arXiv subjects

M. A. Garrett

Publications and source records attributed to M. A. Garrett.

131 records · Page 8Linked to original sources

arXiv1999

A VLBI and MERLIN Survey of faint, compact radio sources

We present preliminary results from a VLBI survey at $λ=6 $cm of a sample of 35 sources with flux densities of 2 - 100 mJy. These sources were selected from the VLA FIRST survey at $λ= 20 $cm, in a 3 degree field around the bright calibrator 1156+295, simply by imposing $S_{20} > 10 $ mJy and $θ< 5$ arcsec. MERLIN observations at $λ6 $cm detected 70/127 of these sources with a threshold of 2 mJy at 50 mas resolution and the closest 35 of these to the calibrator were observed with the VLBA+EVN in snapshot mode at $λ6 $cm. These sources are a mixture of flat and steep-spectrum sources and include: weak flat-spectrum nuclei of large radio galaxies, low power AGN in nearby galaxies and radio quiet quasars. With these short observations, the sensitivity is limited and most appear as either core-jets or simple point sources on the milliarcsec scale. Nonetheless, it is encouraging that with only 10 minutes observation per source, at least 35% of all sources with $S_{20} > 10 $ mJy can be detected and imaged with global 6cm VLBI.

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arXiv1999

Wide-field VLBI Imaging

We discuss the technique of Wide-field imaging as it applies to Very Long Baseline Interferometry (VLBI). In the past VLBI data sets were usually averaged so severely that the field-of-view was typically restricted to regions extending a few hundred milliarcseconds from the phase centre of the field. Recent advances in data analysis techniques, together with increasing data storage capabilities, and enhanced computer processing power, now permit VLBI images to be made whose angular size represents a significant fraction of an individual antenna's primary beam. This technique has recently been successfully applied to several large separation gravitational lens systems, compact Supernova Remnants in the starburst galaxy M82, and two faint radio sources located within the same VLA FIRST field. It seems likely that other VLBI observing programmes might benefit from this wide-field approach to VLBI data analysis. With the raw sensitivity of global VLBI set to improve by a factor 4-5 over the coming few years, the number of sources that can be detected in a given field will rise considerably. In addition, a continued progression in VLBI's ability to image relatively faint and extended low brightness temperature features (such as hot-spots in large-scale astrophysical jets) is also to be expected. As VLBI sensitivity approaches the $μ$Jy level, a wide-field approach to data analysis becomes inevitable.

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arXiv1999

Changes in the angular separation of the lensed images PKS 1830-211 NE & SW

We present 8 epochs of 7 mm dual-polarization VLBA observations of the gravitational lens system PKS 1830-211 made over the course of 14 weeks. Clear changes in the relative positions of the cores of up to $80 μ$arcseconds ($μ$as) were observed between epochs (each separated by $\approx 2$ weeks). A comparison with previous 7 mm VLBA maps shows that the separation of the cores has changed by almost $280 μ$as over 12 months. This leads us to conclude that changes in the brightness distribution of the mm-VLBI core of the background source must be occurring rapidly. This is the first clear observation of significant radio source evolution in a gravitational lens system. It is also the first time that changes in source structure have been detected in a distant extra-galactic source on such short time-scales. This is partly accounted for by the magnification provided by the lens system.

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arXiv1999

Milliarcsec-scale polarisation observations of the gravitational lens B1422+231

We present polarisation observations of the gravitational lens system B1422+231 made at 8.4 GHz using the VLBA and the 100m telescope at Effelsberg. All four images of the quasar show structure on the milliarcsec scale. The three bright images show tangential stretching as expected from lens models. Some basic properties of gravitational lensing are exhibited by this system. The surface brightness of images A and B are the same and the parity reversal expected in image B is revealed, for the first time, by polarisation observations. There is a large differential Faraday rotation between images A and B.

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arXiv1997

A MERLIN and VLBI Survey of Faint Compact Radio Sources

We have selected a field from the VLA FIRST Survey which is typical in all aspects except one: it contains a bright, but extremely compact VLBI calibrator - J1159+291. 127 unresolved FIRST sources with $S_{FIRST}\ge 10$ mJy lie within $2.5^{\circ}$ of this calibrator. $λ6$ cm MERLIN observations with a resolution $\sim 60$ mas detect around half the sources. These detections form the basis of a sample of faint but compact radio sources which are ideally suited to follow-up VLBI observations.

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