Westerbork Northern Sky Survey Research Articles

Strange and Odd Morphology Extragalactic Radio Sources (STROMERSs): A Faint Images of the Radio Sky at Twenty-Centimeters (FIRST) Look at the Strange and Odd Radio Sources

We report the identification of an extremely rare and peculiar set of irregular radio sources, termed “STROMERSs” (STRange and Odd Morphology Extragalactic Radio Sources).ingThe irregular radio sources with very anomalous morphologies that make them exceptionally different from all the known classes and subclasses of irregular radio sources are detected as STROMERSs. A thorough search for this class of sources from the Very Large Array (VLA) Faint Images of the Radio Sky at Twenty-Centimeters (FIRST) gave a total of nine such candidates. We checked the corresponding morphology of the identified sources in other frequency surveys. We found a detectable radio emission for all of the nine sources in the NRAO VLA Sky Survey (NVSS) at 1.4 GHz and in the TIFR GMRT Sky Survey (TGSS) at 150 MHz, while the same was found for only three sources in the Westerbork Northern Sky Survey (WENSS) at 625 MHz. However, the strange morphology was not found in all of those other survey images. We also characterized the sources with their corresponding physical parameters like optical counterpart, size, spectral index, and radio luminosity. ingThe estimated spectral values of the sources indicated that the STROMERSs were most likely radio galaxies. The presence of any nearby galaxy clusters for the STROMERSs was also checked.

Orientation and Accretion in a Representative Sample of Active Galactic Nuclei

Abstract We highlight a representative sample of active galactic nuclei selected independent of orientation. The defining characteristic of the selection is sophisticated matching between the 0.1 < z < 0.6 Sloan Digital Sky Survey quasars from the Seventh Data Release to the Westerbork Northern Sky Survey at 325 MHz and the subsequent application of a spatially integrated radio spectral luminosity density cut. The resulting sample is complete down to the limiting luminosity and unbiased by orientation. Compared to orientation samples in the literature this approach yields less bias with redshift, relatively more lobe-dominated sources including those with radio lobes and no visible core, and a distribution of radio core dominance that is consistent with expectations from a uniform distribution of inclinations with solid angle. We measure properties of the optical spectra, and use the sample to investigate the orientation dependence of the velocity width of the broad Hβ emission line. We recover the known orientation dependence, but the sharp envelope of previous studies where only edge-on sources display the broadest lines, is absent. Scatter in this diagram is not attributable solely to black hole mass, Eddington ratio, or contamination in the sample from compact steep-spectrum sources. A physical framework for quasar beaming and a disk-like broad-line region can describe the representative sample when it is expanded to include additional parameters, in particular jet properties and the broad-line region velocity field. These points serve to illustrate the critical role of sample selection in the interpretation of observable properties as indicators of physical parameters of quasar central engines.

MRG: The Miscellaneous Radio Galaxies from the FIRST Survey

Miscellaneous Radio Galaxies (MRGs) are very rare kind of radio galaxies that exhibit unusual and different jet alignment and orientation from a typical radio galaxy. The peculiar and unique morphology of this type of radio source makes them a special case of study. In this paper, we report the identification of fifteen MRGs from VLA Faint Images of the Radio Sky at Twenty-Centimeters (VLA FIRST) Survey. The MRGs are identified manually by visual inspection of the FIRST database (December, 2017). The individual radio morphology of each sources are cross-checked at different frequencies from other surveys like TIFR GMRT Sky Survey at 150 MHz and Westerbork Northern Sky Survey at 325 MHz. We also identify the associated optical counterparts from the Sloan Digital Sky Survey (SDSS), where available. The different physical parameters like spectral index and radio luminosity of these MRGs are also estimated. An overview on the origin of these unique sources are also drawn.

The LOFAR Long-Baseline Calibrator Survey Classification

Abstract Data manipulation of the LOFAR Long-Baseline Calibrator Survey (LBCS) and the Westerbork Northern Sky Survey (WENSS) catalogues are carried out in the present study. The aim of the study is to make calibrator classification statistics plots and estimations for further observations and upcoming stations. First, mean flux densities of LBCS calibrators against declination and observed station baseline length to the tied core station are plotted and the classified sources are marked. Second, we provide the designation – naming it the success rate – for the number of sources with the correlated signal against all the LBCS catalogue sources. Third, there is a trend in mean peak flux densities between stations – longer the baseline, higher mean peak flux density. Finally, estimations for upcoming and recent stations are made and some results are not encouraging.

Radio relic and the diffuse emission trail discovered in low-mass galaxy cluster Abell 1697

We report the discovery of a putative radio relic, 830 kpc in length, and found towards the outskirts of galaxy cluster Abell 1697 (z = 0.181), using the LOFAR Two Meter Sky Survey at 144 MHz. An X-ray-inferred mass of M500X-ray = 2.9−0.7+0.8 × 1014 M⊙ places Abell 1697 among the least massive relic hosts. The relic is also detected at 325 MHz in the Westerbork Northern Sky Survey and at 1.4 GHz in the NRAO VLA Sky Survey, with an average spectral index of α(144, 325, 1400 MHz) = − 0.98 ± 0.01, and magnetic field of Beq ∼ 0.6 μG. This relic, located in the north-east periphery of the cluster, is 300 kpc wide, exhibits a gradual spectral steepening across the width (α144 MHz1.4 GHz(inj) = −0.70 ± 0.11 to α144 MHz1.4 GHz(edge) = −1.19 ± 0.15), as well as indications of a co-spatial X-ray (ROSAT) shock and the radio relic emission. The radio power of the relic is P1.4 GHz = 8.5 ± 1.1 × 1023 W Hz−1, which is found to be in good agreement with the expected empirical correlation between the radio power and largest linear size of relics. The relic is trailed by extended (790 × 550 kpc) diffuse radio emission towards the cluster centre, which is likely an ultra-steep spectrum (α144 MHz1.4 GHz < −1.84) radio source. This structure is also found to be older by at least 190 Myr, has a very low surface brightness of 0.3 μJy arcsec−2 and magnetic field Beq ∼ 0.8 μG, similar to that of a radio phoenix. Finally, we discuss the possible mechanisms responsible for the relic and the trailing diffuse radio emission, invoking re-acceleration due to wake turbulence, as well as the revival of fossil electrons in the remnant radio lobes of active galactic nuclei by the cluster merger shocks.

Advanced stream search for galaxy clusters with multifrequency microwave data

Based on the data from the Westerbork Northern Sky Survey performed at a frequency of 325 MHz in the range of right ascensions 0h ≤ α < 2h and declinations 29o < δ < 78o and using multifrequency Planck maps, we selected candidate objects with the Sunyaev-Zeldovich effect. The list of the most probable candidates includes 381 sources. It is shown that the search for such objects can be accelerated by using a priori data on the negative level of fluctuations in the CMB map with remote low multipoles in the direction to radio sources.

Search for Candidate Objects with the Sunyaev–Zeldovich Effect in the Radio Source Vicinities—galaxies: clusters: general

Based on the data from the Westerbork Northern Sky Survey performed at a frequency of 325 MHz in the range of right ascensions 0h ≤ α < 2h and declinations 29° < δ < 78° and using multi-frequency Planck maps, we selected candidate objects with the Sunyaev–Zeldovich effect. The list of the most probable candidates includes 381 sources. It is shown that the search for such objects can be accelerated by using a priori data on the negative level of fluctuations in the CMB map with removed low multipoles in the direction to radio sources.

In quest of non-thermal signatures in early-type stars

A reduced fraction of luminous, early-type stars in binaries has provided some of the most interesting sources in modern high-energy astrophysics. A fingerprint of the capability of these systems to accelerate particles up to TeV energies is the associated detection of non-thermal, synchrotron emission often in the radio domain. Here we aim to identify new early-type, luminous stars where energetic, non-thermal processes are at work to enable future comparative studies based on an extended sample. Moreover, these objects also appear as very interesting targets for future gamma-ray observatories such as the Cherenkov Telescope Array. We have designed a methodology to search for new examples of these interesting sources in order to enlarge the extremely reduced population currently known. Our search procedure is described in this paper, together with a practical application using public databases and catalogues currently available (Luminous Stars in the Northern Milky Way, NRAO VLA Sky Survey, and Westerbork Northern Sky Survey). Optical and radio interferometric follow-up observations of selected candidate stars were conducted to better assess their properties. Although no new discoveries of this kind have been achieved yet, the observational data analyzed in this work does provide some interesting side results, such as characterizing the spectroscopic and photometric properties of two early-type stars originally selected as candidates to be explored, namely TYC4051-1277-1 and TYC3594-2269-1. Unexpectedly, this last one was also found to be a new DAO-type white dwarf star instead of a non-degenerate star. In addition, both stars seem to display different optical periods based on our photometric monitoring.

INTEGRATED RADIO CONTINUUM SPECTRA OF GALAXIES

We investigate the spectral shape of the total continuum radiation, between 74 MHz and 5 GHz (400 to 6 cm in wavelength), for a large sample of bright galaxies. We take advantage of the overlapping survey coverage of the VLA Low-Frequency Sky Survey, the Westerbork Northern Sky Survey, the NRAO VLA Sky Survey, and the Green Bank 6 cm survey to achieve significantly better resolution, sensitivity, and sample size compared to prior efforts of this nature. For our sample of 250 bright galaxies we measure a mean spectral index, ${\alpha}$, of -0.69 between 1.4 and 4.85 GHz, -0.55 between 325 MHz and 1.4 GHz, and -0.45 between 74 and 325 MHz, which amounts to a detection of curvature in the mean spectrum. The magnitude of this curvature is approximately ${\Delta \alpha}$ = -0.2 per logarithmic frequency decade when fit with a generalized function having constant curvature. No trend in low frequency spectral flattening versus galaxy inclination is evident in our data, suggesting that free-free absorption is not a satisfying explanation for the observed curvature. The ratio of thermal to non-thermal emission is estimated by two independent methods, (1) using the IRAS far-IR fluxes, and (2) with the value of the total spectral index. Method (1) results in a distribution of 1.4 GHz thermal fractions of 9% ${\pm}$ 3%, which is consistent with previous studies, while method (2) produces a mean 1.4 GHz thermal fraction of 51% with dispersion 26%. The highly implausible values produced by method (2) indicate that the sum of typical power-law thermal and non-thermal components is not a viable model for the total spectral index between 325 and 1.4 GHz.

THE LOW-FREQUENCY RADIO CATALOG OF FLAT-SPECTRUM SOURCES

A well known property of the gamma-ray sources detected by COS-B in the 1970s, by the Compton Gamma-ray Observatory in the 1990s and recently by the Fermi observations is the presence of radio counterparts, in particular for those associated to extragalactic objects. This observational evidence is the basis of the radio-gamma-ray connection established for the class of active galactic nuclei known as blazars. In particular, the main spectral property of the radio counterparts associated with gamma-ray blazars is that they show a flat spectrum in the GHz frequency range. Our recent analysis dedicated to search blazar-like candidates as potential counterparts for the unidentified gamma-ray sources (UGSs) allowed us to extend the radio-gamma-ray connection in the MHz regime. We also showed that below 1 GHz blazars maintain flat radio spectra. Thus on the basis of these new results, we assembled a low-frequency radio catalog of flat spectrum sources built by combining the radio observations of the Westerbork Northern Sky Survey (WENSS) and of the Westerbork in the southern hemisphere (WISH) catalog with those of the NRAO Very Large Array Sky survey (NVSS). This could be used in the future to search for new, unknown blazar-like counterparts of the gamma-ray sources. First we found NVSS counterparts of WSRT radio sources and then we selected flat spectrum radio sources according to a new spectral criterion specifically defined for radio observations performed below 1 GHz. We also described the main properties of the catalog listing 28358 radio sources and their logN-logS distributions. Finally a comparison with with the Green Bank 6-cm radio source catalog has been performed to investigate the spectral shape of the low-frequency flat spectrum radio sources at higher frequencies.

Unveiling the nature of the unidentified gamma-ray sources: blazar counterparts at low radio frequencies

AbstractAbout one third of the gamma-ray sources detected by Fermi have still no firmly established counterpart at lower energies. Here we propose a new approach to find candidate counterparts for the unidentified gamma-ray sources (UGSs) based on the 325 MHz radio survey performed with Westerbork Synthesis Radio Telescope (WSRT) in the northern hemisphere. First we investigate the low-frequency radio properties of blazars, the largest known population of gamma-ray sources; then we search for sources with similar radio properties combining the information derived from the Westerbork Northern Sky Survey (WENSS) with those of the NRAO VLA Sky survey (NVSS). We present a list of candidate counterparts for 32 UGSs with at least one counterpart in the WENSS. We also performed an extensive research in literature to look for infrared and optical counterparts of the gamma-ray blazar candidates selected with the low-frequency radio observations to confirm their nature. On the basis of our multifrequency research we identify 23 new gamma-ray blazar candidates out of 32 UGSs investigated. I will also present the first analysis of very low frequency radio emission of blazars based on the recent Very Large Array Low-Frequency Sky Survey (VLSS) at 74 MHz. I show that blazars present radio flat spectra when evaluated at 74 MHz, about an order of magnitude in frequency lower than previous analyses. The implications of these findings in the contest of the blazars – radio galaxies connection will be discussed.

BLAZAR SPECTRAL PROPERTIES AT 74 MHz

Blazars are the most extreme class of active galactic nuclei (AGNs). Despite a previous investigation at 102 MHz for a small sample of BL Lacs and our recent analysis of blazars detected in the Westerbork Northern Sky Survey (WENSS), a systematic study of the blazar spectral properties at frequencies below 100 MHz has been never carried out. In this paper, we present the first analysis of the radio spectral behavior of blazars based on the recent Very Large Array Low-Frequency Sky Survey (VLSS) at 74 MHz. We search for blazar counterparts in the VLSS catalog confirming that they are detected at 74 MHz. We then show that blazars present radio flat spectra {(i.e., radio spectral indices ~0.5)} when evaluated also about an order of magnitude in frequency lower than previous analyses. Finally, we discuss the implications of our findings in the context of the blazars - radio galaxies connection since the low frequency radio data provide a new diagnostic tool to verify the expectations of the unification scenario for radio-loud active galaxies.

Cosmic radio dipole from NVSS and WENSS

We use linear estimators to determine the magnitude and direction of the cosmic radio dipole from the NRAO VLA Sky Survey (NVSS) and the Westerbork Northern Sky Survey (WENSS). We show that special attention has to be given to the issues of bias due to shot noise, incomplete sky coverage and masking of the Milky Way. We compare several different estimators and show that conflicting claims in the literature can be attributed to the use of different estimators. We find that the NVSS and WENSS estimates of the cosmic radio dipole are consistent with each other and with the direction of the cosmic microwave background (CMB) dipole. We find from the NVSS a dipole amplitude of $(1.6 \pm 0.6) \times 10^{-2}$ in direction $(\mathrm{RA}, \mathrm{dec})= (154^\circ \pm 21^\circ, -2^\circ \pm 21^\circ)$. This amplitude exceeds the one expected from the CMB by a factor of about 3 and is inconsistent with the assumption of a pure kinetic origin of the radio dipole at 99.5% CL.

UNVEILING THE NATURE OF THE UNIDENTIFIED GAMMA-RAY SOURCES. III. GAMMA-RAY BLAZAR-LIKE COUNTERPARTS AT LOW RADIO FREQUENCIES

About one third of the gamma-ray sources listed in the second Fermi LAT catalog (2FGL) have no firmly established counterpart at lower energies so being classified as unidentified gamma-ray sources (UGSs). Here we propose a new approach to find candidate counterparts for the UGSs based on the 325 MHz radio survey performed with Westerbork Synthesis Radio Telescope (WSRT) in the northern hemisphere. First we investigate the low-frequency radio properties of blazars, the largest known population of gamma-ray sources; then we search for sources with similar radio properties combining the information derived from the Westerbork Northern Sky Survey (WENSS) with those of the NRAO VLA Sky survey (NVSS). We present a list of candidate counterparts for 32 UGSs with at least one counterpart in the WENSS. We also performed an extensive research in literature to look for infrared and optical counterparts of the gamma-ray blazar candidates selected with the low-frequency radio observations to confirm their nature. On the basis of our multifrequency research we identify 23 new gamma-ray blazar candidates out of 32 UGSs investigated. Comparison with previous results on the UGSs are also presented. Finally, we speculate on the advantages on the use of the low-frequency radio observations to associate UGSs and to search for gamma-ray pulsar candidates.

Orientation effects in quasar spectra: the broad- and narrow-line regions

We use the SDSS, along with the NVSS and the WENSS to define a sample of 746 radio-loud quasars and measure their 330MHz to 1.4GHz spectral indexes. Following previous authors we take the spectral index as an indicator of the orientation towards the quasars with more pole-on sources having flatter spectral indexes. We use this proxy for orientation to investigate the effect observing angle may have on optical spectra. Quasars with flatter spectral indexes tend to be brighter. However, we find no indication of reddening in steep-spectrum objects to indicate obscuration by a torus as a possible explanation. Nor do we find increased redddening in the flat-spectrum sources which could imply a contribution from jet-related emission. We reproduce a previously-described anti-correlation between the width of the MgII line and radio spectral index indicating a disk-like geometry for the MgII BLR. In contrast to previous authors we find no such correlation for the CIV line suggesting a more isotropic high-ionisation BLR. Both the [OII] and [OIII] narrow lines have more flux in steep spectrum sources while the [OII]/[OIII] flux ratio is lower in these sources. To describe both of these effects we propose a simple geometric model in which the NLR exists primarily on the surface of optically thick clouds facing the active nucleus and the NLR is stratified such that higher-ionisation lines are found preferentially closer to the nucleus. Quantitatively we find that orientation may effect the observed strength of narrow lines, as well as ratios between lines, by a factor of ~2. These findings have implications for the use of narrow emission lines to estimate bolometric luminosities, as well as comparisons between narrow line luminosity functions for type 1s and type 2s, and the potential of emission-line diagnostic diagrams as an accurate tool with which to distinguish classes of AGN.

Deep low-frequency observations with the Giant Metrewave Radio Telescope: a search for relic radio emission

We present deep multifrequency observations using the Giant Metrewave Radio Telescope at 153, 244, 610 and 1260 MHz of a field centered on J0916+6348, to search for evidence of fossil radio lobes which could be due to an earlier cycle of episodic activity of the parent galaxy, as well as halos and relics in clusters of galaxies. We do not find any unambiguous evidence of episodic activity in a list of 374 sources, suggesting that such activity is rare even in relatively deep low-frequency observations. We examine the spectra of all the sources by combining our observations with those from the Westerbork Northern Sky Survey, NRAO VLA Sky Survey and the Faint Images of the Radio Sky at Twenty-centimeters survey. Considering only those which have measurements at a minimum of three different frequencies, we find that almost all sources are consistent with a straight spectrum with a median spectral index, $\alpha\sim$0.8 (S$(\nu)\propto\nu^{-\alpha}$), which appears steeper than theoretical expectations of the injection spectral index. We identify 14 very steep-spectrum sources with $\alpha\geq$1.3. We examine their optical fields and discuss the nature of some of these sources.

A UNIFIED CATALOG OF RADIO OBJECTS DETECTED BY NVSS, FIRST, WENSS, GB6, AND SDSS

We construct a catalog of radio sources detected by the Green Bank 6 cm survey GB6, Faint Images of the Radio Sky at Twenty cm (FIRST) and NRAO VLA Sky Survey (NVSS) (20 cm), and Westerbork Northern Sky Survey (WENSS) (92 cm) radio surveys, and the Sloan Digital Sky Survey (SDSS) optical survey. The 2.7 million entries in the publicly-available master catalog are comprised of the closest three FIRST to NVSS matches (within 30'') and vice versa, and unmatched sources from each survey. Entries are supplemented by data from the other radio and optical surveys, where available. All objects with even a small probability of physical association are included, such that catalog users can easily implement their own selection criteria for data analysis. We perform data analysis in the ~ 3000 deg2 region of sky where the surveys overlap, which contains 140,000 NVSS-FIRST sources, of which 64,000 are detected by WENSS and 12,000 by GB6. About one third of each sample is detected by the SDSS. An automated classification method based on 20 cm fluxes defines three radio morphology classes: complex, resolved, and compact. Radio color-magnitude-morphology diagrams for these classes show structure suggestive of strong underlying physical correlations. Complex and resolved sources tend to have a steep spectral slope (α ~ –0.8) that is nearly constant from 6 to 92 cm, while the compact class (unresolved on ~5'' scale by FIRST) contains a significant number of flat-spectrum (α ~ 0) sources. In the optically-detected sample, quasars dominate the flat-spectrum compact sources while steep-spectrum and resolved objects contain substantial numbers of both quasars and galaxies. Differential radio counts of quasars and galaxies are similar at bright flux levels (>100 mJy at 20 cm), while at fainter levels the quasar counts are significantly reduced below galaxy counts. The optically-undetected sample is strongly biased toward steep-spectrum sources. In samples of quasars and galaxies with SDSS spectra (2885 and 1288 respectively), we find that radio properties such as spectral slope, morphology, and radio loudness are correlated with optical color and luminosity.

A Deep, High‐Resolution Survey of the Low‐Frequency Radio Sky

We report on the first wide-field, very long baseline interferometry (VLBI) survey at 90 cm. The survey area consists of two overlapping 28 deg^2 fields centred on the quasar J0226+3421 and the gravitational lens B0218+357. A total of 618 sources were targeted in these fields, based on identifications from Westerbork Northern Sky Survey (WENSS) data. Of these sources, 272 had flux densities that, if unresolved, would fall above the sensitivity limit of the VLBI observations. A total of 27 sources were detected as far as 2 arcdegrees from the phase centre. The results of the survey suggest that at least 10% of moderately faint (S~100 mJy) sources found at 90 cm contain compact components smaller than ~0.1 to 0.3 arcsec and stronger than 10% of their total flux densities. A ~90 mJy source was detected in the VLBI data that was not seen in the WENSS and NRAO VLA Sky Survey (NVSS) data and may be a transient or highly variable source that has been serendipitously detected. This survey is the first systematic (and non-biased), deep, high-resolution survey of the low-frequency radio sky. It is also the widest field of view VLBI survey with a single pointing to date, exceeding the total survey area of previous higher frequency surveys by two orders of magnitude. These initial results suggest that new low frequency telescopes, such as LOFAR, should detect many compact radio sources and that plans to extend these arrays to baselines of several thousand kilometres are warranted.

The cosmic evolution of low-luminosity radio sources from the Sloan Digital Sky Survey Data Release 1

In this paper we constrain the evolution in the comoving space density for low-luminosity [typically Fanaroff–Riley type I (FRI)] radio sources as a function of cosmic epoch by matching deep radio surveys [Westerbork Northern Sky Survey (WENSS), Faint Images of the Radio Sky at Twenty-centimetres (FIRST) and NRAO VLA Sky Survey (NVSS)] with the Sloan Digital Sky Survey (SDSS). This results in 1014 matched radio sources covering an effective area of 0.217 sr, 686 with L325 MHz < 1025 W Hz−1 sr−1, which is an order of magnitude larger than any previous study at these luminosities. Using the non-parametric V/Vmax test we show that low-luminosity radio sources evolve differently from their more powerful, predominantly Fanaroff–Riley type II (FRII), counterparts. Indeed, we find that the lower luminosity population is consistent with a constant comoving space density with redshift, as opposed to the strong positive evolution for the more powerful sources.

A Wide‐Field, Low‐Frequency Radio Survey of the Field of M31. I. Construction and Statistical Analysis of the Source Catalog

We present here the results of a 325 MHz radio survey of M31, conducted with the A configuration of the Very Large Array. The survey covered an area of 7.6 deg2, and a total of 405 radio sources between ≲6'' and 170'' in extent were mapped with a resolution of 6'' and a 1 σ sensitivity of ~0.6 mJy beam-1. For each source, its morphological class, major axis θM, minor axis θm, position angle θPA, peak flux I, integrated flux density S, spectral index α, and spectral curvature parameter φ were calculated. A comparison of the flux and radial distribution—both in the plane of the sky and in the plane of M31—of these sources with those of the XMM-Newton Large-Scale Structure Survey and Westerbork Northern Sky Survey revealed that a vast majority of sources detected are background radio galaxies. As a result of this analysis, we expect that only a few sources are intrinsic to M31. These sources are identified and discussed in an accompanying paper.