Pulsars, transients and relativistic astrophysics

Abstract:

Circinus X-1 is a neutron star X-ray binary system with an interesting and at times puzzling behaviour over a broad range of frequencies, specifically in the X-ray and radio bands. The system seems to harbour the most relativistic outflow (likely oriented close to the line of sight) observed so far within the Milky Way. It lies within a radio synchrotron nebula and has variable radio flux densities at cm wavelengths. The radio flares associated to the orbital phase zero reached up to 1 Jy in the late '70s, then have been observed at the tens of mJy level until recently; in 2007 January, Circinus X-1 seemed to have entered a very active radio flaring state. Here we present a sample of the 4.8 and 8.6 GHz radio observations made with the Australia Telescope Compact Array, covering 10 years time period. The entire data set comprised 41 epochs, unequally spread in time between 1996 and 2006. We investigate the long-term changes in the brightness, morphology and spectrum of the radio structures. We have detected linear polarisation in a third of the epochs and a good case for Faraday rotation in one epoch. The analysis reveals structural changes in the radio emission at time scales as short as days. Clear evidence for a counter-jet was found in a few epochs.

Abstract:

With the advent of facilities enabling wide-field monitoring of the dynamic radio sky, new areas of parameter space will be opened up for exploration. Such monitoring will be done primarily at low frequencies, in order to maximise the available field of view. One class of radio sources known to be highly variable at GHz frequencies are the so-called 'microquasars', X-ray binaries with relativistic jets. To date however, their low-frequency behaviour has not been well constrained by observations. I will present some of the first attempts to measure their low-frequency properties, showing wide-field images made from data taken with the 74-MHz system on the Very Large Array (VLA) and also the Low Frequency Front Ends (LFFEs), the new suite of low-frequency (117-175 MHz) receivers on the Westerbork Synthesis Radio Telescope (WSRT). I will show results including the low-frequency spectra of the three X-ray binaries SS 433, GRS 1915+105 and Cygnus X-3, a low-frequency study of the W 50 nebula surrounding SS 433, a search for synchrotron lobes inflated by the jets of GRS 1915+105, and the evolution of the May 2006 outburst of Cygnus X-3 at MHz frequencies.

Abstract:

We present a model for the radio emission from radio-quiet quasar nuclei. We show that a thermal origin for the high brightness temperature, flat spectrum point sources (known as radio "cores") is possible provided that the emitting region is hot and optically thin. We hence demonstrate that optically thin bremsstrahlung from a slow, dense disk wind can make a significant contribution to the observed levels of radio core emission. This is a much more satisfactory explanation, particularly for sources where there is no evidence of a jet, than a sequence of self-absorbed synchrotron components that collectively conspire to give a flat spectrum. Furthermore, such core phenomena are already observed directly via milliarcsecond radio imaging of the Galactic microquasar SS 433 and the active galaxy NGC 1068. We contend that radio-emitting disk winds must be operating at some level in radio-loud quasars and radio galaxies as well (although in these cases, observations of the radio cores are frequently contaminated/dominated by synchrotron emission from jet knots). This interpretation of radio core emission mandates mass accretion rates that are substantially higher than Eddington. Moreover, acknowledgment of this mass-loss mechanism as an AGN feedback process has important implications for the input of energy and hot gas into the intergalactic medium (IGM) since it is considerably less directional than that from jets. © 2007. The American Astronomical Society. All rights reserved.

Abstract:

We compare the predictions of simple models for the radio emission from classical novae with the MERLIN radio observations of nova V723 Cas (Nova Cas 1995). Spherically symmetric and ellipsoidal radiative transfer models are implemented in order to generate synthetic emission maps. These are then convolved with an accurate representation of the uv coverage of MERLIN. The parameters and geometry of the shell model are based on those returned by fitting models to the observed light curve. This allows direct comparison of the model images with the nine 6-cm MERLIN images of V723 Cas. It is found that the seemingly complex structure (clumping, apparent rotation) evident in the observations can actually be reproduced with a simple spherical emission model. The simulations show that a 24-h track greatly reduces the instrumental effects and the synthetic radio map is a closer representation of the true (model) sky brightness distribution. It is clear that interferometric arrays with sparse uv coverage (e.g. MERLIN, VLBA) will be more prone to these instrumental effects especially when imaging ring-like objects with time-dependent structure variations. A modelling approach such as that adopted here is essential when interpreting observations. © 2007 RAS.