Parsec-scale structure of quasars: dawn of the golden age?

Half a century after their discovery, the study of quasars remains one of the most fascinating intellectual challenges in astronomy. Quasars are laboratories for everything from relativity to magnetohydrodynamics and are perhaps the best available probes for cosmology. A tremendous amount has been learned about quasars and yet many of the most fundamental questions about their physics remain open. Parsec-scale observations have played an indispensable role in building up our current understanding of quasars; virtually everything we know about quasars depends on such observations. However, the finest hour for parsec scale observations may be just beginning. This is partly due to the development of highly reliable VLBI networks (which is continuing) but mostly due to the unprecedented availability of multiepoch, simultaneous, broadband observations that have long been the `holy grail' for quasar researchers.Comment: Accepted for publication in the Bulletin of the Astronomical Society of India (20 pages, 3 figures

High Resolution Rapid Response observations of compact radio sources with the Ceduna Hobart Interferometer (CHI)

Context. Frequent, simultaneous observations across the electromagnetic spectrum are essential to the study of a range of astrophysical phenomena including Active Galactic Nuclei. A key tool of such studies is the ability to observe an object when it flares i.e. exhibits a rapid and significant increase in its flux density. Aims. We describe the specific observational procedures and the calibration techniques that have been developed and tested to create a single baseline radio interferometer that can rapidly observe a flaring object. This is the only facility that is dedicated to rapid high resolution radio observations of an object south of -30 degrees declination. An immediate application is to provide rapid contemporaneous radio coverage of AGN flaring at {\gamma}-ray frequencies detected by the Fermi Gamma-ray Space Telescope. Methods. A single baseline interferometer was formed with radio telescopes in Hobart, Tasmania and Ceduna, South Australia. A software correlator was set up at the University of Tasmania to correlate these data. Results. Measurements of the flux densities of flaring objects can be made using our observing strategy within half an hour of a triggering event. These observations can be calibrated with amplitude errors better than 15%. Lower limits to the brightness temperatures of the sources can also be calculated using CHI.Comment: 6 pages, 6 figures, 1 table. Accepted for publication in A&

X-ray monitoring of the radio and gamma-ray loud Narrow-Line Seyfert 1 Galaxy PKS 2004-447

We present preliminary results of the X-ray analysis of XMM-Newton and Swift observations as part of a multi-wavelength monitoring campaign in 2012 of the radio-loud narrow line Seyfert 1 galaxy PKS 2004-447. The source was recently detected in gamma-rays by Fermi/LAT among only four other galaxies of that type. The 0.5-10 keV X-ray spectrum is well-described by a simple absorbed powerlaw (photon index ~ 1.6). The source brightness exhibits variability on timescales of months to years with indications for spectral variability, which follows a 'bluer-when-brighter' behaviour, similar to blazars.Comment: Proceedings for the 'Jet 2013' conference. Includes 3 pages, 3 figure

Finding Extremely Compact Sources Using the ASKAP VAST Survey

VLBI observations of intraday variable (IDV) quasars found in the MASIV (Micro-Arcsecond Scintillation-Induced Variability) 5 GHz VLA Survey of 500 flat-spectrum sources in the northern sky have shown that these sources are extremely compact, often unresolved, on milliarcsecond scales, and more core-dominated than their non-IDV counterparts. VAST: an ASKAP Survey for Variables and Slow Transients, proposes to observe 10,000 square degrees of southern sky daily for 2 years in the VAST-Wide survey component. This is expected to reveal of order 30,000 compact sources brighter than 10 mJy showing refractive interstellar scintillation (the cause of centimeter-wavelength IDV) at the survey frequency of about 1.4 GHz. Many of these sources may be suitable astrometric calibrators for VLBI at higher frequencies

The peculiar variable X-ray spectrum of the active galactic nucleus PKS 2005-489

PKS 2005-489 is a well-known, bright southern BL Lac object that has been detected up to TeV energies. In a low-flux state it exhibits the expected multiwavelength double-peaked spectrum in the radio -- $\gamma$-ray band. The high-flux state shows extreme flux variations in the X-ray band with a hardening as well as a peculiar curved feature in the spectrum. Thus far, PKS 2005-489 is the only source to exhibit such a feature. To study the X-ray variability further, we obtained the first hard X-ray spectrum of the source with NuSTAR. We compare quasi-simultaneous radio, optical, UV, soft and hard X-ray, and $\gamma$-ray data of PKS 2005-489 to archival data in order to study its broadband behavior. We find a very consistent quiet state in the SED, with little variation in spectral shape or flux between the 2012 and 2020 data. A possible explanation for the peculiar X-ray spectrum in the flaring state is an additional component in the jet, possibly accelerated via magnetic reconnection, that is not co-spatial to the low-flux state emission region