The luminous X-ray hotspot in 4C 74.26: synchrotron or inverse-Compton emission?
ArXiv 0705.1339 (2007)
Abstract:
We report the discovery of an X-ray counterpart to the southern radio hotspot of the largest-known radio quasar 4C 74.26 (whose redshift is z=0.104). Both XMM-Newton and Chandra images reveal the same significant (10arcsec, i.e. 19kpc) offset between the X-ray hotspot and the radio hotspot imaged with MERLIN. The peak of the X-ray emission may be due to synchrotron or inverse-Compton emission. If synchrotron emission, the hotspot represents the site of particle acceleration and the offset arises from either the jet exhibiting Scheuer's `dentist's drill' effect or a fast spine having less momentum than the sheath surrounding it, which creates the radio hotspot. If the emission arises from the inverse-Compton process, it must be inverse-Compton scattering of the CMB in a decelerating relativistic flow, implying that the jet is relativistic (Gamma >= 2) out to a distance of at least 800kpc. Our analysis, including optical data from the Liverpool Telescope, rules out a background AGN for the X-ray emission and confirms its nature as a hotspot, making it the most X-ray luminous hotspot yet detected.Low-frequency radio observations of Galactic X-ray binary systems
Proceedings of Science 56 (2007)
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.On the origin of radio core emission in radio-quiet quasars
Astrophysical Journal 668:2 PART 2 (2007)
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.Determining the nature of the faint X-ray source population near the Galactic Centre
REV MEX AST ASTR 29 (2007) 54-56
Abstract:
We present results of a multi-wavelength program to study the faint discrete X-ray source population discovered by Chandra in the Galactic Centre (GC). From IR imaging obtained with the VLT we identify candidate K-band counterparts to 75% of the X-ray sources in our sample. By combining follow-up VLT K-band spectroscopy of a subset of these candidate counterparts with the magnitude limits of our photometric survey, we suggest that only a small percentage of the sources are HMXBs, while the majority are likely to be canonical LMXBs and CVs at the distance of the GC. In addition, we present our discovery of highly structured small-scale (5-15 '') extinction towards the Galactic Centre. This is the finest-scale extinction study of the Galactic Centre to date.Galactic centre X-ray sources
AIP CONF PROC 924 (2007) 893-896