Low Frequency Radio Constraints on the Synchrotron Cosmic Web

We present a search for the synchrotron emission from the synchrotron cosmic web by cross correlating 180MHz radio images from the Murchison Widefield Array with tracers of large scale structure (LSS). We use two versions of the radio image covering $21.76\times 21.76$ degrees with point sources brighter than 0.05 Jy subtracted, with and without filtering of Galactic emission. As tracers of the LSS we use the Two-Micron-All-Sky-Survey (2MASS) and the Widefield InfraRed Explorer (WISE) redshift catalogues to produce galaxy number density maps. The cross correlation functions all show peak amplitudes at zero degrees, decreasing with varying slopes towards zero correlation over a range of one degree. The cross correlation signals include components from point source, Galactic, and extragalactic diffuse emission. We use models of the diffuse emission from smoothing the density maps with Gaussians of sizes 1-4 Mpc to find limits on the cosmic web components. From these models we find surface brightness 99.7 per cent upper limits in the range of 0.09-2.20 mJy beam$^{-1}$ (average beam size of 2.6 arcmin), corresponding to 0.01-0.30 mJy arcmin$^{-2}$. Assuming equipartition between energy densities of cosmic rays and the magnetic field, the flux density limits translate to magnetic field strength limits of 0.03-1.98 $\mu$G, depending heavily on the spectral index. We conclude that for a 3$\sigma$ detection of 0.1 $\mu$G magnetic field strengths via cross correlations, image depths of sub-mJy to sub-$\mu$Jy are necessary. We include discussion on the treatment and effect of extragalactic point sources and Galactic emission, and next steps for building on this work.Comment: 25 pages, 18 figures, 4 tables, accepted for publication in MNRA

Archival VLBA observations of the Cygnus A Nuclear Radio Transient (Cyg A-2) Strengthen the Tidal Disruption Event Interpretation

We have analyzed archival VLBA data for Cygnus A between 2002 and 2013, to search for radio emission from the transient discovered in 2015 by \citet{per18} approximately 0.4\arcsec~ from the nucleus of Cygnus A (Cyg A-2). \citet{per18} use VLA and VLBA archival data (between 1989 and 1997) to show that the transient rises in flux density by a factor of at least five in less than approximately 20 years. With the additional data presented here, we revise the rise time to between approximately four years and six years, based on a new detection of the source at 15.4 GHz from October 2011. Our results strengthen the interpretation of Cyg A-2 as the result of a Tidal Disruption Event (TDE), as we can identify the location of the compact object responsible for the TDE and can estimate the angular expansion speed of the resulting radio emitting structures, equivalent to an apparent expansion speed of $<0.9c$. While our results are consistent with recent X-ray analyses, we can rule out a previously suggested date of early 2013 for the timing of the TDE. We favour a timing between early 2009 and late 2011. Applying the model of \citet{nak11}, we suggest a TDE causing a mildly relativistic outflow with a (density-dependent) total energy $>10^{49}$ erg. Due to the improved temporal coverage of our archival measurements, we find that it is unlikely that Cyg A-2 has previously been in a high luminosity radio state over the last 30 years.Comment: Eight pages, three figures, accepted to ApJ Letter

Star-Formation in the Ultraluminous Infrared Galaxy F00183-7111

We report the detection of molecular CO(1-0) gas in F00183-7111, one of the most extreme Ultra-Luminous Infrared Galaxies known, with the Australia Telescope Compact Array. We measure a redshift of 0.3292 for F00183-7111 from the CO(1-0) line and estimate the mass of the molecular gas in 00183 to be 1 $\times$ 10$^{10}$ M$_{\odot}$. We find that F00183-7111 is predominately powered by the AGN and only $\sim$14 per cent of the total luminosity is contributed by star-formation (SFR $\sim$220 M$_{\odot}$ yr$^{-1}$). We also present an optical image of F00183-7111, which shows an extension to the East. We searched for star-formation in this extension using radio continuum observations but do not detect any. This suggests that the star-formation is likely to be predominately nuclear. These observations provide additional support for a model in which the radio emission from ULIRGs is powered by an intense burst of star-formation and by a radio-loud AGN embedded in its nucleus, both triggered by a merger of gas-rich galaxies.Comment: 5 pages, 2 figures, Accepted for publication in MNRAS Letters Accepted 2014 January 19. Received 2013 December 30; in original form 2013 November 2

Radio Detections During Two State Transitions of the Intermediate Mass Black Hole HLX-1

Relativistic jets are streams of plasma moving at appreciable fractions of the speed of light. They have been observed from stellar mass black holes ($\sim$3$-$20 solar masses, M$_\odot$) as well as supermassive black holes ($\sim$10$^6$$-$10$^9$ M$_\odot$) found in the centres of most galaxies. Jets should also be produced by intermediate mass black holes ($\sim$10$^2$$-$10$^5$ M$_\odot$), although evidence for this third class of black hole has until recently been weak. We report the detection of transient radio emission at the location of the intermediate mass black hole candidate ESO 243-49 HLX-1, which is consistent with a discrete jet ejection event. These observations also allow us to refine the mass estimate of the black hole to be between $\sim$9 $\times$10$^{3}$ M$_\odot$ and $\sim$9 $\times$10$^{4}$ M$_\odot$.Comment: 13 pages, includes supplementary online information. Published in Science in August 201