LOFAR Observations of 4C+19.44: On the Discovery of Low-frequency Spectral Curvature in Relativistic Jet Knots

Harris, D. E.; Moldón, J.; Oonk, J. R. R.; Massaro, F.; Paggi, A.; Deller, Adam T.; Godfrey, L.; Morganti, Raffaella; Jorstad, Svetlana G.

doi:10.3847/1538-4357/ab01ff

Cited by 12 publications

(11 citation statements)

References 61 publications

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“…More recently, jet energy dissipation and a change in acceleration mechanism have been put forward as alternative explanations (Godfrey et al 2009). Harris et al (2019) discovered spectral curvature in a blazar using LOFAR long-baseline observations and showed how radio observations in the MHz energy range can improve estimates of source parameters such as the equipartition magnetic field.…”

Section: The Radio Core Of Sdssj08mentioning

confidence: 99%

First constraints on the AGN X-ray luminosity function atz~ 6 from an eROSITA-detected quasar

Wolf

Nandra

Salvato

et al. 2021

A&A

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Context. High-redshift quasars signpost the early accretion history of the Universe. The penetrating nature of X-rays enables a less absorption-biased census of the population of these luminous and persistent sources compared to optical/near-infrared colour selection. The ongoing SRG/eROSITA X-ray all-sky survey offers a unique opportunity to uncover the bright end of the high-z quasar population and probe new regions of colour parameter space. Aims. We searched for high-z quasars within the X-ray source population detected in the contiguous ~140 deg2 field observed by eROSITA during the performance verification phase. With the purpose of demonstrating the unique survey science capabilities of eROSITA, this field was observed at the depth of the final all-sky survey. The blind X-ray selection of high-redshift sources in a large contiguous, near-uniform survey with a well-understood selection function can be directly translated into constraints on the X-ray luminosity function (XLF), which encodes the luminosity-dependent evolution of accretion through cosmic time. Methods. We collected the available spectroscopic information in the eFEDS field, including the sample of all currently known optically selected z > 5.5 quasars and cross-matched secure Legacy DR8 counterparts of eROSITA-detected X-ray point-like sources with this spectroscopic sample. Results. We report the X-ray detection of eFEDSU J083644.0+005459, an eROSITA source securely matched to the well-known quasar SDSS J083643.85+005453.3 (z = 5.81). The soft X-ray flux of the source derived from eROSITA is consistent with previous Chandra observations. The detection of SDSS J083643.85+005453.3 allows us to place the first constraints on the XLF at z > 5.5 based on a secure spectroscopic redshift. Compared to extrapolations from lower-redshift observations, this favours a relatively flat slope for the XLF at z ~ 6 beyond L*, the knee in the luminosity function. In addition, we report the detection of the quasar with LOFAR at 145 MHz and ASKAP at 888 MHz. The reported flux densities confirm a spectral flattening at lower frequencies in the emission of the radio core, indicating that SDSS J083643.85+005453.3 could be a (sub-) gigahertz peaked spectrum source. The inferred spectral shape and the parsec-scale radio morphology of SDSS J083643.85+005453.3 indicate that it is in an early stage of its evolution into a large-scale radio source or confined in a dense environment. We find no indications for a strong jet contribution to the X-ray emission of the quasar, which is therefore likely to be linked to accretion processes. Conclusions. Our results indicate that the population of X-ray luminous AGNs at high redshift may be larger than previously thought. From our XLF constraints, we make the conservative prediction that eROSITA will detect ~90 X-ray luminous AGNs at redshifts 5.7 < z < 6.4 in the full-sky survey (De+RU). While subject to different jet physics, both high-redshift quasars detected by eROSITA so far are radio-loud; a hint at the great potential of combined X-ray and radio surveys for the search of luminous high-redshift quasars.

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Section: The Radio Core Of Sdssj08mentioning

confidence: 99%

First constraints on the AGN X-ray luminosity function atz~ 6 from an eROSITA-detected quasar

Wolf

Nandra

Salvato

et al. 2021

A&A

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“…The LOFAR telescope includes 13 international stations, providing baselines of up to 1989 km which translates to an angular resolution of 0.27 at 150 MHz. Although these international stations have always been available, there have only been a handful of sub-arcsecond studies with the telescope (Moldón et al 2014;Jackson et al 2016;Morabito et al 2016;Varenius et al 2015Varenius et al , 2016Ramírez-Olivencia et al 2018;Harris et al 2019;Kappes et al 2019). This is mainly due to the fact that the calibration with the full international LOFAR telescope is technically challenging.…”

Section: Introductionmentioning

confidence: 99%

Unmasking the history of 3C 293 with LOFAR sub-arcsecond imaging

Kukreti

Morganti

Shimwell

et al. 2022

A&A

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Active galactic nuclei (AGNs) show episodic activity, which can be evident in galaxies that exhibit restarted radio jets. These restarted jets can interact with their environment, leaving signatures on the radio spectral energy distribution. Tracing these signatures is a powerful way to explore the life of radio galaxies. This requires resolved spectral index measurements over a broad frequency range including low frequencies. We present such a study for the radio galaxy 3C 293, which has long been thought to be a restarted galaxy on the basis of its radio morphology. Using the International LOFAR telescope (ILT) we probed spatial scales as fine as ∼0.2 at 144 MHz, and to constrain the spectrum we combined these data with Multi-Element Radio Linked Interferometer Network (MERLIN) and Very Large Array (VLA) archival data at frequencies up to 8.4 GHz that have a comparable resolution. In the inner lobes (∼2 kpc), we detect the presence of a spectral turnover that peaks at ∼225 MHz and is most likely caused by free-free absorption from the rich surrounding medium. We confirm that these inner lobes are part of a jet-dominated young radio source (spectral age 0.17 Myr), which is strongly interacting with the rich interstellar medium (ISM) of the host galaxy. The diffuse emission surrounding these lobes on scales of up to ∼4.5 kpc shows steeper spectral indices (∆α ∼ 0.2-0.5, S ∝ ν −α ) and a spectral age of 0.27 Myr. The outer lobes (extending up to ∼100 kpc) have a spectral index of α ∼ 0.6-0.8 from 144-4850 MHz with a remarkably uniform spatial distribution and only mild spectral curvature (∆α 0.2). We propose that intermittent fuelling and jet flow disruptions are powering the mechanisms that keep the spectral index in the outer lobes from steepening and maintain the spatial uniformity of the spectral index. Overall, it appears that 3C 293 has gone through multiple (two to three) epochs of activity. This study adds 3C 293 to the new sub-group of restarted galaxies with short interruption time periods. This is the first time a spatially resolved study has been performed that simultaneously studies a young source as well as the older outer lobes at such low frequencies. This illustrates the potential of the International LOFAR telescope to expand such studies to a larger sample of radio galaxies.

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“…This has been demonstrated with both the LOFAR high band antenna (HBA; 120-240 MHz) and low band antenna (LBA; 30-78 MHz) for individual sources. All but one of the published studies conducted with the international stations use the HBA (Moldón et al 2015;Jackson et al 2016;Varenius et al 2015Varenius et al , 2016Ramírez-Olivencia et al 2018;Harris et al 2019;Kappes et al 2019). Excellent examples of the scientific potential include a study of M82 by Varenius et al (2015), where the authors investigated the properties of the radio spectra at 154 MHz in the nuclear region of the nearby galaxy, detecting seven previously uncatalogued compact objects, including supernova remnants, and providing spatially resolved information for fitting the radio spectral energy distributions (SEDs) of the supernova remnants.…”

Section: Introductionmentioning

confidence: 99%

Sub-arcsecond imaging with the International LOFAR Telescope

Morabito

Jackson

Mooney

et al. 2022

A&A

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The International LOFAR Telescope is an interferometer with stations spread across Europe. With baselines of up to ∼ 2,000 km, LOFAR has the unique capability of achieving sub-arcsecond resolution at frequencies below 200 MHz. However, it is technically and logistically challenging to process LOFAR data at this resolution. To date only a handful of publications have exploited this capability. Here we present a calibration strategy that builds on previous high-resolution work with LOFAR. It is implemented in a pipeline using mostly dedicated LOFAR software tools and the same processing framework as the LOFAR Two-metre Sky Survey (LoTSS). We give an overview of the calibration strategy and discuss the special challenges inherent to enacting high-resolution imaging with LOFAR, and describe the pipeline, which is publicly available, in detail. We demonstrate the calibration strategy by using the pipeline on P205+55, a typical LoTSS pointing with an 8 hour observation and 13 international stations. We perform in-field delay calibration, solution referencing to other calibrators in the field, self-calibration of these calibrators, and imaging of example directions of interest in the field. We find that for this specific field and these ionospheric conditions, dispersive delay solutions can be transferred between calibrators up to ∼1.5 degrees away, while phase solution transferral works well over ∼1 degree. We also demonstrate a check of the astrometry and flux density scale with the in-field delay calibrator source. Imaging in 17 directions, we find the restoring beam is typically ∼0.3 ×0.2 although this varies slightly over the entire 5 square degree field of view. We find we can achieve ∼80 to 300 µJy bm −1 image rms noise, which is dependent on the distance from the phase centre; typical values are ∼ 90 µJy bm −1 for the 8 hour observation with 48 MHz of bandwidth. Seventy percent of processed sources are detected, and from this we estimate that we should be able to image roughly 900 sources per LoTSS pointing. This equates to ∼3 million sources in the northern sky, which LoTSS will entirely cover in the next several years. Future optimisation of the calibration strategy for efficient post-processing of LoTSS at high resolution (LoTSS-HR) makes this estimate a lower limit.

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LOFAR Observations of 4C+19.44: On the Discovery of Low-frequency Spectral Curvature in Relativistic Jet Knots

Cited by 12 publications

References 61 publications

First constraints on the AGN X-ray luminosity function atz~ 6 from an eROSITA-detected quasar

First constraints on the AGN X-ray luminosity function atz~ 6 from an eROSITA-detected quasar

Unmasking the history of 3C 293 with LOFAR sub-arcsecond imaging

Sub-arcsecond imaging with the International LOFAR Telescope

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