- ID:
- ivo://CDS.VizieR/J/A+A/631/A49
- Title:
- LOFAR images of blazar S5 0836+710
- Short Name:
- J/A+A/631/A49
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The emission and proper motion of the terminal hotspots of active galactic nucleus (AGN) jets can be used as a powerful probe of the intergalactic medium. However, measurements of hotspot advance speeds in active galaxies are difficult, especially in the young universe, because of the low angular velocities and the low brightness of distant radio galaxies. Our goal is to study the termination of an AGN jet in the young universe and to deduce physical parameters of the jet and the intergalactic medium. We used the LOw Frequency ARray (LOFAR) to image the long-wavelength radio emission of the high-redshift blazar S5 0836+710 on arcsecond scales between 120MHz and 160MHz. The LOFAR image shows a compact unresolved core and a resolved emission region about 1.5 arcsec to the southwest of the radio core. This structure is in general agreement with previous higher-frequency radio observations with the Multi-Element Radio-Linked Interferometer Network (MERLIN) and the Very Large Array (VLA). The southern component shows a moderately steep spectrum with a spectral index of about >~-1,and the spectral index of the core is flat to slightly inverted. In addition, we detect for the first time a resolved steep-spectrum halo with a spectral index of about -1 surrounding the core. The arcsecond-scale radio structure of S5 0836+710 can be understood as a Faranoff-Riley (FR) II radio galaxy observed at a small viewing angle. The southern component can be interpreted as the region of the approaching jet's terminal hotspot, and the halo like diffuse component near the core can be interpreted as the counter-hotspot region. From the differentialDoppler boosting of both features, we can derive the hotspot advance speed to (0.01-0.036)c. Ata constant advance speed, the derived age of the source would exceed the total lifetime of such a powerful FR II radio galaxy substantially. Thus, the hotspot advance speed must have been higher in the past, in agreement with a scenario in which the originally highly relativistic jet has lost collimation as a result of instability growth and has transformed into an only mildly relativistic flow. Our data suggest that the density of the intergalactic medium around this distant (z=2.22) AGN could be substantially higher than the values typically found in less distant FR II radio galaxies.
Number of results to display per page
Search Results
- ID:
- ivo://CDS.VizieR/J/A+A/658/A4
- Title:
- LOFAR imaging of Arp299 at 150MHz
- Short Name:
- J/A+A/658/A4
- Date:
- 22 Feb 2022
- Publisher:
- CDS
- Description:
- We present the spatially resolved emission of Arp 299 revealed by 150 MHz international baseline Low-Frequency Array (LOFAR) and 1.4, 5.0, and 8.4GHz Very Large Array (VLA) observations. Nowadays, LOFAR allows to compare its resolution with the VLA ones. These will allow us to obtain pixel-by-pixel spectral imdex maps and local radio SEDs. With this information we recover the nature of the ISM around the main structures (either clumpy or continuous), as well as magnetic fields. The 150MHz image also shows structures never seen before, pointing out that these radio-frequencies observations are a perfect tool to detect them.
- ID:
- ivo://CDS.VizieR/J/A+A/648/A104
- Title:
- LOFAR LBA Sky Survey. I.
- Short Name:
- J/A+A/648/A104
- Date:
- 22 Feb 2022
- Publisher:
- CDS
- Description:
- LOFAR is the only radio telescope that is presently capable of high-sensitivity, high-resolution (i.e. <1mJy/b and <15") observations at ultra-low frequencies (<100MHz). To utilise these capabilities, the LOFAR Surveys Key Science Project is undertaking a large survey to cover the entire northern sky with Low Band Antenna (LBA) observations. The LOFAR LBA Sky Survey (LoLSS) aims to cover the entire northern sky with 3170 pointings in the frequency range 42-66MHz, at a resolution of 15-arcsec and at a sensitivity of 1mJy/beam (1{sigma}. Here we outline the survey strategy, the observational status, the current calibration techniques, and briefly describe several scientific motivations. We also describe the preliminary public data release. The preliminary images were produced using a fully automated pipeline that aims to correct all direction-independent effects in the data. Whilst the direction-dependent effects, such as those from the ionosphere, are not yet corrected, the images presented in this work are still 10 times more sensitive than previous surveys available at these low frequencies. The preliminary data release covers 740deg^2^ around the HETDEX spring field region at a resolution of 47" with a median noise level of 5mJy/beam. The images and the catalogue with 25247 sources are publicly released. We demonstrate that the system is capable of reaching an rms noise of 1mJy/beam and the resolution of 15" once direction-dependent effects are corrected for. LoLSS will provide the ultra-low-frequency information for hundreds of thousands of radio sources, providing critical spectral information and producing a unique dataset that can be used for a wide range of science topics such as: the search for high redshift galaxies and quasars, the study of the magnetosphere of exoplanets, and the detection of the oldest populations of cosmic-rays in galaxies, clusters of galaxies, and from AGN activity.
- ID:
- ivo://CDS.VizieR/J/A+A/658/A2
- Title:
- LOFAR Long-Baseline Calibrator Survey. II.
- Short Name:
- J/A+A/658/A2
- Date:
- 22 Feb 2022
- Publisher:
- CDS
- Description:
- The Low-Frequency Array (LOFAR) Long-Baseline Calibrator Survey (LBCS) was conducted between 2014 and 2019 in order to provide a set of calibrators for the LOFAR array. Here we present the complete survey, for which a preliminary analysis was published in 2016 using 20% of the survey data. The final catalogue consists of 30007 observations of 24713 sources in the northern sky, selected for a combination of bright low-frequency radio flux density and flat spectral index using existing surveys (WENSS, VLSS and MSSS). Approximately 1 calibrator per square degree is identified by the detection of compact flux, for declinations north of 30{deg} and a considerably lower density south of this point. The catalogue contains indicators of degree of correlated flux on baselines between the Dutch core and each of the international stations for all of the observations. Use of the VLBA calibrator list, together with statistical arguments by comparison with flux densities from lower-resolution catalogues, allow us to establish a rough flux density scale for the LBCS observations, so that LBCS statistics can be used to estimate compact fluxes, on scales between 300mas and 2-arcseconds, for sources observed in the survey.
- ID:
- ivo://CDS.VizieR/J/A+A/574/A73
- Title:
- LOFAR long baselines at 140MHz Calibrators
- Short Name:
- J/A+A/574/A73
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- An efficient means of locating calibrator sources for international LOw Frequency ARray (LOFAR) is developed and used to determine the average density of usable calibrator sources on the sky for subarcsecond observations at 140MHz. We used the multi-beaming capability of LOFAR to conduct a fast and computationally inexpensive survey with the full international LOFAR array. Sources were preselected on the basis of 325MHz arcminute-scale flux density using existing catalogues. By observing 30 different sources in each of the 12 sets of pointings per hour, we were able to inspect 630 sources in two hours to determine if they possess a sufficiently bright compact component to be usable as LOFAR delay calibrators.
- ID:
- ivo://CDS.VizieR/J/A+A/598/A104
- Title:
- LOFAR Two-metre Sky Survey
- Short Name:
- J/A+A/598/A104
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The LOFAR Two-metre Sky Survey (LoTSS) is a deep 120-168MHz imaging survey that will eventually cover the entire Northern sky. Each of the 3170 pointings will be observed for 8 hrs, which, at most declinations, is sufficient to produce ~5-arcsec resolution images with a sensitivity of ~0.1mJy/beam and accomplish the main scientific aims of the survey which are to explore the formation and evolution of massive black holes, galaxies, clusters of galaxies and large-scale structure. Due to the compact core and long baselines of LOFAR, the images provide excellent sensitivity to both highly extended and compact emission. For legacy value, the data are archived at high spectral and time resolution to facilitate subarcsecond imaging and spectral line studies. In this paper we provide an overview of the LoTSS. We outline the survey strategy, the observational status, the current calibration techniques, a preliminary data release, and the anticipated scientific impact. The preliminary images that we have released were created using a fully-automated but direction-independent calibration strategy and are significantly more sensitive than those produced by any existing large-area low-frequency survey. In excess of 44,000 sources are detected in the images that have a resolution of 25-arcsec, typical noise levels of less than 0.5mJy/beam, and cover an area of over 350 square degrees in the region of the HETDEX Spring Field (right ascension 10:45:00 to 15:30:00 and declination 45:00:00 to 57:00:00).
- ID:
- ivo://CDS.VizieR/J/A+A/648/A2
- Title:
- LOFAR Two-metre Sky Survey Deep Fields DR1
- Short Name:
- J/A+A/648/A2
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The LOFAR Two-metre Sky Survey (LoTSS) will cover the full northern sky and, additionally, aims to observe the LoTSS deep fields to a noise level of <~10uJy/beam over several tens of square degrees in areas that have the most extensive ancillary data. This paper presents the ELAIS-N1 deep field, the deepest of the LoTSS deep fields to date. With an effective observing time of 163.7 hours, it reaches a root mean square (RMS) noise level of <~20uJy/beam in the central region (and below 30uJy/beam over 10 square degrees). The resolution is 6 arcsecs and 84862 radio sources were detected in the full area (68 square degrees) with 74127 sources in the highest quality area at less than 3 degrees from the pointing centre. The observation reaches a sky density of more than 5000 sources per square degree in the central region (5 square degrees).We present the calibration procedure, which addresses the special configuration of some observations and the extended bandwidth covered (115 to 177MHz; central frequency 146.2MHz) compared to standard LoTSS. We also describe the methods used to calibrate the flux density scale using cross-matching with sources detected by other radio surveys in the literature. We find the flux density uncertainty related to the flux density scale to be 6.5 per cent. By studying the variations of the flux density measurements between different epochs, we show that relative flux density calibration is reliable out to about a 3 degree radius, but that additional flux density uncertainty is present for all sources at about the 3 per cent level; this is likely to be associated with residual calibration errors, and is shown to be more significant in datasets with poorer ionosphere conditions. We also provide intra-band spectral indices, which can be useful to detect sources with unusual spectral properties. The final uncertainty in the flux densities is estimated to be 10 per cent for ELAIS-N1.
- ID:
- ivo://CDS.VizieR/J/A+A/648/A3
- Title:
- LOFAR Two-metre Sky Survey Deep Fields DR1
- Short Name:
- J/A+A/648/A3
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present the source associations, cross-identifications, and multi-wavelength properties of the faint radio source population detected in the deep tier of the LOFAR Two Metre Sky Survey (LoTSS): the LoTSS Deep Fields. The first LoTSS Deep Fields data release consists of deep radio imaging at 150MHz of the ELAIS-N1, Lockman Hole, and Bootes fields, down to RMS sensitives of around 20, 22, and 32uJy/beam, respectively. These fields are some of the best studied extra-galactic fields in the northern sky, with existing deep, wide-area panchromatic photometry from X-ray to infrared wavelengths, covering a total of ~26 square degrees. We first generated improved multi-wavelength catalogues in ELAIS-N1 and Lockman Hole; combined with the existing catalogue for Bootes, we present forced, matched aperture photometry for over 7.2 million sources across the three fields. We identified multi-wavelength counterparts to the radio detected sources, using a combination of the Likelihood Ratio method and visual classification, which greatly enhances the scientific potential of radio surveys and allows for the characterisation of the photometric redshifts and the physical properties of the host galaxies. The final radio-optical cross-matched catalogue consists of 81951 radio-detected sources, with counterparts identified and multi-wavelength properties presented for 79820 (>97%) sources. We also examine the properties of the host galaxies, and through stacking analysis find that the radio population with no identified counterpart is likely dominated by AGN at z~3-4. This dataset contains one of the largest samples of radio-selected star-forming galaxies and active galactic nuclei (AGN) at these depths, making it ideal for studying the history of star-formation, and the evolution of galaxies and AGN across cosmic time.
- ID:
- ivo://CDS.VizieR/J/A+A/622/A1
- Title:
- LOFAR Two-metre Sky Survey DR1 source catalog
- Short Name:
- J/A+A/622/A1
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The LOFAR Two-metre Sky Survey (LoTSS) is an ongoing sensitive, high-resolution 120-168MHz survey of the entire northern sky for which observations are now 20% complete. We present our first full-quality public data release. For this data release 424 square degrees, or 2% of the eventual coverage, in the region of the HETDEX Spring Field (right ascension 10h45m00s to 15h30m00s and declination 45{deg}00'00'' to 57{deg}00'00'') were mapped using a fully automated direction-dependent calibration and imaging pipeline that we developed. A total of 325694 sources are detected with a signal of at least five times the noise, and the source density is a factor of ~10 higher than the most sensitive existing very wide-area radio-continuum surveys. The median sensitivity is S_144MHz_=71{mu}Jy/beam and the point-source completeness is 90% at an integrated flux density of 0.45mJy. The resolution of the images is 6'' and the positional accuracy is within 0.2''. This data release consists of a catalogue containing location, flux, and shape estimates together with 58 mosaic images that cover the catalogued area. In this paper we provide an overview of the data release with a focus on the processing of the LOFAR data and the characteristics of the resulting images. In two accompanying papers we provide the radio source associations and deblending and, where possible, the optical identifications of the radio sources together with the photometric redshifts and properties of the host galaxies. These data release papers are published together with a further ~20 articles that highlight the scientific potential of LoTSS.
- ID:
- ivo://CDS.VizieR/J/A+A/659/A1
- Title:
- LOFAR Two-metre Sky Survey (LoTSS) DR2
- Short Name:
- J/A+A/659/A1
- Date:
- 25 Feb 2022 07:37:08
- Publisher:
- CDS
- Description:
- In this data release from the ongoing LOw-Frequency ARray (LOFAR) Two-metre Sky Survey (LoTSS) we present 120-168MHz images covering 27% of the northern sky. Our coverage is split into two regions centred at approximately 12h45m +44{deg}30' and 1h00m +28{deg}00' and spanning 4178 and 1457 square degrees respectively. The images were derived from 3,451 hrs (7.6 PB) of LOFAR High Band Antenna data which were corrected for the direction-independent instrumental properties as well as direction-dependent ionospheric distortions during extensive, but fully automated, data processing. A catalogue of 4396228 radio sources is derived from our total intensity (Stokes I) maps, where the majority of these have never been detected at radio wavelengths before. At 6"resolution, our full bandwidth Stokes I continuum maps with a central frequency of 144MHz have: a median rms sensitivity of 83mJy/beam; a flux density scale accuracy of approximately 10%; an astrometric accuracy of 0.2"; and we estimate the point-source completeness to be 90% at a peak brightness of 0.8mJy/beam. By creating three 16MHz bandwidth images across the band we are able to measure the in-band spectral index of many sources, albeit with an error on the derived spectral index of >+/-0.2 which is a consequence of our flux-density scale accuracy and small fractional bandwidth. Our circular polarisation (Stokes V) 20" resolution 120-168 MHz continuum images have a median rms sensitivity of 95uJy/beam, and we estimate a Stokes I to Stokes V leakage of 0.056%. Our linear polarisation (Stokes Q and Stokes U) image cubes consist of 480x97.6kHz wide planes and have a median rms sensitivity per plane of 10.8mJy/beam at 4' and 2.2mJy/beam at 20"; we estimate the Stokes I to Stokes Q/U leakage to be approximately 0.2%. Here we characterise and publicly release our Stokes I, Q, U and V images in addition to the calibrated uv-data to facilitate the thorough scientific exploitation of this unique dataset.
