S. T. Garrington

A systematic asymmetry in the polarization properties of double radio sources with one jet

It has long been a puzzle why high-luminosity extragalactic radio sources, which are symmetric in many respects, have jets on one side only1.The jets may be intrinsically asymmetric, because dissipation is stronger on one side or because they flip alternately between the two lobes they supply2, or the one-sidedness may be only apparent, the result of Doppler beaming in twin relativistic jets3. So far, attempts to distinguish observationally between these two possibilities have been inconclusive4. Following a suggestion by Laing5, we report here new observations which establish that almost invariably the jet side depolarizes less rapidly with increasing wavelength than the opposite side. If the jet one-sidedness is intrinsic, this depolarization most probably occurs internally, but if the one-sidedness is due to Doppler beaming the depolarization must occur in a foreground screen. Further detailed polarization observations, which can determine whether the depolarization in such sources is internal or external, should therefore settle whether jets are intrinsically or only apparently one-sided.

An asymmetric shock wave in the 2006 outburst of the recurrent nova RS Ophiuchi

Nova outbursts take place in binary star systems comprising a white dwarf and either a low-mass Sun-like star or, as in the case of the recurrent nova RS Ophiuchi, a red giant. Although the cause of these outbursts is known to be thermonuclear explosion of matter transferred from the companion onto the surface of the white dwarf, models of the previous (1985) outburst of RS Ophiuchi failed to adequately fit the X-ray evolution and there was controversy over a single-epoch high-resolution radio image, which suggested that the remnant was bipolar rather than spherical as modelled. Here we report the detection of spatially resolved structure in RS Ophiuchi from two weeks after its 12 February 2006 outburst. We track an expanding shock wave as it sweeps through the red giant wind, producing a remnant similar to that of a type II supernova but evolving over months rather than millennia. As in supernova remnants, the radio emission is non-thermal (synchrotron emission), but asymmetries and multiple emission components clearly demonstrate that contrary to the assumptions of spherical symmetry in models of the 1985 explosion, the ejection is jet-like, collimated by the central binary whose orientation on the sky can be determined from these observations.

The Coordinated Radio and Infrared Survey for High-mass Star Formation. II. Source Catalog

The CORNISH project is the highest resolution radio continuum survey of the Galactic plane to date. It is the 5 GHz radio continuum part of a series of multi-wavelength surveys that focus on the northern GLIMPSE region (10° < l < 65°), observed by the Spitzer satellite in the mid-infrared. Observations with the Very Large Array in B and BnA configurations have yielded a 1.″5 resolution Stokes I map with a root mean square noise level better than 0.4 mJy beam -1 . Here we describe the data-processing methods and data characteristics, and present a new, uniform catalog of compact radio emission. This includes an implementation of automatic deconvolution that provides much more reliable imaging than standard CLEANing. A rigorous investigation of the noise characteristics and reliability of source detection has been carried out. We show that the survey is optimized to detect emission on size scales up to 14″ and for unresolved sources the catalog is more than 90% complete at a flux density of 3.9 mJy. We have detected 3062 sources above a 7σ detection limit and present their ensemble properties. The catalog is highly reliable away from regions containing poorly sampled extended emission, which comprise less than 2% of the survey area. Imaging problems have been mitigated by down-weighting the shortest spacings and potential artifacts flagged via a rigorous manual inspection with reference to the Spitzer infrared data. We present images of the most common source types found: H II regions, planetary nebulae, and radio galaxies. The CORNISH data and catalog are available online at http://cornish.leeds.ac.uk.

High-resolution studies of radio sources in the Hubble Deep and Flanking Fields

18 days of MERLIN data and 42 h of A-array VLA data at 1.4 GHz have been combined to image a 10-arcmin field centred on the Hubble Deep Field (HDF). This area also includes the Hubble Flanking Fields (HFF). A complete sample of 92 radio sources with S 1.4 > 40 μJy was detected using the VLA data alone and then imaged with the MERLIN+VLA combination. The combined images offer (i) higher angular resolution (synthesized beams of diameter 0.2-0.5 arcsec), (ii) improved astrometric accuracy, and (iii) improved sensitivity compared with VLA-only data. The images are amongst the most sensitive yet made at 1.4GHz, with rms noise levels of 3.3 μJy beam -1 in the 0.2-arcsec images. Virtually all the sources are resolved, with angular sizes in the range 0.2 to 3 arcsec. The central 3-arcmin square was imaged separately to search for sources down to 27 μJy. No additional sources were detected, indicating that sources fainter than 40 μJy are heavily resolved with MERLIN and must have typical angular sizes > 0.5 arcsec. Radio sources associated with compact galaxies have been used to align the HDF, the HFF and a larger CFHT optical field to the radio-based International Celestial Reference Frame. The HST optical fields have been registered to 70 per cent of the sources are starburst-type systems associated with major disc galaxies in the redshift range 0.3-1.3. Chandra detections are associated with 55 of the 92 radio sources, but their X-ray flux densities do not appear to be correlated with the radio flux densities or morphologies. The most recent submillimetre results on the HDF and HFF do not provide any unambiguous identifications with these latest radio data, except for HDF8550.1, but suggest at least three strong candidates.

The Coordinated Radio and Infrared Survey for High-Mass Star Formation (The CORNISH Survey). I. Survey Design

We describe the motivation, design and implementation of the CORNISH survey, an arcsecond resolution radio continuum survey of the inner Galactic plane at 5GHz using the Karl G. Jansky Very Large Array (VLA). It is a blind survey co-ordinated with the northern Spitzer GLIMPSE I region covering 10 o < l <65 o and |b| <1 o at similar resolution. We discuss in detail the strategy that we employed to control the shape of the synthesised beam across this survey that covers a wide range of fairly low declinations. Two snapshots separated by 4 hours in hour angle kept the beam elongation to less that 1.5 over 75% of the survey area and less than 2 over 98% of the survey. The prime scientific motivation is to provide an unbiased survey for ultra-compact H II regions to study this key phase in massive star formation. A sensitivity around 2mJy will allow the automatic distinction between radio loud and quiet mid-IR sources found in the Spitzer surveys. This survey has many legacy applications beyond star formation including evolved stars, active stars and binaries, and extragalactic sources. The CORNISH survey for compact ionized sources complements other Galactic plane surveys that target diffuse and non-thermal sources as well as atomic and molecular phases to build up a complete picture of the ISM in the Galaxy.

Light Curves and Radio Structure of the 1999 September Transient Event in V4641 Sagittarii (=XTE J1819-254=SAX J1819.3-2525)

We report on radio observations of the 1999 September event of the X-ray transient V4641 Sgr (=XTE J1819-254=SAX J1819.3-2525). This event was extremely rapid in its rise and decay across radio, optical, and X-ray wavelengths; the X-rays rose to 12 crab within 8 hr and faded to below 0.1 crab in less than 2 hr. Radio observations were made with seven telescopes during the first day following the onset of the strong X-ray event, revealing a strong radio source that was detected for 3 further weeks by the more sensitive telescopes. The radio source was resolved even in the first Very Large Array (VLA) images (September 16.027 UT), being ~025 long with an axis ratio of at least 10 : 1. The total flux density decayed by a factor of ~4 over the first day, and by September 17.94 UT the radio emission was confined to a slowly decaying, marginally resolved remnant located at one side of the early elongated emission. The H I absorption spectrum gives a minimum kinematic distance of about 400 pc; various other arguments suggest that the true distance is not much greater than this. The inferred proper motions for the early extended emission (04-11 day-1) correspond to v/c ~ 1.0-3.2 (d/0.5 kpc), and this together with the radio morphology argues that this is a relativistic jet source like GRS 1915+105 and GRO J1655-40. The proper motion of the late-time remnant is at least 100 times smaller. One simple interpretation posits the ejection of a single short-lived jet segment, followed by a more slowly decaying, optically thin jet segment ejection. These two components can explain both the multifrequency radio light curves and the radio images. The most likely parameters for the fast-jet system with net-averaged proper motion of ~04 day-1, assuming d = 0.5 kpc, are v ~ 0.85c and i ~ 63°, where i is the inclination to the line of sight. The corresponding apparent velocities are 1.4c and 0.6c for the approaching and receding jets, making V4641 Sgr the closest superluminal jet source known.

Mass Loss and Jet Outflow in the Orion Nebula Proplyd LV 2

We have obtained Hubble Space Telescope (HST) Space Telescope Imaging Spectrometer high-resolution spectra of the Orion proplyd LV 2 in the C III doublet at 1906.68 and 1908.73 ?. New images at the 6 cm wavelength with MERLIN complement earlier HST images at a similar spatial resolution. This object is one of the closest proplyds to ?1 Ori C, the source of the photoionizing and photoevaporating radiation. Combining the spectra with the HST images and detailed theoretical models has allowed a determination of the mass-loss rate as 8.2 ? 10-7 M? yr-1 ?10%. This rate of mass loss is used to address the conundrum of the continued existence of proplyds. Even though they should be photoevaporated in only about 105 yr, there is no evidence for their destruction. It is concluded that the only explanation is that the age of ?1 Ori C is less than 105 yr. These spectra and previously unpublished ground-based spectra in [O III] also show the presence of a monopolar microjet, redshifted by about 100 km s-1 with respect to the systemic velocity. This jet is more visible in the 6 cm MERLIN images than in HST images, and this image together with the spectra are used to determine the flow parameters for the jet. Our spectra also include the stand-off shock that lies between LV 2 and ?1 Ori C. This is the result of the high-velocity wind coming from the hot star ?1 Ori C with the low-velocity wind coming from the proplyd. As expected, this shock is at rest with respect to the two objects.

Global very long-baseline interferometry observations of compact radio sources in M82

Observations of the starburst galaxy, M82, have been made with a 20-station global VLBI array at λ18cm. Maps are presented of the brightest young supernova remnants (SNR) in M82 and the wide-field mapping techniques used in making images over a field of view of ∼1 arcminute with 3 milliarcsecond resolution are discussed. A limit has been placed on the power law deceleration of the young SNR, 43.31+592 with an index greater than 0.73 ± 0.11 from observations with the European VLBI Network. Using the global array we have resolved compact knots of radio emission in the source which, with future global observations, will enable better constraints to be placed on the expansion parameters of this SNR. The latest global observations have also provided high resolution images of the most compact radio source in M82, 41.95+575. We determine an upper limit to the radial expansion rate along the major axis of 2000 km s. However, the new images also show structure resembling that of collimated ejection which brings into question the previous explanation of the source as being due to the confinement of a supernova by a high density circumstellar medium. It is apparent that we are now able to image the brightest supernova remnants in M82 with a linear scale which allows direct comparison with galactic SNR such as Cassiopeia A.

AGN and starbursts at high redshift: High resolution EVN radio observations of the Hubble Deep Field

We present deep, wide-field European VLBI Network (EVN) 1.6 GHz observations of the Hubble Deep Field (HDF) region with a resolution of 0.025 arcseconds. Above the 210 μ Jy/beam (

An evolution of the infrared-radio correlation at very low flux densities?

5\sigma