arXiv subjects

J. Dennett-Thorpe

Publications and source records attributed to J. Dennett-Thorpe.

13 recordsLinked to original sources

arXiv2018

Constraining the radio jet proper motion of the high-redshift quasar J2134-0419 at z=4.3

To date, PMN J2134-0419 (at a redshift z=4.33) is the second most distant quasar known with a milliarcsecond-scale morphology permitting direct estimates of the jet proper motion. Based on two-epoch observations, we constrained its radio jet proper motion using the very long baseline interferometry (VLBI) technique. The observations were conducted with the European VLBI Network (EVN) at 5 GHz on 1999 November 26 and 2015 October 6. We imaged the central 10-pc scale radio jet emission and modeled its brightness distribution. By identifying a jet component at both epochs separated by 15.86 yr, a proper motion of mu=0.035 +- 0.023 mas/yr is found. It corresponds to an apparent superluminal speed of beta_a=4.1 +- 2.7 c . Relativistic beaming at both epochs suggests that the jet viewing angle with respect to the line of sight is smaller than 20 deg, with a minimum bulk Lorentz factor Gamma=4.3. The small value of the proper motion is in good agreement with the expectations from the cosmological interpretation of the redshift and the current cosmological model. Additionally we analyzed archival Very Large Array observations of J2143-0419 and found indication of a bent jet extending to ~30 kpc.

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arXiv2003

The depolarisation properties of powerful radio sources: breaking the radio-power vs. redshift degeneracy

We define 3 samples of extragalactic radio sources of type FRII, containing 26 objects in total. The control sample consists of 6C and 7C sources with radio powers of around 10e27 W\Hz at 151 MHz and redshifts of around z=1. The other samples contain 3CRR sources with either comparable redshifts but radio powers about a decade larger or with comparable radio powers but redshifts around z = 0.4. We use these samples to investigate the possible evolution of their depolarisation and rotation measure properties with redshift and radio power independently. We used VLA data for all sources at 4800 MHz and two frequencies within the 1400 MHz band, either from our own observations or from the archive. We present maps of the total intensity flux, polarised flux, depolarisation, spectral index, rotation measure and magnetic field direction where not previously published. Radio cores were detected in twelve of the twenty-six radio sources. Fourteen of the sources show a strong Laing-Garrington effect but almost all of the sources show some depolarisation asymmetry. All sources show evidence for an external Faraday screen being responsible for the observed depolarisation. We find that sources at higher redshift are more strongly depolarised. Rotation measure shows no trend with either redshift or radio power, however variations in the rotation measure across individual sources increase with the redshift of the sources but do not depend on their radio power.

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arXiv2003

Annual modulation in the scattering of J1819+3845: peculiar plasma velocity and anisotropy

We present two years of monitoring observations of the extremely variable quasar J1819+3845. We observe large yearly changes in the timescale of the variations (from ~ 1 hour to ~ 10 hours at 5GHz). This annual effect can only be explained if the variations are caused by a propagation effect, and thus affected by the Earth's relative speed through the projected intensity pattern. To account for this effect, the scattering plasma must have a transverse velocity with respect to the local standard of rest. The velocity calculated from these observations is in good agreement with that obtained from a two telescope delay experiment (Dennett-Thorpe & de Bruyn 2001). We also show that either the source itself is elongated, or that the scattering plasma is anisotropic, with an axial ratio of >6:1. As the source is extended on scales relevant to the scattering phenomenon, it seems plausible that the anisotropy is due to the source itself, but this remains to be investigated. From the scintillation characteristics we find that the scattering material is a very strong, thin scatterer within ~ten parsecs. We determine a source size at 5GHz of 100 to 900microarcsecs, and associated brightness temperatures of 10^{10} to 10^{12}K.

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arXiv2002

Interstellar scintillation as the origin of rapid radio variability in the quasar J1819+3845

Quasars shine brightly due to the liberation of gravitational energy as matter falls onto a supermassive black hole in the centre of a galaxy. Variations in the radiation received from active galactic nuclei (AGN) are studied at all wavelengths, revealing the tiny dimensions of the region and the processes of fuelling the black hole. Some AGN are variable at optical and shorter wavelengths, and display radio outbursts over years and decades. These AGN often also show faster variations at radio wavelengths (intraday variability, IDV) which have been the subject of much debate. The simplest explanation, supported by a correlation in some sources between the optical (intrinsic) and faster radio variations, is that the rapid radio variations are intrinsic. However, this explanation implies physically difficult brightness temperatures, suggesting that the variations may be due to scattering of the incident radiation in the interstellar medium of our Galaxy. Here we present results which show unambiguously that the variations in one extreme case are due to interstellar scintillation. We also measure the transverse velocity of the scattering material, revealing a surprising high velocity plasma close to the Solar System.

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arXiv2001

Jet reorientation in AGN: two winged radio galaxies

Winged, or X-shaped, radio sources form a small class of morphologically peculiar extragalactic sources. We present multi-frequency radio observations of two such sources. We derive maximum ages since any re-injection of fresh particles of 34 and 17Myr for the wings of 3C223.1 and 3C403, respectively, based on the lack of synchrotron and inverse Compton losses. On morphological grounds we favour an explanation in terms of a fast realignment of the jet axis which occurred within a few Myr. There is no evidence for merger activity, and the host galaxies are found to reside in no more than poor cluster environments. A number of puzzling questions remain about those sources: in particular, although the black hole could realign on sufficiently short timescales, the origin of the realignment is unknown.

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arXiv2001

EVN observations of low-luminosity flat-spectrum AGNs

We present and discuss the results of VLBI (EVN) observations of three low-luminosity (P(5 GHz)<10^25 W/Hz) Broad Emission Line AGNs carefully selected from a sample of flat spectrum radio sources (CLASS). Based on the total and the extended radio power at 5 GHz and at 1.4 GHz respectively, these objects should be technically classified as radio-quiet AGN and thus the origin of their radio emission is not clearly understood. The VLBI observations presented in this paper have revealed compact radio cores which imply a lower limit on the brightness temperature of about 3X10^8 K. This result rules out a thermal origin for the radio emission and strongly suggests an emission mechanism similar to that observed in more powerful radio-loud AGNs. Since, by definition, the three objects show a flat (or inverted) radio spectrum between 1.4 GHz and 8.4 GHz, the observed radio emission could be relativistically beamed. Multi-epoch VLBI observations can confirm this possibility in two years time.

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arXiv2000

The microarcsecond quasar J1819+3845

We present new WSRT observations of the micro-arcsecond quasar J1819+3845. All short term variations are attributed to interstellar scintillation of a source which is at most 30 micro-arcseconds in diameter. The timescale of the modulations changes over the year, which we interpret as due to a peculiar velocity of the scattering medium. The scintillation behaviour can be used to determine sub-structure in the source.

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arXiv2000

On the viewing angle to broad-lined radio galaxies

We address the nature of broad-lined radio galaxies, in particular their radio axis orientation, using new, matched resolution, dual frequency radio observations of a sample of twelve nearby broad-lined extragalactic 3C objects. Radio spectral index and depolarisation asymmetries indicate that these objects have a preferred orientation with respect to the observer. In addition, the spectral asymmetries are suggestive of lower Doppler factors in the broad-lined radio galaxies when compared to 3C quasars. This is in agreement with their optical properties, and leads to the conclusion that some objects are lower powered versions-at similar lines of sight-of the more distant quasars, whereas others are at larger angles to the line of sight.

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arXiv2000

Flat radio-spectrum galaxies and BLLacs: Part I: core properties

This paper concerns the relationship of BLLacs and flat spectrum weak emission-line galaxies. We compare the weak emission-line galaxies and the BLLacs in a sample of 57 flat spectrum objects (Marcha et al. 1996), using high-frequency radio and non-thermal optical flux densities, spectral indices and polarization properties. We consider whether objects which are not `traditional' BLLacs-due to their larger emission line strengths, and larger CaII spectral breaks-are simply starlight diluted BLLacs. Their broad-band spectral properties are consistent with this interpretation, but their radio polarization may indicate more subtle effects. Comparison of the weak emission-line galaxies and the BLLacs shows that, on average, the former have steeper spectra between 8 and 43GHz, and are less polarized at 8.4GHz. This is consistent with many of the weak-lined objects being at larger angles to the line of sight than the BLLacs. In addition to this population, we indicate a number of the weak emission-line galaxies which may be `hidden BLLacs': relativistically boosted objects very close to the line of sight with an apparently weak AGN.

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arXiv1999

The discovery of a microarcsecond quasar: J1819+3845

We report on the discovery of a source which exhibits over 300% amplitude changes in radio flux density on the period of hours. This source, J1819+3845, is the most extremely variable extragalactic source known in the radio sky. We believe these properties are due to interstellar scintillation, and show that the source must emit at least 55% of its flux density within a radius of <16 microarcseconds at 5GHz. The apparent brightness temperature is > 5.10^{12} K, and the source may be explained by a relativistically moving source with a Doppler factor ~15. The scattering occurs predominantly in material only a few tens of parsecs from the earth, which explains its unusually rapid variability. If the source PKS 0405-385 (Kedziora-Chudczer et al 1997) is similarly affected by local scattering material, Doppler factors of ~1000 are not required to explain this source. The discovery of a second source whose properties are well modeled by interstellar scintillation strengthens the argument for this as the cause for much of the variations seen in intra-day variables (IDV).

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arXiv1999

The CERES Astronomical Database

The CERES Astronomical Archive was created initially in order to make the large amount of data that were collected from the two surveys, the Jodrell-VLA Astrometric Survey (JVAS) and the Cosmic Lens All-Sky Survey (CLASS), easily accessible to the partner members of the Consortium for European Research on Extragalactic Surveys (CERES) through the web. Here we describe the web based archive of the 15,000 flat spectrum radio sources. There is a wealth of information that one can access including downloading the raw and processed data for each of the sources. We also describe the search engines that were developed so that a) one can search through the archive for an object or a sample of objects by setting specific criteria and b) select new samples. Since new data are continually gathered on the same sources from follow up programs, an automatic update engine was created so that the new information and data can be added easily in the archive.

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arXiv1998

Asymmetry of jets, lobe size and spectral index in radio galaxies and quasars

We investigate the correlations between spectral index, jet side and extent of the radio lobes for a sample of nearby FRII radio galaxies. In Dennett-Thorpe et al. (1997) we studied a sample of quasars and found that the high surface brightness regions had flatter spectra on the jet side (explicable as a result of Doppler beaming) whilst the extended regions had spectral asymmetries dependent on lobe length. Unified schemes predict that asymmetries due to beaming will be much smaller in narrow-line radio galaxies than in quasars: we therefore investigate in a similar manner, a sample of radio galaxies with detected jets. We find that spectral asymmetries in these objects are uncorrelated with jet sidedness at all brightness levels, but depend on relative lobe volume. Our results are not in conflict with unified schemes, but suggest that the differences between the two samples are due primarily to power or redshift, rather than to orientation. We also show directly that hotspot spectra steepen as a function of radio power or redshift. Whilst a shift in observed frequency due to the redshift may account for some of the steepening, it cannot account for all of it, and a dependence on radio power is required.

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arXiv1997

Asymmetry of jets, lobe length and spectral index in quasars

The less depolarized lobe of a radio source is generally the lobe containing the jet (Laing-Garrington correlation) but the less depolarized lobe is also generally that with the flatter radio spectrum (Liu-Pooley correlation). Both effects are strong; taken together they would imply a correlation between jet side and lobe spectral index, i.e. between an orientation-dependent feature and one which is intrinsic. We test this prediction using detailed spectral imaging of a sample of quasars with well-defined jets and investigate whether the result can be reconciled with the standard interpretation of one-sided jets in terms of relativistic aberration. Our central finding is that the spectrum of high surface brightness regions is indeed flatter on the jet side, but that the spectrum of low surface brightness regions is flatter on the side with the longer lobe. We discuss possible causes for these correlations and favour explanations in terms of relativistic bulk motion in the high surface brightness regions and differential synchrotron ageing in the extended lobe material.

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