arXiv subjects

S. T. Garrington

Publications and source records attributed to S. T. Garrington.

42 records · Page 3Linked to original sources

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

Gigahertz Peaked Spectrum sources from the Jodrell Bank-VLA Astrometric Survey

Observations with MERLIN 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 deg. < δ< 75 deg. These sources were selected from JVAS and other catalogues to have convex spectra between 1.4 and 8.4 GHz, characteristic of Gigahertz Peaked Spectrum (GPS) sources. We have confirmed 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 identified quasars in the sample have large ($z\ga 2$) redshifts.

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arXiv1998

MERLIN observations of relativistic ejections from GRS 1915+105

We present high resolution MERLIN radio images of multiple relativistic ejections from GRS 1915+105 in 1997 October / November. The observations were made at a time of complex radio behaviour, corresponding to multiple optically-thin outbursts and several days of rapid radio flux oscillations. The radio imaging resolved four major ejection events from the system. As previously reported from earlier VLA observations of the source, we observe apparent superluminal motions resulting from intrinsically relativistic motions of the ejecta. However, our measured proper motions are significantly greater than those observed on larger angular scales with the VLA. Under the assumption of an intrinsically symmetric ejection, we can place an upper limit on the distance to GRS 1915+105 of 11.2 +/- 0.8 kpc. Solutions for the velocities unambiguously require a higher intrinsic speed by about 0.1c than that derived from the earlier VLA observations, whilst the angle to the line-of-sight is not found to be significantly different. At a distance of 11 kpc, we obtain solutions of v = 0.98 (-0.05,+0.02)c and theta = 66 +/- 2 degrees. The jet also appears to be curved on a scale which corresponds to a period of around 7 days. We observe significant evolution of the linear polarisation of the approaching component, with large rotations in position angle and a general decrease in fractional polarisation. The power input into the formation of the jet is very large, >10^38 erg/s at 11 kpc for a pair plasma. If the plasma contains a cold proton for each electron, then the mass outflow rate, >10^18 g/sec is comparable to inflow rates previously derived from X-ray spectral fits.

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arXiv1998

MERLIN observations of GRS 1915+105 : a progress report

We present a progress report on MERLIN radio imaging of a radio outburst from GRS 1915+105. The major ejection occurred at the end of an approximately 20-day `plateau' state, characterised by low/hard X-ray fluxes and a relatively strong flat-spectrum radio component. Apparent superluminal motions have been mapped with unprecedented resolution, and imply higher velocities in the jet than previously derived.

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arXiv1998

Deep radio observations of 3C324 and 3C368: evidence for jet-cloud interactions

High resolution, deep radio images are presented for two distant radio galaxies, 3C324 (z=1.206) and 3C368 (z=1.132), which are both prime examples of the radio-optical alignment effect seen in powerful radio galaxies with redshifts z > 0.6. Radio observations were made using the Very Large Array in A-array configuration at 5 and 8 GHz, and using the MERLIN array at 1.4 and 1.65 GHz. Radio spectral index, radio polarisation, and rotation measure maps are presented for both sources. Radio core candidates are detected in each source, and by aligning these with the centroid of the infrared emission the radio and the optical/infrared images can be related astrometrically with 0.1 arcsec accuracy. In each source the radio core is located at a minimum of the optical emission, probably associated with a central dust lane. Both sources also exhibit radio jets which lie along the directions of the bright strings of optical knots seen in high resolution Hubble Space Telescope images. The northern arm of 3C368 shows a close correlation between the radio and optical emission, whilst along the jet direction of 3C324 the bright radio and optical knots are co-linear but not co-spatial. These indicate that interactions between the radio jet and its environment play a key role in producing the excess ultraviolet emission of these sources, but that the detailed mechanisms vary from source to source. 3C368 is strongly depolarised and has an average rest-frame rotation measure of a few hundred rad/m^2, reaching about 1000 rad/m^2 close to the most depolarised regions. 3C324 has weaker depolarisation, and an average rest-frame rotation measure of between 100 and 200 rad/m^2. Both sources show large gradients in their rotation measure structures, with variations of up to 1000 rad/m^2 over distances of about 10 kpc.

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arXiv1995

HST and MERLIN Observations of the Jet in 3C273

We present red and blue images of the jet of the quasar 3C273 obtained with the WFPC2 on the Hubble Space Telescope as well as a new radio map made with the MERLIN array. The images are of significantly better quality than that of previous data. The two maps are aligned to an accuracy of 0.020$''$; this accuracy is achieved because both the quasar and the jet are contained in both the radio and optical images. The start of the optical jet is marked by an elongated knot which appears identical at radio and optical wavelengths. Other knots in the optical jet correspond to narrow oblique features within the radio outline. The total width of the smooth emission in the optical jet is~$0.7''$; the FWHM of the optical knots is~$0.3''$. The knots may trace the current location of a narrow, perhaps helical jet lying within the outlines of the older radio cocoon.

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