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Lovell, J. E. J.

Publications and source records attributed to Lovell, J. E. J..

18 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS2016

The Gamma-Ray Emitting Radio-Loud Narrow-Line Seyfert 1 Galaxy PKS 2004-447 II. The Radio View

Context. gamma-ray-detected radio-loud narrow-line Seyfert 1 (gamma-NLS1) galaxies constitute a small but interesting sample of the gamma-ray-loud AGN. The radio-loudest gamma-NLS1 known, PKS2004447, is located in the southern hemisphere and is monitored in the radio regime by the multiwavelength monitoring programme TANAMI. Aims. We aim for the first detailed study of the radio morphology and long-term radio spectral evolution of PKS2004447, which are essential for understanding the diversity of the radio properties of gamma-NLS1s. Methods. The TANAMI VLBI monitoring program uses the Australian Long Baseline Array (LBA) and telescopes in Antarctica, Chile, New Zealand, and South Africa to monitor the jets of radio-loud active galaxies in the southern hemisphere. Lower resolution radio flux density measurements at multiple radio frequencies over four years of observations were obtained with the Australia Telescope Compact Array (ATCA). Results. The TANAMI VLBI image at 8.4GHz shows an extended one-sided jet with a dominant compact VLBI core. Its brightness temperature is consistent with equipartition, but it is an order of magnitude below other gamma-NLS1s with the sample value varying over two orders of magnitude. We find a compact morphology with a projected large-scale size 11 kpc and a persistent steep radio spectrum with moderate flux-density variability. Conclusions. PKS2004447 appears to be a unique member of the gamma-NLS1 sample. It exhibits blazar-like features, such as a flat featureless X-ray spectrum and a core-dominated, one-sided parsec-scale jet with indications for relativistic beaming. However, the data also reveal properties atypical for blazars, such as a radio spectrum and large-scale size consistent with compact-steep-spectrum (CSS) objects, which are usually associated with young radio sources. These characteristics are unique among all gamma-NLS1s and extremely rare among gamma-ray-loud AGN.

galaxies: active / galaxies: individual: PKS 2004-↗
NASA NTRS2012

Dual-Frequency Observations of 140 Compact, Flat-Spectrum Active Galactic Nuclei for Scintillation-Induced Variability

The 4.9 GHz Micro-Arcsecond Scintillation-Induced Variability (MASIV) Survey detected a drop in Interstellar Scintillation (ISS) for sources at red shifts z > or approx. 2, indicating an apparent increase in angular diameter or a decrease in flux density of the most compact components of these sources, relative to their extended emission. This can result from intrinsic source size effects or scatter broadening in the Intergalactic Medium (IGM) , in excess of the expected (1+z)1/2 angular diameter scaling of brightness temperature limited sources resulting from cosmological expansion. We report here 4.9 GHz and 8.4 GHz observations and data analysis for a sample of 140 compact, fiat-spectrum sources which may allow us to determine the origin of this angular diameter-redshift relation by exploiting their different wavelength dependences. In addition to using ISS as a cosmological probe, the observations provide additional insight into source morphologies and the characteristics of ISS. As in the MASIV Survey, the variability of the sources is found to be significantly correlated with line-of-sight H(alpha) intensities, confirming its link with ISS. For 25 sources, time delays of about 0.15 to 3 days are observed between the scintillation patterns at both frequencies, interpreted as being caused by a shift in core positions when probed at different optical depths. Significant correlation is found between ISS amplitudes and source spectral index; in particular, a large drop in ISS amplitudes is observed at alpha < -0.4 confirming that steep spectrum sources scintillate less. We detect a weakened redshift dependence of ISS at 8.4 GHz over that at 4.9 GHz, with the mean variance at 4-day timescales reduced by a factor of 1.8 in the z > 2 sources relative to the z < 2 sources, as opposed to the factor of 3 decrease observed at 4.9 GHz. This suggests scatter broadening in the IGM, but the interpretation is complicated by subtle selection effects that will be explored further in a follow-up paper.

Koay, J. Y.↗
NASA NTRS2012

Why Do Compact Active Galactic Nuclei at High Redshift Twinkle Less?

The fraction of compact active galactic.nuclei (AGNs) that exhibit interstellar scintillation (ISS) at radio wavelengths, as well as their scintillation amplitudes, have been found to decrease significantly for sources at redshifts z approx greater than 2. This can be attributed to an increase in the angular sizes of the mu-as-scale cores or a decrease in the flux densities of the compact mu-as cores relative to that of the mas-scale components with increasing redshift, possibly arising from (1) the space-time curvature of an expanding Universe, (2) AGN evolution, (3) source selection biases, (4) scatter broadening in the ionized intergalactic medium (IGM), or (5) gravitational lensing. We examine the frequency scaling of this redshift dependence of ISS to determine its origin, using data from a dual-frequency survey of ISS of 128 sources at 0 approx < z approx < 4. We present a novel method of analysis which accounts for selection effects in the source sample. We determine that the redshift dependence of ISS is partially linked to the steepening of source spectral indices (alpha (sup 8.4, sub 4.9)) with redshift, caused either by selection biases or AGN evolution, coupled with weaker ISS in the alpha (sup 8.4, sub 4.9) < -0.4 sources. Selecting only the -0.4 < alpha (sup 8.4, sub 4.9) < 0.4 sources, we find that the redshift dependence of ISS is still significant, but is not significantly steeper than the expected (1 + z)(exp 0.5) scaling of source angular sizes due to cosmological expansion for a brightness temperature and flux-limited sample of sources. We find no significant evidence for scatter broadening in the IGM, ruling it out as the main cause of the redshift dependence of ISS. We obtain an upper limit to IGM scatter broadening of approx. < 110 mu-as at 4.9 GHz with 99% confidence for all lines of sight, and as low as approx. < 8 mu-as for sight-lines to the most compact, approx 10 mu-as sources.

Koay, J. Y.↗
NASA NTRS2012

PKS 2123-463: A Confirmed Gamma-ray Blazar at High Redshift

The flat spectrum radio quasar (FSRQ) PKS 2123−463 was associated in the First Fermi-LAT source catalog with the gamma-ray source 1FGL J2126.1−4603, but when considering the full first two years of Fermi observations, no gamma-ray source at a position consistent with this FSRQ was detected, and thus PKS 2123−463 was not reported in the Second Fermi-LAT source catalog. On 2011 December 14 a gamma-ray source positionally consistent with PKS 2123−463 was detected in flaring activity by Fermi-LAT. This activity triggered radio-to-X-ray observations by the Swift, GROND, ATCA, Ceduna, and KAT-7 observatories. Results of the localization of the gamma-ray source over 41 months of Fermi-LAT operation are reported here in conjunction with the results of the analysis of radio, optical, UV and X-ray data collected soon after the gamma-ray flare. The strict spatial association with the lower energy counterpart together with a simultaneous increase of the activity in optical, UV, X-ray and gamma-ray bands led to a firm identification of the gamma-ray source with PKS 2123−463. A new photometric redshift has been estimated as z = 1.46 +/- 0.05 using GROND and Swift/UVOT observations, in rough agreement with the disputed spectroscopic redshift of z = 1.67. We fit the broadband spectral energy distribution with a synchrotron/external Compton model. We find that a thermal disk component is necessary to explain the optical/UV emis- sion detected by Swift/UVOT. This disk has a luminosity of 1.8x1046 erg s−1, and a fit to the disk emission assuming a Schwarzschild (i.e., nonrotating) black hole gives a mass of 2 x 109 M(solar mass). This is the first black hole mass estimate for this source.

Gamma-ray blazar↗
NASA NTRS2011

Dual-Frequency VLBI Study of Centaurus A on Sub-Parsec Scales: The Highest-Resolution View of an Extragalactic Jet

Centaurus A is the closest active galactic nucleus. High resolution imaging using Very Long Baseline Interferometry (VLBI) enables us to study the spectral and kinematic behavior of the radio jet-<:ounterjet system on sub-parsec scales, providing essential information for jet emission and formation models. Aims. Our aim is to study the structure and spectral shape of the emission from the central-parsec region of Cen A. Methods. As a target of the Southern Hemisphere VLBI monitoring program TANAMI (Tracking Active Galactic Nuclei with Millliarcsecond Interferometry), VLBI observations of Cen A are made regularly at 8.4 and 22.3 GHz with the Australian Long Baseline Array (LBA) and associated telescopes in Antarctica, Chile, and South Africa. Results. The first dual-frequency images of this source are presented along with the resulting spectral index map. An angular resolution of 0.4 mas x 0.7 mas is achieved at 8.4 GHz, corresponding to a linear scale of less than 0.013 pc. Hence, we obtain the highest resolution VLBI image of Cen A, comparable to previous space-VLBI observations. By combining with the 22.3 GHz image, we present the corresponding dual-frequency spectral index distribution along the sub-parsec scale jet revealing the putative emission regions for recently detected y-rays from the core region by Fermi/LAT. Conclusions. We resolve the innermost structure of the milliarcsecond scale jet and counter jet system of Cen A into discrete components. The simultaneous observations at two frequencies provide the highest resolved spectral index map of an AGN jet allowing us to identify up to four possible sites as the origin of the high energy emission. Key words. galaxies: active galaxies: individual (Centaurus A, NGC 5128) - galaxies: jets - techniques: high angular resolution

Mueller, C.↗
NASA NTRS2000

Analysis of the PKS0637-752 X-Ray Jet System

The X-ray jet emitted from the quasar PKS0637-752 is the largest and most luminous detected to date. It extends 10 arcsec west of the nucleus, and is coincident with GHz radio jet emission in this region. If the individual X-ray/optical/radio knots in this jet were detected as isolated objects, they might be classified as BL Lac objects or quasars. We present a detailed analysis of the Chandra observations of this system, including a search for an intra-cluster medium to confine the jet, and limits to X-ray emission from the eastern radio knot and past the bend of the radio jet to the northwest. The projection in the plane of the sky implies that the X-ray jet is an aligned extension of the pc-scale jet, for which VLBI/VSOP observations show super-luminal motion with an apparent velocity of 11c. In this case, the X-ray jet is of order 1 Mpc in length, and explaining the X-ray emission mechanism presents new challenges.

Schwartz, D. A.↗
NASA NTRS1998

Interstellar Optics

We discuss the effects of finite source size on the diffraction pattern produced by scattering in a thin screen, particularly as applied to radio-wave scattering, by density fluctuations in the interstellar plasma.

Interstellar Optics Scattering techniques telescop↗
NASA NTRS1996

Interstellar Broadening of Images in the Gravitational Lens Pks 1830-211

The remarkably strong radio gravitational lens PKS 1830-211 consists of a one arcsecond diameter Einstein ring with two bright compact (milliarcsecond) components located on opposite sides of the ring. We have obtained 22 GHz VLBA data on this source to determine the intrinsic angular sizes of the compact components. Previous VLBI observations at lower frequencies indicate that the brightness temperatures of these components are significantly lower than 10(exp 10) K (Jauncey, et al. 1991), less than is typical for compact synchrotron radio sources and less than is implied by the short timescales of flux density variations. A possible explanation is that interstellar scattering is broadening the apparent angular size of the source and thereby reducing the observed brightness temperature. Our VLBA data support this hypothesis. At 22 GHz the measured brightness temperature is at least 10(exp 11) K, and the deconvolved 2 size of the core in the southwest compact component is proportional to upsilon(sup -2) between 1.7 and 22 GHz. VLBI observations at still higher frequencies should be unaffected by interstellar scattering.

Jones, D. L.↗
NASA NTRS1996

VLBI Observations of Southern EGRET Identifications: PKS 0208-512, PKS 0521-365 and PKS 0537-441 - 1

We present high-resolution very long baseline interferometry images of three southern radio sources that the Energetic Gamma-Ray Experiment Telescope (EGRET), on board the Compton Gamma Ray Observatory, has identified as greater than 100 MeV gamma-ray sources. These are the first results in a continuing program of VLBI observations of southern EGRET identifications. For two of these sources, PKS 0208-512 (at 4.851 GHz) and PKS 0537-441 (at 4.851 and 8.418 GHz), the images represent first-epoch observations. For the remaining lower redshift object, PKS 0521-365, we present images from three epochs at 4.851 GHz and an image from one further epoch at 8.418 GHz, spanning approximately 1 yr. We discuss the need for further extensive VLBI observations of EGRET-identified radio sources.

Tingay, S. J.↗
NASA NTRS1996

Jet Propulsion Laboratories in Space: The Production of Astrophysical Jets by Magnetic Accretion Disks, and a Super-Eddington Wind Model for the Early Stages of GROJ1455-40

Measurements of the properties of gravitational lenses have the power to tell us what sort of universe we live in. The brightest known radio Einstein ring/gravitational lens PKS 1830-211, whilst obscured by our galaxy at optical wavelengths, has recently provided a lensing galaxy redshift of 0.89 through the detection of a new absorption feature, most likely due to neutral hydrogen in a second redshift system at z=0.19.

PKS 1830-211↗
NASA NTRS1996

PKS 1830-211: A Possible Compound Gravitational Lens

Measurements of the properties of gravitational lenses have the power to tell us what sort of universe we live in. The brightest known radio Einstein ring/gravitational lens PKS 1830-211, whilst obscured by our Galaxy at optical wavelengths, has recently provided a lensing galaxy redshift of 0.89 through the detection of molecular absorption in the millimetre waveband.

new absorption feature lensing galaxy redshift sec↗
NASA NTRS1996

Interstellar Broadening of Images in the Gravitational Lens PKS 1830-211

The remarkably strong radio gravitational lens PKS 1830-211 consists of a one arcsecond diameter Einstein ring with two bright compact (milliarcsecond) components located on opposite sides of the ring. We have obtained 22GHz VLBA data on this source to determine the intrinsic angular sizes of the compact components.

galaxies↗
NASA NTRS1995

VLBI Phase-Referenced Observations on Southern Hemisphere HIPPARCOS Radio Start

Presented are multiepoch Very Long Baseline Interferometry (VLBI) observations on Southern Hemisphere radio stars phase-referenced to background radio sources. The differential astrometry analysis results in high-precision determinations of proper motions and parallaxes. The astrophysical implications and astrometric consequences of these results are discussed.

Radio Emission Stars VLBI Radio Astronomy Astrophy↗