Generation Of Low-frequency Radio Telescopes Research Articles

Radio jets: Properties, life and impact

AbstractOur view of the properties of extragalactic radio jets and the impact they have on the host galaxy has expanded in the recent years. This has been possible thanks to the data from new or upgraded radio telescopes. This review briefly summarises the current status of the field and describes some of the exciting recent results and the surprises they have brought. In particular, the physical properties of radio jets as function of their radio power will be discussed together with the advance made in understanding the life-cycle of radio sources. The evolutionary stage (e.g. newly born, dying, restarted) of the radio AGN can be derived from their morphology and properties of the radio spectra. The possibilities offered by the new generation of low-frequency radio telescopes make it possible to derive (at least to first order) the time-scale spent in each phase. The presence of a cycle of activity ensures a recurrent impact of the radio jets on their surrounding inter-stellar and inter-galactic medium and, therefore, their relevance for AGN feedback. The last part is dedicated to the recent results showing the effect of jets on the surrounding galactic medium. The predictions made by numerical simulations on the impact of a radio jet (and in particular a newly born jet) on a clumpy medium describe well what is seen by the observations. The high resolution studies of jet-driven outflows of cold gas (and molecular) has provided new important addition both in term of quantifying the impact of the outflows and their relevance for feedback as well as for providing an unexpected view of the physical conditions of the gas under these extreme conditions.

Constraining coherent low-frequency radio flares from compact binary mergers

ABSTRACT The presence and detectability of coherent radio emission from compact binary mergers (containing at least one neutron star) remains poorly constrained due to large uncertainties in the models. These compact binary mergers may initially be detected as short gamma-ray bursts or via their gravitational wave emission. Several radio facilities have developed rapid response modes enabling them to trigger on these events and search for this emission. For this paper, we constrain this coherent radio emission using the deepest available constraints for GRB 150424A, which were obtained via a triggered observation with the Murchison Widefield Array. We then expand this analysis to determine the properties of magnetar merger remnants that may be formed via a general population of binary neutron star mergers. Our results demonstrate that many of the potential coherent emission mechanisms that have been proposed for such events can be detected or very tightly constrained by the complementary strategies used by the current generation of low-frequency radio telescopes.

Polarized point sources in the LOFAR Two-meter Sky Survey: A preliminary catalog

The polarization properties of radio sources at very low frequencies (<200 MHz) have not been widely measured, but the new generation of low-frequency radio telescopes, including the Low Frequency Array (LOFAR: a Square Kilometre Array Low pathfinder), now gives us the opportunity to investigate these properties. In this paper, we report on the preliminary development of a data reduction pipeline to carry out polarization processing and Faraday tomography for data from the LOFAR Two-meter Sky Survey (LOTSS) and present the results of this pipeline from the LOTSS preliminary data release region (10h45m–15h30m right ascension, 45°–57° declination, 570 square degrees). We have produced a catalog of 92 polarized radio sources at 150 MHz at 4.′3 resolution and 1 mJy rms sensitivity, which is the largest catalog of polarized sources at such low frequencies. We estimate a lower limit to the polarized source surface density at 150 MHz, with our resolution and sensitivity, of 1 source per 6.2 square degrees. We find that our Faraday depth measurements are in agreement with previous measurements and have significantly smaller errors. Most of our sources show significant depolarization compared to 1.4 GHz, but there is a small population of sources with low depolarization indicating that their polarized emission is highly localized in Faraday depth. We predict that an extension of this work to the full LOTSS data would detect at least 3400 polarized sources using the same methods, and probably considerably more with improved data processing.

LOFAR 150-MHz observations of SS 433 and W 50

We present LOFAR high-band data over the frequency range 115-189 MHz for the X-ray binary SS 433, obtained in an observing campaign from 2013 February - 2014 May. Our results include a deep, wide-field map, allowing a detailed view of the surrounding supernova remnant W 50 at low radio frequencies, as well as a light curve for SS 433 determined from shorter monitoring runs. The complex morphology of W 50 is in excellent agreement with previously published higher-frequency maps; we find additional evidence for a spectral turnover in the eastern wing, potentially due to foreground free-free absorption. Furthermore, SS 433 is tentatively variable at 150 MHz, with both a debiased modulation index of 11 per cent and a $\chi^2$ probability of a flat light curve of $8.2 \times 10^{-3}$. By comparing the LOFAR flux densities with contemporaneous observations carried out at 4800 MHz with the RATAN-600 telescope, we suggest that an observed $\sim$0.5-1 Jy rise in the 150-MHz flux density may correspond to sustained flaring activity over a period of approximately six months at 4800 MHz. However, the increase is too large to be explained with a standard synchrotron bubble model. We also detect a wealth of structure along the nearby Galactic plane, including the most complete detection to date of the radio shell of the candidate supernova remnant G 38.7-1.4. This further demonstrates the potential of supernova remnant studies with the current generation of low-frequency radio telescopes.

Faraday tomography of the local interstellar medium with LOFAR: Galactic foregrounds towards IC 342

The new generation of low-frequency radio telescopes, such as the Low Frequency Array (LOFAR: a Square Kilometre Array-low pathfinder), provides advancements in our capability of probing Galactic magnetism through low-frequency polarimetry. Maps of diffuse polarized radio emission and Faraday rotation can be used to infer properties of, and trace structure in, the magnetic fields in the ISM. However, to date very little of the sky has been probed at high angular and Faraday depth resolution. We observed a 5x5 degree region centred on the nearby galaxy IC342 using LOFAR in the frequency range 115-178 MHz at 4 arcmin resolution and performed Faraday tomography to detect foreground Galactic polarized synchrotron emission separated by Faraday depth (different amounts of Faraday rotation). Our Faraday depth cube shows rich polarized structure, with up to 30 K of polarized emission at 150 MHz. We detect two overlapping diffuse polarized features that are clearly separated in Faraday depth. Faraday-thick structures at such low frequencies would be too strongly depolarized to explain the observations and are therefore rejected. Only Faraday thin structures will not be strongly depolarized; producing such structures requires localized variations in the ratio of synchrotron emissivity to Faraday depth per unit distance, which can arise from several physical phenomena, such as a transition between regions of ionized and neutral gas. We conclude that the observed polarized emission is Faraday thin, and propose that the emission originates from two neutral clouds in the local ISM. We have modeled the Faraday rotation for this line of sight and estimated that the line of sight component of magnetic field of the local ISM for this direction varies between -0.86 and +0.12 uG. We propose that this may be a useful method for mapping magnetic fields within the local ISM.

Low-radio-frequency eclipses of the redback pulsar J2215+5135 observed in the image plane with LOFAR.

The eclipses of certain types of binary millisecond pulsars (i.e. 'black widows' and 'redbacks') are often studied using high-time-resolution, 'beamformed' radio observations. However, they may also be detected in images generated from interferometric data. As part of a larger imaging project to characterize the variable and transient sky at radio frequencies<200MHz, we have blindly detected the redback system PSR J2215+5135 as a variable source of interest with the Low-Frequency Array (LOFAR). Using observations with cadences of two weeks - six months, we find preliminary evidence that the eclipse duration is frequency dependent (∝ν-0.4), such that the pulsar is eclipsed for longer at lower frequencies, in broad agreement with beamformed studies of other similar sources. Furthermore, the detection of the eclipses in imaging data suggests an eclipsing medium that absorbs the pulsed emission, rather than scattering it. Our study is also a demonstration of the prospects of finding pulsars in wide-field imaging surveys with the current generation of low-frequency radio telescopes.

Calibrating high-precision Faraday rotation measurements for LOFAR and the next generation of low-frequency radio telescopes (Corrigendum)

Calibrating high-precision Faraday rotation measurements for LOFAR and the next generation of low-frequency radio telescopes (Corrigendum)

Calibrating high-precision Faraday rotation measurements for LOFAR and the next generation of low-frequency radio telescopes

Faraday rotation measurements using the current and next generation of low-frequency radio telescopes will provide a powerful probe of astronomical magnetic fields. However, achieving the full potential of these measurements requires accurate removal of the time-variable ionospheric Faraday rotation contribution. We present ionFR, a code that calculates the amount of ionospheric Faraday rotation for a specific epoch, geographic location, and line-of-sight. ionFR uses a number of publicly available, GPS-derived total electron content maps and the most recent release of the International Geomagnetic Reference Field. We describe applications of this code for the calibration of radio polarimetric observations, and demonstrate the high accuracy of its modeled ionospheric Faraday rotations using LOFAR pulsar observations. These show that we can accurately determine some of the highest-precision pulsar rotation measures ever achieved. Precision rotation measures can be used to monitor rotation measure variations - either intrinsic or due to the changing line-of-sight through the interstellar medium. This calibration is particularly important for nearby sources, where the ionosphere can contribute a significant fraction of the observed rotation measure. We also discuss planned improvements to ionFR, as well as the importance of ionospheric Faraday rotation calibration for the emerging generation of low-frequency radio telescopes, such as the SKA and its pathfinders.

VLBI observations of Jupiter with the initial test station of LOFAR and the Nançay decametric array

Aims. To demonstrate and test the capability of the next generation of low-frequency radio telescopes to perform high resolution observations across intra-continental baselines. Jupiter’s strong burst emission is used to perform broadband full signal cross-correlations on time intervals of up to hundreds of milliseconds. Methods. Broadband VLBI observations at about 20 MHz on a baseline of ∼50 000 wavelengths were performed to achieve arcsecond angular resolution. lofar’s Initial Test Station (lofar/its, The Netherlands) and the Nancay Decametric Array (nda, France) digitize the measured electric field with 12 bit and 14 bit in a 40 MHz baseband. The fine structure in Jupiter’s signal was used for data synchronization prior to correlation on the time-series data. Results. Strong emission from Jupiter was detected during snapshots of a few seconds and detailed features down to microsecond time-scales were identified in dynamic spectra. Correlations of Jupiter’s burst emission returned strong fringes on 1 ms time-scales over channels as narrow as a hundred kilohertz bandwidth. Conclusions. Long baseline interferometry is confirmed at low frequencies, in spite of phase shifts introduced by variations in ionospheric propagation characteristics. Phase coherence was preserved over tens to hundreds of milliseconds with a baseline of ∼700 km. No significant variation with time was found in the correlations and an estimate for the fringe visibility of 1, suggested that the source was not resolved. The upper limit on the source region size of Jupiter Io-B S-bursts corresponds to an angular resolution of ∼3 arcsec. Adding remote stations to the lofar network at baselines up to thousand kilometers will provide 10 times higher resolution down to an arcsecond.

Antennas for the next generation of low-frequency radio telescopes

The next generation of large telescopes for radio astronomy at frequencies below 100 MHz will consist of tens of thousands of wide-band dipole-like antennas, each individually instrumented with a receiver and combined using digital signal processing. At these frequencies, the sensitivity of a telescope is limited by Galactic noise, with the result that even simple dipoles can deliver extraordinary useable bandwidth. In this paper the necessary characteristics for these antennas are explained, some bounds on performance are developed, and a few candidate designs are analyzed. It is shown that antenna systems consisting of simple wire dipoles, a 360 K active balun, and a long coaxial feedline can achieve Galactic noise-limited performance over large portions of the range 10-100 MHz. It is further shown that when these antennas are used as elements in a compact array, their Galactic noise-limited characteristics are not significantly affected.

Deflection of Pulsar Signal Reveals Compact Structures in the Galaxy

We have detected a strong deflection of radio waves from the pulsar PSR B0834+06 in scintillation observations. Interference between the undeflected pulsar image and deflected subimages allows single dish interferometry of the interstellar medium with sub-milliarcsecond resolution. We infer the presence of scattering structure(s) similar to those that are thought to cause Extreme Scattering Events in quasar flux monitoring programs: size ~ 0.2 AU (an angular size of 0.1 mas) with an electron density of > 10^3 compared to the warm ionized medium. The deflectors are nearly stationary in a scattering screen that is thin (< 5% of the pulsar-observer distance in extent), is located 70% of the way from the Earth to the pulsar, and has been seen consistently in observations dating back 20 years. The pulsar scans the scattering screen at a velocity of 110 km/s with a detection radius of 15 mas. Pulsar observations such as these -- particularly with a new generation of low-frequency radio telescopes with large collecting areas -- hold promise for improving constraints on the poorly understood physical characteristics and space density of the deflecting structures. Such observations may also prove useful in correcting deviations the deflectors produce in high-precision timing of millarcsecond pulsars.

The Growth of HiiRegions During Reionization

Recently, there has been a great deal of interest in understanding the reionization of hydrogen in the intergalactic medium (IGM). One of the major outstanding questions is how this event proceeds on large scales. Motivated by numerical simulations, we develop a model for the growth of H II regions during the reionization era. We associate ionized regions with large-scale density fluctuations and use the excursion set formalism to model the resulting size distribution. We then consider ways in which to characterize the morphology of ionized regions. We show how to construct the power spectrum of fluctuations in the neutral hydrogen field. The power spectrum contains definite features from the H II regions that should be observable with the next generation of low-frequency radio telescopes through surveys of redshifted 21 cm emission from the reionization era. Finally, we also consider statistical descriptions beyond the power spectrum and show that our model of reionization qualitatively changes the distribution of neutral gas in the IGM.

Statistical Probes of Reionization with 21 Centimeter Tomography

We consider the degree to which 21 cm tomography of the high-redshift universe can distinguish different ionization histories. Using a new analytic model for the size distribution of H II regions that associates these ionized bubbles with large-scale galaxy overdensities, we compute the angular power spectrum and other statistical properties of the 21 cm brightness temperature during reionization. We show that the H II regions imprint features on the power spectrum that allow us to separate histories with discrete bubbles from those with partial uniform ionization (by, for example, X-rays). We also show that double reionization scenarios will modify the morphology of the bubbles in ways that depend on the mechanism through which the first generation of sources shuts off. If, for example, the transition occurs globally at a fixed redshift, the first generation imprints a persistent feature on the 21 cm power spectrum. Finally, we compare our model to one in which voids are ionized first. While the power spectra of these two models are qualitatively similar, we show that the underlying distributions of neutral hydrogen differ dramatically and suggest that other statistical tests can distinguish them. The next generation of low-frequency radio telescopes will have the sensitivity to distinguish all of these models and strongly constrain the history and morphology of reionization.

The 21 Centimeter Forest: Radio Absorption Spectra as Probes of Minihalos before Reionization

We study the absorption along lines of sight toward high-z radio sources caused by the 21 cm transition of neutral hydrogen in the intergalactic medium (IGM) before reionization. Using semianalytic methods, we compute the number density of observable features caused by both minihalos (bound objects that are unable to cool efficiently because of their small virial temperatures) and protogalactic disks. We show that both sets of features should be observable by the next generation of low-frequency radio telescopes, including the Low Frequency Array and the Square Kilometer Array, provided that sufficiently bright background sources exist. The statistics of minihalo absorption features seen along lines of sight to radio-loud quasars offer a way to measure the evolution of the radiation background and the IGM temperature with cosmic time. Intersections with disks are much less common but cause significantly deeper absorption features that would be visible in the spectra of both radio-loud quasars and gamma-ray bursts (GRBs). The absorption feature caused by H I in the host galaxy of a GRB should be observable, offering a route to determine spectroscopically the burst redshift.