Richard A. Perley

Time delay for the gravitational lens system B0218+357

Measurement of the time delay between multiple images of a gravitational lens system is potentially an accurate method of determining the Hubble constant over cosmological distances. One of the most promising candidates for an application of this technique is the system B0218+357, which was found in the Jodrell Bank/VLA Astrometric Survey (JVAS). This system consists of two images of a compact radio source, separated by 335 milliarcsec, and an Einstein ring which can provide a strong constraint on the mass distribution in the lens. We present here the results of a three-month VLA monitoring campaign at two frequencies. The data are of high quality, and both images show clear variations in total flux density, percentage polarization and polarization position angle at both frequencies. The time delay between the variations in the two images has been calculated using a chi-squared minimization to be 10.5 +/- 0.4 d at 95 per cent confidence, with the error being derived from Monte Carlo simulations of the light curves. Although mass modelling of the system is at a preliminary stage, taking the lensing galaxy to be a singular isothermal ellipsoid and using the new value for the time delay, we obtain a value for the Hubble constant of 69(-19)(+13) km s(-1) Mpc(-1), again at 95 per cent confidence.

Magnetic Fields in the Hydra A Cluster

Sensitive, high-resolution VLA polarimetric observations are presented for the radio galaxy Hydra A, an outstanding example of a high-luminosity radio source embedded within a cooling flow cluster. We find extremely high Faraday rotation measures (RMs), throughout the central regions of this source, with a strong asymmetry between the north and south lobes. Simple models for the high RMs are discussed and compared with the observations. To explain the north-south asymmetry in RM requires a magnetic field of strength ∼6 μG organized over a scale of ∼100 kpc. On smaller scales we find a tangled magnetic field with a strength of ∼30 μG

THE TWO-COMPONENT RADIO LUMINOSITY FUNCTION OF QUASI-STELLAR OBJECTS: STAR FORMATION AND ACTIVE GALACTIC NUCLEUS

Despite decades of study, it remains unclear whether there are distinct radio-loud and radio-quiet populations of quasi-stellar objects (QSOs). Early studies were limited by inhomogeneous QSO samples, inadequate sensitivity to probe the radio-quiet population, and degeneracy between redshift and luminosity for flux-density-limited samples. Our new 6 GHz EVLA observations allow us for the first time to obtain nearly complete (97%) radio detections in a volume-limited color-selected sample of 179 QSOs more luminous than M_i = -23 from the Sloan Digital Sky Survey (SDSS) Data Release Seven in the narrow redshift range 0.2 23.5, and (c) the total number of SDSS QSOs in our volume-limited sample. We show that the RLF can be explained as a superposition of two populations, dominated by AGNs at the bright end and star formation in the QSO host galaxies at the faint end.

Where have all the cluster halos gone

A new LF (330 MHz) VLA image of the Perseus cluster confirms the presence of a miniradio halo with diameter of about 430 kpc (H0 = 75 km/s Mpc) surrounding 3C 84. A careful comparison with the Coma cluster shows that there is no evidence for a similar, very extended halo in Perseus despite the large number of cluster radio galaxies which could power such a halo. These two clusters represent two classes of radio halos which differ by the absence (Coma) or presence (Perseus) of cooling inflows. It is argued that smaller halos as in Perseus result form insufficient clusterwide magnetic fields. A simple model is presented which suggests that cooling flows can suppress the diffusion of turbulently amplified B-fields outward from the cluster core. Such a suppression leads to the development of minihalos which are confined to the cores of cooling flow clusters. 25 refs.

Origin of the structures and polarization in the classical double 3C 219

Scaled-array VLA observations of the classical double radio galaxy 3C 219 at 1.4, 1.5, 1.7, and 4.9 GHz with 1.4″ resolution have been used to derive the spectral and polarimetric properties of the source. The extended emission of the lobes is filamentary, as in other well-resolved radio galaxies. A second type of filamentation, spatially independent of the total intensity filaments, is found in the depolarization distribution of the source. The depolarization filaments are associated with strong local gradients in the rotation measure and may indicate that a clumpy magnetoionic medium surrounds the radio galaxy

Very large array imaging of five Fanaroff-Riley II 3CR radio galaxies

This paper presents high resolution, high dynamic range Very Large Array (VLA) radio images of five luminous classical double radio galaxies (RGs) at 5 GHz. We detect the radio cores of at least four of these RGs. A long, continuous radio jet is seen only in one source, 3C 22, in contrast to the 100% jet detection in a comparable sample of QSRs. However, there are candidate jets or jet components seen in three other RGs. Overall, the jets in these RGs are less prominent, relative to the lobes, than those in the quasar sample. We also compare the polarization properties of these RGs at three different wavelengths: 3.6,6, and 20 cm. Moderate depolarization asymmetries between 6 and 20 cm are found between the lobes of two RGs, 3C 22 and 3C 324

Tracing Electron Beams in the Sun's Corona with Radio Dynamic Imaging Spectroscopy

We report observations of type III radio bursts at decimeter wavelengths (type IIIdm bursts)—signatures of suprathermal electron beams propagating in the low corona—using the new technique of radio dynamic imaging spectroscopy provided by the recently upgraded Karl G. Jansky Very Large Array. For the first time, type IIIdm bursts were imaged with high time and frequency resolution over a broad frequency band, allowing electron beam trajectories in the corona to be deduced. Together with simultaneous hard X-ray and extreme ultraviolet observations, we show that these beams emanate from an energy release site located in the low corona at a height below ~15 Mm, and propagate along a bundle of discrete magnetic loops upward into the corona. Our observations enable direct measurements of the plasma density along the magnetic loops, and allow us to constrain the diameter of these loops to be less than 100 km. These overdense and ultra-thin loops reveal the fundamentally fibrous structure of the Suns corona. The impulsive nature of the electron beams, their accessibility to different magnetic field lines, and the detailed structure of the magnetic release site revealed by the radio observations indicate that the localized energy release is highly fragmentary in time and space, supporting a bursty reconnection model that involves secondary magnetic structures for magnetic energy release and particle acceleration.

Inverse-Compton emission from the lobes of 3C 353

X-ray emission due to inverse-Compton scattering of microwave background photons by electrons in the lobes of powerful radio galaxies has now been seen in a large number of objects. Combining an inverse-Compton model for the lobe X-ray emission with information obtained from radio synchrotron emission provides a method of constraining the electron population and magnetic field energy density, which cannot be ac complished using the radio data alone. Using six frequencies of new and archival radio data and new XMM-Newtonobservations of the Fanaroff & Riley class II radio galaxy 3C 353, we show that inverse-Compton emission is detected in the radio lobes of this source at a lev el consistent with what is seen in other objects. We argue that variations in the X-ray/radi o ratio in the brighter eastern lobe require positionally varying magnetic field strength. We al so examine the X-ray nucleus and the cluster, Zw 1819.1-0108, spatially and spectrally.

Ionospheric corrections for VLA observations using Local GPS data

Abstract : We have conducted an experiment to evaluate the usefulness of ionospheric data produced using the Global Positioning System (GPS) for making Faraday rotation and interferometer phase corrections at the NRAO Very Large Array (VLA). Four GPS receivers were installed at the VLA site -- one at the array center and one at the end of each arm. A simple ionospheric model consisting of a vertical TEC, a horizontal gradient, and the azimuth of that gradient was developed and fitted to the GPS Total Electron Content (TEC) data from each receiver. The model was then used to predict the TEC in the observing direction. Ionospheric Faraday rotation and phase gradients were then estimated and compared with VLA measurements taken at frequencies of 322 and 333 MHz. We find that we can normally make Faraday rotation corrections that are accurate to less than or approximately 2 deg, although one unexplained discrepancy remains. The interferometer phase shifts caused by large-scale (greater than or approx. 1000 km) ionospheric structures can be predicted by our model. However the phase shifts caused by smaller (less than or approx. 100 km) structures can be estimated only when the direction of observation lies within a few degrees of one of the GPS satellites.

3C 129 at 90 Centimeters: Evidence for a Radio Relic?

We present a new wide-field map of the radio galaxy 3C 129 and its companion galaxy 3C 129.1 at ? = 90 cm. We see a distinct steep-spectrum feature near the head of 3C 129, extending in a direction perpendicular to the radio tails. We propose that this Crosspiece might consist of fossil radio plasma, which has been reenergized by the compression of the bow shock of the supersonically moving galaxy 3C 129. One possible origin of the fossil radio plasma could be the tail of a nearby head-tail radio galaxy. We discuss the implications of and give testable predictions for this scenario.