Alok Ranjan Patnaik

The Identification of EGRET Sources with Flat-Spectrum Radio Sources

We present a method to assess the reliability of the identification of EGRET sources with extragalactic radio sources. We verify that EGRET is detecting the blazar class of active galactic nuclei (AGNs). However, many published identifications are found to be questionable. We provide a table of 42 blazars that we expect to be robust identifications of EGRET sources. This includes one previously unidentified EGRET source, the lensed AGN PKS 1830-210, near the direction of the Galactic center. We provide the best available positions for 16 more radio sources that are also potential identifications for previously unidentified EGRET sources. All high Galactic latitude EGRET sources (|b| > 3?) that demonstrate significant variability can be identified with flat-spectrum radio sources. This suggests that EGRET is not detecting any type of AGN other than blazars. This identification method has been used to establish with 99.998% confidence that the peak ?-ray flux of a blazar is correlated with its average 5 GHz radio flux. An even better correlation is seen between ?-ray flux and the 2.29 GHz flux density measured with VLBI at the base of the radio jet. Also, using high-confidence identifications, we find that the radio sources identified with EGRET sources have greater correlated VLBI flux densities than the parent population of flat radio spectrum sources.

PMN J0134-0931: A gravitationally lensed quasar with unusual radio morphology

The radio-loud quasar J0134-0931 was discovered to have an unusual morphology during our search for gravitational lenses. In Very Large Array and MERLIN images, there are five compact components with a maximum separation of 07. All of these components have the same spectral index from 5 to 43 GHz. In a Very Long Baseline Array image at 1.7 GHz, a curved arc of extended emission joins two of the components in a manner suggestive of gravitational lensing. At least two of the radio components have near-infrared counterparts. We argue that this evidence implies that J0134-0931 is a gravitational lens, although we have not been able to devise a plausible model for the foreground gravitational potential. Like several other radio-loud lenses, the background source has an extraordinarily red optical counterpart.

Hubble Space Telescope observations of the gravitational lens system B1422+231

Hubble Space Telescope observations of the gravitational lens system B1422+231 are presented. The spectra of the four components are virtually identical, although there are differences among the Lyα emission-line equivalent widths that might be due to microlensing. There are also significant differences between the component flux density ratios measured at radio and optical wavelengths. High-resolution imaging reveals the lens close to the faintest quasar image. The lens has photometric properties consistent with an elliptical galaxy at z ~ 0.4. The addition of new constraints to simple lensing models affirms the B1422+231 system as a suitable tool for an eventual direct measure of the Hubble constant.

PMN J1632?0033: A New Gravitationally Lensed Quasar

We report the discovery of a gravitationally lensed quasar resulting from our survey for lenses in the southern sky. Radio images of J1632-0033 with the Very Large Array and the Australia Telescope Compact Array exhibit two compact, flat-spectrum components with separation 147 and flux density ratio 13.2. Images with the Hubble Space Telescope reveal the optical counterparts to the radio components and also the lens galaxy. An optical spectrum of the bright component obtained with the first Magellan telescope reveals quasar emission lines at redshift 3.42. Deeper radio images with the Multi-Element Radio Linked Interferometry Network and the Very Long Baseline Array reveal a faint third radio component located near the center of the lens galaxy, which is either a third image of the background quasar or faint emission from the lens galaxy.

Changes in the measured image separation of the gravitational lens system PKS 1830-211

We present eight epochs of 43-GHz, dual-polarization VLBA observations of the gravitational lens system PKS 1830-211, made over fourteen weeks. A bright, compact ‘core’ and a faint extended ‘jet’ are clearly seen in maps of both lensed images at all eight epochs. The relative separation of the radio centroid of the cores (as measured on the sky) changes by up to 87 μ as between subsequent epochs. A comparison with the previous 43-GHz VLBA observations made 8 months earlier shows even larger deviations in the separation of up to 201 μas. The measured changes are most likely produced by changes in the brightness distribution of the background source, enhanced by the magnification of the lens. A relative magnification matrix that is applicable on the milliarcsecond scale has been determined by relating two vectors (the ‘core–jet’ separations and the offsets of the polarized and total intensity emission) in the two lensed images. The determinant of this matrix, −1.13 ( ± 0.61), is in good agreement with the measured flux density ratio of the two images. The matrix predicts that the 10-mas-long jet, which is clearly seen in previous 15- and 8.4-GHz VLBA observations, should correspond to a 4-mas-long jet trailing to the south-east of the south-western image. The clear non-detection of this trailing jet is a strong evidence for substructure in the lens and may require more realistic lens models to be invoked, such as that of Nair & Garrett.

The new gravitational lens system B1030+074

We report the discovery of a new double image gravitational lens system B1030+074 which was found during the Jodrell Bank VLA Astrometric Survey (JVAS). We have collected extensive radio data on the system using the VLA, MERLIN, the EVN and the VLBA and optical observations using WFPC2 on the HST. The lensed images are separated by 1.56 arcseconds and their flux density ratio at centimetric wavelengths is approximately 14:1 although the ratio is slightly frequency dependent and the images appear to be time variable. The HST pictures show both the lensed images and the lensing galaxy close to the weaker image. The lensing galaxy has substructure which could be a spiral arm or an interacting galaxy.

Gigahertz Peaked Spectrum sources from the Jodrell Bank–VLA Astrometric Survey - I. Sources in the region

Observations with MERLIN 1 at 408 MHz have been used to establish the low-frequency part of the spectra of more than a hundred compact radio sources taken from the part of the Jodrell Bank{VLA Astrometric Survey limited by 35 75. These sources were selected from JVAS and other catalogues to have con- vex spectra between 1.4 and 8.4 GHz, characteristic of Gigahertz Peaked Spectrum (GPS) sources. We have con- rmed convex shapes of the spectra of 76 objects (one half of our initial candidates) thereby yielding the largest genuine sample of GPS sources compiled so far. Seven of 17 identied quasars in the sample have large (z > 2) redshifts.

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We report the discovery of a new double-image gravitational lens, resulting from our search for lenses in the southern sky. Radio source PMN J2004-1349 is composed of two compact components separated by 113 in VLA, MERLIN, and VLBA images. The components have a flux ratio of 1:1 at radio frequencies ranging from 5 to 22.5 GHz. The I-band optical counterpart is also an equal double, with roughly the same separation and position angle as the radio double. Upon subtraction of the components from the I-band image, we identify a dim pattern of residuals as the lens galaxy. While the present observations are sufficient to establish that PMN J2004-1349 is a gravitational lens, additional information will be necessary (such as the redshifts of the galaxy and quasar and precise astrometry and photometry of the lens galaxy) before constructing detailed mass models.

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A time delay of 12±3 days has been measured for the lens system 0218+357 using VLA 15GHz polarization observations.

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III Zw 2 is a spiral galaxy with an optical spectrum and faint extended radio structure typical of a Seyfert galaxy, but also with an extremely variable, blazar-like radio core. We have now discovered a new radio flare in which the source has brightened more than 20-fold within less than 2 yr. A broadband radio spectrum between 1.4 and 666 GHz shows a textbook-like synchrotron spectrum peaking at 43 GHz, with a self-absorbed synchrotron spectral index +2.5 at frequencies below 43 GHz and an optically thin spectral index -0.75 at frequencies above 43 GHz. The outburst spectrum can be well fitted by two homogenous, spherical components with equipartition sizes of 0.1 and 0.2 pc at 43 and 15 GHz and with magnetic fields of 0.4 and 1 G. VLBA observations at 43 GHz confirm this double structure and these sizes. Timescale arguments suggest that the emitting regions are shocks which are continuously accelerating particles. This could be explained by a frustrated jet scenario with very compact hot spots. Similar millimeter-peaked spectrum sources could have escaped our attention because of their low flux density at typical survey frequencies and their strong variability.