arcs.Results.In this initial set of 31 sources, 15 allow for the full determination of the scintillation properties; nine of these show detectable scintillation arcs at 120–180 MHz. Eight of the observed sources show unresolved scintillation; and the final eight do not display diffractive scintillation. Some correlation between scintillation detectability and pulsar brightness and a dispersion measure is apparent, although no clear cut-off values can be determined. Our measurements across a large fractional bandwidth allow a meaningful test of the frequency scaling of scintillation parameters, uncorrupted by influences from refractive scintillation variations.Conclusions.Our results indicate the powerful advantage and great potential of ISS studies at low frequencies and the complex dependence of scintillation detectability on parameters such as pulsar brightness and interstellar dispersion. This work provides the first installment of a larger-scale census and longer-term monitoring of ISS effects at low frequencies