arXiv subjects

T. W. B. Muxlow

Publications and source records attributed to T. W. B. Muxlow.

At least 91 records · Page 5Linked to original sources

arXiv2005

Six months of mass outflow and inclined rings in the ejecta of V1494 Aql

V1494 Aql was a very fast nova which reached a visual maximum of about 4.0 by the end of 1999 December 3. We report observations from 4 to 284 days after discovery, including sub-mm- and cm-band fluxes, a single MERLIN image, and optical spectroscopy in the 410 to 700 nm range. The extent of the radio continuum emission is consistent with a recent lower distance estimate of 1.6 kpc. We conclude that the optical and radio emission arises from the same expanding ejecta. We show that these observations are not consistent with simple kinematical spherical shell models used in the past to explain the rise and fall of the radio flux density in these objects. The resolved remnant structure is consistent with an inclined ring of enhanced density within the ejecta. Optical spectroscopy indicates likely continued mass ejection for over 195 days, with the material becoming optically thin in the visible sometime between 195 and 285 days after outburst.

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arXiv2004

OH in Messier 82

Several new main line OH masers have been detected in the nearby starburst galaxy M82. Eight masers have been detected to 5 sigma, six of which are new detections. Observations covering both the 1665 and 1667 MHz lines with both the Very Large Array (VLA) and the Multi-Element Radio Linked Interferometer Network (MERLIN) have been used to accurately measure the positions and velocities of these features. Following analysis of the data, another six objects below 5 sigma, but with velocities consistent with the distribution inferred from the more certain detections, have been detected. These are classified as possible detections.

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arXiv2004

A detailed study of the nuclear region of Mrk 273

We present 5 GHz EVN+MERLIN observations of the nuclear region of the ultra luminous infrared galaxy Mrk 273. These observations confirms the detection, on the mas scale, of two resolved component labelled as N and SE in the literature. We use published VLBA observations at 1.4 GHz, at the same resolution, to derive spectral index information of component N and SE and discuss these findings in relation with different hypothesis (compact starburst or AGN) for the origin of the radio emission.

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arXiv2004

Radio sources at low Galactic latitudes

We present high-resolution radio observations of a sample of 65 radio sources at low Galactic latitudes. The sources were all observed at 5 GHz with the Very Large Array A-array. MERLIN observations at 5 GHz of the ultracompact HII region G34.26+0.15 and one of the extragalactic sources, B1857-000, are also presented, as are GMRT observations of HI in the direction of three sources, B1801-203, B1802-196 and B1938+229. These observations were made with the objectives of (i) finding compact components suitable for studying the effects of interstellar scattering at lower frequencies, (ii) identifying high surface-brightness lobes of background radio sources to probe the Galactic magnetic field on different scales via polarization observations, and (iii) searching for young supernova remnants. We discuss the nature of the sources found to have shell or shell-like structure and exhibiting both thermal and non-thermal spectra. Of the remaining sources, B1749-281 is coincident within the positional errors of a known pulsar, not detected earlier at 5 GHz. The rest are likely to be background extragalactic objects.

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arXiv2004

MERLIN observations of Stephan's Quintet

We present MERLIN L-band images of the compact galaxy group, Stephan's Quintet. The Seyfert 2 galaxy, NGC 7319, the brightest member of the compact group, is seen to have a triple radio structure typical of many extra-galactic radio sources which have a flat spectrum core and two steep spectrum lobes with hot spots. The two lobes are asymmetrically distributed on opposite sides of the core along the minor axis of the galaxy. Ultraviolet emission revealed in a high resolution HRC/ACS HST image is strongly aligned with the radio plasma and we interpret the intense star formation in the core and north lobe as an event induced by the collision of the north radio jet with over-dense ambient material. In addition, a re-mapping of archive VLA L-band observations reveals more extended emission along the major axis of the galaxy which is aligned with the optical axis. Images formed from the combined MERLIN and archive VLA data reveal more detailed structure of the two lobes and hot spots.

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arXiv2004

A possible radio supernova in the outer part of NGC 3310

As part of an on-going radio supernova monitoring program, we have discovered a variable, compact steep spectrum radio source ~65 arcsec (~4 kpc) from the centre of the starburst galaxy NGC 3310. If the source is at the distance of NGC 3310, then its 5 GHz luminosity is ~3 x 10^{19} WHz^-1. The source luminosity, together with its variability characteristics, compact structure (<17 mas) and its association with a group of HII regions, leads us to propose that it is a previously uncatalogued type II radio supernova. A search of archival data also shows an associated X-ray source with a luminosity similar to known radio supernova.

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arXiv2004

Second Epoch Global VLBI Observations of Compact Radio Sources in the M82 Starburst Galaxy

We have presented the results of a second epoch of global Very Long Baseline Interferometry observations, taken on 23 February 2001 at a wavelength of 18 cm, of the central kiloparsec of the nearby starburst galaxy Messier 82. These observations were aimed at studying the structural and flux evolution of some of the compact radio sources in the central region that have been identified as supernova remnants. The objects 41.95+575 and 43.31+592 have been studied, expansion velocities of 2500 +/- 1200 km/s and 7350 +/- 2100 km/s respectively have been derived. Flux densities of 31.1 +/- 0.3 mJy and 17.4 +/- 0.3 mJy have been measured for the two objects. These results are consistent with measurements and predictions from previous epochs.

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arXiv2003

A radio jet in the prototypical symbiotic star Z And?

As part of a multi-wavelength campaign to observe the 2000--2002 outburst of the prototypical symbiotic star Z Andromedae, we observed this object six times each with the Multi-Element Radio Linked Interferometer Network (MERLIN) and Very Large Array (VLA). The radio flux varied significantly during the course of the optical outburst at all three observation frequencies (1.4, 5, and 15 GHz). A jet-like extension was present in the 2001 September MERLIN image and appeared to be aligned perpendicularly to the plane of the binary orbit. Assuming that the ejection took place at the beginning of the optical outburst, the 0.06 arcsec separation between the peak of the extended emission and the central core implies that the ejected material was moving with a velocity of \~400 km/s. This extended emission faded on a timescale of ~months and was not detected at any other epoch. We consider the implications of jets being a component of a ``prototypical'' symbiotic system and compare properties of the observed jet of Z And with those of the jets in X-ray binary systems.

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arXiv2003

3C459: A highly asymmetric radio galaxy with a starburst

Multifrequency radio observations of the radio galaxy 3C459 using MERLIN, VLA and the EVN, and an optical HST image using the F702W filter are presented. The galaxy has a very asymmetric radio structure, a high infrared luminosity and a young stellar population. The eastern component of the double-lobed structure is brighter, much closer to the nucleus and is significantly less polarized than the western one. This is consistent with the jet on the eastern side interacting with dense gas, which could be due to a merged companion or dense cloud of gas. The HST image of the galaxy presented here exhibits filamentary structures, and is compared with the MERLIN 5-GHz radio map. EVN observations of the prominent central component, which has a steep radio spectrum, show a strongly curved structure suggesing a bent or helical radio jet. The radio structure of 3C459 is compared with other highly asymmetric, Fanaroff-Riley II radio sources, which are also good candidates for studying jet-cloud interactions. Such sources are usually of small linear size and it is possible that the jets are interacting with clouds of infalling gas that fuel the radio source.

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arXiv2002

Radio asymmetry in 3C99

The N-galaxy 3C99, a Compact Steep-spectrum Source showing a triple asymmetric structure, has been observed with several arrays of radio telescopes at sub-arcsecond resolution. New images from MERLIN, European VLBI Network and the Very Long Baseline Array show that the source components detected in the central region of 3C99 have steep spectral indices, making it difficult to determine which of them is the core. The asymmetric radio structure of 3C99 is explained using the Scheuer-Baldwin continuous streaming model with a rather large difference in the interstellar medium density on the two sides of the central region. The age of 3C99 has been estimated; the source is found younger than 10^6 years.

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arXiv2002

A Parsec-Scale Study of the 5/15 GHz Spectral Indices of the Compact Radio Sources in M82

Observations of the starburst galaxy, M82, have been made with the VLA in its A-configuration at 15 GHz and MERLIN at 5 GHz enabling a spectral analysis of the compact radio structure on a scale of < 0.1'' (1.6 pc). Crucial to these observations was the inclusion of the Pie Town VLBA antenna, which increased the resolution of the VLA observations by a factor of ~2. A number of the weaker sources are shown to have thermal spectra and are identified as HII regions with emission measures ~10^7 cm^-6 pc. Some of the sources appear to be optically thick at 5 GHz implying even higher emission measures of ~10^8 cm^-6 pc. The number of compact radio sources in M82 whose origin has been determined is now 46, of which 30 are supernova related and the remaining 16 are HII regions. An additional 15 sources are noted, but have yet to be identified, meaning that the total number of compact sources in M82 is at least 61. Also, it is shown that the distribution of HII regions is correlated with the large-scale ionised gas distribution, but is different from the distribution of supernova remnants. In addition, the brightest HII region at (B1950) 09h 51m 42.21s +69 54' 59.2'' shows a spectral index gradient across its resolved structure which we attribute to the source becoming optically thick towards its centre.

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arXiv2002

Discovery of the host galaxy of HDF850.1, the brightest sub-mm source in the Hubble Deep Field

Despite extensive observational efforts, the brightest sub--mm source in the Hubble Deep Field, HDF850.1, has failed to yield a convincing optical/infrared identification almost 4 years after its discovery. This failure is all the more notable given the availability of supporting multi-frequency data of unparalleled depth, and sub-arcsec positional accuracy for the sub-mm/mm source. Consequently, HDF850.1 has become a test case of the possibility that the most violently star-forming objects in the universe are too red and/or distant to be seen in the deepest optical images. Here we report the discovery of the host galaxy of HDF850.1. This object has been revealed by careful analysis of a new, deep K-prime image of the HDF obtained with the Subaru 8.2-m telescope. Its reality is confirmed by a similar analysis of the HST NICMOS F160W image of the same region. This object is extremely faint (K=23.5), clumpy (on sub-arcsec scales) and very red (I-K > 5.2; H-K = 1.4 +- 0.35). The likelihood that it is the correct identification is strongly reinforced by a reanalysis of the combined MERLIN+VLA 1.4-GHz map of the field which yields a new radio detection of HDF850.1 only 0.1 arcsec from the new near-ir counterpart, and with sufficient positional accuracy to exclude all previously considered alternative optical candidates. We have calculated new confidence limits on the estimated redshift of HDF850.1 and find z = 4.1 +- 0.5. We also calculate that the flux density of HDF850.1 has been boosted by a factor of ~3 through lensing by the intervening elliptical 3-586.0, consistent with predictions that a small but significant fraction of blank-field sub-mm sources are lensed by foreground galaxies. We discuss the wider implications of these results for the sub-mm population and cosmic star-formation history.

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arXiv2002

Variable circular polarisation associated with relativistic ejections from GRS 1915+105

We report the discovery of variable circularly polarised radio emission associated with relativistic ejections from GRS 1915+105, based on observations with the Australia Telescope Compact Array (ATCA) and the Multi-Element Radio-Linked Interferometer Network (MERLIN). Following a radio flare in 2001 January, significant and variable circular polarisation, at a fractional level of 0.2-0.4%, was measured with ATCA at four frequencies between 1-9 GHz. Following an additional outburst 65 days later in 2001 March, further ATCA observations measured a comparable sign and level of circular polarisation at two frequencies. At this second epoch, contemporaneous MERLIN observations directly imaged a relativistic ejection event and allowed us to confidently associate both the circularly and linearly polarised emission with the relativistic ejecta, allowing a detailed measurement of the full polarisation properties in the optically thin phase. The fractional circular polarisation spectrum appears to flatten at higher frequencies in 2001 January, when there is strong evidence for multiple components at different optical depths. While we cannot conclusively distinguish between synchrotron or propagation-induced conversion as the origin of the circularly polarised component, we do not consider that coherent or birefringent scintillation mechanisms are likely. The implication is therefore that the ejections from GRS 1915+105 are associated with a significant population of low-energy electrons, with associated consequences for the energetics of relativistic ejection events. [abridged]

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arXiv2001

Initial low/hard state, multiple jet ejections and X-ray/radio correlations during the outburst of XTE J1859+226

We have studied the 1999 soft X-ray transient outburst of XTE J1859+226 at radio and X-ray wavelengths. The event was characterised by strong variability in the disc, corona and jet - in particular, a number of radio flares (ejections) took place and seemed well-correlated with hard X-ray events. Apparently unusual for the `canonical soft' X-ray transient, there was an initial period of low/hard state behaviour during the rise from quiescence but prior to the peak of the main outburst - we show that not only could this initial low/hard state be an ubiquitous feature of soft X-ray transient outbursts but that it could also be extremely important in our study of outburst mechanisms.

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arXiv2001

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

VLBI now has the capability of imaging relatively large fields-of-view (~ 5 arcmin sq) with sub-mJy detection limits. In principle such observations can distinguish between starburst and AGN activity in cosmologically distant sources. We present the first deep, wide-field European VLBI Network (EVN) 1.6 GHz observations of faint radio sources in the Hubble Deep Field (HDF) region. The EVN clearly (> 5sigma) detects 2 radio sources in a field that encompasses the HDF and part of the Hubble Flanking Fields (HFF). The sources detected are: VLA J123644+621133 (a z=1.01, low-luminosity FR-I radio source located within the HDF itself) and VLA J123642+621331 (a dust enshrouded, optically faint, z=4.424 starburst system). A third radio source, J123646+621404, is detected at the 4sigma level. The observations presented here, suggest that the radio emission in the dusty starburst, VLA J123642+621331, arises mainly from AGN activity, suggesting that in this system, both AGN and starburst activity co-exist. The prospects for future VLBI deep fields are briefly discussed.

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arXiv2000

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 microJy/beam (5sigma) detection level, the EVN clearly detects two radio sources in a field that encompasses the HDF and part of the Hubble Flanking Fields (HFF). The sources detected are: VLA J123644+621133 (a z=1.013, low-luminosity FR-I radio source located within the HDF itself) and VLA J123642+621331 (a dust enshrouded, optically faint, z=4.424 starburst system). A third radio source, J123646+621404, is detected at the 4sigma level. The VLBI detections of all three sources suggest that most of the radio emission of these particular sources (including the dusty starburst) is generated by an embedded AGN.

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arXiv2000

MERLIN/VLA imaging of the gravitational lens system B0218+357

Gravitational lenses offer the possibility of accurately determining the Hubble parameter (H_0) over cosmological distances, and B0218+357 is one of the most promising systems for an application of this technique. In particular this system has an accurately measured time delay (10.5+/-0.4 d; Biggs et al. 1999) and preliminary mass modelling has given a value for H_0 of 69 +13/-19 km/s/Mpc. The error on this estimate is now dominated by the uncertainty in the mass modelling. As this system contains an Einstein ring it should be possible to constrain the model better by imaging the ring at high resolution. To achieve this we have combined data from MERLIN and the VLA at a frequency of 5 GHz. In particular MERLIN has been used in multi-frequency mode in order to improve substantially the aperture coverage of the combined data set. The resulting map is the best that has been made of the ring and contains many new and interesting features. Efforts are currently underway to exploit the new data for lensing constraints using the LensClean algorithm (Kochanek & Narayan 1992).

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arXiv2000

Starburst activity in a ROSAT Narrow Emission-Line Galaxy

We present multiwaveband photometric and optical spectropolarimetric observations of the R=15.9 narrow emission line galaxy R117_A which lies on the edge of the error circle of the ROSAT X-ray source R117 (from McHardy et al 1998). The overall spectral energy distribution of the galaxy is well modelled by a combination of a normal spiral galaxy and a moderate-strength burst of star formation. The far infra-red and radio emission is extended along the major axis of the galaxy, indicating an extended starburst. On positional grounds, the galaxy is a good candidate for the identification of R117 and the observed X-ray flux is very close to what would be expected from a starburst of the observed far infra-red and radio fluxes. Although an obscured high redshift QSO cannot be entirely ruled out as contributing some fraction of the X-ray flux, we find no candidates to K=20.8 within the X-ray errorbox and so conclude that R117_A is responsible for a large fraction, if not all, of the X-ray emission from R117. Searches for indicators of an obscured AGN in R117_A have so far proven negative; deep spectropolarimetric observations show no signs of broad lines to a limit of one per cent and, for the observed far infra-red and radio emission, we would expect a ten times greater X-ray flux if the overall emission were powered by an AGN. We therefore conclude that the X-ray emission from R117 is dominated by starburst emission from the galaxy R117_A.

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