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(Abridged) Apertif is a phased-array feed system for WSRT, providing forty instantaneous beams over 300 MHz of bandwidth. A dedicated survey program started on 1 July 2019, with the last observations taken on 28 February 2022. We describe the release of data products from the first year of survey operations, through 30 June 2020. We focus on defining quality control metrics for the processed data products. The Apertif imaging pipeline, Apercal, automatically produces non-primary beam corrected continuum images, polarization images and cubes, and uncleaned spectral line and dirty beam cubes for each beam of an Apertif imaging observation. For this release, processed data products are considered on a beam-by-beam basis within an observation. We validate the continuum images by using metrics that identify deviations from Gaussian noise in the residual images. If the continuum image passes validation, we release all processed data products for a given beam. We apply further validation to the polarization and line data products. We release all raw observational data from the first year of survey observations, for a total of 221 observations of 160 independent target fields, covering approximately one thousand square degrees of sky. Images and cubes are released on a per beam basis, and 3374 beams are released. The median noise in the continuum images is 41.4 uJy/bm, with a slightly lower median noise of 36.9 uJy/bm in the Stokes V polarization image. The median angular resolution is 11.6"/sin(Dec). The median noise for all line cubes, with a spectral resolution of 36.6 kHz, is 1.6 mJy/bm, corresponding to a 3-sigma HI column density sensitivity of 1.8 x 10^20 atoms cm^-2 over 20 km/s (for a median angular resolution of 24" x 15"). We also provide primary beam images for each individual Apertif compound beam. The data are made accessible using a Virtual Observatory interface., Comment: Accepted for publication in A&A, updated Figure 1

We analyse five early science datasets from the APERture Tile in Focus (Apertif) phased array feed system to verify the polarisation capabilities of Apertif in view of future larger data releases. We aim to characterise the source population of the polarised sky in the L-Band using polarised source information in combination with IR and optical data. We use automatic routines to generate full field-of-view Q- and U-cubes and perform RM-Synthesis, source finding, and cross-matching with published radio, optical, and IR data to generate polarised source catalogues. SED-fitting routines were used to determine photometric redshifts, star-formation rates, and galaxy masses. IR colour information was used to classify sources as AGN or star-forming-dominated and early- or late-type. We surveyed an area of 56deg$^2$ and detected 1357 polarised source components in 1170 sources. The fraction of polarised sources is 10.57% with a median fractional polarisation of 4.70$\pm$0.14%. We confirmed the reliability of the Apertif measurements by comparing them with polarised cross-identified NVSS sources. Average RMs of the individual fields lie within the error of the best Milky Way foreground measurements. All of our polarised sources were found to be dominated by AGN activity in the radio regime with most of them being radio-loud (79%) and of the FRII class (87%). The host galaxies of our polarised source sample are dominated by intermediate disc and star-forming disc galaxies. The contribution of star formation to the radio emission is on the order of a few percent for $\approx$10% of the polarised sources while for $\approx$90% it is completely dominated by the AGN. We do not see any change in fractional polarisation for different star-formation rates of the AGN host galaxies., Comment: 24 pages, 21 figures

Apertif (APERture Tile In Focus) is one of the Square Kilometre Array (SKA) pathfinder facilities. The Apertif project is an upgrade to the 50-year-old Westerbork Synthesis Radio Telescope (WSRT) using phased-array feed technology. The new receivers create 40 individual beams on the sky, achieving an instantaneous sky coverage of 6.5 square degrees. The primary goal of the Apertif Imaging Survey is to perform a wide survey of 3500 square degrees (AWES) and a medium deep survey of 350 square degrees (AMES) of neutral atomic hydrogen (up to a redshift of 0.26), radio continuum emission and polarisation. Each survey pointing yields 4.6 TB of correlated data. The goal of Apercal is to process this data and fully automatically generate science ready data products for the astronomical community while keeping up with the survey observations. We make use of common astronomical software packages in combination with Python based routines and parallelisation. We use an object oriented module-based approach to ensure easy adaptation of the pipeline. A Jupyter notebook based framework allows user interaction and execution of individual modules as well as a full automatic processing of a complete survey observation. If nothing interrupts processing, we are able to reduce a single pointing survey observation on our five node cluster with 24 physical cores and 256 GB of memory each within 24h keeping up with the speed of the surveys. The quality of the generated images is sufficient for scientific usage for 44 % of the recorded data products with single images reaching dynamic ranges of several thousands. Future improvements will increase this percentage to over 80 %. Our design allowed development of the pipeline in parallel to the commissioning of the Apertif system.

We describe the APERture Tile In Focus (Apertif) system, a phased array feed (PAF) upgrade of the Westerbork Synthesis Radio Telescope which has transformed this telescope into a high-sensitivity, wide field-of-view L-band imaging and transient survey instrument. Using novel PAF technology, up to 40 partially overlapping beams can be formed on the sky simultaneously, significantly increasing the survey speed of the telescope. With this upgraded instrument, an imaging survey covering an area of 2300 deg2 is being performed which will deliver both continuum and spectral line data sets, of which the first data has been publicly released. In addition, a time domain transient and pulsar survey covering 15,000 deg2 is in progress. An overview of the Apertif science drivers, hardware and software of the upgraded telescope is presented, along with its key performance characteristics., Comment: 29 pages, 42 figures, accepted for publication by A&A

We present the serendipitous detection of the two main OH maser lines at 1667 and 1665 MHz associated with IRAS 10597+5926 at z = 0.19612 in the untargeted Apertif Wide-area Extragalactic Survey (AWES), and the subsequent measurement of the OH 1612 MHz satellite line in the same source. With a total OH luminosity of log(L/L_Sun) = 3.90 +/- 0.03, IRAS 10597+5926 is the fourth brightest OH megamaser (OHM) known. We measure a lower limit for the 1667/1612 ratio of R_1612 > 45.9 which is the highest limiting ratio measured for the 1612 MHz OH satellite line to date. OH satellite line measurements provide a potentially valuable constraint by which to compare detailed models of OH maser pumping mechanisms. Optical imaging shows the galaxy is likely a late-stage merger. Based on published infrared and far ultraviolet fluxes, we find that the galaxy is an ultra luminous infrared galaxy (ULIRG) with log(L_TIR/L_Sun) = 12.24, undergoing a star burst with an estimated star formation rate of 179 +/- 40 M_Sun/yr. These host galaxy properties are consistent with the physical conditions responsible for very bright OHM emission. Finally, we provide an update on the predicted number of OH masers that may be found in AWES, and estimate the total number of OH masers that will be detected in each of the individual main and satellite OH 18 cm lines., Comment: 9 pages, 4 figures. Accepted for publication in A&A

The recently discovered colliding-wind binary (CWB) Apep has been shown to emit luminously from radio to X-rays, with the emission driven by a binary composed of two Wolf-Rayet (WR) stars of one carbon-sequence (WC8) and one nitrogen-sequence (WN4-6b). Mid-infrared imaging revealed a giant spiral dust plume that is reminiscent of a pinwheel nebula but with additional features that suggest Apep is a unique system. We have conducted observations with the Australian Long Baseline Array to resolve Apep's radio emission on milliarcsecond scales, allowing us to relate the geometry of the wind-collision region to that of the spiral plume. The observed radio emission shows a bow-shaped structure, confirming its origin as a wind-collision region. The shape and orientation of this region is consistent with being originated by the two stars and with being likely dominated by the stronger wind of the WN4-6b star. This shape allowed us to provide a rough estimation of the opening angle of $\sim 150^\circ$ assuming ideal conditions. The orientation and opening angle of the emission also confirms it as the basis for the spiral dust plume. We also provide estimations for the two stars in the system to milliarcsecond precision. The observed radio emission, one order of magnitude brighter and more luminous than any other known non-thermal radio-emitting CWB, confirms it is produced by an extremely powerful wind collision. Such a powerful wind-collision region is consistent with Apep being a binary composed of two WR stars, so far the first unambiguously confirmed system of its kind., Comment: 9 pages, 3 figures, accepted for publication in MNRAS

Supermassive black holes at the centres of galaxies can cycle through periods of activity and quiescence. Characterising the duty cycle of active galactic nuclei is crucial for understanding the impact of the energy they release on the host galaxy. For radio AGN, this can be done by identifying dying (remnant) and restarted radio galaxies from their radio spectral properties. Using the combination of images at 1400 MHz produced by Apertif, the new phased-array feed receiver installed on the Westerbork Synthesis Radio Telescope, and images at 150 MHz provided by LOFAR, we have derived resolved spectral index images (at a resolution of ~15 arcsec) for all the sources within ~6 deg^2 area of the Lockman Hole region. We were able to select 15 extended radio sources with emission (partly or entirely) characterised by extremely steep spectral indices (steeper than 1.2). These objects represent radio sources in the remnant or the restarted phases of their life cycle. Our findings suggest this cycle to be relatively fast. They also show a variety of properties relevant for modelling the evolution of radio galaxies. For example, the restarted activity can occur while the remnant structure from a previous phase of activity is still visible. This provides constraints on the duration of the 'off' (dying) phase. In extended remnants with ultra-steep spectra at low frequencies, the activity likely stopped a few hundred megayears ago, and they correspond to the older tail of the age distribution of radio galaxies, in agreement with simulations of radio source evolution. We find remnant radio sources with a variety of structures (from double-lobed to amorphous), suggesting different types of progenitors. The present work sets the stage for exploiting low-frequency spectral index studies of extended sources by taking advantage of the large areas common to the LOFAR and the Apertif surveys., Comment: Accepted for publication in A&A. This paper is part of the 1st data release of the LoTSS Deep Fields. 17 pages, 10 figures

Massive outflows of neutral atomic hydrogen (HI) have been observed in absorption in a number of radio galaxies and are considered a signature of AGN feedback. These outflows on kpc-scales have not been investigated in great detail as they require high-angular resolution observations to be spatially resolved. In some radio AGN, they are likely the result of the radio jets interacting with the interstellar medium. We have used a global VLBI array to map the HI outflow in a small sample of young and restarted radio galaxies which we previously observed with the VLA and the WSRT at lower resolution. Here, we report on our findings for 4C52.37 and 3C293 and we discuss the sample including the previously published sources 4C12.50 and 3C236. For 4C52.37, we present the first-ever HI VLBI observation which recovered the majority of the outflowing HI gas in form of clouds towards the central 100pc of the AGN. The clouds are blue-shifted by up to 600km/s with respect to the systemic velocity. 3C293 is largely resolved out in our VLBI observation, but we detect, towards the VLBI core, some outflowing HI gas blueshifted with respect to the systemic velocity by up to 300km/s. We also find indications that the HI outflow is extended. Overall, we find that the fraction of HI gas recovered by our VLBI observation varies significantly within our sample. In all cases we find evidence for a clumpy structure of both the outflowing and the quiescent gas, consistent with predictions from numerical simulations. The outflows include at least a component of relatively compact clouds (10^4-10^5Msun) often observed already at a few tens of pc (in projection) from the core. We also find indications that the HI outflow might have a diffuse component, especially in larger sources. Our results support the interpretation that we observe these AGNs at different stages in the evolution of the jet-ISM interaction., Comment: 15 pages, 9 figures, accepted for publication in Astronomy & Astrophysics

Flares in radio-loud AGN are thought to be associated with the injection of fresh plasma into the compact jet base. Such flares are usually strongest and appear earlier at shorter radio wavelengths. Hence, VLBI at mm-wavelengths is best suited to study the earliest structural changes of compact jets associated with emission flares. We study the morphological changes of the parsec-scale jet in the nearby (z=0.049) gamma-ray bright radio galaxy 3C111 following a flare that developed into a major radio outburst in 2007. We analyse three successive observations of 3C111 at 86 GHz with the Global mm-VLBI Array (GMVA) between 2007 and 2008 which yield a very high angular resolution of ~45muas. In addition, we make use of single-dish radio flux density measurements from the F-GAMMA and POLAMI programmes, archival single-dish and VLBI data. We resolve the flare into multiple plasma components with a distinct morphology resembling a bend in an otherwise remarkably straight jet. The flare-associated features move with apparent velocities of ~4.0c to ~4.5c and can be traced also at lower frequencies in later epochs. Near the base of the jet, we find two bright features with high brightness temperatures up to ~10^11K, which we associate with the core and a stationary feature in the jet. The flare led to multiple new jet components indicative of a dynamic modulation during the ejection. We interpret the bend-like feature as a direct result of the outburst which makes it possible to trace the transverse structure of the jet. In this scenario, the components follow different paths in the jet stream consistent with expectations for a spine-sheath structure, which is not seen during intermediate levels of activity. The possibility of coordinated multiwavelength observations during a future bright radio flare in 3C111 makes this source an excellent target for probing the radio-gamma-ray connection., Comment: 11 pages, 7 figures, accepted for publication by Astronomy & Astrophysics

The propagation of radio waves from distant compact radio sources through turbulent interstellar plasma in our Galaxy causes these sources to twinkle, a phenomenon called interstellar scintillation. Such scintillations are a unique probe of the micro-arcsecond structure of radio sources as well as of the sub-AU-scale structure of the Galactic interstellar medium. Weak scintillations (i.e. an intensity modulation of a few percent) on timescales of a few days or longer are commonly seen at centimetre wavelengths and are thought to result from the line-of-sight integrated turbulence in the interstellar plasma of the Milky Way. So far, only three sources were known that show more extreme variations, with modulations at the level of some dozen percent on timescales shorter than an hour. This requires propagation through nearby (d <~10 pc) anomalously dense (n_e ~10^2 cm^-3) plasma clouds. Here we report the discovery with Apertif of a source (J1402+5347) showing extreme (~50%) and rapid variations on a timescale of just 6.5 minutes in the decimetre band (1.4 GHz). The spatial scintillation pattern is highly anisotropic, with a semi-minor axis of about 20,000 km. The canonical theory of refractive scintillation constrains the scattering plasma to be within the Oort cloud. The sightline to J1402+5347, however, passes unusually close to the B3 star Alkaid (eta UMa) at a distance of 32 pc. If the scintillations are associated with Alkaid, then the angular size of J1402+5347 along the minor axis of the scintels must be smaller than ~10 micro arcsec yielding an apparent brightness temperature for an isotropic source of >~ 10^ 14K. }, Comment: Accepted for Astronomy and Astrophysics Letters

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

We report the detection of a bright fast radio burst, FRB\,191108, with Apertif on the Westerbork Synthesis Radio Telescope (WSRT). The interferometer allows us to localise the FRB to a narrow $5\arcsec\times7\arcmin$ ellipse by employing both multibeam information within the Apertif phased-array feed (PAF) beam pattern, and across different tied-array beams. The resulting sight line passes close to Local Group galaxy M33, with an impact parameter of only 18\,kpc with respect to the core. It also traverses the much larger circumgalactic medium of M31, the Andromeda Galaxy. We find that the shared plasma of the Local Group galaxies could contribute $\sim$10\% of its dispersion measure of 588\,pc\,cm$^{-3}$. FRB\,191108 has a Faraday rotation measure of +474\,$\pm\,3$\,rad\,m$^{-2}$, which is too large to be explained by either the Milky Way or the intergalactic medium. Based on the more moderate RMs of other extragalactic sources that traverse the halo of M33, we conclude that the dense magnetised plasma resides in the host galaxy. The FRB exhibits frequency structure on two scales, one that is consistent with quenched Galactic scintillation and broader spectral structure with $\Delta\nu\approx40$\,MHz. If the latter is due to scattering in the shared M33/M31 CGM, our results constrain the Local Group plasma environment. We found no accompanying persistent radio sources in the Apertif imaging survey data.

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: Few Active Galactic Nuclei (AGN) reveal a double sided jet system. However, these are crucial to understand basic physical properties of extragalactic jets. Aims: We address the questions whether jets in AGN are symmetric in nature, how well they are collimated on small scales, and how do they evolve with time. Methods: We monitored the sub-parsec scale morphology of NGC1052 with the VLBA at 43GHz from 2005 to 2009. Results: A detailed study of 29 epochs show an asymmetry between both jets. A kinematic analysis of the outflows reveals higher apparent velocities for the eastern jet as compared to the western one: beta_ej = 0.529+-0.038 and beta_wj = 0.343+-0.037, respectively. Contradictory to previous studies, we find higher flux densities for the western jet. The distribution of brightness temperature and jet width features well collimated jets up to 1mas distance to the dynamic center and a nearly conical outflow further outwards. Combining flux density ratios and velocities of the jet flow, we were not able to find a combination of intrinsic velocity and inclination angle of the jets that is consistent for all observations, which is contradictory to symmetrically evolving jets. Spectral index maps between quasi-simultaneous 22GHz and 43GHz observations support the existence of an optically thick absorber covering the innermost 1.6 mas around the 43GHz central feature and an optically thin jet emission with a spectral index of <= -1. Conclusions: Our results fit into a picture in which we expect larger internal energy and/or magnetic flux in the western jet and higher kinetic energy in the eastern jet. Previous observations at lower frequencies found slower velocities of the moving jet features as compared to this work. Considering the different velocities at different areas we suggest a spine-sheath structure with a faster inner layer and a slower outer layer., Comment: Accepted by Astronomy & Astrophysics

The energetic feedback that is generated by radio jets in active galactic nuclei (AGNs) has been suggested to be able to produce fast outflows of atomic hydrogen (HI) gas that can be studied in absorption at high spatial resolution. We have used the Very Large Array (VLA) and a global very-long-baseline-interferometry (VLBI) array to locate and study in detail the HI outflow discovered with the Westerbork Synthesis Radio Telescope (WSRT) in the re-started radio galaxy 3C 236. We confirm, from the VLA data, the presence of a blue-shifted wing of the HI with a width of $\sim1000\mathrm{\,km\,s^{-1}}$. This HI outflow is partially recovered by the VLBI observation. In particular, we detect four clouds with masses of $0.28\text{-}1.5\times 10^4M_\odot$ with VLBI that do not follow the regular rotation of most of the HI. Three of these clouds are located, in projection, against the nuclear region on scales of $\lesssim 40\mathrm{\,pc}$, while the fourth is co-spatial to the south-east lobe at a projected distance of $\sim270\mathrm{\,pc}$. Their velocities are between $150$ and $640\mathrm{\,km\,s^{-1}}$ blue-shifted with respect to the velocity of the disk-related HI. These findings suggest that the outflow is at least partly formed by clouds, as predicted by some numerical simulations and originates already in the inner (few tens of pc) region of the radio galaxy. Our results indicate that all of the outflow could consist of many clouds with perhaps comparable properties as the ones detected, distributed also at larger radii from the nucleus where the lower brightness of the lobe does not allow us to detect them. However, we cannot rule out the presence of a diffuse component of the outflow. The fact that 3C 236 is a low excitation radio galaxy, makes it less likely that the optical AGN is able to produce strong radiative winds leaving the radio jet as the main driver for the HI outflow., Comment: 13 pages, 5 figures, accepted for publication in Astronomy & Astrophysics

PKS 1718$-$649 is one of the closest and most comprehensively studied candidates of a young active galactic nucleus (AGN) that is still embedded in its optical host galaxy. The compact radio structure, with a maximal extent of a few parsecs, makes it a member of the group of compact symmetric objects (CSO). Its environment imposes a turnover of the radio synchrotron spectrum towards lower frequencies, also classifying PKS 1718$-$649 as gigahertz-peaked radio spectrum (GPS) source. Its close proximity has allowed the first detection of extended X-ray emission in a GPS/CSO source with Chandra that is for the most part unrelated to nuclear feedback. However, not much is known about the nature of this emission. By co-adding all archival Chandra data and complementing these datasets with the large effective area of XMM-Newton, we are able to study the detailed physics of the environment of PKS 1718$-$649. Not only can we confirm that the bulk of the $\lesssim$kiloparsec-scale environment emits in the soft X-rays, but we also identify the emitting gas to form a hot, collisionally ionized medium. While the feedback of the central AGN still seems to be constrained to the inner few parsecs, we argue that supernovae are capable of producing the observed large-scale X-ray emission at a rate inferred from its estimated star formation rate., Comment: 5 pages, 2 figures, 2 tables, accepted for publication by A&A

We aim to better understand the dynamics within relativistic magneto-hydrodynamical flows in the extreme environment and close vicinity of supermassive black holes. To do so, we analyze the peculiar radio galaxy 3C 111, for which long-term polarimetric observations are available. We make use of the high spatial resolution of the VLBA network and the MOJAVE monitoring program, which provides high data quality also for single sources and allows us to study jet dynamics on parsec scales in full polarization with an evenly sampled time-domain. We additionally consider data from the IRAM 30-m Telescope as well as the SMA. Jet properties such as the electric vectors, the (polarized) flux density, feature size, and brightness temperature, describe a complex evolution of the polarized jet. The electric vector position angles (EVPAs) of features traveling down the jet perform a large and smooth rotation of $\gtrsim 180^{\circ}$ across a distance of about 20 pc. In contrast, the EVPAs are strongly variable within the first parsecs of the jet. We find a tendency towards transverse EVPAs across the jet with a local anomaly of aligned vectors in between. The transverse extent of the flow decreases coincident with a jump in brightness temperature around where we observe the EVPAs to turn into alignment with the jet flow. Also the gradients of the feature size and particle density with distance steepen in that region. We interpret the propagating polarized features with shocks and the observed local anomalies with the interaction of these shocks with a recollimation shock of the underlying flow. Together with a sheared magnetic field, this shock-shock interaction can explain the large rotation of the EVPA. The superimposed variability of the EVPAs close to the core is likely related to a clumpy Faraday screen, which also contributes significantly to the observed EVPA rotation in that region., Comment: 19 pages, 22 figures, 2 tables, accepted for publication by A&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 a summary of the campaign of remote observations that supported the European Space Agency's Rosetta mission. Telescopes across the globe (and in space) followed comet 67P/Churyumov-Gerasimenko from before Rosetta's arrival until nearly the end of mission in September 2016. These provided essential data for mission planning, large-scale context information for the coma and tails beyond the spacecraft, and a way to directly compare 67P with other comets. The observations revealed 67P to be a relatively `well behaved' comet, typical of Jupiter family comets and with activity patterns that repeat from orbit-to-orbit. Comparison between this large collection of telescopic observations and the in situ results from Rosetta will allow us to better understand comet coma chemistry and structure. This work is just beginning as the mission ends -- in this paper we present a summary of the ground-based observations and early results, and point to many questions that will be addressed in future studies., Comment: Author prepared version; final published version available at journal. 22 pages

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

The extragalactic VHE gamma-ray sky is rich in blazars. These are jetted active galactic nuclei viewed at a small angle to the line-of-sight. Only a handful of objects viewed at a larger angle are known so far to emit above 100 GeV. Multi-wavelength studies of such objects up to the highest energies provide new insights into the particle and radiation processes of active galactic nuclei. We report the results from the first multi-wavelength campaign observing the TeV detected nucleus of the active galaxy IC 310, whose jet is observed at a moderate viewing angle of 10 deg - 20 deg. The multi-instrument campaign was conducted between 2012 Nov. and 2013 Jan., and involved observations with MAGIC, Fermi, INTEGRAL, Swift, OVRO, MOJAVE and EVN. These observations were complemented with archival data from the AllWISE and 2MASS catalogs. A one-zone synchrotron self-Compton model was applied to describe the broad-band spectral energy distribution. IC 310 showed an extraordinary TeV flare at the beginning of the campaign, followed by a low, but still detectable TeV flux. Compared to previous measurements, the spectral shape was found to be steeper during the low emission state. Simultaneous observations in the soft X-ray band showed an enhanced energy flux state and a harder-when-brighter spectral shape behaviour. No strong correlated flux variability was found in other frequency regimes. The broad-band spectral energy distribution obtained from these observations supports the hypothesis of a double-hump structure. The harder-when-brighter trend in the X-ray and VHE emission is consistent with the behaviour expected from a synchrotron self-Compton scenario. The contemporaneous broad-band spectral energy distribution is well described with a one-zone synchrotron self-Compton model using parameters that are comparable to those found for other gamma-ray-emitting misaligned blazars., Comment: 15 pages, 10 figures. Accepted for publication in A&A

Context. The majority of bright extragalactic gamma-ray sources are blazars. Only a few radio galaxies have been detected by Fermi/LAT. Recently, the GHz-peaked spectrum source PKS 1718-649 was confirmed to be gamma-ray bright, providing further evidence for the existence of a population of gamma-ray loud, compact radio galaxies. A spectral turnover in the radio spectrum in the MHz to GHz range is a characteristic feature of these objects, which are thought to be young due to their small linear sizes. The multiwavelength properties of the gamma-ray source PMN J1603-4904 suggest that it is a member of this source class. Aims. The known radio spectrum of PMN J1603-4904 can be described by a power law above 1 GHz. Using observations from the Giant Metrewave Radio Telescope (GMRT) at 150, 325, and 610 MHz, we investigate the behaviour of the spectrum at lower frequencies to search for a low-frequency turnover. Methods. Data from the TIFR GMRT Sky Survey (TGSS ADR) catalogue and archival GMRT observations were used to construct the first MHz to GHz spectrum of PMN J1603-4904. Results. We detect a low-frequency turnover of the spectrum and measure the peak position at about 490 MHz (rest-frame), which, using the known relation of peak frequency and linear size, translates into a maximum linear source size of ~1.4 kpc. Conclusions. The detection of the MHz peak indicates that PMN J1603-4904 is part of this population of radio galaxies with turnover frequencies in the MHz to GHz regime. Therefore it can be considered the second, confirmed object of this kind detected in gamma-rays. Establishing this gamma-ray source class will help to investigate the gamma-ray production sites and to test broadband emission models., Comment: 5 pages, A&A (Letters) accepted

Near-Eddington accretion rates onto low-mass black holes are thought to be a prime driver of the multi-wavelength properties of the narrow-line Seyfert 1 (NLS1) population of active galactic nuclei (AGN). Orientation effects have repeatedly been considered as another important factor involved, but detailed studies have been hampered by the lack of measured viewing angles towards this type of AGN. Here we present multi-epoch, 15 GHz VLBA images (MOJAVE program) of the radio-loud and Fermi/LAT-detected NLS1 galaxy 1H 323+342. These are combined with single-dish, multi-frequency radio monitoring of the source's variability, obtained with the Effelsberg 100-m and IRAM 30-m telescopes, in the course of the F-GAMMA program. The VLBA images reveal 6 components with apparent peeds of ~1 to ~7 c, and one quasi-stationary feature. Combining the obtained apparent jet speed ($\beta_{app}$) and variability Doppler factor ($D_{var}$) estimates together with other methods, we constrain the viewing angle towards 1H 0323+342 to $\theta \leq 4 - 13$ deg. Using literature values of $\beta_{app}$ and $D_{var}$, we also deduce a viewing angle of $\leq$ 8-9 deg towards another radio- and {\gamma}-ray-loud NLS1, namely SBS 0846+513., Comment: 16 pages, 3 figures, accepted for publication in Research in Astronomy and Astrophysics

While one of the IAU's missions is to "serve as the internationally recognized authority for assigning designations to celestial bodies and surface features on them", the participation of the public in the naming of celestial objects has been a little-known, but decade-long tradition of the IAU. While reiterating its opposition to having the public pay to give a name to an exoplanet, the IAU Executive Committee nonetheless recognized the right of organizations to invite public, international exoplanet naming or voting campaigns. To this end, clear selection rules were to be defined by the IAU, inviting mutual collaboration, the goal being to sanction the campaign and officially approve the resulting names, for the sake of boosting the public's interest in astronomy and at the same time reaffirm the authority of the IAU. In no way were these names supposed to supersede the designations in use by professional astronomers. Since the field of exoplanet research was still "virgin" for naming but is now becoming scientifically mature (with hundreds of well-characterized, confirmed objects) and so appealing to the public's interest and imagination, IAU decided to organize a naming contest as the leader of a new project with a potentially high worldwide impact, the first since the International Year of Astronomy in 2009, called "NameExoWorlds"., Comment: Transactions IAU, Volume XXIXA, Proc. XXIXA IAU General Assembly (Honolulu, USA), August 2015, Cambridge University Press

Supermassive black holes (SMBH) are essential for the production of jets in radio-loud active galactic nuclei (AGN). Theoretical models based on Blandford & Znajek extract the rotational energy from a Kerr black hole, which could be the case for NGC1052, to launch these jets. This requires magnetic fields of the order of $10^3\,$G to $10^4\,$G. We imaged the vicinity of the SMBH of the AGN NGC1052 with the Global Millimetre VLBI Array and found a bright and compact central feature, smaller than 1.9 light days (100 Schwarzschild radii) in radius. Interpreting this as a blend of the unresolved jet bases, we derive the magnetic field at 1 Schwarzschild radius to lie between 200 G and ~80000 G consistent with Blandford & Znajek models., Comment: 8 pages, 7 figures, accepted by Astronomy & Astrophysics

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

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

Very-high-energy $\gamma$-ray observations of the active galaxy IC 310 with the MAGIC telescopes have revealed fast variability with doubling time scales of less than 4.8min. This implies that the emission region in IC 310 is smaller than 20% of the gravitational radius of the central supermassive black hole with a mass of $3\times 10^8 M_\odot$, which poses serious questions on the emission mechanism and classification of this enigmatic object. We report on the first quasi-simultaneous multi-frequency VLBI observations of IC 310 conducted with the EVN. We find a blazar-like one-sided core-jet structure on parsec scales, constraining the inclination angle to be less than $\sim 20^\circ$ but very small angles are excluded to limit the de-projected length of the large-scale radio jet., Comment: 4 pages, proceedings of the 12th European VLBI Network Symposium and Users Meeting - EVN 2014, 7-10 October 2014, Cagliari, Italy. Published online in PoS, ID.109