arXiv subjects

T. W. Shimwell

Publications and source records attributed to T. W. Shimwell.

At least 127 records · Page 7Linked to original sources

arXiv2019

LOFAR early-time search for coherent radio emission from GRB 180706A

The nature of the central engines of gamma-ray bursts (GRBs) and the composition of their relativistic jets are still under debate. If the jets are Poynting flux dominated rather than baryon dominated, a coherent radio flare from magnetic re-connection events might be expected with the prompt gamma-ray emission. There are two competing models for the central engines of GRBs; a black hole or a newly formed milli-second magnetar. If the central engine is a magnetar it is predicted to produce coherent radio emission as persistent or flaring activity. In this paper, we present the deepest limits to date for this emission following LOFAR rapid response observations of GRB 180706A. No emission is detected to a 3$σ$ limit of 1.7 mJy beam$^{-1}$ at 144 MHz in a two-hour LOFAR observation starting 4.5 minutes after the gamma-ray trigger. A forced source extraction at the position of GRB 180706A provides a marginally positive (1 sigma) peak flux density of $1.1 \pm 0.9$ mJy. The data were time-sliced into different sets of snapshot durations to search for FRB like emission. No short duration emission was detected at the location of the GRB. We compare these results to theoretical models and discuss the implications of a non-detection.

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arXiv2019

A LOFAR-IRAS cross-match study: the far-infrared radio correlation and the 150-MHz luminosity as a star-formation rate

Aims. We aim to study the far-infrared radio correlation (FIRC) at 150 MHz in the local Universe (at a median redshift z~0:05) and improve the use of the rest-frame 150-MHz luminosity, L150, as a star-formation rate (SFR) tracer, which is unaffected by dust extinction. Methods. We cross-match the 60-um selected Revised IRAS Faint Source Survey Redshift (RIFSCz) catalogue and the 150-MHz selected LOFAR value-added source catalogue in the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX) Spring Field. We estimate L150 for the cross-matched sources and compare it with the total infrared (IR) luminosity, LIR, and various SFR tracers. Results. We find a tight linear correlation between log L150 and log LIR for star-forming galaxies, with a slope of 1.37. The median qIR value (defined as the logarithm of the LIR to L150 ratio) and its rms scatter of our main sample are 2.14 and 0.34, respectively. We also find that log L150 correlates tightly with the logarithm of SFR derived from three different tracers, i.e., SFR_Halpha based on the Halpha line luminosity, SFR_60 based on the rest-frame 60-um luminosity and SFR_IR based on LIR, with a scatter of 0.3 dex. Our best-fit relations between L150 and these SFR tracers are, log L150 (Lsun) = 1.35(0.06) x log SFR_Halpha (Msun/yr) + 3.20(0.06), log L150 (Lsun) = 1.31(0.05) x log SFR_60 (Msun/yr) + 3.14(0.06), and log L150 (Lsun) = 1.37(0.05) x log SFR_IR (Msun/yr) + 3.09(0.05), which show excellent agreement with each other.

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arXiv2019

Particle acceleration in a nearby galaxy cluster pair: the role of cluster dynamics

Diffuse radio emission associated with the intra-cluster medium (ICM) is observed in a number of merging galaxy clusters. It is currently believed that in mergers a fraction of the kinetic energy is channeled into non-thermal components, such as turbulence, cosmic rays and magnetic fields, that may lead to the formation of giant synchrotron sources in the ICM. Studying merging galaxy clusters in different evolutionary phases is fundamental to understanding the origin of radio emission in the ICM. We observed the nearby galaxy cluster pair RXC J1825.3+3026 ($z\sim0.065$) and CIZA J1824.1+3029 ($z\sim0.071$) at 120-168 MHz with the LOw Frequency ARray (LOFAR) and made use of a deep (240 ks) XMM-Newton dataset to study the non-thermal and thermal properties of the system. RXC J1825.3+3026 is in a complex dynamical state, with a primary on-going merger in the E-W direction and a secondary later stage merger with a group of galaxies in the SW, while CIZA J1824.1+3029 is dynamically relaxed. These two clusters are in a pre-merger phase. We report the discovery of a Mpc-scale radio halo with a low surface brightness extension in RXC J1825.3+3026 that follows the X-ray emission from the cluster center to the remnant of a galaxy group in the SW. This is among the least massive systems and the faintest giant radio halo known to date. Contrary to this, no diffuse radio emission is observed in CIZA J1824.1+3029 nor in the region between the pre-merger cluster pair. The power spectra of the X-ray surface brightness fluctuations of RXC J1825.3+3026 and CIZA J1824.1+3029 are in agreement with the findings for clusters exhibiting a radio halo and the ones where no radio emission has been detected, respectively. We provide quantitative support to the idea that cluster mergers play a crucial role in the generation of non-thermal components in the ICM.

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arXiv2019

LOFAR discovery of a radio halo in the high-redshift galaxy cluster PSZ2 G099.86+58.45

In this Letter, we report the discovery of a radio halo in the high-redshift galaxy cluster PSZ2 G099.86+58.45 ($z=0.616$) with the LOw Frequency ARray (LOFAR) at 120-168 MHz. This is one of the most distant radio halos discovered so far. The diffuse emission extends over $\sim$ 1 Mpc and has a morphology similar to that of the X-ray emission as revealed by XMM-Newton data. The halo is very faint at higher frequencies and is barely detected by follow-up 1-2 GHz Karl G.~Jansky Very Large Array (JVLA) observations, which enable us to constrain the radio spectral index to be $α\leq 1.5-1.6$, i.e.; with properties between canonical and ultra-steep spectrum radio halos. Radio halos are currently explained as synchrotron radiation from relativistic electrons that are re-accelerated in the intra-cluster medium (ICM) by turbulence driven by energetic mergers. We show that in such a framework radio halos are expected to be relatively common at $\sim150$ MHz ($\sim30-60\%$) in clusters with mass and redshift similar to PSZ2 G099.86+58.45; however, at least 2/3 of these radio halos should have steep spectrum and thus be very faint above $\sim 1$ GHz frequencies. Furthermore, since the luminosity of radio halos at high redshift depends strongly on the magnetic field strength in the hosting clusters, future LOFAR observations will also provide vital information on the origin and amplification of magnetic fields in galaxy clusters.

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arXiv2019

LOFAR first look at the giant radio galaxy 3C 236

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 have 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.

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arXiv2019

Revisiting the Fanaroff-Riley dichotomy and radio-galaxy morphology with the LOFAR Two-Metre Sky Survey (LoTSS)

The relative positions of the high and low surface brightness regions of radio-loud active galaxies in the 3CR sample were found by Fanaroff and Riley to be correlated with their luminosity. We revisit this canonical relationship with a sample of 5805 extended radio-loud AGN from the LOFAR Two-Metre Sky Survey (LoTSS), compiling the most complete dataset of radio-galaxy morphological information obtained to date. We demonstrate that, for this sample, radio luminosity does *not* reliably predict whether a source is edge-brightened (FRII) or centre-brightened (FRI). We highlight a large population of low-luminosity FRIIs, extending three orders of magnitude below the traditional FR break, and demonstrate that their host galaxies are on average systematically fainter than those of high-luminosity FRIIs and of FRIs matched in luminosity. This result supports the jet power/environment paradigm for the FR break: low-power jets may remain undisrupted and form hotspots in lower mass hosts. We also find substantial populations that appear physically distinct from the traditional FR classes, including candidate restarting sources and ``hybrids''. We identify 459 bent-tailed sources, which we find to have a significantly higher SDSS cluster association fraction (at $z<0.4$) than the general radio-galaxy population, similar to the results of previous work. The complexity of the LoTSS faint, extended radio sources demonstrates the need for caution in the automated classification and interpretation of extended sources in modern radio surveys, but also reveals the wealth of morphological information such surveys will provide and its value for advancing our physical understanding of radio-loud AGN.

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arXiv2019

NGC 326: X-shaped no more

We present new 144-MHz LOFAR observations of the prototypical `X-shaped' radio galaxy NGC 326, which show that the formerly known wings of the radio lobes extend smoothly into a large-scale, complex radio structure. We argue that this structure is most likely the result of hydrodynamical effects in an ongoing group or cluster merger, for which pre-existing X-ray and optical data provide independent evidence. The large-scale radio structure is hard to explain purely in terms of jet reorientation due to the merger of binary black holes, a previously proposed explanation for the inner structure of NGC 326. For this reason, we suggest that the simplest model is one in which the merger-related hydrodynamical processes account for all the source structure, though we do not rule out the possibility that a black hole merger has occurred. Inference of the black hole-black hole merger rate from observations of X-shaped sources should be carried out with caution in the absence of deep, sensitive low-frequency observations. Some X-shaped sources may be signposts of cluster merger activity, and it would be useful to investigate the environments of these objects more generally.

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arXiv2019

Scalability Model for the LOFAR Direction Independent Pipeline

LOFAR is a leading aperture synthesis telescope operated in the Netherlands with stations across Europe. The LOFAR Two-meter Sky Survey (LoTSS) will produce more than 3000 14 TB data sets, mapping the entire northern sky at low frequencies. The data produced by this survey is important for understanding the formation and evolution of galaxies, supermassive black holes and other astronomical phenomena. All of the LoTSS data needs to be processed by the LOFAR Direction Independent (DI) pipeline, prefactor. Understanding the performance of this pipeline is important when trying to optimize the throughput for large projects, such as LoTSS and other deep surveys. Making a model of its completion time will enable us to predict the time taken to process large data sets, optimize our parameter choices, help schedule other LOFAR processing services, and predict processing time for future large radio telescopes. We tested the prefactor pipeline by scaling several parameters, notably number of CPUs, data size and size of calibration sky model. We present these results as a comprehensive model which will be used to predict processing time for a wide range of processing parameters. We also discover that smaller calibration models lead to significantly faster calibration times, while the calibration results do not significantly degrade in quality. Finally, we validate the model and compare predictions with production runs from the past six months, quantifying the performance penalties incurred by processing on a shared cluster. We conclude by noting the utility of the results and model for the LoTSS Survey, LOFAR as a whole and for other telescopes.

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arXiv2019

A single fast radio burst localized to a massive galaxy at cosmological distance

Fast Radio Bursts (FRBs) are brief radio emissions from distant astronomical sources. Some are known to repeat, but most are single bursts. Non-repeating FRB observations have had insufficient positional accuracy to localize them to an individual host galaxy. We report the interferometric localization of the single pulse FRB 180924 to a position 4 kpc from the center of a luminous galaxy at redshift 0.3214. The burst has not been observed to repeat. The properties of the burst and its host are markedly different from the only other accurately localized FRB source. The integrated electron column density along the line of sight closely matches models of the intergalactic medium, indicating that some FRBs are clean probes of the baryonic component of the cosmic web.

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arXiv2019

A radio ridge connecting two galaxy clusters in a filament of the cosmic web

Galaxy clusters are the most massive gravitationally bound structures in the Universe. They grow by accreting smaller structures in a merging process that produces shocks and turbulence in the intra-cluster gas. We observed a ridge of radio emission connecting the merging galaxy clusters Abell 0399 and Abell 0401 with the Low Frequency Array (LOFAR) at 140 MHz. This emission requires a population of relativistic electrons and a magnetic field located in a filament between the two galaxy clusters. We performed simulations to show that a volume-filling distribution of weak shocks may re-accelerate a pre-existing population of relativistic particles, producing emission at radio wavelengths that illuminates the magnetic ridge.

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arXiv2019

A Massive Cluster at z = 0.288 Caught in the Process of Formation: The Case of Abell 959

The largest galaxy clusters are observed still to be forming through major cluster-cluster mergers, often showing observational signatures such as radio relics and giant radio haloes. Using LOFAR Two-meter Sky Survey data, we present new detections of both a radio halo (with a spectral index of $α_{143}^{1400}=1.48^{+0.06}_{-0.23}$) and a likely radio relic in Abell 959, a massive cluster at a redshift of z=0.288. Using a sample of clusters with giant radio haloes from the literature (80 in total), we show that the radio halo in A959 lies reasonably well on the scaling relations between the thermal and non-thermal power of the system. Additionally, we find evidence that steep-spectrum haloes tend to reside in clusters with high X-ray luminosities relative to those expected from cluster LM scaling relations, indicating that such systems may preferentially lie at an earlier stage of the merger, consistent with the theory that some steep-spectrum haloes result from low-turbulence mergers. Lastly, we find that halo systems containing radio relics tend to lie at lower X-ray luminosities, relative to those expected from cluster LM scaling relations, for a given halo radio power than those without relics, suggesting that the presence of relics indicates a later stage of the merger, in line with simulations.

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arXiv2018

Untangling cosmic magnetic fields: Faraday tomography at metre wavelengths with LOFAR

The technique of Faraday tomography is a key tool for the study of magnetised plasmas in the new era of broadband radio polarisation observations. In particular, observations at metre-wavelengths provide significantly better Faraday depth accuracies compared to traditional cm-wavelength observations. However, the effect of Faraday depolarisation makes the polarised signal very challenging to detect at metre wavelengths (MHz frequencies). In this work, Faraday tomography is used to characterise the Faraday rotation properties of polarised sources found in data from the LOFAR Two-Metre Sky Survey (LoTSS). Of the 76 extragalactic polarised sources analysed here, we find that all host a radio-loud AGN. The majority of the sources ($\sim$64%) are large FRII radio galaxies with a median projected linear size of 710 kpc and median radio luminosity at 144 MHz of $4\times10^{26}$ W Hz$^{-1}$ (with $\sim$13% of all sources having a linear size $>1$ Mpc). In several cases, both hotspots are detected in polarisation at an angular resolution of $\sim$20". One such case allowed a study of intergalactic magnetic fields on scales of 3.4 Mpc. Other detected source types include an FRI radio galaxy and at least 8 blazars. Most sources display simple Faraday spectra, however, we highlight one blazar that displays a complex Faraday spectrum, with two close peaks in the Faraday dispersion function.

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arXiv2018

CHANG-ES XII: A LOFAR and VLA view of the edge-on star-forming galaxy NGC 3556

Low-frequency radio continuum studies of star-forming edge-on galaxies can help to further understand how cosmic-ray electrons (CRe) propagate through the interstellar medium into the halo and how this is affected by energy losses and magnetic fields. Observations with the Very Large Array (VLA) from Continuum Haloes in Nearby Galaxies - an EVLA Survey (CHANG-ES) are combined with those with the LOw Frequency ARray (LOFAR) from the LOFAR Two-metre Sky Survey (LoTSS ) to identify the prevailing mode of cosmic-ray transport in the edge-on spiral galaxy NGC 3556. We mapped the radio spectral index, magnetic field strength, and orientation using VLA 1.5 and 6 GHz and LOFAR 144 MHz data, and we fit 1D cosmic-ray propagation models to these maps using Spinnaker (Spectral Index Numerical Analysis of K(c)osmic-ray electron radio emission) and its interactive wrapper Spinteractive. We find that the spectral index in the galactic midplane is, as expected for young CRe, alpha ~ -0.7 and steepens towards the halo of the galaxy as a consequence of spectral ageing. The intensity scale heights are about 1.4 and 1.9 kpc for the thin disc, and 3.3 and 5.9 kpc for the thick disc at 1.5 GHz and 144 MHz, respectively. While pure diffusion cannot explain our data, advection can, particularly if we assume a linearly accelerating wind. Our best-fitting model has an initial speed of 123 km s^-1 in the galactic midplane and reaches the escape velocity at heights between 5 kpc and 15 kpc above the disc, depending on the assumed dark matter halo of the galaxy. This galactic wind scenario is corroborated by the existence of vertical filaments seen both in the radio continuum and in H alpha in the disc-halo interface and of a large-scale reservoir of hot, X-ray emitting gas in the halo. Radio haloes show the existence of galactic winds, possibly driven by cosmic rays, in typical star-forming spiral galaxies.

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arXiv2018

Exploring the properties of low-frequency radio emission and magnetic fields in a sample of compact galaxy groups using the LOFAR Two-Metre Sky Survey (LoTSS)

We use the LOFAR Two-metre Sky Survey (LoTSS) Data Release I to identify the groups of galaxies (and individual galaxies) from the Hickson Compact Groups and Magnitude Limited Compact Groups samples that emit at the frequency of 150\,MHz, characterise their radio emission (extended or limited to the galaxies), and compare new results to earlier observations and theoretical predictions. The detection of 73 systems (and 7 more -- probably) out of 120, of which as many as 17 show the presence of extended radio structures, confirms the previous hypothesis of the common character of the magnetic field inside galaxy groups and its detectability. In order to investigate the future potential of low-frequency radio studies of galaxy groups, we also present a more detailed insight into four radio-emitting systems, for which the strength of the magnetic field inside their intergalactic medium (IGM) is calculated. The estimated values are comparable to that found inside star-forming galaxies, suggesting a dynamical and evolutionary importance of the magnetic field in galaxy groups.

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arXiv2018

LoTSS DR1: Double-double radio galaxies in the HETDEX field

Double-double radio galaxies (DDRGs) represent a short but unique phase in the life-cycle of some of the most powerful radio-loud active galactic nuclei (RLAGN). These galaxies display large-scale remnant radio plasma in the intergalactic medium left behind by a past episode of active galactic nuclei (AGN) activity, and meanwhile, the radio jets have restarted in a new episode. The knowledge of what causes the jets to switch off and restart is crucial to our understanding of galaxy evolution, while it is important to know if DDRGs form a host galaxy dichotomy relative to RLAGN. We utilised the LOFAR Two-Metre Sky Survey DR1, using a visual identification method to compile a sample of morphologically selected candidate DDRGs, showing two pairs of radio lobes. To confirm the restarted nature in each of the candidate sources, we obtained follow-up observations with the VLA at higher resolution to observe the inner lobes or restarted jets, the confirmation of which created a robust sample of 33 DDRGs. We created a comparison sample of 777 RLAGN from the DR1 catalogue, and compared the optical and infrared magnitudes and colours of their host galaxies. We find that there is no statistically significant difference in the brightness of the host galaxies between double-doubles and single-cycle RLAGN. The DDRG and RLAGN samples also have similar distributions in WISE mid-infrared colours, indicating similar ages of stellar populations and dust levels in the hosts of DDRGs. We conclude that DDRGs and 'normal' RLAGN are hosted by galaxies of the same type, and that DDRG activity is simply a normal part of the life cycle of RLAGN. Restarted jets, particularly for the class of low-excitation radio galaxies, rather than being a product of a particular event in the life of a host galaxy, must instead be caused by smaller scale changes, such as in the accretion system surrounding the black hole.

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arXiv2018

Ultra-steep spectrum emission in the merging galaxy cluster Abell 1914

A number of radio observations have revealed the presence of large synchrotron-emitting sources associated with the intra-cluster medium. There is strong observational evidence that the emitting particles have been (re-)accelerated by shocks and turbulence generated during merger events. The particles that are accelerated are thought to have higher initial energies than those in the thermal pool but the origin of such mildly relativistic particles remains uncertain and needs to be further investigated. The galaxy cluster Abell 1914 is a massive galaxy cluster in which X-ray observations show clear evidence of merging activity. We carried out radio observations of this cluster with the LOw Frequency ARay (LOFAR) at 150 MHz and the Giant Metrewave Radio Telescope (GMRT) at 610 MHz. We also analysed Very Large Array (VLA) 1.4 GHz data, archival GMRT 325 MHz data, CFHT weak lensing data and Chandra observations. Our analysis shows that the ultra-steep spectrum source (4C38.39; $α\lesssim -2$), previously thought to be part of a radio halo, is a distinct source with properties that are consistent with revived fossil plasma sources. Finally, we detect some diffuse emission to the west of the source 4C38.39 that could belong to a radio halo.

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arXiv2018

The LOFAR Two-metre Sky Survey - II. First data release

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$^\circ$00$'$00$''$ to 57$^\circ$00$'$00$''$) were mapped using a fully automated direction-dependent calibration and imaging pipeline that we developed. A total of 325,694 sources are detected with a signal of at least five times the noise, and the source density is a factor of $\sim 10$ higher than the most sensitive existing very wide-area radio-continuum surveys. The median sensitivity is S$_{\rm 144 MHz} = 71\,μ$Jy beam$^{-1}$ 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 $\sim$20 articles that highlight the scientific potential of LoTSS.

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arXiv2018

The LOFAR Two-metre Sky Survey IV. First Data Release: Photometric redshifts and rest-frame magnitudes

The LOFAR Two-metre Sky Survey (LoTSS) is a sensitive, high-resolution 120-168 MHz survey of the Northern sky. The LoTSS First Data Release (DR1) presents 424 square degrees of radio continuum observations over the HETDEX Spring Field (10h45m00s $<$ right ascension $<$ 15h30m00s and 45$^\circ$00$'$00$'$ $<$ declination $<$ 57$^\circ$00$'$00$''$) with a median sensitivity of 71$μ$Jy/beam and a resolution of 6$''$. In this paper we present photometric redshifts (photo-$z$) for 94.4% of optical sources over this region that are detected in the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) 3$π$ steradian survey. Combining the Pan-STARRS optical data with mid-infrared photometry from the Wide-field Infrared Survey Explorer, we estimate photo-$z$s using a novel hybrid photometric redshift methodology optimised to produce the best possible performance for the diverse sample of radio continuum selected sources. For the radio-continuum detected population, we find an overall scatter in the photo-$z$ of 3.9% and an outlier fraction ($\left | z_{\rm{phot}} - z_{\rm{spec}} \right | / (1+z_{\rm{spec}}) > 0.15$) of 7.9%. We also find that, at a given redshift, there is no strong trend in photo-$z$ quality as a function of radio luminosity. However there are strong trends as a function of redshift for a given radio luminosity, a result of selection effects in the spectroscopic sample and/or intrinsic evolution within the radio source population. Additionally, for the sample of sources in the LoTSS First Data Release with optical counterparts, we present rest-frame optical and mid-infrared magnitudes based on template fits to the consensus photometric (or spectroscopic when available) redshift.

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