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
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.
Abstract
Eruptive activity in the solar corona can often lead to the propagation of shock waves. In the radio domain the primary signature of such shocks are type II radio bursts, observed in dynamic ...spectra as bands of emission slowly drifting toward lower frequencies over time. These radio bursts can sometimes have an inhomogeneous and fragmented fine structure, but the cause of this fine structure is currently unclear. Here we observe a type II radio burst on 2019 March 20th using the New Extension in Nançay Upgrading LOFAR, a radio interferometer observing between 10–85 MHz. We show that the distribution of size scales of density perturbations associated with the type II fine structure follows a power law with a spectral index in the range of
α
= −1.7 to −2.0, which closely matches the value of −5/3 expected of fully developed turbulence. We determine this turbulence to be upstream of the shock, in background coronal plasma at a heliocentric distance of ∼2
R
⊙
. The observed inertial size scales of the turbulent density inhomogeneities range from ∼62 Mm to ∼209 km. This shows that type II fine structure and fragmentation can be due to shock propagation through an inhomogeneous and turbulent coronal plasma, and we discuss the implications of this on electron acceleration in the coronal shock.
Context. The magnetized solar system planets are strong radio emitters and theoretical studies suggest that the radio emission from nearby exoplanets in close-in orbits could reach intensity levels ...103–107 times higher than Jupiter’s decametric emission. Detection of exoplanets in the radio domain would open up a brand new field of research, however, currently there are no confirmed detections at radio frequencies. Aims. We investigate the radio emission from Jupiter, scaled such that it mimics emission coming from an exoplanet, with low-frequency beam-formed observations using LOFAR. The goals are to define a set of observables that can be used as a guideline in the search for exoplanetary radio emission and to measure effectively the sensitivity limit for LOFAR beam-formed observations. Methods. We observe “Jupiter as an exoplanet” by dividing a LOFAR observation of Jupiter by a down-scaling factor and adding this observation to beam-formed data of the “sky background”. Then we run this artificial dataset through our total intensity (Stokes-I) and circular polarization (Stokes-V) processing and post-processing pipelines and determine up to which down-scaling factor Jupiter is still detected in the dataset. Results. We find that exoplanetary radio bursts can be detected at 5 pc if the circularly polarized flux is 105 times stronger than the typical level of Jupiter’s radio bursts during active emission events (~4 × 105 Jy). Equivalently, circularly polarized radio bursts can be detected up to a distance of 20 pc (encompassing the known exoplanets 55 Cnc, Tau Boötis, and Upsilon Andromedae) assuming the level of emission is 105 times stronger than the peak flux of Jupiter’s decametric burst emission (~6 × 106 Jy).
Pulsar scintillation studies with LOFAR Wu, Ziwei; Verbiest, Joris P. W.; Main, Robert A. ...
Astronomy and astrophysics (Berlin),
07/2022, Letnik:
663
Journal Article
Recenzirano
Odprti dostop
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.
ABSTRACT
Propagation effects are one of the main sources of noise in high-precision pulsar timing. For pulsars below an ecliptic latitude of 5°, the ionized plasma in the solar wind can introduce ...dispersive delays of order $100\, \mu \mathrm{s}$ around solar conjunction at an observing frequency of 300 MHz. A common approach to mitigate this assumes a spherical solar wind with a time-constant amplitude. However, this has been shown to be insufficient to describe the solar wind. We present a linear, Gaussian-process piecewise Bayesian approach to fit a spherical solar wind of time-variable amplitude, which has been implemented in the pulsar software run_enterprise. Through simulations, we find that the current EPTA+InPTA data combination is not sensitive to such variations; however, solar wind variations will become important in the near future with the addition of new InPTA data and data collected with the low-frequency LOFAR telescope. We also compare our results for different high-precision timing data sets (EPTA+InPTA, PPTA, and LOFAR) of 3 ms pulsars (J0030+0451, J1022+1001, J2145−0450), and find that the solar-wind amplitudes are generally consistent for any individual pulsar, but they can vary from pulsar to pulsar. Finally, we compare our results with those of an independent method on the same LOFAR data of the three millisecond pulsars. We find that differences between the results of the two methods can be mainly attributed to the modelling of dispersion variations in the interstellar medium, rather than the solar wind modelling.
ABSTRACT
Interstellar scattering (ISS) of radio pulsar emission can be used as a probe of the ionized interstellar medium (IISM) and causes corruptions in pulsar timing experiments. Two types of ISS ...phenomena (intensity scintillation and pulse broadening) are caused by electron density fluctuations on small scales (< 0.01 au). Theory predicts that these are related, and both have been widely employed to study the properties of the IISM. Larger scales (∼1 – 100 au) cause measurable changes in dispersion and these can be correlated with ISS observations to estimate the fluctuation spectrum over a very wide scale range. IISM measurements can often be modelled by a homogeneous power-law spatial spectrum of electron density with the Kolmogorov (−11/3) spectral exponent. Here, we aim to test the validity of using the Kolmogorov exponent with PSR J0826+2637. We do so using observations of intensity scintillation, pulse broadening and dispersion variations across a wide fractional bandwidth (20–180 MHz). We present that the frequency dependence of the intensity scintillation in the high-frequency band matches the expectations of a Kolmogorov spectral exponent, but the pulse broadening in the low-frequency band does not change as rapidly as predicted with this assumption. We show that this behaviour is due to an inhomogeneity in the scattering region, specifically that the scattering is dominated by a region of transverse size ∼40 au. The power spectrum of the electron density, however, maintains the Kolmogorov spectral exponent from spatial scales of 5 × 10−6 au to ∼100 au.
The LOFAR Transients Pipeline Swinbank, John D.; Staley, Tim D.; Molenaar, Gijs J. ...
Astronomy and computing,
06/2015, Letnik:
11, Številka:
Part A
Journal Article
Recenzirano
Odprti dostop
Current and future astronomical survey facilities provide a remarkably rich opportunity for transient astronomy, combining unprecedented fields of view with high sensitivity and the ability to access ...previously unexplored wavelength regimes. This is particularly true of LOFAR, a recently-commissioned, low-frequency radio interferometer, based in the Netherlands and with stations across Europe. The identification of and response to transients is one of LOFAR’s key science goals. However, the large data volumes which LOFAR produces, combined with the scientific requirement for rapid response, make automation essential. To support this, we have developed the LOFAR Transients Pipeline, or TraP. The TraP ingests multi-frequency image data from LOFAR or other instruments and searches it for transients and variables, providing automatic alerts of significant detections and populating a lightcurve database for further analysis by astronomers. Here, we discuss the scientific goals of the TraP and how it has been designed to meet them. We describe its implementation, including both the algorithms adopted to maximize performance as well as the development methodology used to ensure it is robust and reliable, particularly in the presence of artefacts typical of radio astronomy imaging. Finally, we report on a series of tests of the pipeline carried out using simulated LOFAR observations with a known population of transients.
This past decade has seen tremendous advancements in the study of extrasolar planets. Observations are now made with increasing sophistication from both ground- and space-based instruments, and ...exoplanets are characterized with increasing precision. There is a class of particularly interesting exoplanets that reside in the habitable zone, which is defined as the area around a star where the planet is capable of supporting liquid water on its surface. Planetary systems around M dwarfs are considered to be prime candidates to search for life beyond the Solar System. Such planets are likely to be tidally locked and have close-in habitable zones. Theoretical calculations also suggest that close-in exoplanets are more likely to have weaker planetary magnetic fields, especially in the case of super-Earths. Such exoplanets are subjected to a high flux of galactic cosmic rays (GCRs) due to their weak magnetic moments. GCRs are energetic particles of astrophysical origin that strike the planetary atmosphere and produce secondary particles, including muons, which are highly penetrating. Some of these particles reach the planetary surface and contribute to the radiation dose. Along with the magnetic field, another factor governing the radiation dose is the depth of the planetary atmosphere. The higher the depth of the planetary atmosphere, the lower the flux of secondary particles will be on the surface. If the secondary particles are energetic enough, and their flux is sufficiently high, the radiation from muons can also impact the subsurface regions, such as in the case of Mars. If the radiation dose is too high, the chances of sustaining a long-term biosphere on the planet are very low. We have examined the dependence of the GCR-induced radiation dose on the strength of the planetary magnetic field and its atmospheric depth, and found that the latter is the decisive factor for the protection of a planetary biosphere.
ABSTRACT Radio observations of young stellar objects (YSOs) enable the study of ionized plasma outflows from young protostars via their free-free radiation. Previous studies of the low-mass young ...system T Tau have used radio observations to model the spectrum and estimate important physical properties of the associated ionized plasma (local electron density, ionized gas content, and emission measure). However, without an indication of the low-frequency turnover in the free-free spectrum, these properties remain difficult to constrain. This paper presents the detection of T Tau at 149 MHz with the Low Frequency Array (LOFAR)-the first time a YSO has been observed at such low frequencies. The recovered total flux indicates that the free-free spectrum may be turning over near 149 MHz. The spectral energy distribution is fitted and yields improved constraints on local electron density ( cm−3), ionized gas mass ( ), and emission measure ( pc cm−6).
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