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The typically large distances, extinction, and crowding of Galactic supermassive star clusters have so far hampered the identification of their very low mass members, required to extend our understanding of star and planet formation, and early stellar evolution, to starburst. This situation has now evolved thanks to the James Webb Space Telescope (JWST), and its unmatched resolution and sensitivity in the infrared. In this paper, the third of the series of the Extended Westerlund 1 and 2 Open Clusters Survey (EWOCS), we present JWST/NIRCam and JWST/MIRI observations of the supermassive star cluster Westerlund 1. These observations are specifically designed to unveil the cluster members down to the BD mass regime, and to allow us to select and study the protoplanetary disks and to study the mutual feedback between the cluster members and the surrounding environment. Westerlund 1 was observed as part of JWST GO-1905 for 23.6 hours. The data have been reduced using the JWST calibration pipeline, together with specific tools necessary to remove artifacts. Source identification and photometry were performed with DOLPHOT. The MIRI images show a plethora of different features. Diffuse nebular emission is observed around the cluster, which is typically composed of myriads of droplet-like features pointing toward the cluster center or the group of massive stars surrounding the WR star W72/A. A long pillar is also observed in the NW. The MIRI images also show resolved shells and outflows surrounding the M-type RSG W20, W26, W75, and W237, the sgB[e] star W9 and the YHG W4. The color-magnitude diagrams built using the NIRCam photometry show a clear cluster sequence, which is marked in its upper part by the 1828 NIRCam stars with X-ray counterparts. NIRCam observations using the F115W filter have reached the 23.8 mag limit with 50\% completeness (roughly corresponding to a 0.06 Msol brown dwarf)., Comment: Accepted for publication in Astronomy & Astrophysics

A plasma scenario with negative triangularity (NT) shaping is achieved on MAST-U for the first time. While edge localized modes (ELMs) are eventually suppressed as the triangularity is decreased below $\delta$ < -0.06, an extended period of H-mode operation with Type-III ELMs is sustained at less negative $\delta$ even through access to the second stability region for ideal ballooning modes is closed. This documents a qualitative difference from the ELM-free access conditions documented in NT scenarios on conventional aspect ratio machines. The electron temperature at the pedestal top drops across the transition to ELM-free operation, but a steady rise in core temperature as $\delta$ is decreased allows for similar normalized beta in the ELM-free NT and H-mode positive triangularity shapes.

Studies of the stellar and accretion properties of classical T Tauri stars (CTTS) require comparison with photospheric spectral templates. Here we aim at expanding the currently available grid of wide-wavelength coverage observed spectra of non-accreting stars with additional new spectra and an interpolation method that allows us to obtain a continuous grid of low resolution spectra ranging from spectral type G8 to M9.5, while also mitigating observational uncertainties. This interpolated grid is then implemented in the self-consistent method to derive stellar and accretion properties of CTTS. With the new templates, we aim to estimate a lower limit on the accretion luminosities that can be obtained through a study of the UV excess emission using observed templates. We analyse the molecular photospheric features present in the VLT/X-Shooter spectra of the targets to perform a spectral classification, including estimates of their extinction. We apply a non-parametric fitting method to the full grid of observed templates to obtain an interpolated grid of templates. We use the uncertainties on our interpolated grid to estimate a lower limit on the accretion luminosity that we can measure with this method. We find that the measurable accretion luminosities ranges from $\sim 2.7$ dex lower than the stellar luminosity in M5.5 stars to $\sim 1.3$ dex lower for G8 stars. For young stars with masses of $\sim 1M_{\odot}$ and ages of 3-6 Myr this limit translates into an observational limit of mass accretion rate on the order of $10^{-10} \rm M_{\odot}/yr$. The implementation of an interpolated grid of observed templates allows us to better disentangle degenerate solutions, leading to a more reliable estimate of accretion rates in young accreting stars., Comment: Accepted to A&A, Version before changes by the language editor

Nuclear fusion could offer clean, abundant energy. However, managing the immense power exhausted from the core fusion plasma towards the divertor remains a major challenge. This is compounded in emerging compact reactor designs which promise more cost-effective pathways towards commercial fusion energy. Alternative divertor configurations (ADCs) are a potential solution to this challenge. In this work, we demonstrate exhaust control in ADCs for the first time, on MAST-U. We employ a novel diagnostic strategy for the neutral gas buffer which shields the target. Our work shows that ADCs tackle key risks and uncertainties in realising fusion energy: 1) an enlarged operating window which 2) improves exhaust control through the absorption of transients which can remove the neutral shield and damage the divertor, 3) isolation of each divertor from other reactor regions, enabling combined control. This showcases real-world benefits of alternative divertors for effective heat load management and control in reactors.

$\kappa$CrB is a ${\sim}2.5\,$Gyr old K1 sub-giant star, with an eccentric exo-Jupiter at ${\sim}2.8\,$au and a debris disc at tens of au. We present ALMA Band 6 ($1.3\,$mm) and HST scattered light ($0.6\,\mu$m) images, demonstrating $\kappa$CrB's broad debris disc, covering an extent $50{-}180\,$au in the millimetre (peaking at $110\,$au), and $51{-}280\,$au in scattered light (peaking at $73\,$au). By modelling the millimetre emission, we estimate the dust mass as ${\sim}0.016\,M{\oplus}$, and constrain lower-limit planetesimal sizes as $D_{\rm{max}}{>}1\,$km and the planetesimal belt mass as $M_{\rm{disc}}{>}1\,M_{\oplus}$. We constrain the properties of an outer body causing a linear trend in 17 years of radial velocity data to have a semi-major axis $8{-}66\,$au and a mass $(0.4{-}120)\,M_{\rm{Jup}}$. There is a large inner cavity seen in the millimetre emission, which we show is consistent with carving by such an outer massive companion with a string of lower mass planets. Our scattered light modelling shows that the dust must have a high anisotropic scattering factor ($g{\sim}0.8{-}0.9$) but an inclination ($i{\sim}30{-}40\,$degrees) that is inferred to be significantly lower than the $i{\sim}61\,$degrees millimetre inclination. The origin of such a discrepancy is unclear, but could be caused by a misalignment in the micron and millimetre sized dust. We place an upper limit on the CO gas mass of $M_{\rm{CO}}{<}(4.2{-}13){\times}10^{-7}\,M_{\oplus}$, and show this to be consistent with levels expected from planetesimal collisions, or from CO-ice sublimation as $\kappa$CrB begins its giant branch ascent., Comment: Accepted for publication in MNRAS. 21 pages, 14 figures

We present \textit{ALMA} 1.3 mm and 0.86 mm observations of the nearby (17.34 pc) F9V star q1 Eri (HD 10647, HR 506). This system, with age ${\sim}1.4$ Gyr, hosts a ${\sim}2$ au radial velocity planet and a debris disc with the highest fractional luminosity of the closest 300 FGK type stars. The \textit{ALMA} images, with resolution ${\sim}0.5''$, reveal a broad (34{-}134 au) belt of millimeter emission inclined by $76.7{\pm}1.0$ degrees with maximum brightness at $81.6{\pm}0.5$ au. The images reveal an asymmetry, with higher flux near the southwest ansa, which is also closer to the star. Scattered light observed with the Hubble Space Telescope is also asymmetric, being more radially extended to the northeast. We fit the millimeter emission with parametric models and place constraints on the disc morphology, radius, width, dust mass, and scale height. We find the southwest ansa asymmetry is best fitted by an extended clump on the inner edge of the disc, consistent with perturbations from a planet with mass $8 M_{\oplus} {-} 11 M_{\rm Jup}$ at ${\sim}60$ au that may have migrated outwards, similar to Neptune in our Solar System. If the measured vertical aspect ratio of $h{=}0.04{\pm}0.01$ is due to dynamical interactions in the disc, then this requires perturbers with sizes ${>}1200$ km. We find tentative evidence for an 0.86 mm excess within 10 au, $70{\pm}22\, \mu$Jy, that may be due to an inner planetesimal belt. We find no evidence for CO gas, but set an upper bound on the CO gas mass of $4{\times}10^{-6}$ M$_{\oplus}$ ($3\,\sigma$), consistent with cometary abundances in the Solar System., Comment: Accepted for publication in MNRAS. Paper: 21 pages, appendix: 4 pages. 16 figures

We observed the K7 class III star NO Lup in an ALMA survey of the 1-3 Myr Lupus association and detected circumstellar dust and CO gas. Here we show that the J = 3-2 CO emission is both spectrally and spatially resolved, with a broad velocity width ${\sim}19$kms$^{-1}$ for its resolved size ${\sim}1''$ (${\sim}130$ au). We model the gas emission as a Keplerian disc, finding consistency, but only with a central mass of ${\sim}11M_{\odot}$, which is implausible given its spectral type and X-Shooter spectrum. A good fit to the data can also be found by modelling the CO emission as outflowing gas with a radial velocity ${\sim}22$kms$^{-1}$. We interpret NO Lup's CO emission as the first imaged class III circumstellar disc with outflowing gas. We conclude that the CO is continually replenished, but cannot say if this is from the break-up of icy planetesimals or from the last remnants of the protoplanetary disc. We suggest further work to explore the origin of this CO, and its higher than expected velocity in comparison to photoevaporative models., Comment: 6 pages, 4 figures. Accepted for publication in MNRAS following peer review (this is a pre-copyedited, author-produced PDF version)

Class III stars are those in star forming regions without large non-photospheric infrared emission, suggesting recent dispersal of their protoplanetary disks. We observed 30 class III stars in the 1-3 Myr Lupus region with ALMA at ${\sim}856\mu$m, resulting in 4 detections that we attribute to circumstellar dust. Inferred dust masses are $0.036{-}0.093M_\oplus$, ${\sim}1$ order of magnitude lower than any previous measurements; one disk is resolved with radius ${\sim}80$ au. Two class II sources in the field of view were also detected, and 11 other sources, consistent with sub-mm galaxy number counts. Stacking non-detections yields a marginal detection with mean dust mass ${\sim}0.0048M_\oplus$. We searched for gas emission from the CO J=3-2 line, and present its detection to NO Lup inferring a gas mass ($4.9 {\pm} 1.1$) ${\times}10^{-5} M_\oplus$ and gas-to-dust ratio $1.0{\pm}0.4$. Combining our survey with class II sources shows a gap in the disk mass distribution from $0.09{-}2M_\oplus$ for ${>}0.7M_\odot$ Lupus stars, evidence of rapid dispersal of mm-sized dust from protoplanetary disks. The class III disk mass distribution is consistent with a population model of planetesimal belts that go on to replenish the debris disks seen around main sequence stars. This suggests that planetesimal belt formation does not require long-lived protoplanetary disks, i.e., planetesimals form within ${\sim}$2 Myr. While all 4 class III disks are consistent with collisional replenishment, for two the gas and/or mid-IR emission could indicate primordial circumstellar material in the final stages of protoplanetary disk dispersal. Two class III stars without sub-mm detections exhibit hot emission that could arise from ongoing planet formation processes inside ${\sim}1$ au., Comment: 30 pages, 20 figures. This is a pre-copyedited, author-produced PDF of an article accepted for publication in MNRAS following peer review

Following our study of the radio and high-energy properties of $\gamma$-ray-emitting radio galaxies, here we investigate the kinematic and spectral properties of the parsec-scale jets of radio galaxies that have not yet been detected by Fermi-LAT. We take advantage of the regular VLBI observations provided by the TANAMI monitoring program, and explore the kinematic properties of six $\gamma$-ray-faint radio galaxies. We include publicly available VLBI kinematics of $\gamma$-ray-quiet radio galaxies monitored by the MOJAVE program and perform a Fermi-LAT analysis, deriving upper limits. We combine these results with those from our previous paper to construct the largest sample of radio galaxies with combined VLBI and $\gamma$-ray measurements to date. We find superluminal motion up to $\beta_\mathrm{app}=3.6$ in the jet of PKS 2153$-$69. We find a clear trend of higher apparent speed as a function of distance from the jet core on scales of $\sim10^5\,R_s$, corresponding to the end of the collimation and acceleration zone in nearby radio galaxies. We find evidence of subluminal apparent motion in the jets of PKS 1258$-$321 and IC 4296, and no measurable motion for PKS 1549$-$79, PKS 1733$-$565 and PKS 2027$-$308. We compare the VLBI properties of $\gamma$-ray-detected and undetected radio galaxies, and find significantly different distributions of median core flux density, and, possibly, of median core brightness temperature. We find a significant correlation between median core flux density and $\gamma$-ray flux, but no correlation with typical Doppler boosting indicators such as median core brightness temperature and core dominance. Our study suggests that high-energy emission from radio galaxies is related to parsec-scale radio emission from the inner jet, but is not driven by Doppler boosting effects, in contrast to the situation in their blazar counterparts., Comment: Main text: 15 pages, 12 figures, 6 tables. Appendix: 14 pages, 20 figures, 13 tables. Accepted for publication in Astronomy and Astrophysics

In the framework of the TANAMI multi-wavelength and VLBI monitoring, we study the evolution of the parsec-scale radio emission in radio galaxies in the southern hemisphere and their relationship to the $\gamma$-ray properties. In this first paper, we focus on Fermi-LAT-detected sources. We perform a kinematic analysis for five $\gamma$-ray detected radio galaxies using multi-epoch 8.4 GHz VLBI images, deriving limits on intrinsic jet parameters. We analyzed Fermi-LAT data in order to study possible connections between the $\gamma$-ray properties and the pc-scale jets of Fermi-LAT-detected radio galaxies, both in terms of variability and average properties. We discuss the individual source results and draw preliminary conclusions on sample properties including published VLBI results from the MOJAVE survey, with a total of fifteen sources. We find that the first $\gamma$-ray detection of Pictor A might be associated with the passage of a new VLBI component through the radio core. For the peculiar AGN PKS 0521-36, we detect subluminal parsec-scale jet motions, and we confirm the presence of fast $\gamma$-ray variability in the source down to timescales of 6 hours. We robustly confirm the presence of significant superluminal motion, up to $\beta_{app}\sim$3, in the jet of the TeV radio galaxy PKS 0625-35. Finally, we place a lower limit on the age of the Compact Symmetric Object (CSO) PKS 1718-649. We draw some preliminary conclusions on the relationship between pc-scale jets and $\gamma$-ray emission in radio galaxies. We find that the VLBI core flux density correlates with the $\gamma$-ray flux, as seen in blazars. On the other hand, the $\gamma$-ray luminosity does not show any dependence on the core brightness temperature and core dominance, two indicators of Doppler boosting, suggesting that $\gamma$-ray emission in radio galaxies is not driven by orientation-dependent effects., Comment: Main text: 22 pages, 22 figures, 11 tables. Appendix: 19 pages, 10 figures, 9 tables. Accepted for publication in Astronomy and Astrophysics

Context. Radio occultation is a technique used to study planetary atmospheres by means of the refraction and absorption of a spacecraft carrier signal through the atmosphere of the celestial body of interest, as detected from a ground station on Earth. This technique is usually employed by the deep space tracking and communication facilities (e.g., NASA's Deep Space Network (DSN), ESA's Estrack). Aims. We want to characterize the capabilities of the Planetary Radio Interferometry and Doppler Experiment (PRIDE) technique for radio occultation experiments, using radio telescopes equipped with Very Long Baseline Interferometry (VLBI) instrumentation. Methods. We conducted a test with ESA's Venus Express (VEX), to evaluate the performance of the PRIDE technique for this particular application. We explain in detail the data processing pipeline of radio occultation experiments with PRIDE, based on the collection of so-called open-loop Doppler data with VLBI stations, and perform an error propagation analysis of the technique. Results. With the VEX test case and the corresponding error analysis, we have demonstrated that the PRIDE setup and processing pipeline is suited for radio occultation experiments of planetary bodies. The noise budget of the open-loop Doppler data collected with PRIDE indicated that the uncertainties in the derived density and temperature profiles remain within the range of uncertainties reported in previous Venus' studies. Open-loop Doppler data can probe deeper layers of thick atmospheres, such as that of Venus, when compared to closed-loop Doppler data. Furthermore, PRIDE through the VLBI networks around the world, provides a wide coverage and range of large antenna dishes, that can be used for this type of experiments.

We present Chandra X-ray imaging of a flux-limited sample of flat spectrum radio-emitting quasars with jet-like structure. X-rays are detected from 59% of 56 jets. No counterjets were detected. The core spectra are fitted by power law spectra with photon index $\Gamma_x$ whose distribution is consistent with a normal distribution with mean 1.61{+0.04}{-0.05} and dispersion 0.15{+0.04}{-0.03}. We show that the distribution of $\alpha_{rx}$, the spectral index between the X-ray and radio band jet fluxes, fits a Gaussian with mean 0.974 $\pm$ 0.012 and dispersion 0.077 $\pm$ 0.008. We test the model in which kpc-scale X-rays result from inverse Compton scattering of cosmic microwave background photons off the jet's relativistic electrons (the IC-CMB model). In the IC-CMB model, a quantity Q computed from observed fluxes and the apparent size of the emission region depends on redshift as $(1+z)^{3+\alpha}$. We fit $Q \propto (1+z)^{a}$, finding $a = 0.88 \pm 0.90$ and reject at 99.5% confidence the hypothesis that the average $\alpha_{rx}$ depends on redshift in the manner expected in the IC-CMB model. This conclusion is mitigated by lack of detailed knowledge of the emission region geometry, which requires deeper or higher resolution X-ray observations. Furthermore, if the IC-CMB model is valid for X-ray emission from kpc-scale jets, then the jets must decelerate on average: bulk Lorentz factors should drop from about 15 to 2-3 between pc and kpc scales. Our results compound the problems that the IC-CMB model has in explaining the X-ray emission of kpc-scale jets., Comment: 41 pages, 15 figures, accepted for publication in the Astrophysical Journal Supplements

We investigate the relationship between 5 GHz interstellar scintillation (ISS) and 15 GHz intrinsic variability of compact, radio-selected AGNs drawn from the Microarcsecond Scintillation-Induced Variability (MASIV) Survey and the Owens Valley Radio Observatory (OVRO) blazar monitoring program. We discover that the strongest scintillators at 5 GHz (modulation index, $m_5 \geq 0.02$) all exhibit strong 15 GHz intrinsic variability ($m_{15} \geq 0.1$). This relationship can be attributed mainly to the mutual dependence of intrinsic variability and ISS amplitudes on radio core compactness at $\sim 100\, \mu$as scales, and to a lesser extent, on their mutual dependences on source flux density, arcsec-scale core dominance and redshift. However, not all sources displaying strong intrinsic variations show high amplitude scintillation, since ISS is also strongly dependent on Galactic line-of-sight scattering properties. This observed relationship between intrinsic variability and ISS highlights the importance of optimizing the observing frequency, cadence, timespan and sky coverage of future radio variability surveys, such that these two effects can be better distinguished to study the underlying physics. For the full MASIV sample, we find that Fermi-detected gamma-ray loud sources exhibit significantly higher 5 GHz ISS amplitudes than gamma-ray quiet sources. This relationship is weaker than the known correlation between gamma-ray loudness and the 15 GHz variability amplitudes, most likely due to jet opacity effects., Comment: 18 pages, 13 figures, 5 tables, resubmitted to MNRAS after minor revision

We present an approach to testing the gravitational redshift effect using the RadioAstron satellite. The experiment is based on a modification of the Gravity Probe A scheme of nonrelativistic Doppler compensation and benefits from the highly eccentric orbit and ultra-stable atomic hydrogen maser frequency standard of the RadioAstron satellite. Using the presented techniques we expect to reach an accuracy of the gravitational redshift test of order $10^{-5}$, a magnitude better than that of Gravity Probe A. Data processing is ongoing, our preliminary results agree with the validity of the Einstein Equivalence Principle., Comment: Submitted to Physics Letters A

TANAMI is a multiwavelength program monitoring active galactic nuclei (AGN) south of -30deg declination including high-resolution Very Long Baseline Interferometry (VLBI) imaging, radio, optical/UV, X-ray and gamma-ray studies. We have previously published first-epoch 8.4GHz VLBI images of the parsec-scale structure of the initial sample. In this paper, we present images of 39 additional sources. The full sample comprises most of the radio- and gamma-ray brightest AGN in the southern quarter of the sky, overlapping with the region from which high-energy (>100TeV) neutrino events have been found. We characterize the parsec-scale radio properties of the jets and compare with the quasi-simultaneous Fermi/LAT gamma-ray data. Furthermore, we study the jet properties of sources which are in positional coincidence with high-energy neutrino events as compared to the full sample. We test the positional agreement of high-energy neutrino events with various AGN samples. Our observations yield the first images of many jets below -30deg declination at milliarcsecond resolution. We find that gamma-ray loud TANAMI sources tend to be more compact on parsec-scales and have higher core brightness temperatures than gamma-ray faint jets, indicating higher Doppler factors. No significant structural difference is found between sources in positional coincidence with high-energy neutrino events and other TANAMI jets. The 22 gamma-ray brightest AGN in the TANAMI sky show only a weak positional agreement with high-energy neutrinos demonstrating that the >100TeV IceCube signal is not simply dominated by a small number of the $\gamma$-ray brightest blazars. Instead, a larger number of sources have to contribute to the signal with each individual source having only a small Poisson probability for producing an event in multi-year integrations of current neutrino detectors., Comment: A&A accepted (edited version), 21 pages, 13 figures

We present results from a multiwavelength study of the blazar PKS 1954-388 at radio, UV, X-ray, and gamma-ray energies. A RadioAstron observation at 1.66 GHz in June 2012 resulted in the detection of interferometric fringes on baselines of 6.2 Earth-diameters. This suggests a source frame brightness temperature of greater than 2x10^12 K, well in excess of both equipartition and inverse Compton limits and implying the existence of Doppler boosting in the core. An 8.4 GHz TANAMI VLBI image, made less than a month after the RadioAstron observations, is consistent with a previously reported superluminal motion for a jet component. Flux density monitoring with the Australia Telescope Compact Array confirms previous evidence for long-term variability that increases with observing frequency. A search for more rapid variability revealed no evidence for significant day-scale flux density variation. The ATCA light-curve reveals a strong radio flare beginning in late 2013 which peaks higher, and earlier, at higher frequencies. Comparison with the Fermi gamma-ray light-curve indicates this followed ~9 months after the start of a prolonged gamma-ray high-state -- a radio lag comparable to that seen in other blazars. The multiwavelength data are combined to derive a Spectral Energy Distribution, which is fitted by a one-zone synchrotron-self-Compton (SSC) model with the addition of external Compton (EC) emission., Comment: 12 pages, 7 figures

Observations of the frequencies of different rotational transitions of the methanol molecule have provided the most sensitive probe to date for changes in the proton-to-electron mass ratio, over space and time. Using methanol absorption detected in the gravitational lens system PKS B1830-211, changes in the proton-to-electron ratio over the last 7.5 billion years have been constrained to a fractional change less than 1.1e-07. Molecular absorption systems at cosmological distances present the best opportunity for constraining or measuring changes in the fundamental constants of physics over time, however, we are now at the stage where potential differences in the morphology of the absorbing systems and the background source, combined with their temporal evolution, provide the major source of uncertainty in some systems. Here we present the first milliarcsecond resolution observations of the molecular absorption system towards PKS B1830-211. We have imaged the absorption from the 12.2-GHz transition of methanol (which is redshifted to 6.45 GHz) toward the southwestern component and show that it is possibly offset from the peak of the continuum emission and partially resolved on milliarcsecond scales. Future observations of other methanol transitions with similar angular resolution offer the best prospects for reducing systematic errors in investigations of possible changes in the proton-to-electron mass ratio on cosmological scales., Comment: 8 pages, 3 figures, accepted for publication in MNRAS

The discovery of extraterrestrial very-high-energy neutrinos by the IceCube collaboration has launched a quest for the identification of their astrophysical sources. Gamma-ray blazars have been predicted to yield a cumulative neutrino signal exceeding the atmospheric background above energies of 100 TeV, assuming that both the neutrinos and the gamma-ray photons are produced by accelerated protons in relativistic jets. Since the background spectrum falls steeply with increasing energy, the individual events with the clearest signature of being of an extraterrestrial origin are those at PeV energies. Inside the large positional-uncertainty fields of the first two PeV neutrinos detected by IceCube, the integrated emission of the blazar population has a sufficiently high electromagnetic flux to explain the detected IceCube events, but fluences of individual objects are too low to make an unambiguous source association. Here, we report that a major outburst of the blazar PKS B1424-418 occurred in temporal and positional coincidence with the third PeV-energy neutrino event (IC35) detected by IceCube. Based on an analysis of the full sample of gamma-ray blazars in the IC35 field and assuming a photo-hadronic emission model, we show that the long-term average gamma-ray emission of blazars as a class is in agreement with both the measured all-sky flux of PeV neutrinos and the spectral slope of the IceCube signal. The outburst of PKS B1424-418 has provided an energy output high enough to explain the observed PeV event, indicative of a direct physical association., Comment: Accepted for publication in Nature Physics on Mar 3rd, 2016 (arXiv version is the original submitted manuscript as per NatPhys regulations), 38 pages (including 10 pages of supplementary material), 3 figures

Using high-resolution radio imaging with VLBI techniques, the TANAMI program has been observing the parsec-scale radio jets of southern (declination south of -30{\deg}) gamma-ray bright AGN simultaneously with Fermi/LAT monitoring of their gamma-ray emission. We present the radio and gamma-ray properties of the TANAMI sources based on one year of contemporaneous TANAMI and Fermi/LAT data. A large fraction (72%) of the TANAMI sample can be associated with bright gamma-ray sources for this time range. Association rates differ for different optical classes with all BL Lacs, 76% of quasars and just 17% of galaxies detected by the LAT. Upper limits were established on the gamma-ray flux from TANAMI sources not detected by LAT. This analysis led to the identification of three new Fermi sources whose detection was later confirmed. The gamma-ray and radio luminosities are related by $L_\gamma \propto L_r^{0.89+-0.04}$. The brightness temperatures of the radio cores increase with the average gamma-ray luminosity, and the presence of brightness temperatures above the inverse Compton limit implies strong Doppler boosting in those sources. The undetected sources have lower gamma/radio luminosity ratios and lower contemporaneous brightness temperatures. Unless the Fermi/LAT-undetected blazars are strongly gamma-ray-fainter than the Fermi/LAT-detected ones, their gamma-ray luminosity should not be significantly lower than the upper limits calculated here., Comment: 12 pages, 11 figures, 3 tables, Accepted for publication in Astronomy & Astrophysics

Gamma-ray detected radio-loud narrow-line Seyfert 1 (g-NLS1) galaxies constitute a small but interesting sample of the gamma-ray loud AGN. The radio-loudest g-NLS1 known, PKS 2004-447, is located in the southern hemisphere and is monitored in the radio regime by the multiwavelength monitoring program TANAMI. We aim for the first detailed study of the radio morphology and long-term radio spectral evolution of PKS 2004-447, which are essential to understand the diversity of the radio properties of g-NLS1s. The TANAMI VLBI monitoring program uses the Australian Long Baseline Array (LBA) and telescopes in Antarctica, Chile, New Zealand, and South Africa to monitor the jets of radio-loud active galaxies in the southern hemisphere. Lower resolution radio flux density measurements at multiple radio frequencies over four years of observations were obtained with the Australia Telescope Compact Array (ATCA). The TANAMI VLBI image at 8.4 GHz shows an extended one-sided jet with a dominant compact VLBI core. Its brightness temperature is consistent with equipartition, but it is an order of magnitude below other g-NLS1s with the sample value varying over two orders of magnitude. We find a compact morphology with a projected large-scale size <11 kpc and a persistent steep radio spectrum with moderate flux-density variability. PKS 2004-447 appears to be a unique member of the g-NLS1 sample. It exhibits blazar-like features, such as a flat featureless X-ray spectrum and a core dominated, one-sided parsec-scale jet with indications for relativistic beaming. However, the data also reveal properties atypical for blazars, such as a radio spectrum and large-scale size consistent with Compact-Steep-Spectrum (CSS) objects, which are usually associated with young radio sources. These characteristics are unique among all g-NLS1s and extremely rare among gamma-ray loud AGN., Comment: 11 pages, 4 figures, accepted by Astronomy & Astrophysics

The source(s) of the neutrino excess reported by the IceCube Collaboration is unknown. The TANAMI Collaboration recently reported on the multiwavelength emission of six bright, variable blazars which are positionally coincident with two of the most energetic IceCube events. Objects like these are prime candidates to be the source of the highest-energy cosmic rays, and thus of associated neutrino emission. We present an analysis of neutrino emission from the six blazars using observations with the ANTARES neutrino telescope.The standard methods of the ANTARES candidate list search are applied to six years of data to search for an excess of muons --- and hence their neutrino progenitors --- from the directions of the six blazars described by the TANAMI Collaboration, and which are possibly associated with two IceCube events. Monte Carlo simulations of the detector response to both signal and background particle fluxes are used to estimate the sensitivity of this analysis for different possible source neutrino spectra. A maximum-likelihood approach, using the reconstructed energies and arrival directions of through-going muons, is used to identify events with properties consistent with a blazar origin.Both blazars predicted to be the most neutrino-bright in the TANAMI sample (1653$-$329 and 1714$-$336) have a signal flux fitted by the likelihood analysis corresponding to approximately one event. This observation is consistent with the blazar-origin hypothesis of the IceCube event IC14 for a broad range of blazar spectra, although an atmospheric origin cannot be excluded. No ANTARES events are observed from any of the other four blazars, including the three associated with IceCube event IC20. This excludes at a 90\% confidence level the possibility that this event was produced by these blazars unless the neutrino spectrum is flatter than $-2.4$., Comment: 15 pages, 3 figures, 2 tables

Context. Blazars are a subset of active galactic nuclei (AGN) with jets that are oriented along our line of sight. Variability and spectral energy distribution (SED) studies are crucial tools for understanding the physical processes responsible for observed AGN emission. Aims. We report peculiar behaviour in the bright gamma-ray blazar PKS 1424-418 and use its strong variability to reveal information about the particle acceleration and interactions in the jet. Methods. Correlation analysis of the extensive optical coverage by the ATOM telescope and nearly continuous gamma-ray coverage by the Fermi Large Area Telescope is combined with broadband, time-dependent modeling of the SED incorporating supplemental information from radio and X-ray observations of this blazar. Results. We analyse in detail four bright phases at optical-GeV energies. These flares of PKS 1424-418 show high correlation between these energy ranges, with the exception of one large optical flare that coincides with relatively low gamma-ray activity. Although the optical/gamma-ray behaviour of PKS 1424-418 shows variety, the multiwavelength modeling indicates that these differences can largely be explained by changes in the flux and energy spectrum of the electrons in the jet that are radiating. We find that for all flares the SED is adequately represented by a leptonic model that includes inverse Compton emission from external radiation fields with similar parameters. Conclusions. Detailed studies of individual blazars like PKS 1424-418 during periods of enhanced activity in different wavebands are helping us identify underlying patterns in the physical parameters in this class of AGN., Comment: accepted for publication in A&A

Centaurus A is the closest radio-loud active galaxy. Very Long Baseline Interferometry (VLBI) enables us to study the jet-counterjet system on milliarcsecond (mas) scales, providing essential information for jet emission and propagation models. We study the evolution of the central parsec jet structure of Cen A over 3.5 years. The proper motion analysis of individual jet components allows us to constrain jet formation and propagation and to test the proposed correlation of increased high energy flux with jet ejection events. Cen A is an exceptional laboratory for such detailed study as its proximity translates to unrivaled linear resolution, where 1 mas corresponds to 0.018 pc. The first 7 epochs of high-resolution TANAMI VLBI observations at 8 GHz of Cen A are presented, resolving the jet on (sub-)mas scales. They show a differential motion of the sub-pc scale jet with significantly higher component speeds further downstream where the jet becomes optically thin. We determined apparent component speeds within a range of 0.1c to 0.3c, as well as identified long-term stable features. In combination with the jet-to-counterjet ratio we can constrain the angle to the line of sight to ~12{\deg} to 45{\deg}. The high resolution kinematics are best explained by a spine-sheath structure supported by the downstream acceleration occurring where the jet becomes optically thin. On top of the underlying, continuous flow, TANAMI observations clearly resolve individual jet features. The flow appears to be interrupted by an obstacle causing a local decrease in surface brightness and a circumfluent jet behavior. We propose a jet-star interaction scenario to explain this appearance. The comparison of jet ejection times with high X-ray flux phases yields a partial overlap of the onset of the X-ray emission and increasing jet activity, but the limited data do not support a robust correlation., Comment: 16 pages, 10 figures, A&A, 569, L115 (accepted 23th June, published online 1st October)

The IceCube Collaboration has announced the discovery of a neutrino flux in excess of the atmospheric background. Due to the steeply falling atmospheric background spectrum, events at PeV energies are most likely of extraterrestrial origin. We present the multiwavelength properties of the six radio brightest blazars positionally coincident with these events using contemporaneous data of the TANAMI blazar sample, including high-resolution images and spectral energy distributions. Assuming the X-ray to {\gamma}-ray emission originates in the photoproduction of pions by accelerated protons, the integrated predicted neutrino luminosity of these sources is large enough to explain the two detected PeV events., Comment: accepted to A&A, 2 June

We investigate the nature and classification of PMNJ1603-4904, a bright radio source close to the Galactic plane, which is associated with one of the brightest hard-spectrum gamma-ray sources detected by Fermi/LAT. It has previously been classified as a low-peaked BL Lac object based on its broadband emission and the absence of optical emission lines. Optical measurements, however, suffer strongly from extinction and the absence of pronounced short-time gamma-ray variability over years of monitoring is unusual for a blazar. We are combining new and archival multiwavelength data in order to reconsider the classification and nature of this unusual gamma-ray source. For the first time, we study the radio morphology at 8.4GHz and 22.3GHz, and its spectral properties on milliarcsecond (mas) scales, based on VLBI observations from the TANAMI program. We combine the resulting images with multiwavelength data in the radio, IR, optical/UV, X-ray, and gamma-ray regimes. PMNJ1603-4904 shows a symmetric brightness distribution at 8.4GHz on mas-scales, with the brightest, and most compact component in the center of the emission region. The morphology is reminiscent of a Compact Symmetric Object (CSO). Such objects have been predicted to produce gamma-ray emission but have not been detected as a class by Fermi/LAT so far. Sparse (u, v)-coverage at 22.3GHz prevents an unambiguous modeling of the source morphology. IR measurements reveal an excess in the spectral energy distribution (SED), which can be modeled with a blackbody with a temperature of about 1600K, and which is usually not present in blazar SEDs. The VLBI data and the shape of the SED challenge the current blazar classification. PMNJ1603-4904 seems to be either a highly peculiar BL Lac object or a misaligned jet source. In the latter case, the intriguing VLBI structure opens room for a possible classification as a gamma-ray bright CSO., Comment: 11 pages, 7 figures, accepted for publication in Astronomy & Astrophysics

The Fermi Gamma-ray Space Telescope has been detecting a wealth of sources where the multi-wavelength counterpart is either inconclusive or missing altogether. We present a combination of factors that can be used to identify multi-wavelength counterparts to these Fermi unassociated sources. This approach was used to select and investigate seven bright, high-latitude unassociated sources with radio, UV, X-ray and gamma-ray observations. As a result, four of these sources are candidates to be active galactic nuclei (AGN), and one to be a pulsar, while two do not fit easily into these known categories of sources. The latter pair of extra-ordinary sources might reveal a new category subclass or a new type of gamma-ray emitters. These results altogether demonstrate the power of a multi-wavelength approach to illuminate the nature of unassociated Fermi sources., Comment: 9 pages, 2 figures, 1 Table. Accepted for publication in ApJ

The AuScope geodetic Very Long Baseline Interferometry array consists of three new 12 m radio telescopes and a correlation facility in Australia. The telescopes at Hobart (Tasmania), Katherine (Northern Territory) and Yarragadee (Western Australia) are co-located with other space geodetic techniques including Global Navigation Satellite Systems (GNSS) and gravity infrastructure, and in the case of Yarragadee, Satellite Laser Ranging (SLR) and Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) facilities. The correlation facility is based in Perth (Western Australia). This new facility will make significant contributions to improving the densification of the International Celestial Reference Frame in the Southern Hemisphere, and subsequently enhance the International Terrestrial Reference Frame through the ability to detect and mitigate systematic error. This, combined with the simultaneous densification of the GNSS network across Australia will enable the improved measurement of intraplate deformation across the Australian tectonic plate. In this paper we present a description of this new infrastructure and present some initial results, including telescope performance measurements and positions of the telescopes in the International Terrestrial Reference Frame. We show that this array is already capable of achieving centimetre precision over typical long-baselines and that network and reference source systematic effects must be further improved to reach the ambitious goals of VLBI2010., Comment: Journal of Geodesy, March 2013

We report the detection of four water masers within the Small Magellanic Cloud (SMC); two discovered with the 70-m Tidbinbilla radio telescope, and two discovered with the Australia Telescope Compact Array (ATCA). Precise positions of all four masers have been derived from ATCA observations, and the characteristics of each water maser have been monitored over a period of several years. Sensitive observations towards two previously detected water masers reported in the literature failed to detect any emission. The detected water masers show evidence of higher levels of temporal variability than equivalent Galactic sources, and one of the features associated with NGC346 IR1 shows an acceleration of 9.6 km/s yr^-1 over a 31 day period. Sensitive targeted observations for methanol and OH masers failed to detect any accompanying emission - in the case of methanol perhaps highlighting an under abundance in the SMC, consistent with expectations due to lower metallicity. The water masers are both bright and compact making them excellent targets for Very Long Baseline Interferometry (VLBI) observations, which can potentially measure the proper motion of the SMC (~1 - 2 mas yr^-1) with temporal baselines of ~12 months. Such observations would utilise sources associated with only the current epoch of star formation and hence have several advantages over alternative methods.

We present 18 GHz Australia Telescope Compact Array imaging of the Mpc-scale quasar jet PKS 0637-752 with angular resolution ~0.58 arcseconds. We draw attention to a spectacular train of quasi-periodic knots along the inner 11 arcseconds of the jet, with average separation ~1.1 arcsec (7.6 kpc projected). We consider two classes of model to explain the periodic knots: those that involve a static pattern through which the jet plasma travels (e.g. stationary shocks); and those that involve modulation of the jet engine. Interpreting the knots as re-confinement shocks implies the jet kinetic power Q ~ 10^{46} erg/s, but the constant knot separation along the jet is not expected in a realistic external density profile. For models involving modulation of the jet engine, we find that the required modulation period is 2 x 10^3 yr < \tau < 3 x 10^5 yr. The lower end of this range is applicable if the jet remains highly relativistic on kpc-scales, as implied by the IC/CMB model of jet X-ray emission. We suggest that the quasi-periodic jet structure in PKS 0637-752 may be analogous to the quasi-periodic jet modulation seen in the microquasar GRS 1915+105, believed to result from limit cycle behaviour in an unstable accretion disk. If variations in the accretion rate are driven by a binary black hole, the predicted orbital radius is 0.7 < a < 30 pc, which corresponds to a maximum angular separation of ~0.1 - 5 mas., Comment: 5 Pages, 2 Figures

The flat spectrum radio quasar (FSRQ) PKS 2123-463 was associated in the First Fermi-LAT source catalog with the gamma-ray source 1FGL J2126.1-4603, but when considering the full first two years of Fermi observations, no gamma-ray source at a position consistent with this FSRQ was detected, and thus PKS 2123-463 was not reported in the Second Fermi-LAT source catalog. On 2011 December 14 a gamma-ray source positionally consistent with PKS 2123-463 was detected in flaring activity by Fermi-LAT. This activity triggered radio-to-X-ray observations by the Swift, GROND, ATCA, Ceduna, and KAT-7 observatories. Results of the localization of the gamma-ray source over 41 months of Fermi-LAT operation are reported here in conjunction with the results of the analysis of radio, optical, UV and X-ray data collected soon after the gamma-ray flare. The strict spatial association with the lower energy counterpart together with a simultaneous increase of the activity in optical, UV, X-ray and gamma-ray bands led to a firm identification of the gamma-ray source with PKS 2123-463. A new photometric redshift has been estimated as z = 1.46+/-0.05 using GROND and Swift/UVOT observations, in rough agreement with the disputed spectroscopic redshift of z = 1.67. We fit the broadband spectral energy distribution with a synchrotron/external Compton model. We find that a thermal disk component is necessary to explain the optical/UV emission detected by Swift/UVOT. This disk has a luminosity of about 1.8x10^46 erg/s, and a fit to the disk emission assuming a Schwarzschild (i.e., nonrotating) black hole gives a mass of about 2x10^9 solar masses. This is the first black hole mass estimate for this source., Comment: 9 pages, 5 figures, 5 tables. Accepted for publication in MNRAS. arXiv admin note: text overlap with arXiv:1207.3092

We present a detailed study of the X-ray, optical and radio emission from the jet, lobes and core of the quasar PKS 2101-490 as revealed by new Chandra, HST and ATCA images. We extract the radio to X-ray spectral energy distributions from seven regions of the 13 arcsecond jet, and model the jet X-ray emission in terms of Doppler beamed inverse Compton scattering of the cosmic microwave background (IC/CMB) for a jet in a state of equipartition between particle and magnetic field energy densities. This model implies that the jet remains highly relativistic hundreds of kpc from the nucleus, with a bulk Lorentz factor Gamma ~ 6 and magnetic field of order 30 microGauss. We detect an apparent radiative cooling break in the synchrotron spectrum of one of the jet knots, and are able to interpret this in terms of a standard one-zone continuous injection model, based on jet parameters derived from the IC/CMB model. However, we note apparent substructure in the bright optical knot in one of the HST bands. We confront the IC/CMB model with independent estimates of the jet power, and find that the IC/CMB model jet power is consistent with the independent estimates, provided that the minimum electron Lorentz factor gamma_min > 50, and the knots are significantly longer than the jet width, as implied by de-projection of the observed knot lengths., Comment: 16 pages, 10 figures, 6 tables

Rapid variability in the radio flux density of the BL Lac object PKSB1144-379 has been observed at four frequencies, ranging from 1.5 to 15 GHz, with the VLA and the University of Tasmania's Ceduna antenna. Intrinsic and line of sight effects were examined as possible causes of this variability, with interstellar scintillation best explaining the frequency dependence of the variability timescales and modulation indices. This scintillation is consistent with a compact source 20-40 microarcseconds, or 0.15-0.3 pc in size. The inferred brightness temperature for PKSB1144-379 (assuming that the observed variations are due to scintillation) is 6.2e12 K at 4.9 GHz, with approximately 10 percent of the total flux in the scintillating component. We show that scintillation surveys aimed at identifying variability timescales of days to weeks are an effective way to identify the AGN with the highest brightness temperatures., Comment: 6 pages, 3 figures, accepted for publication in ApJ Letters

The fraction of compact active galactic nuclei (AGNs) that exhibit interstellar scintillation (ISS) at radio wavelengths, as well as their scintillation amplitudes, have been found to decrease significantly for sources at redshifts z > 2. This can be attributed to an increase in the angular sizes of the \muas-scale cores or a decrease in the flux densities of the compact \muas cores relative to that of the mas-scale components with increasing redshift, possibly arising from (1) the space-time curvature of an expanding Universe, (2) AGN evolution, (3) source selection biases, (4) scatter broadening in the ionized intergalactic medium (IGM) and intervening galaxies, or (5) gravitational lensing. We examine the frequency scaling of this redshift dependence of ISS to determine its origin, using data from a dual-frequency survey of ISS of 128 sources at 0 < z < 4. We present a novel method of analysis which accounts for selection effects in the source sample. We determine that the redshift dependence of ISS is partially linked to the steepening of source spectral indices ({\alpha}^8.4_4.9) with redshift, caused either by selection biases or AGN evolution, coupled with weaker ISS in the {\alpha}^8.4_4.9 < -0.4 sources. Selecting only the -0.4 < {\alpha}^8.4_4.9 < 0.4 sources, we find that the redshift dependence of ISS is still significant, but is not significantly steeper than the expected (1+z)^0.5 scaling of source angular sizes due to cosmological expansion for a brightness temperature and flux-limited sample of sources. We find no significant evidence for scatter broadening in the IGM, ruling it out as the main cause of the redshift dependence of ISS. We obtain an upper limit to IGM scatter broadening of < 110\muas at 4.9 GHz with 99% confidence for all lines of sight, and as low as < 8\muas for sight-lines to the most compact, \sim 10\muas sources., Comment: 38 pages, 13 figures, accepted for publication in The Astrophysical Journal

As a critical part of the Tracking Active Galactic Nuclei with Austral Milliarcsecond Interferometry (TANAMI) program, in November 2007 the Australia Telescope Compact Array (ATCA) started monitoring the radio spectra of a sample of southern hemisphere active galactic nuclei (AGN) that were selected as likely candidates for detection (as well as a control sample) by the Large Area Telescope (LAT) aboard the Fermi Gamma Ray Space Observatory. The initial sample was chosen based on properties determined from AGN detections by the Energetic Gamma Ray Experiment Telescope (EGRET). Most of the initial sample has been detected by Fermi/LAT and with the addition of new detections the sample has grown to include 226 AGN, 133 of which have data for more than one epoch. For the majority of these AGN, our monitoring program provides the only dynamic radio spectra available. The ATCA receiver suite makes it possible to observe several sources at frequencies between 4.5 and 41 GHz in a few hours, resulting in an excellent measure of spectral index at each epoch. By examining how the spectral index changes over time, we aim to investigate the mechanics of radio and gamma-ray emission from AGN jets., Comment: 5 pages, 1 figure, 2012 Fermi & Jansky Proceedings - eConf C1111101

The gamma-ray blazar PKS 0208-512 has shown strong periods of flaring, at all frequencies from radio to gamma-ray. This has led to its inclusion in the TANAMI project, which tracks the jets of southern AGN using VLBI as well as supporting flux density monitoring programs. Time series analysis of the light curves generated by such monitoring is presented and discussed and VLBI maps of the source are used to show the evolution in the jet. A frequency dependent lag is observed between flaring at different radio frequencies which does not appear to correspond to purely optical depth effects. Major flaring at gamma-ray frequencies appears to be preceded by a new component in the jet seen in our VLBI data., Comment: 2012 Fermi & Jansky Proceedings - eConf C1111101

We have used the Australia Telescope Compact Array to measure the absorption from the 2(0) - 3(-1}E 12.2 GHz transition of methanol towards the z=0.89 lensing galaxy in the PKS B 1830-211 gravitational lens system. Comparison of the velocity of the main absorption feature with the published absorption spectrum from the 1(0) - 2(-1)E transition of methanol shows that they differ by -0.6 +/- 1.6 km/s . We can use these observations to constrain the changes in the proton-to-electron mass ratio from z=0.89 to the present to 0.8 +/- 2.1 x 10^-7. This result is consistent, and of similar precision to recent observations at z = 0.68 achieved through comparison of a variety of rotational and inversion transitions, and approximately a factor of 2 better than previous constraints obtained in this source. Future more sensitive observations which incorporate additional rotational methanol transitions offer the prospect of improving current results by a factor of 5-10., Comment: 8 pages, 1 figure, accepted for publication in ApJ Letters

The extreme, intra-hour and > 10% rms flux density scintillation observed in AGNs such as PKS 0405-385, J1819+3845 and PKS 1257-326 at cm wavelengths has been attributed to scattering in highly turbulent, nearby regions in the interstellar medium. Such behavior has been found to be rare. We searched for rapid scintillators among 128 flat spectrum AGNs and analyzed their properties to determine the origin of such rapid and large amplitude radio scintillation. The sources were observed at the VLA at 4.9 and 8.4 GHz simultaneously at two hour intervals over 11 days. We detected six rapid scintillators with characteristic time-scales of < 2 hours, none of which have rms variations > 10%. We found strong lines of evidence linking rapid scintillation to the presence of nearby scattering regions, estimated to be < 12 pc away for ~ 200 muas sources and < 250 pc away for ~ 10 muas sources. We attribute the scarcity of rapid and large amplitude scintillators to the requirement of additional constraints, including large source compact fractions. J1819+3845 was found to display ~ 2% rms variations at ~ 6 hour time-scales superposed on longer > 11 day variations, suggesting that the highly turbulent cloud responsible for its extreme scintillation has moved away, with its scintillation now caused by a more distant screen ~ 50 to 150 pc away., Comment: 5 pages, 3 figures, accepted for publication in Astronomy and Astrophysics

The 4.9 GHz Micro-Arcsecond Scintillation-Induced Variability (MASIV) Survey detected a drop in Interstellar Scintillation (ISS) for sources at redshifts z > 2, indicating an apparent increase in angular diameter or a decrease in flux density of the most compact components of these sources, relative to their extended emission. This can result from intrinsic source size effects or scatter broadening in the Intergalactic Medium (IGM), in excess of the expected (1+z)^0.5 angular diameter scaling of brightness temperature limited sources due to cosmological expansion. We report here 4.9 GHz and 8.4 GHz observations and data analysis for a sample of 140 compact, flat-spectrum sources which may allow us to determine the origin of this angular diameter-redshift relation by exploiting their different wavelength dependences. In addition to using ISS as a cosmological probe, the observations provide additional insight into source morphologies and the characteristics of ISS. As in the MASIV Survey, the variability of the sources is found to be significantly correlated with line-of-sight H-alpha intensities, confirming its link with ISS. For 25 sources, time delays of about 0.15 to 3 days are observed between the scintillation patterns at both frequencies, interpreted as being caused by a shift in core positions when probed at different optical depths. Significant correlation is found between ISS amplitudes and source spectral index; in particular, a large drop in ISS amplitudes is observed at spectral indices of < -0.4 confirming that steep spectrum sources scintillate less. We detect a weakened redshift dependence of ISS at 8.4 GHz over that at 4.9 GHz, with the mean variance at 4-day timescales reduced by a factor of 1.8 in the z > 2 sources relative to the z < 2 sources, as opposed to the factor of 3 decrease observed at 4.9 GHz. This suggests scatter broadening in the IGM., Comment: 30 pages, 14 figures, accepted for publication in the Astronomical Journal

We present deep {\it HST, Chandra, VLA} and {\it ATCA} images of the jets of PKS 0208--512 and PKS 1202--262, which were found in a {\it Chandra} survey of a flux-limited sample of flat-spectrum radio quasars with jets (see Marshall et al., 2005). We discuss in detail their X-ray morphologies and spectra. We find optical emission from one knot in the jet of PKS 1202--262 and two regions in the jet of PKS 0208--512. The X-ray emission of both jets is most consistent with external Comptonization of cosmic microwave background photons by particles within the jet, while the optical emission is most consistent with the synchrotron process. We model the emission from the jet in this context and discuss implications for jet emission models, including magnetic field and beaming parameters., Comment: 13 pages, 11 figures, ApJ in press

Centaurus A is the closest active galactic nucleus. High resolution imaging using Very Long Baseline Interferometry (VLBI) enables us to study the spectral and kinematic behavior of the radio jet-counterjet system on sub-parsec scales, providing essential information for jet emission and formation models. Our aim is to study the structure and spectral shape of the emission from the central-parsec region of Cen A. As a target of the Southern Hemisphere VLBI monitoring program TANAMI (Tracking Active Galactic Nuclei with Milliarcsecond Interferometry), VLBI observations of Cen A are made regularly at 8.4 and 22.3 GHz with the Australian Long Baseline Array (LBA) and associated telescopes in Antarctica, Chile, and South Africa. The first dual-frequency images of this source are presented along with the resulting spectral index map. An angular resolution of 0.4 mas x 0.7 mas is achieved at 8.4 GHz, corresponding to a linear scale of less than 0.013 pc. Hence, we obtain the highest resolution VLBI image of Cen A, comparable to previous space-VLBI observations. By combining with the 22.3 GHz image, which has been taken without contributing transoceanic baselines at somewhat lower resolution, we present the corresponding dual-frequency spectral index distribution along the sub-parsec scale jet revealing the putative emission regions for recently detected gamma-rays from the core region by Fermi/LAT. We resolve the innermost structure of the milliarcsecond scale jet and counterjet system of Cen A into discrete components. The simultaneous observations at two frequencies provide the highest resolved spectral index map of an AGN jet allowing us to identify multiple possible sites as the origin of the high energy emission., Comment: 5 pages, 3 figures (1 color); A&A, accepted

We present results from continued Chandra X-ray imaging and spectroscopy of a flux-limited sample of flat spectrum radio-emitting quasars with jet-like extended structure. X-rays are detected from 24 of the 39 jets observed so far. We compute the distribution of alpha_rx, the spectral index between the X-ray and radio bands, showing that it is broad, extending at least from 0.8 to 1.2. While there is a general trend that the radio brightest jets are detected most often, it is clear that predicting the X-ray flux from the radio knot flux densities is risky so a shallow X-ray survey is the most effective means for finding jets that are X-ray bright. We test the model in which the X-rays result from inverse Compton (IC) scattering of cosmic microwave background (CMB) photons by relativistic electrons in the jet moving with high bulk Lorentz factor nearly along the line of sight. Depending on how the jet magnetic fields vary with z, the observed X-ray to radio flux ratios do not follow the redshift dependence expected from the IC-CMB model. For a subset of our sample with known superluminal motion based on VLBI observations, we estimate the angle of the kpc-scale jet to the line of sight by considering the additional information in the bends observed between pc- and kpc-scale jets. These angles are sometimes much smaller than estimates based on the IC-CMB model with a Lorentz factor of 15, indicating that these jets may decelerate significantly from pc scales to kpc scales., Comment: Accepted for publication in the ApJ supplements. 48 pages, with 9 figures, 7 tables, and 19 color images

We have studied the archetypal Gigahertz Peaked Spectrum radio galaxy, PKS 1934-638, using the Australian Long Baseline Array, augmented with two new telescopes that greatly improve the angular resolution of the array. These VLBI observations represent the first scientific results from a new antenna in NZ and the first antenna of the Australian SKA Pathfinder (ASKAP). A compact double radio source, PKS 1934-638, has been monitored over a period of 40 years, and the observation described here provides the latest datum, eight years after the previous observation, to aid in the study of the long-term evolution of the source structure. We take advantage of these new long baselines to probe PKS 1934-638 at the relatively low frequency of 1.4 GHz, in order to examine the effects of optical depth on the structure of the radio source. Optical depth effects, resulting in the observation of frequency dependent structure, may have previously been interpreted in terms of an expansion of the source as a function of time. Expansion and frequency dependent effects are important to disentangle in order to estimate the age of PKS 1934-638. We show that frequency dependent structure effects are likely to be important in PKS 1934-638 and present a simple two-dimensional synchrotron source model in which opacity effects due to synchrotron self-absorption are taken into account. Evidence for expansion of the radio source over 40 years is therefore weak, with consequences for the estimated age of the radio source., Comment: 18 pages, 5 figures. Accepted for publication in AJ

We introduce the TANAMI program (Tracking Active Galactic Nuclei with Austral Milliarcsecond Interferometry) which is monitoring an initial sample of 43 extragalactic jets located south of -30 degrees declination at 8.4 GHz and 22 GHz since 2007. All aspects of the program are discussed. First epoch results at 8.4 GHz are presented along with physical parameters derived therefrom. We present first epoch images for 43 sources, some observed for the first time at milliarcsecond resolution. Parameters of these images as well as physical parameters derived from them are also presented and discussed. These and subsequent images from the TANAMI survey are available at http://pulsar.sternwarte.uni-erlangen.de/tanami/ We obtain reliable, high dynamic range images of the southern hemisphere AGN. All the quasars and BL Lac objects in the sample have a single-sided radio morphology. Galaxies are either double-sided, single-sided or irregular. About 28% of the TANAMI sample has been detected by LAT during its first three months of operations. Initial analysis suggests that when galaxies are excluded, sources detected by LAT have larger opening angles than those not detected by LAT. Brightness temperatures of LAT detections and non-detections seem to have similar distributions. The redshift distributions of the TANAMI sample and sub-samples are similar to those seen for the bright gamma-ray AGN seen by LAT and EGRET but none of the sources with a redshift above 1.8 have been detected by LAT., Comment: 35 pages, 18 figures