scholarly journals Blazars in the LOFAR Two-Metre Sky Survey first data release

2019 ◽  
Vol 622 ◽  
pp. A14 ◽  
Author(s):  
S. Mooney ◽  
J. Quinn ◽  
J. R. Callingham ◽  
R. Morganti ◽  
K. Duncan ◽  
...  
Keyword(s):  
Spectral Index ◽  
Low Frequency ◽  
Value Added ◽  
Low Frequencies ◽  
Sample Distribution ◽  
Sky Survey ◽  
Data Release ◽  
Frequency Regime ◽  
Source Confusion ◽  
Bl Lacs

Historically, the blazar population has been poorly understood at low frequencies because survey sensitivity and angular resolution limitations have made it difficult to identify megahertz counterparts. We used the LOFAR Two-Metre Sky Survey (LoTSS) first data release value-added catalogue (LDR1) to study blazars in the low-frequency regime with unprecedented sensitivity and resolution. We identified radio counterparts to all 98 known sources from the Third Fermi-LAT Point Source Catalogue (3FGL) or Roma-BZCAT Multi-frequency Catalogue of Blazars (5th edition) that fall within the LDR1 footprint. Only the 3FGL unidentified γ-ray sources (UGS) could not be firmly associated with an LDR1 source; this was due to source confusion. We examined the redshift and radio luminosity distributions of our sample, finding flat-spectrum radio quasars (FSRQs) to be more distant and more luminous than BL Lacertae objects (BL Lacs) on average. Blazars are known to have flat spectra in the gigahertz regime but we found this to extend down to 144 MHz, where the radio spectral index, α, of our sample is −0.17 ± 0.14. For BL Lacs, α = −0.13 ± 0.16 and for FSRQs, α = −0.15 ± 0.17. We also investigated the radio-to-γ-ray connection for the 30 γ-ray-detected sources in our sample. We find Pearson’s correlation coefficient is 0.45 (p = 0.069). This tentative correlation and the flatness of the spectral index suggest that the beamed core emission contributes to the low-frequency flux density. We compare our sample distribution with that of the full LDR1 on colour-colour diagrams, and we use this information to identify possible radio counterparts to two of the four UGS within the LDR1 field. We will refine our results as LoTSS continues.

scholarly journals The origin of radio emission in broad absorption line quasars: Results from the LOFAR Two-metre Sky Survey

2019 ◽  
Vol 622 ◽  
pp. A15 ◽  
Author(s):  
L. K. Morabito ◽  
J. H. Matthews ◽  
P. N. Best ◽  
G. Gürkan ◽  
M. J. Jarvis ◽  
...  
Keyword(s):  
Radio Emission ◽  
Absorption Line ◽  
Low Frequency ◽  
Value Added ◽  
Sloan Digital Sky Survey ◽  
Broad Absorption ◽  
Low Frequencies ◽  
Broad Absorption Line ◽  
Radio Detection ◽  
Sky Survey

We present a study of the low-frequency radio properties of broad absorption line quasars (BALQSOs) from the LOFAR Two-metre Sky-Survey Data Release 1 (LDR1). The value-added LDR1 catalogue contains Pan-STARRS counterparts, which we match with the Sloan Digital Sky Survey (SDSS) DR7 and DR12 quasar catalogues. We find that BALQSOs are twice as likely to be detected at 144 MHz than their non-BAL counterparts, and BALQSOs with low-ionisation species present in their spectra are three times more likely to be detected than those with only high-ionisation species. The BALQSO fraction at 144 MHz is constant with increasing radio luminosity, which is inconsistent with previous results at 1.4 GHz, indicating that observations at the different frequencies may be tracing different sources of radio emission. We cross-match radio sources between the Faint Images of the Radio Sky at Twenty Centimeters (FIRST) survey and LDR1, which provides a bridge via the LDR1 Pan-STARRS counterparts to identify BALQSOs in SDSS. Consequently we expand the sample of BALQSOs detected in FIRST by a factor of three. The LDR1-detected BALQSOs in our sample are almost exclusively radio-quiet (log(R144 MHz) < 2), with radio sizes at 144 MHz typically less than 200 kpc; these radio sizes tend to be larger than those at 1.4 GHz, suggesting more extended radio emission at low frequencies. We find that although the radio detection fraction increases with increasing balnicity index (BI), there is no correlation between BI and either low-frequency radio power or radio-loudness. This suggests that both radio emission and BI may be linked to the same underlying process, but are spatially distinct phenomena.

scholarly journals Giant radio galaxies in the LOFAR Two-metre Sky Survey

2020 ◽  
Vol 635 ◽  
pp. A5 ◽  
Author(s):  
P. Dabhade ◽  
H. J. A. Röttgering ◽  
J. Bagchi ◽  
T. W. Shimwell ◽  
M. J. Hardcastle ◽  
...  
Keyword(s):  
Spectral Index ◽  
Radio Galaxy ◽  
Radio Galaxies ◽  
Large Sample ◽  
Search Techniques ◽  
Sky Survey ◽  
Data Release ◽  
Cluster Environment ◽  
First Time

Giant radio galaxies (GRGs) are a subclass of radio galaxies, which have grown to megaparsec scales. GRGs are much rarer than normal-sized radio galaxies (< 0.7 Mpc) and the reason for their gigantic sizes is still debated. Here, we report on the biggest sample of GRGs identified to date. These objects were found in the LOFAR Two-metre Sky Survey first data release images, which cover a 424 deg2 region. Of the 239 GRGs found, 225 are new discoveries. The GRGs in our sample have sizes ranging from 0.7 Mpc to 3.5 Mpc and have redshifts (z) between 0.1 and 2.3. Seven GRGs have sizes above 2 Mpc and one has a size of ∼3.5 Mpc. The sample contains 40 GRGs hosted by spectroscopically confirmed quasars. Here, we present the search techniques employed and the resulting catalogue of the newly discovered large sample of GRGs along with their radio properties. In this paper, we also show for the first time that the spectral index of GRGs is similar to that of normal-sized radio galaxies, indicating that most of the GRG population is not dead or is not similar to a remnant-type radio galaxy. We find that 20 out of 239 GRGs in our sample are located at the centres of clusters and we present our analysis on their cluster environment and radio morphology.

scholarly journals A seismic vertical vibrator driven by linear synchronous motors

Geophysics ◽  
2015 ◽  
Vol 80 (2) ◽  
pp. EN57-EN67 ◽  
Author(s):  
Rik Noorlandt ◽  
Guy Drijkoningen ◽  
Johan Dams ◽  
Rob Jenneskens
Keyword(s):  
Low Frequency ◽  
Field Tests ◽  
Seismic Source ◽  
Neutral Position ◽  
Low Frequencies ◽  
Synchronous Motors ◽  
Frequency Regime ◽  
Linear Synchronous Motors ◽  
Spring Mechanism ◽  
The Individual

A linear synchronous motor (LSM) is an electric motor that can produce large controllable forces and is therefore suitable as a driving engine for a seismic vibrator. This motor consists of two independent elements, a magnet track and a coil track, allowing practically unlimited motor displacements. This makes the LSM very suitable for expanding the source frequency band to the lower frequencies in which larger strokes are needed. In contrast to hydraulic engines, the LSM performs equally well over the whole frequency range, making possible a smaller amount of signal distortion, especially at the low frequencies. To find the feasibility of an LSM-driven vibrator, we successfully designed and built a multi-LSM prototype vibrator of some 1200 kg. We addressed the synchronization between the individual motor tracks and the different motors. To lower the energy consumption, a spring mechanism was implemented that delivered the force needed to lift the vibrator mass to its neutral position. The resonance belonging to this spring mechanism was successfully suppressed with the help of a position feedback control that also suppressed the temperature effects. The seismic data acquired in the field tests proved that the prototype LSM vibrator acted very well as a seismic source. It has no trouble generating pseudorandom sweeps, and even given its limited size, it generated signals within the low-frequency regime, down to 2 Hz, rather easily.

scholarly journals First look at the giant radio galaxy 3C 236 with LOFAR

2019 ◽  
Vol 628 ◽  
pp. A69 ◽  
Author(s):  
A. Shulevski ◽  
P. D. Barthel ◽  
R. Morganti ◽  
J. J. Harwood ◽  
M. Brienza ◽  
...  
Keyword(s):  
Spectral Index ◽  
Radio Galaxy ◽  
Ambient Medium ◽  
Low Frequency ◽  
Low Frequencies ◽  
North West ◽  
Link Type ◽  
The North ◽  
Morphological Differences ◽  
Inner Region

We have examined the giant radio galaxy 3C 236 using LOFAR at 143 MHz down to an angular resolution of 7″, in combination with observations at higher frequencies. We used the low frequency data to derive spectral index maps with the highest resolution yet at these low frequencies. We confirm a previous detection of an inner hotspot in the north-west lobe and for the first time observe that the south-east lobe hotspot is in fact a triple hotspot, which may point to an intermittent source activity. Also, the spectral index map of 3C 236 shows that the spectral steepening at the inner region of the northern lobe is prominent at low frequencies. The outer regions of both lobes show spectral flattening, in contrast with previous high frequency studies. We derive spectral age estimates for the lobes, as well as particle densities of the IGM at various locations. We propose that the morphological differences between the lobes are driven by variations in the ambient medium density as well as the source activity history.

scholarly journals Dipolar Excitation of a Perfectly Electrically Conducting Spheroid in a Lossless Medium at the Low-Frequency Regime

2018 ◽  
Vol 2018 ◽  
pp. 1-20 ◽  
Author(s):  
Panayiotis Vafeas
Keyword(s):  
Electric Fields ◽  
Vector Fields ◽  
Dimensional Space ◽  
Scattering Problem ◽  
Low Frequency ◽  
Main Idea ◽  
Wave Number ◽  
Low Frequencies ◽  
Static Approximation ◽  
Frequency Regime

The electromagnetic vector fields, which are scattered off a highly conductive spheroid that is embedded within an otherwise lossless medium, are investigated in this contribution. A time-harmonic magnetic dipolar source, located nearby and operating at low frequencies, serves as the excitation primary field, being arbitrarily orientated in the three-dimensional space. The main idea is to obtain an analytical solution of this scattering problem, using the appropriate system of spheroidal coordinates, such that a possibly fast numerical estimation of the scattered fields could be useful for real data inversion. To this end, incident and scattered as well as total fields are written in a rigorous low-frequency manner in terms of positive integral powers of the real-valued wave number of the exterior environment. Then, the Maxwell-type problem is converted to interconnected Laplace’s or Poisson’s equations, complemented by the perfectly conducting boundary conditions on the spheroidal object and the necessary radiation behavior at infinity. The static approximation and the three first dynamic contributors are sufficient for the present study, while terms of higher orders are neglected at the low-frequency regime. Henceforth, the 3D scattering boundary value problems are solved incrementally, whereas the determination of the unknown constant coefficients leads either to concrete expressions or to infinite linear algebraic systems, which can be readily solved by implementing standard cut-off techniques. The nonaxisymmetric scattered magnetic and electric fields follow and they are obtained in an analytical compact fashion via infinite series expansions in spheroidal eigenfunctions. In order to demonstrate the efficiency of our analytical approach, the results are degenerated so as to recover the spherical case, which validates this approach.

scholarly journals A low frequency study of linear polarization in radio galaxies

2020 ◽  
Author(s):  
V H Mahatma ◽  
M J Hardcastle ◽  
J Harwood ◽  
S P O’Sullivan ◽  
G Heald ◽  
...  
Keyword(s):  
Flux Density ◽  
Angular Size ◽  
Low Frequency ◽  
Radio Galaxies ◽  
Low Frequencies ◽  
Sky Survey ◽  
Linearly Polarized ◽  
Low Mass ◽  
The Magnetic Field ◽  
Cent Detection

Abstract Radio galaxies are linearly polarized – an important property that allows us to infer the properties of the magnetic field of the source and its environment. However at low frequencies, Faraday rotation substantially depolarizes the emission, meaning that comparatively few polarized radio galaxies are known at low frequencies. Using the LOFAR Two Metre Sky Survey at 150 MHz and at 20 arcsec resolution, we select 342 radio galaxies brighter than 50 mJy and larger than 100 arcsec in angular size, of which 67 are polarized (18 per cent detection fraction). These are predominantly Fanaroff Riley type II (FR-II) sources. The detection fraction increases with total flux density, and exceeds 50 per cent for sources brighter than 1 Jy. We compare the sources in our sample detected by LOFAR to those also detected in NVSS at 1400 MHz, and find that our selection bias toward bright radio galaxies drives a tendency for sources depolarized between 1400 and 150 MHz to have flatter spectra over that frequency range than those that remain polarized at 150 MHz. By comparing observed rotation measures with an analytic model we find that we are preferentially sensitive to sources in low mass environments. We also infer that sources with one polarized hotspot are inclined by a small angle to the line of sight, while sources with hotspots in both lobes lie in the plane of the sky. We conclude that low frequency polarization in radio galaxies is related to a combination of environment, flux density and jet orientation.

scholarly journals Placing LOFAR-detected quasars in C iv emission space: implications for winds, jets and star formation

2021 ◽  
Vol 502 (3) ◽  
pp. 4154-4169
Author(s):  
Amy L Rankine ◽  
James H Matthews ◽  
Paul C Hewett ◽  
Manda Banerji ◽  
Leah K Morabito ◽  
...  
Keyword(s):  
Star Formation ◽  
Low Frequency ◽  
Sloan Digital Sky Survey ◽  
Baldwin Effect ◽  
Ultraviolet Emission ◽  
X Ray ◽  
Bolometric Luminosity ◽  
Radio Detection ◽  
Sky Survey ◽  
Data Release

ABSTRACT We present an investigation of the low-frequency radio and ultraviolet properties of a sample of ≃10 500 quasars from the Sloan Digital Sky Survey Data Release 14, observed as part of the first data release of the Low-Frequency-Array Two-metre Sky Survey. The quasars have redshifts 1.5 &lt; z &lt; 3.5 and luminosities $44.6 \lt \log _{10}\left(L_{\text{bol}}/\rm{erg\,s}^{-1}\right) \lt 47.2$. We employ ultraviolet spectral reconstructions based on an independent component analysis to parametrize the C iv λ1549-emission line that is used to infer the strength of accretion disc winds, and the He ii λ1640 line, an indicator of the soft X-ray flux. We find that radio-detected quasars are found in the same region of C iv blueshift versus equivalent-width space as radio-undetected quasars, but that the loudest, most luminous and largest radio sources exist preferentially at low C iv blueshifts. Additionally, the radio-detection fraction increases with blueshift whereas the radio-loud fraction decreases. In the radio-quiet population, we observe a range of He ii equivalent widths as well as a Baldwin effect with bolometric luminosity, whilst the radio-loud population has mostly strong He ii, consistent with a stronger soft X-ray flux. The presence of strong He ii is a necessary but not sufficient condition to detect radio-loud emission suggesting some degree of stochasticity in jet formation. Using energetic arguments and Monte Carlo simulations, we explore the plausibility of winds, compact jets, and star formation as sources of the radio quiet emission, ruling out none. The existence of quasars with similar ultraviolet properties but differing radio properties suggests, perhaps, that the radio and ultraviolet emission is tracing activity occurring on different time-scales.

scholarly journals The Power of Low Frequencies: Faraday Tomography in the Sub-GHz Regime

Galaxies ◽  
2018 ◽  
Vol 6 (4) ◽  
pp. 112 ◽  
Author(s):  
Cameron Van Eck
Keyword(s):  
Magnetic Fields ◽  
Interstellar Medium ◽  
Frequency Dependence ◽  
Faraday Rotation ◽  
Low Frequency ◽  
Low Frequencies ◽  
Polarized Emission ◽  
Frequency Regime ◽  
Nearby Galaxies ◽  
Murchison Widefield Array

Faraday tomography, the study of the distribution of extended polarized emission by strength of Faraday rotation, is a powerful tool for studying magnetic fields in the interstellar medium of our Galaxy and nearby galaxies. The strong frequency dependence of Faraday rotation results in very different observational strengths and limitations for different frequency regimes. I discuss the role these effects take in Faraday tomography below 1 GHz, emphasizing the 100–200 MHz band observed by the Low Frequency Array and the Murchison Widefield Array. With that theoretical context, I review recent Faraday tomography results in this frequency regime, and discuss expectations for future observations.

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

2018 ◽  
Vol 613 ◽  
pp. A58 ◽  
Author(s):  
C. L. Van Eck ◽  
M. Haverkorn ◽  
M. I. R. Alves ◽  
R. Beck ◽  
P. Best ◽  
...  
Keyword(s):  
Low Frequency ◽  
Small Population ◽  
Point Sources ◽  
Source Surface ◽  
Radio Sources ◽  
Low Frequencies ◽  
Sky Survey ◽  
Region 10 ◽  
New Generation ◽  
Right Ascension

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.

scholarly journals The UTR-2 Very Low Frequency Sky Survey and Its Main Results

1998 ◽  
Vol 179 ◽  
pp. 100-102
Author(s):  
K.P. Sokolov
Keyword(s):  
High Frequency ◽  
Surface Brightness ◽  
Low Frequency ◽  
Source Population ◽  
Low Frequencies ◽  
Physical Conditions ◽  
The Past ◽  
Sky Survey ◽  
Low Surface Brightness ◽  
Compact Sources

During the past decade there has been a dramatic increase in the amount of high-frequency (v > 1000 MHz) data currently underlying the studies of bright compact sources (Tb ∼ 1011–12K, l ≤ 1 kpc) with flat spectra. But in order to determine physical conditions inside extragalactic radio sources at different stages of their evolution the studies of old extended (l ∼ 100 kpc) sources with low surface brightness and steep spectra which constitute the dominant radio source population at very low frequencies (v ≪ 100 MHz) are also needed. These sources are known to represent the final stage in the evolution of extragalactic objects.

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