arXiv subjects

G. B. Taylor

Publications and source records attributed to G. B. Taylor.

At least 181 records · Page 10Linked to original sources

arXiv2003

Observations of HI Absorbing Gas in Compact Radio Sources at Cosmological Redshifts

We present an overview of the occurrence and properties of atomic gas associated with compact radio sources at redshifts up to z=0.85. Searches for HI 21cm absorption were made with the Westerbork Synthesis Radio Telescope at UHF-high frequencies (725-1200 MHz). Detections were obtained for 19 of the 57 sources with usable spectra (33%). We have found a large range in line depths, from tau=0.16 to tau<=0.001. There is a substantial variety of line profiles, including Gaussians of less than 10km/s, to more typically 150km/s, as well as irregular and multi-peaked absorption profiles, sometimes spanning several hundred km/s. Assuming uniform coverage of the entire radio source, we obtain column depths of atomic gas between 1e19 and 3.3e21(Tsp/100K)(1/f)cm^(-2). There is evidence for significant gas motions, but in contrast to earlier results at low redshift, there are many sources in which the HI velocity is substantially negative (up to v=-1420km/s) with respect to the optical redshift, suggesting that in these sources the atomic gas, rather than falling into the centre, may be be flowing out, interacting with the jets, or rotating around the nucleus.

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arXiv2003

Motion Statistics in the CJ Survey -- The Status in October 2002

In the Caltech-Jodrell Survey of bright flat-spectrum radio sources, 3-5 epochs have now been observed for nearly all 293 sources; a uniquely large sample. The derivation of component motions has not yet been completed; it is complicated by, for example, variability and jet curvature. Nevertheless, some basic results are clear. The average apparent velocity in CJF quasars is near 3c, while for radio galaxies and BL Lacs it is around 1c. The distribution of velocities is broad, indicating a broad distribution of jet Lorentz factors, with many low values, and/or a decoupling of the Lorentz factors between the cores and the moving jet components, due to bending, speed changes, or due to pattern motions (shocks). A hint has emerged at this workshop that the average apparent velocities might be lower at lower radio frequencies; this will need careful verification.

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arXiv2003

A View through Faraday's fog: Parsec scale rotation measures in AGN

Rotation measure observations of 9 quasars, 4 BL Lacertae objects, and 3 radio galaxies are presented. The rest frame rotation measures in the cores of the quasars and the jets of the radio galaxy M87 are several thousand radians per meter squared. The BL Lacertae objects and the jets of the quasars have rest frame rotation measures of a few hundred radians per meter squared. A nuclear rotation measure of 500 radians per meter squared in the rest frame is suggested as the dividing line between quasar and BL Lacertae objects. The substantial rotation measures of the BL Lacertae objects and quasars cast doubt on the previous polarization position angle investigations of these objects at frequencies of 15 GHz or less. BL Lacertae itself has a rotation measure that varies in time, similar to the behavior observed for the quasars 3C273 and 3C279. A simple model with magnetic fields of 40 microgauss or less can account for the observed rotation measures.

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arXiv2003

VLBI Polarimetry of 177 Sources from the Caltech-Jodrell Bank Flat-spectrum Survey

We present VLBA observations and a statistical analysis of 5 GHz VLBI polarimetry data from 177 sources in the Caltech-Jodrell Bank flat-spectrum (CJF) survey. The CJF survey, a complete, flux-density-limited sample of 293 extragalactic radio sources, gives us the unique opportunity to compare a broad range of source properties for quasars, galaxies and BL Lacertae objects. We focus primarily on jet properties, specifically the correlation between the jet axis angle and the polarization angle in the core and jet. A strong correlation is found for the electric vector polarization angle in the cores of quasars to be perpendicular to the jet axis. Contrary to previous claims, no correlation is found between the jet polarization angle and the jet axis in either quasars or BL Lac objects. With this large, homogeneous sample we are also able to investigate cosmological issues and AGN evolution.

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arXiv2003

The Broadband Afterglow of GRB980703

We present radio observations of the afterglow of the bright gamma-ray burst GRB980703 made between one day and one year after the burst. These data are combined with published late-time radio measurements and existing optical, near-infrared (NIR) and X-ray observations to create a comprehensive broadband dataset for modeling the physical parameters of the outflow. While a wind-stratified medium cannot be ruled out statistically, it requires a high fraction of the shock energy in the electrons, and so is not favored on theoretical grounds. Instead, the data are consistent with a fireball model in which the ejecta are collimated and expanding into a constant density medium. The radio data cannot be fit with an isotropic shock but instead require a jet break at ~ 3.5 days, not seen at optical wavelengths due to the presence of a a bright host galaxy. The addition of the full radio dataset constrains the self-absorption frequency, giving an estimate of the circumburst density of n ~ 30 cm^-3, a value which differs substantially from previous estimates. This result is consistent with the growing number of GRB afterglows for which broadband modeling yields n ~= 0.1-100 cm^-3, with a typical value ~ 10 cm^-3.

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arXiv2002

Lobe Advance Velocities in the Extragalactic Compact Symmetric Object 4C 31.04

We report on the results of a two epoch study of the low power Compact Symmetric Object 4C 31.04. Observations performed with the VLBA at 5 GHz in 1995 and 2000 have yielded images of this source at milliarcsecond angular resolution. A central core is detected, with bright compact hot spots and extended lobes on both sides. Model-fitting and other analysis of the data (brightness profile, difference map) clearly indicate that the source is expanding. We estimate the velocity of this expansion to be (0.085 +/- 0.016) mas/yr, i.e. (0.33 +/- 0.06) c in both hot spots. Assuming a constant expansion velocity, we estimate the kinematic age of the source at 550 yrs. We also study the spectral index using VLBA observations at 1.3 GHz and MERLIN at 22 GHz. The derived spectral age is 3000-5000 years in equipartition conditions. The two estimates are discussed and found to be in agreement, given present uncertainties.

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arXiv2002

VLBA Continuum and H I Absorption Observations of the Ultra-Luminous Infrared Galaxy IRAS 17208-0014

We present phase-referenced VLBI observations of the radio continuum emission from, and the neutral hydrogen 21 cm absorption toward, the Ultra-Luminous Infrared Galaxy IRAS 17208-0014. The observations were carried out at 1362 MHz using the Very Long Baseline Array, including the phased Very Large Array as an element. The high-resolution radio continuum images reveal a nuclear starburst region in this galaxy, which is composed of diffuse emission approximately 670 x 340 pc on the plane of the sky, and a number of compact sources. These sources are most likely to be clustered supernova remnants and/or luminous radio supernovae. Their brightness temperatures range over (2.2-6.6) x 10^{5} K, with radio spectral luminosities between (1-10) x 10^{21} W Hz^{-1}. The total VLBI flux density of the starburst region is ~52 mJy, which is about 50% of the total flux density detected with the VLA at arcsecond resolution. For this galaxy, we derive a massive star formation rate of ~84pm13 M{_\odot} yr^{-1}, and a supernova rate of ~4pm1 yr^{-1}. H I absorption is detected in multiple components with optical depths ranging between 0.3 and 2.5, and velocity widths between 58 and 232 km s^{-1}. The derived column densities, assuming T_{s}=100 K, range over (10-26) x 10^{21} cm^{-2}. The H I absorption shows a strong velocity gradient of 453 km s^{-1} across 0.36 arcsec (274 pc). Assuming Keplerian motion, the enclosed dynamical mass is about 2.3 x 10^9 sin^{-2}i M{_\odot}, comparable to the enclosed dynamical mass estimated from CO observations.

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arXiv2002

Magnetic Fields in Clusters of Galaxies Obtained Through the Study of Faraday Rotation Measures

We are developing a new approach to investigate cluster magnetic field strengths and structures. It is based on the comparison of simulated Rotation Measure and radio halo images, obtained from 3-dimensional multi-scale cluster magnetic fields models, with observations. This approach is applied to A2255 in which wide diffuse radio emissions and Rotation Measure images show the evidence of cluster magnetic fields on both large and small scales.

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arXiv2002

Continued Radio Monitoring of the Gamma Ray Burst 991208

We present radio observations of the afterglow of the bright gamma-ray burst GRB 991208 at frequencies of 1.4, 4.9 and 8.5 GHz, taken between two weeks and 300 days after the burst. The well-sampled radio light curve at 8.5 GHz shows that the peak flux density peaked about 10 days after the burst and decayed thereafter as a power-law t^-1.07. This decay rate is more shallow than the optical afterglow with t^-2.2, which was measured during the first week. These late-time data are combined with extensive optical, millimeter and centimeter measurements and fitted to the standard relativistic blast wave model. In agreement with previous findings, we find that an isotropic explosion in a constant density or wind-blown medium cannot explain these broadband data without modifying the assumption of a single power-law slope for the electron energy distribution. A jet-like expansion provides a reasonable fit to the data. In this case, the flatter radio light curve compared to the optical may be due to emission from an underlying host galaxy, or due to the blastwave making a transition to non-relativistic expansion. The model that best represents the data is a free-form model in which it is assumed that the broadband emission originates from a synchrotron spectrum, while the time-evolution of the break frequencies and peak flux density are solved for explicitly. Although the decay indices for most of the synchrotron parameters are similar to the jet model, the evolution of the cooling break is unusually rapid, and therefore requires some non-standard evolution in the shock. (abridged)

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arXiv2002

Parsec Scale Properties of Radio Sources

We present a new complete sample of radio sources selected from the B2 and 3CR catalogues, with no bias respect to the jet velocity and orientation. Using preliminary data we investigate the parsec scale properties of radio sources with different radio power and kpc scale morphology. We stress the evidence for high velocity pc scale jets in all sources and conclude that the properties of parsec scale jets are similar regardless of the source total power and large scale morphology. Moreover we show two epoch images of two nearby radio galaxies: a low power compact source and a CSO. The comparison of the two epoch images suggests that in both sources the size is increasing with time. We derive a possible advance speed and estimate their dynamic age.

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arXiv2002

X-ray Detection of the Inner Jet in the Radio Galaxy 3C 129

During the course of an investigation on the interaction of the radio galaxy 3C129 and its ambient cluster gas, we found excess X-ray emission aligned with the northern radio jet. The emission extends from the weak X-ray core of the host galaxy approximately 2.5" to the first resolved radio knot. On a smaller scale, we have also detected a weak radio extension in the same position angle with the VLBA. Although all the evidence suggests that Doppler favoritism augments the emission of the northern jet, it is unlikely that the excess X-ray emission is produced by inverse Compton emission. We find many similarities between the 3C129 X-ray jet and recent jet detections from Chandra data of low luminosity radio galaxies. For most of these current detections synchrotron emission is the favored explanation for the observed X-rays.

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arXiv2002

The Broadband Afterglow of GRB980329

We present radio observations of the afterglow of the bright gamma-ray burst GRB980329 made between one month and several years after the burst, a re-analysis of previously published submillimeter data, and late-time optical and near-infrared (NIR) observations of the host galaxy. From the absence of a spectral break in the optical/NIR colors of the host galaxy, we exclude the earlier suggestion that GRB980329 lies at a redshift of z >~5. We combine our data with the numerous multi-wavelength observations of the early afterglow, fit a comprehensive afterglow model to the entire broadband dataset, and derive fundamental physical parameters of the blast-wave and its host environment. Models for which the ejecta expand isotropically require both a high circumburst density and extreme radiative losses from the shock. No low density model (n << 10 cm^{-3}) fits the data. A burst with a total energy of ~ 10^{51} erg, with the ejecta narrowly collimated to an opening angle of a few degrees, driven into a surrounding medium with density ~ 20 cm^{-3}, provides a satisfactory fit to the lightcurves over a range of redshifts.

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arXiv2002

HI Absorption in the Gigamaser Galaxy TXS 2226-184 and the Relation between HI Absorption and Water Emission

We report on the discovery of HI in absorption toward the gigamaser galaxy TXS2226-184 using the VLA. The absorption appears to consist of two components -- one with a width of 125 km/s, and one broader (420 km/s), both toward the compact radio source in the nucleus of the galaxy. Based on these large velocity widths we suggest that the HI absorption is produced in the central parsecs of the galaxy, on a similar scale to that which gives rise to the water maser emission. This brings to eight the number of galaxies known to exhibit both water masers and HI absorption. We explore the relationship between these two phenomena, and present a physically motivated (but unfruitful) search for water maser emission in five radio galaxies known to exhibit strong HI absorption.

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arXiv2002

Faraday Rotation Measures in the Parsec-Scale Jets of the Radio Galaxies M87, 3C 111, and 3C 120

Parsec-scale Faraday rotation measure (RM) maps are presented for the radio galaxies M87, 3C 111, and 3C 120. These Very Long Baseline Array observations were made at 8, 12, and 15 GHz. M87 has an extreme RM distribution that varies from -4000 radians m^-2 to more than 9000 radians m^-2 across a projected distance of 0.3 pc in its jet. M87 has no polarized flux closer than 17 mas from the core. 3C 111 and 3C 120 both show polarized emission in their cores that is consistent with the expectations of unified schemes for these broad-line radio galaxies. 3C 111 has an RM gradient that increases from -200 radians m^-2 four mas from the core to -750 radians m^-2 on the side of the jet closest to the core. 3C 120 has a more moderate RM distribution in the jet of approximately 100 radians m^-2, but this increases by an order of magnitude in the core.

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arXiv2001

Cluster Magnetic Fields

Magnetic fields in galaxy clusters have been measured using a variety of techniques, including: studies of synchrotron relic and halo radio sources within clusters, studies of inverse Compton X-ray emission from clusters, surveys of Faraday rotation measures of polarized radio sources both within and behind clusters, and studies of Cluster Cold Fronts in X-ray images. These measurements imply that most cluster atmospheres are substantially magnetized, with typical field strengths of order 1 microGauss with high areal filling factors out to Mpc radii. There is likely, however, to be considerable variation in field strengths and topologies both within and between clusters, especially when comparing dynamically relaxed clusters to those that have recently undergone a merger. In some locations, such as the cores of cooling flow clusters, the magnetic fields reach levels of 10 - 40 microGauss, and may be dynamically important. In all clusters the magnetic fields have a significant effect on energy transport in the intracluster medium. We also review current theories on the origin of cluster magnetic fields.

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arXiv2001

Radio galaxies and magnetic fields in A514

A514 contains six extended and polarized radio sources located at various projected distances from the cluster center. Here we present a detailed study of these six radio sources in total intensity and polarization using the Very Large Array at 3.6 and 6 cm. Since the radio sources sample different lines of sight across the cluster, an analysis of the Faraday Rotation Measures (RMs) provides information on the strength and the structure of the cluster magnetic field. These sources show a decreasing Faraday Rotation Measure with increasing distance from the cluster center. We estimate the strength of the magnetic field to be ~3-7 uG in the cluster center. From the RM structure across the stronger and more extended sources we estimate the coherence length of the magnetic field to be about 9 kpc at the cluster center.

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arXiv2001

Magnetic fields in the Centaurus cluster

We present multi-frequency VLA observations of the radio galaxy PKS 1246-410 at the center of the Centaurus Cluster, and compare these results to recent Chandra observations. The unusual radio morphology of PKS 1246-410 probably results from interactions with the hot, X-ray emitting gas. This gas, along with cluster magnetic fields, also produces substantial Faraday Rotation Measures towards PKS 1246-410. We discuss these observations in the context of a sample of 14 luminous X-ray clusters with embedded radio galaxies and possible cooling flows. A correlation is found between the cooling flow rate and the maximum Faraday Rotation Measures. Magnetic fields of strength 10-40 uG are found to be common to the centers of clusters with strong cooling flows, and somewhat lower field strengths of 2-10 uG are found in the non cooling-flow clusters.

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arXiv2001

Compact Symmetric Objects as Radio Flux Density Calibrators

We present results from the first intensive monitoring campaign of a sample of Compact Symmetric Objects (CSOs). We observed seven CSOs at 8.5 GHz over a period of eight months, with an average spacing between observations of 2.7 days. Our results show that, as predicted, the flux densities of the CSOs are extremely stable; the mean RMS variability of the sample was 0.7% in flux density. The low variability of the CSOs makes them excellent flux density calibrators at this frequency. We recommend that at least four CSOs be included in any VLA monitoring campaign which requires precise epoch-to-epoch calibration, such as those to measure gravitational lens time delays. The CSO data enable the correction of small systematic errors in the primary flux calibration.

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