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We present 1 - 12 GHz Karl G. Jansky Very Large Array observations of 9 off-nuclear persistent radio sources (PRSs) in nearby (z < 0.055) dwarf galaxies, along with high-resolution European very-long baseline interferometry (VLBI) Network (EVN) observations for one of them at 1.7GHz. We explore the plausibility that these PRSs are associated with fast radio burst (FRB) sources by examining their properties, physical sizes, host-normalized offsets, spectral energy distributions (SEDs), radio luminosities, and light curves, and compare them to those of the PRSs associated with FRBs 20121102A and 20190520B, two known active galactic nuclei (AGN), and one likely AGN in our sample with comparable data, as well as other radio transients exhibiting characteristics analogous to FRB-PRSs. We identify a single source in our sample, J1136+2643, as the most promising FRB- PRS, based on its compact physical size and host-normalized offset. We further identify two sources, J0019+1507 and J0909+5955, with physical sizes comparable to FRB-PRSs, but which exhibit large offsets and flat spectral indices potentially indicative of a background AGN origin. We test the viability of neutron star wind nebulae and hypernebulae models for J1136+2643, and find that the physical size, luminosity, and SED of J1136+2643 are broadly consistent with these models. Finally, we discuss the alternative interpretation that the radio sources are instead powered by accreting massive black holes and outline future prospects and follow-up observations for differentiating between these scenarios., Comment: 25 pages, 7 figures, 3 tables

The existence of accreting supermassive black holes up to billions of solar masses at early cosmological epochs (in the context of this work, redshifts z>=6) requires very fast growth rates which is challenging to explain. The presence of a relativistic jet can be a direct indication of activity and accretion status in active galactic nuclei (AGN), constraining the radiative properties of these extreme objects. However, known jetted AGN beyond z~6 are still very rare. The radio-emitting AGN J2331+1129 has recently been claimed as a candidate BL Lac object at redshift z=6.57, based on its synchrotron-dominated emission spectrum and the lack of ultraviolet/optical emission lines. It is a promising candidate for the highest-redshift blazar known to date. The aim of the observations described here was to support or refute the blazar classification of this peculiar source. We performed high-resolution radio interferometric imaging observations of J2331+1129 using the Very Long Baseline Array at 1.6 and 4.9 GHz in 2022 Feb. The images revealed a compact but slightly resolved, flat-spectrum core feature at both frequencies, indicating that the total radio emission is produced by a compact jet and originates from within a central 10-pc scale region. While these are consistent with the radio properties of a BL Lac object, the inferred brightness temperatures are at least an order of magnitude lower than expected from a Doppler-boosted radio jet, leaving the high-redshift BL Lac identification still an open question., Comment: 5 pages, 1 figure, accepted for publication in Astronomy & Astrophysics as a Letter to the Editor

PSO J334.2028+1.4075 (PSO J334) is a luminous quasar located at redshift z=2.06. The source gained attention when periodic flux density variations were discovered in its optical light curve. These variations were initially interpreted as the variability due to the orbital motion of a supermassive black hole binary (SMBHB) residing in a single circumbinary accretion disk. However, subsequent multiwavelength observations provided evidence against the binary hypothesis as no optical periodicity was found on extended time baselines. On the other hand, detailed radio analysis with the Karl G. Jansky Very Large Array (VLA) and the Very Long Baseline Array (VLBA) revealed a lobe-dominated quasar at kpc scales, and possibly a precessing jet, which could retain PSO J334 as a binary SMBH candidate. We aim to study both the large- and small-scale radio structures in PSO J334 to provide additional evidence for or against the binary scenario. We observed the source at 1.7 GHz with the European Very Long Baseline Interferometry Network (EVN), and at 1.5 and 6.2 GHz with the VLA, at frequencies that complement the previous radio interferometric study. Our images reveal a single component at parsec scales slightly resolved in the southeast-northwest direction and a lobe-dominated quasar at kiloparsec scales with a complex structure. The source morphology and polarization in our VLA maps suggest that the jet is interacting with dense clumps of the ambient medium. While we also observe a misalignment between the inner jet and the outer lobes, we suggest that this is due to the restarted nature of the radio jet activity and the possible presence of a warped accretion disk rather than due to the perturbing effects of a companion SMBH. Our analysis suggests that PSO J334 is most likely a jetted AGN with a single SMBH, and there is no clear evidence of a binary SMBH system in its central engine.

X-ray binaries are known to show state transitions related to accretion rate changes which are often accompanied with dramatic changes in the jet emission. However, it is not clear whether this characteristics of stellar-mass black hole systems can be scaled up to the accretion disk of active galactic nuclei. The Seyfert 1 galaxy, KUG 1141+371 has been showing a steadily increasing X-ray flux since 2007, and exhibited variability behaviour similar to the state transitions observed in X-ray binaries. It was hypothesised to undergo a rapid boost of mass accretion. If the X-ray binary analogy holds then the appearance of jet emission can also be expected in KUG 1141+371. While the source was not detected in the Faint Images of the Radio Sky at Twenty-centimeters in 1994, it appears in the VLA Sky Survey in 2019 and at 22 GHz in a VLA observation in 2018 at mJy flux density level. Our VLBI observations revealed a compact, flat-spectrum radio feature. Its high brightness temperature indicates the radio emission originates from an AGN., Comment: 5 pages, 2 figures; to appear in the proceedings of the 15th European VLBI Network Symposium and Users' Meeting (EVN2022), 11-15 July 2022, University College Cork, Ireland

The powerful high-redshift quasar J2102+6015 (at z=4.575) may provide useful information for studying supermassive black hole growth, galaxy evolution and feedback in the early Universe. The source has so far been imaged with very long baseline interferometry (VLBI) at 2/8 GHz (S/X) bands only, showing complex compact structure. Its total radio spectrum peaks at ~6 GHz in the rest frame. There is no sign of Doppler-boosted jet emission, and the separation of the two major features in its east-west oriented structure spanning ~10 milliarcsec does not change significantly on a timescale longer than a decade. However, VLBI astrometric monitoring observations suggest quasi-periodic (~3 yr) variation in its absolute position. J2102+6015 is presumably a young radio source with jets misaligned with respect to the line of sight. Here we briefly report on our new high-resolution imaging observations made with the European VLBI Network (EVN) at 5 and 22 GHz frequencies in 2021 June, and give an overview of what is currently known about this peculiar distant jetted active galactic nucleus., Comment: 8 pages, 2 figures; submitted to the proceedings of the 15th European VLBI Network Symposium and Users' Meeting (EVN2022), 11-15 July 2022, University College Cork, Ireland

Active galactic nuclei (AGN) have been observed as far as redshift z~7. They are crucial in investigating the early Universe as well as the growth of supermassive black holes at their centres. Radio-loud AGN with their jets seen at a small viewing angle are called blazars and show relativistic boosting of their emission. Thus, their apparently brighter jets are easier to detect in the high-redshift Universe. DES J014132.4-542749.9 is a radio-luminous but X-ray weak blazar candidate at z = 5. We conducted high-resolution radio interferometric observations of this source with the Australian Long Baseline Array at 1.7 and 8.5 GHz. A single, compact radio emitting feature was detected at both frequencies with a flat radio spectrum. We derived the milliarcsecond-level accurate position of the object. The frequency dependence of its brightness temperature is similar to that of blazar sources observed at lower redshifts. Based on our observations, we can confirm its blazar nature. We compared its radio properties with those of two other similarly X-ray-weak and radio-bright AGN, and found that they show very different relativistic boosting characteristics., Comment: 8 pages, 4 figures, accepted for publication in PASA

We present the results from multi-wavelength observations of a transient discovered during the follow-up of S191213g, a gravitational wave (GW) event reported by the LIGO-Virgo Collaboration as a possible binary neutron star merger in a low latency search. This search yielded SN2019wxt, a young transient in a galaxy whose sky position (in the 80\% GW contour) and distance ($\sim$150\,Mpc) were plausibly compatible with the localisation uncertainty of the GW event. Initially, the transient's tightly constrained age, its relatively faint peak magnitude ($M_i \sim -16.7$\,mag) and the $r-$band decline rate of $\sim 1$\,mag per 5\,days appeared suggestive of a compact binary merger. However, SN2019wxt spectroscopically resembled a type Ib supernova, and analysis of the optical-near-infrared evolution rapidly led to the conclusion that while it could not be associated with S191213g, it nevertheless represented an extreme outcome of stellar evolution. By modelling the light curve, we estimated an ejecta mass of $\sim 0.1\,M_\odot$, with $^{56}$Ni comprising $\sim 20\%$ of this. We were broadly able to reproduce its spectral evolution with a composition dominated by helium and oxygen, with trace amounts of calcium. We considered various progenitors that could give rise to the observed properties of SN2019wxt, and concluded that an ultra-stripped origin in a binary system is the most likely explanation. Disentangling electromagnetic counterparts to GW events from transients such as SN2019wxt is challenging: in a bid to characterise the level of contamination, we estimated the rate of events with properties comparable to those of SN2019wxt and found that $\sim 1$ such event per week can occur within the typical GW localisation area of O4 alerts out to a luminosity distance of 500\,Mpc, beyond which it would become fainter than the typical depth of current electromagnetic follow-up campaigns., Comment: By the ENGRAVE collaboration (engrave-eso.org). 35 pages, 20 figures, final version accepted by A&A

The nature of jets in active galactic nuclei (AGN) in the early Universe and their feedback to the host galaxy remain a highly topical question. Observations of the radio structure of high-redshift AGNs enabled by very long baseline interferometry (VLBI) provide indispensable input into studies of their properties and role in the galaxies' evolution. Up to now, only five AGNs at redshift $z > 6$ have been studied with the VLBI technique. VIKJ2318-3113 is a recently discovered quasar at z = 6.44 that has not been imaged with VLBI before the current work. Here we present the first VLBI imaging results of this high-redshift quasar, with the aim of corroborating its high-resolution appearance with the physical model of the object. We carried out VLBI phase-referencing observations of VIKJ2318-3113 using the Very Long Baseline Array at two frequencies, 1.6 and 4.7 GHz, and obtained the first view at the radio structure on the milliarcsecond scale. The source was clearly detected at 1.6 GHz. We found that almost all of its radio emission comes from the pc-scale core region. Our dual-frequency observations constrain the spectral index and brightness temperature of the radio core. Its properties are similar to those of other known high-redshift radio-loud AGNs., Comment: 6 pages, 2 figures, accepted by A&A

Fast Radio Bursts (FRBs) are extremely luminous and brief signals (with duration of milliseconds or even shorter) of extragalactic origin. Despite the fact that hundreds of FRBs have been discovered to date, their nature still remains unclear. Precise localizations of FRBs can unveil their host galaxies and local environments -- and thus shed light on the physical processes that led to the burst production. However, this has only been achieved for a few FRBs to date. The European VLBI Network (EVN) is currently the only instrument capable of localizing FRBs down to the milliarcsecond level. This level of precision was critical to associate the first localized FRB, 20121102A, to a star-forming region in a low-metallicity dwarf galaxy and physically related it to a compact persistent radio source. Analogously, a second repeating FRB, 20180916B, was found to just outside the edge of a prominent star-forming region of a nearby spiral galaxy. The PRECISE project (Pinpointing REpeating ChIme Sources with EVN dishes), starting from 2019, has observed hundreds of hours per year with a subset of EVN telescopes with the goal of localizing repeating FRBs discovered by the CHIME/FRB Collaboration. The ultimate goal of PRECISE is to disentangling the environments where FRBs can be produced. Here we present the state of the art of the FRB field, the PRECISE project, and the localizations achieved until now, which have unveiled a variety of environments where FRBs can be found that challenges the current models., Comment: 8 pages, 1 figure, to appear in the proceedings of the European VLBI Network Mini-Symposium and Users' Meeting 2021, Proceedings of Science, PoS(EVN2021)035

We study milliarcsecond-scale properties of the persistent radio counterpart to FRB 121102 and investigate the spectro-polarimetric properties of a bright burst. For the former, we use European VLBI Network (EVN) observations in 2017 at 1.7 and 4.8 GHz. For the latter, we re-analyse the 1.7-GHz data from the 100-m Effelseberg telescope taken in 2016. These observations predate other polarimetric studies of FRB 121102, and yield the highest burst Faraday rotation measure (RM) to date, RM = 1.27*10^5 rad m^-2, consistent with the decreasing RM trend. The fractional polarization of the burst emission is 15% at 1.7 GHz. This can be reconciled with the high fractional polarization at higher frequencies if the Faraday width of the burst environment is 150 rad m^-2 - a bare 0.1% of the total Faraday rotation. The width may originate from minor non-uniformities in the Faraday screen, or from effects in the emitting region itself. The upper limit on the persistent source size is 1 pc, barely consistent with a young supernova (SN) scenario. The flux variability limit of <10% is not in favor of the young SN scenario, and challenges other interpretations as well. The fractional polarization of the faint persistent source is constrained at <25% at 4.8 GHz ruling out a common origin with the highly polarized individual bursts., Comment: 9 pages, 8 figures; accepted to MNRAS

Very long baseline interferometric (VLBI) localisations of repeating fast radio bursts (FRBs) have demonstrated a diversity of local environments: from nearby star-forming regions to globular clusters. Here we report the VLBI localisation of FRB 20201124A using an ad-hoc array of dishes that also participate in the European VLBI Network (EVN). In our campaign, we detected 18 total bursts from FRB 20201124A at two separate epochs. By combining the visibilities from both observing epochs, we were able to localise FRB 20201124A with a 1-$\sigma$ error of 4.5 milliarcseconds (mas). We use the relatively large burst sample to investigate astrometric accuracy, and find that for $\gtrsim20$ baselines ($\gtrsim7$ dishes) that we can robustly reach milliarcsecond precision even using single-burst data sets. Sub-arcsecond precision is still possible for single bursts, even when only $\sim$ six baselines (four dishes) are available. We explore two methods for determining the individual burst positions: the peaks of the dirty maps and a Gaussian fit to the cross fringe pattern on the dirty maps. We found the latter to be more reliable due to the lower mean and standard deviation in the offsets from the FRB position. Our VLBI work places FRB 20201124A 705$\pm$26 mas (1-$\sigma$ errors) from the optical centre of the host galaxy, and consistent with originating from within the recently-discovered extended radio structure associated with star-formation in the host galaxy. Future high-resolution optical observations, e.g. with Hubble Space Telescope, can determine the proximity of our FRB 20201124A VLBI position to nearby knots of star formation., Comment: submitted, comments welcome

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

Double-peaked emission line AGN (DPAGN) have been regarded as binary black hole candidates. We present here results from parsec-scale radio observations with the Very Long Baseline Array (VLBA) of five DPAGN belonging to the KISSR sample of emission-line galaxies. This work concludes our pilot study of nine type 2 Seyfert and LINER DPAGN from the KISSR sample. In the nine sources, dual compact cores are only detected in the "offset AGN", KISSR 102. The overall incidence of jets however, in the eight sources detected with the VLBA, is $\ge$60%. We find a difference in the "missing flux density" going from the Very Large Array (VLA) to VLBA scales between Seyferts and LINERs, with LINERs showing less missing flux density on parsec-scales. Using the emission-line modeling code, MAPPINGS III, we find that the emission lines are likely to be influenced by jets in 5/9 sources. Jet-medium interaction is the likely cause of the emission-line splitting observed in the SDSS spectra of these sources. Jets in radio-quiet AGN are therefore energetically capable of influencing their parsec- and kpc-scale environments, making them agents of "radio AGN feedback", similar to radio-loud AGN., Comment: 25 pages, 9 tables, 14 figures, accepted for publication in ApJ

Fast radio bursts (FRBs) are extragalactic radio flashes of unknown physical origin. Their high luminosities and short durations require extreme energy densities, like those found in the vicinity of neutron stars and black holes. Studying the burst intensities and polarimetric properties on a wide range of timescales, from milliseconds down to nanoseconds, is key to understanding the emission mechanism. However, high-time-resolution studies of FRBs are limited by their unpredictable activity levels, available instrumentation and temporal broadening in the intervening ionised medium. Here we show that the repeating FRB 20200120E can produce isolated shots of emission as short as about 60 nanoseconds in duration, with brightness temperatures as high as $3\times 10^{41}$ K (excluding relativistic effects), comparable to `nano-shots' from the Crab pulsar. Comparing both the range of timescales and luminosities, we find that FRB 20200120E observationally bridges the gap between known Galactic young pulsars and magnetars, and the much more distant extragalactic FRBs. This suggests a common magnetically powered emission mechanism spanning many orders of magnitude in timescale and luminosity. In this work, we probe a relatively unexplored region of the short-duration transient phase space; we highlight that there likely exists a population of ultra-fast radio transients at nanosecond to microsecond timescales, which current FRB searches are insensitive to., Comment: Submitted. Comments welcome

Fast radio bursts (FRBs) are exceptionally luminous flashes of unknown physical origin, reaching us from other galaxies (Petroff et al. 2019). Most FRBs have only ever been seen once, while others flash repeatedly, though sporadically (Spitler et al. 2016, CHIME/FRB Collaboration et al. 2021). Many models invoke magnetically powered neutron stars (magnetars) as the engines producing FRB emission (Margalit & Metzger 2018, CHIME/FRB Collaboration et al. 2020). Recently, CHIME/FRB announced the discovery (Bhardwaj et al. 2021) of the repeating FRB 20200120E, coming from the direction of the nearby grand design spiral galaxy M81. Four potential counterparts at other observing wavelengths were identified (Bhardwaj et al. 2021) but no definitive association with these sources, or M81, could be made. Here we report an extremely precise localisation of FRB 20200120E, which allows us to associate it with a globular cluster (GC) in the M81 galactic system and to place it ~2pc offset from the optical center of light of the GC. This confirms (Bhardwaj et al. 2021) that FRB 20200120E is 40 times closer than any other known extragalactic FRB. Because such GCs host old stellar populations, this association strongly challenges FRB models that invoke young magnetars formed in a core-collapse supernova as powering FRB emission. We propose, instead, that FRB 20200120E is a highly magnetised neutron star formed via either accretion-induced collapse of a white dwarf or via merger of compact stars in a binary system (Margalit et al. 2019). Alternative scenarios involving compact binary systems, efficiently formed inside globular clusters, could also be responsible for the observed bursts., Comment: Submitted. Comments welcome

The Arecibo Observatory (AO) is a multidisciplinary research and education facility that is recognized worldwide as a leading facility in astronomy, planetary, and atmospheric and space sciences. AO's cornerstone research instrument was the 305-m William E. Gordon telescope. On December 1, 2020, the 305-m telescope collapsed and was irreparably damaged. In the three weeks following the collapse, AO's scientific and engineering staff and the AO users community initiated extensive discussions on the future of the observatory. The community is in overwhelming agreement that there is a need to build an enhanced, next-generation radar-radio telescope at the AO site. From these discussions, we established the set of science requirements the new facility should enable. These requirements can be summarized briefly as: 5 MW of continuous wave transmitter power at 2 - 6 GHz, 10 MW of peak transmitter power at 430 MHz (also at 220MHz under consideration), zenith angle coverage 0 to 48 deg, frequency coverage 0.2 to 30 GHz and increased Field-of-View. These requirements determine the unique specifications of the proposed new instrument. The telescope design concept we suggest consists of a compact array of fixed dishes on a tiltable, plate-like structure with a collecting area equivalent to a 300m dish. This concept, referred to as the Next Generation Arecibo Telescope (NGAT), meets all of the desired specifications and provides significant new science capabilities to all three research groups at AO. This whitepaper presents a sample of the wide variety of the science that can be achieved with the NGAT, the details of the telescope design concept and the need for the new telescope to be located at the AO site. We also discuss other AO science activities that interlock with the NGAT in the white paper., Comment: 82 pages (executive summary 10 pages), 21 figures, Arecibo observatory white paper (Updated with the complete author list and minor edits)

Broad Balmer emission lines in active galactic nuclei (AGN) may display dramatic changes in amplitude, even disappearance and re-appearance in some sources. As a nearby galaxy at a redshift of z = 0.0264, Mrk 590 suffered such a cycle of Seyfert type changes between 2006 and 2017. Over the last fifty years, Mrk 590 also underwent a powerful continuum outburst and a slow fading from X-rays to radio wavelengths with a peak bolometric luminosity reaching about ten per cent of the Eddington luminosity. To track its past accretion and ejection activity, we performed very long baseline interferometry (VLBI) observations with the European VLBI Network (EVN) at 1.6 GHz in 2015. The EVN observations reveal a faint (~1.7 mJy) radio jet extending up to ~2.8 mas (projected scale ~1.4 pc) toward north, and probably resulting from the very intensive AGN activity. To date, such a parsec-scale jet is rarely seen in the known changing-look AGN. The finding of the faint jet provides further strong support for variable accretion as the origin of the type changes in Mrk 590., Comment: 5 pages, 1 figure, accepted for publication in MNRAS Letters

FRB 20180916B is a well-studied repeating fast radio burst source. Its proximity (~150 Mpc), along with detailed studies of the bursts, have revealed many clues about its nature -- including a 16.3-day periodicity in its activity. Here we report on the detection of 18 bursts using LOFAR at 110-188 MHz, by far the lowest-frequency detections of any FRB to date. Some bursts are seen down to the lowest-observed frequency of 110 MHz, suggesting that their spectra extend even lower. These observations provide an order-of-magnitude stronger constraint on the optical depth due to free-free absorption in the source's local environment. The absence of circular polarization and nearly flat polarization angle curves are consistent with burst properties seen at 300-1700 MHz. Compared with higher frequencies, the larger burst widths (~40-160 ms at 150 MHz) and lower linear polarization fractions are likely due to scattering. We find ~2-3 rad/m^2 variations in the Faraday rotation measure that may be correlated with the activity cycle of the source. We compare the LOFAR burst arrival times to those of 38 previously published and 22 newly detected bursts from the uGMRT (200-450 MHz) and CHIME/FRB (400-800 MHz). Simultaneous observations show 5 CHIME/FRB bursts when no emission is detected by LOFAR. We find that the burst activity is systematically delayed towards lower frequencies by ~3 days from 600 MHz to 150 MHz. We discuss these results in the context of a model in which FRB 20180916B is an interacting binary system featuring a neutron star and high-mass stellar companion., Comment: Accepted for publication by ApJL

Fast radio bursts (FRBs) are bright, coherent, short-duration radio transients of as-yet unknown extragalactic origin. FRBs exhibit a wide variety of spectral, temporal and polarimetric properties, which can unveil clues into their emission physics and propagation effects in the local medium. Here we present the high-time-resolution (down to 1 $\mu$s) polarimetric properties of four 1.7-GHz bursts from the repeating FRB 20180916B, which were detected in voltage data during observations with the European VLBI Network (EVN). We observe a range of emission timescales spanning three orders of magnitude, with the shortest component width reaching 3-4 $\mu$s (below which we are limited by scattering). This is the shortest timescale measured in any FRB, to date. We demonstrate that all four bursts are highly linearly polarised ($\gtrsim 80\%$), show no evidence for significant circular polarisation ($\lesssim 15\%$), and exhibit a constant polarisation position angle (PPA) during and between bursts. On short timescales ($\lesssim 100$ $\mu$s), however, there appear to be subtle (few degree) PPA variations across the burst profiles. These observational results are most naturally explained in an FRB model where the emission is magnetospheric in origin, as opposed to models where the emission originates at larger distances in a relativistic shock., Comment: Submitted; comments welcome

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

The merging of supermassive black holes (SMBHs) is a direct consequence of our hierarchical picture of galaxy evolution. It is difficult to track the merging process of SMBHs during mergers of galaxies as SMBHs are naturally difficult to observe. We want to characterise and confirm the presence of two independent active galactic nuclei (AGN) separated by a few kiloparsec in seven strongly interacting galaxies previously selected from the Sloan Digital Sky Survey (SDSS) as Seyfert-Seyfert pairs based on emission-line ratio diagnostics. Optical slit spectra taken with MODS at the Large Binocular Telescope (LBT) are presented to infer the detailed spatial distribution of optical emission lines, and their line ratios and AGN signatures with respect to the host galaxies, thereby quantifying the impact of beam smearing and large fibre apertures on the spectra captured by the SDSS. We find that at most two of the seven targets actually retain a Seyfert-Seyfert dual AGN, whereas the others may be more likely powered by post-AGB stars in retired galaxies or through shocks in the ISM based on spatially resolved optical line diagnostics. The major cause of this discrepancy is a bias caused by the spillover of flux from the primary source in the secondary SDSS fibre which can be more than an order of magnitude at <3" separations. Previously reported extremely low X-ray-to-[\ion{O}{iii}] luminosity ratios may be explained by this misclassification, as can heavily obscured AGN for the primaries. We also find that the nuclei with younger stellar ages host the primary AGN. Studies of close dual AGN selected solely from fibre-based spectroscopy can create severe biases in the sample selection and interpretation of the results. Spatially resolved spectroscopy should ideally be used in the future to characterise such compact systems together with multi-wavelength follow-up observations., Comment: 15 pages, 8 figures, A&A in press

Fast radio bursts (FRBs) are bright, millisecond-duration radio transients originating from extragalactic distances. Their origin is unknown. Some FRB sources emit repeat bursts, ruling out cataclysmic origins for those events. Despite searches for periodicity in repeat burst arrival times on time scales from milliseconds to many days, these bursts have hitherto been observed to appear sporadically, and though clustered, without a regular pattern. Here we report the detection of a $16.35\pm0.15$ day periodicity (or possibly a higher-frequency alias of that periodicity) from a repeating FRB 180916.J0158+65 detected by the Canadian Hydrogen Intensity Mapping Experiment Fast Radio Burst Project (CHIME/FRB). In 38 bursts recorded from September 16th, 2018 through February 4th, 2020, we find that all bursts arrive in a 5-day phase window, and 50% of the bursts arrive in a 0.6-day phase window. Our results suggest a mechanism for periodic modulation either of the burst emission itself, or through external amplification or absorption, and disfavour models invoking purely sporadic processes.

We report the detection of an intriguing parsec-scale radio source in the "offset AGN" candidate, KISSR 102. The elliptical host galaxy includes two optical nuclei at a projected separation of 1.54 kpc, N1 and N2, to the south-east and north-west, respectively. Phase-referenced VLBA observations at 1.5 and 4.9 GHz of this LINER galaxy, have detected double radio components (A and B) at a projected separation of 4.8 parsec at 1.5 GHz, and another partially-resolved double radio structure at 4.9 GHz coincident with the brighter radio component A. These radio detections are confined to the optical nucleus N1. The brightness temperatures of all the detected radio components are high, $\gtrsim10^8$ K, consistent with them being components of a radio AGN. The 1.5-4.9 GHz spectral index is inverted ($\alpha\sim+0.64\pm0.08$) for component A and steep for component B ($\alpha \lesssim-1.6$). The dramatic change in the spectral indices of A and B is inconsistent with it being a typical "core-jet" structure from a single AGN or the mini-lobes of a compact symmetric object. To be consistent with a "core-jet" structure, the jet in KISSR 102 would need to be undergoing strong jet-medium interaction with dense surrounding media resulting in a drastic spectral steepening of the jet. Alternatively, the results could be consistent with the presence of a parsec-scale binary radio AGN, which is the end result of a three-body interaction involving three supermassive black holes in the centre of KISSR 102., Comment: 11 pages, 6 figures, ApJ accepted

Fast radio bursts (FRBs) are brief, bright, extragalactic radio flashes. Their physical origin remains unknown, but dozens of possible models have been postulated. Some FRB sources exhibit repeat bursts. Though over a hundred FRB sources have been discovered to date, only four have been localised and associated with a host galaxy, with just one of the four known to repeat. The properties of the host galaxies, and the local environments of FRBs, provide important clues about their physical origins. However, the first known repeating FRB has been localised to a low-metallicity, irregular dwarf galaxy, and the apparently non-repeating sources to higher-metallicity, massive elliptical or star-forming galaxies, suggesting that perhaps the repeating and apparently non-repeating sources could have distinct physical origins. Here we report the precise localisation of a second repeating FRB source, FRB 180916.J0158+65, to a star-forming region in a nearby (redshift $z = 0.0337 \pm 0.0002$) massive spiral galaxy, whose properties and proximity distinguish it from all known hosts. The lack of both a comparably luminous persistent radio counterpart and a high Faraday rotation measure further distinguish the local environment of FRB 180916.J0158+65 from that of the one previously localised repeating FRB source, FRB 121102. This demonstrates that repeating FRBs have a wide range of luminosities, and originate from diverse host galaxies and local environments., Comment: 61 pages, 11 figures, 3 tables. Published in Nature

Pulsars typically exhibit radio emission in the form of narrow pulses originated from confined regions of their magnetospheres. A potential presence of magnetospherically originated emission outside this region, the so-called off-pulse emission, would challenge the existing theories. Detection of significant off-pulse emission has been reported so far from only two pulsars, B0525+21 and B2045-16, at 325 and 610 MHz. However, the nature of this newly uncovered off-pulse emission remains unclear. To probe its origin we conducted very high resolution radio observations of B0525+21 and B2045-16 with the European VLBI Network (EVN) at 1.39 GHz. Whereas the pulsed emission is detected at a level consistent with previous observations, we report absence of any off-pulse emission above $42$ and $96\ \mathrm{\mu Jy\ beam^{-1}}$ (three times the rms noise levels) for B0525+21 and B2045-16, respectively. Our stringent upper limits imply the off-pulse emission to be less than $0.4$ and $0.3\%$ of the period-averaged pulsed flux density, i.e., much fainter than the previously suggested values of $1$-$10\%$. Since the EVN data are most sensitive to extremely compact angular scales, our results suggest a non-magnetospheric origin for the previously reported off-pulse emission. Presence of extended emission that is resolved out on these milliarcsecond scales still remains plausible. In this case, we constrain the emission to arise from structures with sizes of $\sim (0.61$-$19) \times 10^3\ \mathrm{au}$ for B0525+21 and $\sim (0.48$-$8.3) \times 10^3\ \mathrm{au}$ for B2045-16. These constraints might indicate that the two pulsars are accompanied by compact bow-shock pulsar wind nebulae. Future observations probing intermediate angular scales ($\sim 0.1$-$5\ \mathrm{arcsec}$) will help in clarifying the actual origin of the off-pulse emission., Comment: 6 pages, 4 figures, accepted for publication as a Letter in A&A

Active galactic nuclei are the most luminous persistent (non-transient, even if often variable) objects in the Universe. They are bright in the entire electromagnetic spectrum. Blazars are a special class where the jets point nearly to our line of sight. Because of this special geometry and the bulk relativistic motion of the plasma in the jet, their radiation is enhanced by relativistic beaming. The majority of extragalactic objects detected in gamma-rays are blazars. However, finding their counterparts in other wavebands could be challenging. Here we present the results of our 5-GHz European VLBI Network (EVN) observation of the radio source J1331+2932, a candidate blazar found while searching for possible gamma-ray emission from the stellar binary system DG CVn (Loh et al. 2017). The highest-resolution radio interferometric measurements provide the ultimate tool to confirm the blazar nature of a radio source by imaging compact radio jet structure with Doppler-boosted radio emission, and give the most accurate celestial coordinates as well., Comment: 4 pages, 1 figure, accepted for the proceedings of the 14th European VLBI Network Symposium & Users Meeting (Granada, Spain, 8-11 Oct 2018). Proceedings of Science, PoS(EVN2018)100

Apparently periodic optical variations of the luminous high-redshift (z=2.06) quasar PSO J334.2028+1.4075 led Liu et al. (2015) to interpret the variability as the orbital period of a binary supermassive black hole (SMBH) residing in a single circumbinary accretion disk. The proposed orbital separation was around 0.006 pc, and the possible inspiral time about 7 yr in the rest frame of the quasar. Such objects would be of high interest as the difficult-to-find end products of binary SMBH evolution, and potential sources of low-frequency gravitational waves. However, extending the time baseline of the variability study, Liu et al. (2016) later found that the periodicity of PSO J334.2028+1.4075 does not remain persistent. Foord et al. (2017) did not find evidence for the binary active galactic nucleus scenario based on Chandra X-ray observations. The object has also been studied in detail in the radio (Mooley et al. 2018) with the Karl G. Jansky Very Large Array (VLA) and the Very Long Baseline Array (VLBA), revealing a lobe-dominated quasar at kpc scales, and possibly a precessing jet, which might retain PSO J334.2028+1.4075 as a binary SMBH candidate. Here we report on our 1.7-GHz observation with the European VLBI Network (EVN) which complements the VLBA data taken at higher frequencies, and discuss the current knowledge about the nature of this interesting object., Comment: 4 pages, 1 figure, accepted for the proceedings of the 14th European VLBI Network Symposium & Users Meeting (Granada, Spain, 8-11 Oct 2018). Proceedings of Science, PoS(EVN2018)098

Volonteri et al. (2011) found that the number of radio-loud quasars above redshift 4 calculated from the luminosity function (based upon Swift/BAT observations) is much smaller than the number estimated from the known high-redshift beamed sources, blazars, assuming that for every beamed source with a Lorentz factor of $\Gamma$, statistically $2 \Gamma^2$ non-beamed sources should exist. To explain the missing misaligned (non-beamed) population of high-redshift sources, they proposed various explanations, involving heavy optical obscuration and significantly different Lorentz factors at early cosmological epochs. Our EVN observations targeting high-redshift ($z>4$) blazar candidates revealed 3 sources not showing relativistic beaming, but rather kpc-scale double structures. These three sources have significant radio emission resolved out with the EVN, while they are compact on $\sim 5-10$ arcsec scale. Our dual-frequency ($1.5$ and $5$ GHz) e-MERLIN observations of these three sources revealed a rich morphology, bending jets, and hot spots with possible sites of interaction between the jets and the surrounding medium at intermediate scales., Comment: 6 pages, 3 figures, 1 table, accepted for the Proceedings of the 14th European VLBI Network Symposium & Users Meeting

Ofek (2017) identified FIRST J141918.9+394036 (hereafter FIRST J1419+3940) as a radio source sharing similar properties and host galaxy type to the compact, persistent radio source associated with the first known repeating fast radio burst, FRB 121102. Law et al. (2018) showed that FIRST J1419+3940 is a transient source decaying in brightness over the last few decades. One possible interpretation is that FIRST J1419+3940 is a nearby analogue to FRB 121102 and that the radio emission represents a young magnetar nebula (as several scenarios assume for FRB 121102). Another interpretation is that FIRST J1419+3940 is the afterglow of an `orphan' long gamma-ray burst (GRB). The environment is similar to where most such events are produced. To distinguish between these hypotheses, we conducted radio observations using the European VLBI Network at 1.6 GHz to spatially resolve the emission and to search for millisecond-duration radio bursts. We detect FIRST J1419+3940 as a compact radio source with a flux density of $620 \pm 20\ \mathrm{\mu Jy}$ (on 2018 September 18) and a source size of $3.9 \pm 0.7\ \mathrm{mas}$ (i.e. $1.6 \pm 0.3\ \mathrm{pc}$ given the angular diameter distance of $83\ \mathrm{Mpc}$). These results confirm that the radio emission is non-thermal and imply an average expansion velocity of $(0.10 \pm 0.02)c$. Contemporaneous high-time-resolution observations using the 100-m Effelsberg telescope detected no millisecond-duration bursts of astrophysical origin. The source properties and lack of short-duration bursts are consistent with a GRB jet expansion, whereas they disfavor a magnetar birth nebula., Comment: 8 pages, 4 figures, accepted for publication in ApJL

Fast Radio Bursts (FRBs) are transient sources that emit a single radio pulse with a duration of only a few milliseconds. Since the discovery of the first FRB in 2007, tens of similar events have been detected. However, their physical origin remains unclear, and a number of scenarios even larger than the number of known FRBs has been proposed during these years. The presence of repeating bursts in FRB 121102 allowed us to perform a precise localization of the source with the Very Large Array and the European VLBI Network (EVN). Optical observations with Keck, Gemini and HST unveiled the host to be a low-metallicity star-forming dwarf galaxy located at a redshift of 0.193. The EVN results showed that the bursts are co-located (within a projected separation of $< 40$ pc) to a compact and persistent radio source with a size of $< 0.7$ pc inside a star-forming region. This environment resembles the ones where superluminous supernovae (SLSNe) or long gamma-ray bursts are produced. Although the nature of this persistent source and the origin of the bursts remain unknown, scenarios considering a pulsar/magnetar energizing a young SLSN, or a system with a pulsar/magnetar in the vicinity of a massive black hole are the most plausible ones to date. More recent observations have shown that the bursts from FRB 121102 are almost 100% linearly polarized at an unexpectedly high and variable Faraday rotation measure, that has been observed to date only in vicinities of massive black holes. The bursts are thus likely produced from a neutron star in such environment, although the system can still be explained by a young neutron star embedded in a highly magnetized nebula. Upcoming interferometric searches are expected to report tens of these localizations in the coming years, unveil if this source is representative of the whole population or a particular case, and dramatically boosting the field of FRBs., Comment: 11 pages, 3 figures, proceedings of the 14th European VLBI Network Symposium & Users Meeting, held on 8-11 October 2018 in Granada, Spain

FRB 121102 is the only known repeating fast radio burst source. Here we analyze a wide-frequency-range (1-8 GHz) sample of high-signal-to-noise, coherently dedispersed bursts detected using the Arecibo and Green Bank telescopes. These bursts reveal complex time-frequency structures that include sub-bursts with finite bandwidths. The frequency-dependent burst structure complicates the determination of a dispersion measure (DM); we argue that it is appropriate to use a DM metric that maximizes frequency-averaged pulse structure, as opposed to peak signal-to-noise, and find DM = 560.57 +/- 0.07 pc/cc at MJD 57644. After correcting for dispersive delay, we find that the sub-bursts have characteristic frequencies that typically drift lower at later times in the total burst envelope. In the 1.1-1.7 GHz band, the ~ 0.5-1-ms sub-bursts have typical bandwidths ranging from 100-400 MHz, and a characteristic drift rate of ~ 200 MHz/ms towards lower frequencies. At higher radio frequencies, the sub-burst bandwidths and drift rate are larger, on average. While these features could be intrinsic to the burst emission mechanism, they could also be imparted by propagation effects in the medium local to the source. Comparison of the burst DMs with previous values in the literature suggests an increase of Delta(DM) ~ 1-3 pc/cc in 4 years, though this could be a stochastic variation as opposed to a secular trend. This implies changes in the local medium or an additional source of frequency-dependent delay. Overall, the results are consistent with previously proposed scenarios in which FRB 121102 is embedded in a dense nebula., Comment: Submitted to ApJ; comments welcome

Recently, Saxena et al. (2018) reported the discovery of a possible radio galaxy, J1530$+$1049 at a redshift of z=5.72. We observed the source with the European Very Long Baseline Interferometry Network at $1.7$ GHz. We detected two faint radio features with a separation of $\sim 400$ mas. The radio power calculated from the VLA flux density by Saxena et al. (2018), and the projected source size derived from our EVN data place J1530$+$1049 among the medium-sized symmetric objects (MSOs) which are thought to be young counterparts of radio galaxies (An and Baan 2012). Thus, our finding is consistent with a radio galaxy in an early phase of its evolution as proposed by Saxena et al. (2018)., Comment: 4 pages, 1 figure; accepted by the Research Notes of the American Astronomical Society

The binary neutron star merger event GW170817 was detected through both electromagnetic radiation and gravitational waves. Its afterglow emission may have been produced by either a narrow relativistic jet or an isotropic outflow. High spatial resolution measurements of the source size and displacement can discriminate between these scenarios. We present Very Long Baseline Interferometry observations, performed 207.4 days after the merger, using a global network of 32 radio telescopes. The apparent source size is constrained to be smaller than 2.5 milliarcseconds at the 90% confidence level. This excludes the isotropic outflow scenario, which would have produced a larger apparent size, indicating that GW170817 produced a structured relativistic jet. Our rate calculations show that at least 10% of neutron star mergers produce such a jet., Comment: 28 pages, 10 figures, 2 tables. This is the author's version of the work. It is posted here by permission of the AAAS for personal use, not for redistribution. The definitive version was published in Science , (2019-02-21), doi: 10.1126/science.aau8815

We report on an optical (SDSS) and X-ray (XMM) study of an optically selected sample of four dual AGN systems at projected separations of 30--60~kpc. All sources are detected in the X-ray band (0.3-10 keV); seven objects are optically identified as Seyfert, while one source, optically classified as a LINER, is likely powered by accretion in virtue of its relatively high X-ray luminosity (1.2$\times10^{41}$ erg/s). Six of the eight objects are obscured in X-rays with N$_{\rm H} \geq$ 10$^{23}$ cm$^{-2}$; three of these, whose X-ray spectrum is dominated by a reflection component, are likely Compton-thick (N$_{\rm H} \geq$ 10$^{24}$ cm$^{-2}$). This finding is in agreement with the hypothesis that galaxy encounters are effective in driving gas inflow toward the nuclear region, thus increasing the obscuration. We compare the absorption properties in our dual AGN with those in larger samples observed in X-rays but selected in different ways (optical, IR and hard X-rays). We find that the obscured (N$_{\rm H} \geq$ 10$^{22}$ cm$^{-2}$) AGN fraction within the larger sample is 84$\pm$4 per cent (taking into account the 90 per cent error on the N$_{\rm H}$ measure) up to large pair separations ($\sim$100~kpc). This is statistically higher than the fraction of obscured AGN in isolated galaxies found in X-ray surveys., Comment: 18 pages, 7 figures. Accepted for publication in MNRAS

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

To date, PMN J2134-0419 (at a redshift z=4.33) is the second most distant quasar known with a milliarcsecond-scale morphology permitting direct estimates of the jet proper motion. Based on two-epoch observations, we constrained its radio jet proper motion using the very long baseline interferometry (VLBI) technique. The observations were conducted with the European VLBI Network (EVN) at 5 GHz on 1999 November 26 and 2015 October 6. We imaged the central 10-pc scale radio jet emission and modeled its brightness distribution. By identifying a jet component at both epochs separated by 15.86 yr, a proper motion of mu=0.035 +- 0.023 mas/yr is found. It corresponds to an apparent superluminal speed of beta_a=4.1 +- 2.7 c . Relativistic beaming at both epochs suggests that the jet viewing angle with respect to the line of sight is smaller than 20 deg, with a minimum bulk Lorentz factor Gamma=4.3. The small value of the proper motion is in good agreement with the expectations from the cosmological interpretation of the redshift and the current cosmological model. Additionally we analyzed archival Very Large Array observations of J2143-0419 and found indication of a bent jet extending to ~30 kpc., Comment: 7 pages, 3 figures, 2 tables, accepted for publication in MNRAS

Traditional radio interferometric correlators produce regular-gridded samples of the true $uv$-distribution by averaging the signal over constant, discrete time-frequency intervals. This regular sampling and averaging then translate to be irregular-gridded samples in the $uv$-space, and results in a baseline-length-dependent loss of amplitude and phase coherence, which is dependent on the distance from the image phase centre. The effect is often referred to as "decorrelation" in the $uv$-space, which is equivalent in the source domain to "smearing". This work discusses and implements a regular-gridded sampling scheme in the $uv$-space (baseline-dependent sampling) and windowing that allow for data compression, field-of-interest shaping and source suppression. The baseline-dependent sampling requires irregular-gridded sampling in the time-frequency space i.e. the time-frequency interval becomes baseline-dependent. Analytic models and simulations are used to show that decorrelation remains constant across all the baselines when applying baseline-dependent sampling and windowing. Simulations using MeerKAT telescope and the European Very Long Baseline Interferometry Network show that both data compression, field-of-interest shaping and outer field-of-interest suppression are achieved.

Fast radio bursts (FRBs) are millisecond-duration, extragalactic radio flashes of unknown physical origin. FRB 121102, the only known repeating FRB source, has been localized to a star-forming region in a dwarf galaxy at redshift z = 0.193, and is spatially coincident with a compact, persistent radio source. The origin of the bursts, the nature of the persistent source, and the properties of the local environment are still debated. Here we present bursts that show ~100% linearly polarized emission at a very high and variable Faraday rotation measure in the source frame: RM_src = +1.46 x 10^5 rad m^-2 and +1.33 x 10^5 rad m^-2 at epochs separated by 7 months, in addition to narrow (< 30 mus) temporal structure. The large and variable rotation measure demonstrates that FRB 121102 is in an extreme and dynamic magneto-ionic environment, while the short burst durations argue for a neutron star origin. Such large rotation measures have, until now, only been observed in the vicinities of massive black holes (M_BH > 10^4 MSun). Indeed, the properties of the persistent radio source are compatible with those of a low-luminosity, accreting massive black hole. The bursts may thus come from a neutron star in such an environment. However, the observed properties may also be explainable in other models, such as a highly magnetized wind nebula or supernova remnant surrounding a young neutron star., Comment: Published in Nature: DOI: 10.1038/nature25149

We present a detailed study of the quasar-galaxy pair: J1243+4043 - UGC07904. The sight line of the background quasar ( $z_q$= 1.5266) passes through a region of the galaxy ($z_g$=0.0169) at an impact parameter of 6.9 kpc with high metallicity (0.5 Z$_\odot$) and negligible dust extinction. We detect HI 21-cm absorption from the foreground galaxy at arcsecond and milliarcsecond scales. For typical cold neutral medium (CNM) temperatures in the Milky Way, this 21-cm absorber can be classified as a damped Ly$\alpha$ absorber (DLA). We infer the harmonic mean spin temperature of the gas to be $\sim$400 K and for a simple two-phase medium we estimate the CNM-fraction to be $f_{\rm CNM}$ = 0.27. This is remarkably consistent with the CNM fraction observed in the Galaxy and less than that of high-redshift DLAs. The quasar exhibits a core-jet morphology on milliarcsecond scales, corresponding to an overall extent of $\sim$9 pc at $z_g$. We show that the size of CNM absorbing clouds associated with the foreground galaxy is $>$5 pc and they may be part of cold gas structures that extend beyond $\sim$35 pc. Interestingly, the rotation measure of quasar J1243+4043 is higher than any other source in samples of quasars with high-$z$ DLAs. However, we do not find any detectable differences in RMs and polarization fraction of sight lines with or without high-$z$ ($z\ge2$) DLAs or low-$z$ ($z\le0.3$) 21-cm absorbers. Finally, the foreground galaxy UGC07904 is also part of a galaxy group. We serendipitously detect HI 21-cm emission from four members of the group, and a $\sim$80 kpc long HI bridge connecting two of the other members. The latter, together with the properties of the group members, suggests that the group is a highly interactive environment., Comment: 14 pages, 11 figures, 3 tables. Submitted to MNRAS (referee's comments from Minor revision incorporated)

Narrow-line Seyfert 1 galaxies (NLS1) are an intriguing subclass of active galactic nuclei. Their observed properties indicate low central black hole mass and high accretion rate. The extremely radio-loud NLS1 sources often show relativistic beaming and are usually regarded as younger counterparts of blazars. Recently, the object SDSS J110006.07+442144.3 was reported as a candidate NLS1 source. The characteristics of its dramatic optical flare indicated its jet-related origin. The spectral energy distribution of the object was similar to that of the gamma-ray detected radio-loud NLS1, PMN J0948+0022. Our high-resolution European Very Long Baseline Interferometry Network observations at 1.7 and 5 GHz revealed a compact core feature with a brightness temperature of >~ 10^(10) K. Using the lowest brightness temperature value and assuming a moderate Lorentz factor of ~9 the jet viewing angle is <~ 26 deg. Archival Very Large Array data show a large-scale radio structure with a projected linear size of ~150 kpc reminiscent of double-sided morphology., Comment: 8 pages, 5 figures. Accepted for publication in MNRAS

We present the results from 1.5 and 5 GHz phase-referenced VLBA and 1.5 GHz Karl G. Jansky Very Large Array (VLA) observations of the Seyfert 2 galaxy KISSR1219, which exhibits double peaked emission lines in its optical spectrum. The VLA and VLBA data reveal a one-sided core-jet structure at roughly the same position angles, providing evidence of an AGN outflow. The absence of dual parsec-scale radio cores puts the binary black hole picture in doubt for the case of KISSR1219. The high brightness temperatures of the parsec-scale core and jet components ($>10^6$ K) are consistent with this interpretation. Doppler boosting with jet speeds of $\gtrsim0.55c$ to $\gtrsim0.25c$, going from parsec- to kpc-scales, at a jet inclination $\gtrsim50^\circ$ can explain the jet one-sidedness in this Seyfert 2 galaxy. A blue-shifted broad emission line component in [O {\sc iii}] is also indicative of an outflow in the emission line gas at a velocity of $\sim350$ km s$^{-1}$, while the [O {\sc i}] doublet lines suggest the presence of shock-heated gas. A detailed line ratio study using the MAPPINGS III code further suggests that a shock+precursor model can explain the line ionization data well. Overall, our data suggest that the radio outflow in KISSR1219 is pushing the emission line clouds, both ahead of the jet and in a lateral direction, giving rise to the double peak emission line spectra., Comment: 15 pages, 7 figures, ApJ accepted

We present optical, near- and mid-infrared imaging of the host galaxy of FRB 121102 with the Gemini North telescope, the Hubble Space Telescope and the Spitzer Space Telescope. The FRB 121102 host galaxy is resolved, revealing a bright star forming region located in the outskirts of the irregular, low-metallicity dwarf galaxy. The star forming region has a half-light radius of 0.68 kpc (0.20 arcsec), encompassing the projected location of the compact (<0.7 pc), persistent radio source that is associated with FRB 121102. The half-light diameter of the dwarf galaxy is 5 to 7 kpc, and broadband spectral energy distribution fitting indicates that it has a total stellar mass of M*~10^8 Msun. The metallicity of the host galaxy is low, 12+log10 ([O/H])=8.0+-0.1. The properties of the host galaxy of FRB 121102 are comparable to those of extreme emission line galaxies, also known to host hydrogen-poor superluminous supernovae and long-duration gamma-ray bursts. The projected location of FRB 121102 within the star forming region supports the proposed connection of FRBs with newly born neutron stars or magnetars., Comment: 6 pages, 2 figures, submitted to ApJL; v2 has correct r' band magnitude

We undertook coordinated campaigns with the Green Bank, Effelsberg, and Arecibo radio telescopes during Chandra X-ray Observatory and XMM-Newton observations of the repeating fast radio burst FRB 121102 to search for simultaneous radio and X-ray bursts. We find 12 radio bursts from FRB 121102 during 70 ks total of X-ray observations. We detect no X-ray photons at the times of radio bursts from FRB 121102 and further detect no X-ray bursts above the measured background at any time. We place a 5$\sigma$ upper limit of $3\times10^{-11}$ erg cm$^{-2}$ on the 0.5--10 keV fluence for X-ray bursts at the time of radio bursts for durations $<700$ ms, which corresponds to a burst energy of $4\times10^{45}$ erg at the measured distance of FRB 121102. We also place limits on the 0.5--10 keV fluence of $5\times10^{-10}$ erg cm$^{-2}$ and $1\times10^{-9}$ erg cm$^{-2}$ for bursts emitted at any time during the XMM-Newton and Chandra observations, respectively, assuming a typical X-ray burst duration of 5 ms. We analyze data from the Fermi Gamma-ray Space Telescope Gamma-ray Burst Monitor and place a 5$\sigma$ upper limit on the 10--100 keV fluence of $4\times10^{-9}$ erg cm$^{-2}$ ($5\times10^{47}$ erg at the distance of FRB 121102) for gamma-ray bursts at the time of radio bursts. We also present a deep search for a persistent X-ray source using all of the X-ray observations taken to date and place a 5$\sigma$ upper limit on the 0.5--10 keV flux of $4\times10^{-15}$ erg s$^{-1}$ cm$^{-2}$ ($3\times10^{41}$ erg~s$^{-1}$ at the distance of FRB 121102). We discuss these non-detections in the context of the host environment of FRB 121102 and of possible sources of fast radio bursts in general., Comment: 13 pages, 5 figures, published in ApJ

The binary system AR Sco contains an M star and the only known radio-pulsing white dwarf. The system shows emission from radio to X-rays, likely dominated by synchrotron radiation. The mechanism that produces most of this emission remains unclear. Two competing scenarios have been proposed: Collimated outflows, and direct interaction between the magnetospheres of the white dwarf and the M star. The two proposed scenarios can be tested via very long baseline interferometric radio observations. We conducted a radio observation with the Australian Long Baseline Array (LBA) on 20 Oct 2016 at 8.5 GHz to study the compactness of the radio emission. Simultaneous data with the Australian Telescope Compact Array (ATCA) were also recorded for a direct comparison of the obtained flux densities. AR Sco shows radio emission compact on milliarcsecond angular scales ($\lesssim 0.02\ \mathrm{AU}$, or $4\ \mathrm{R_{\odot}}$). The emission is orbitally modulated, with an average flux density of $\approx 6.5\ \mathrm{mJy}$. A comparison with the simultaneous ATCA data shows that no flux is resolved out on mas scales, implying that the radio emission is produced in this compact region. Additionally, the obtained radio light curves on hour timescales are consistent with the optical light curve. The radio emission in AR Sco is likely produced in the magnetosphere of the M star or the white dwarf, and we see no evidence for a radio outflow or collimated jets significantly contributing to the radio emission., Comment: 4 pages, 2 figures, accepted for publication in A&A

Black hole X-ray binaries undergo occasional outbursts caused by changing inner accretion flows. Here we report high-angular resolution radio observations of the 2013 outburst of the black hole candidate X-ray binary system J1908+094, using data from the VLBA and EVN. We show that following a hard-to-soft state transition, we detect moving jet knots that appear asymmetric in morphology and brightness, and expand to become laterally resolved as they move away from the core, along an axis aligned approximately $-11$\degree\ east of north. We initially see only the southern component, whose evolution gives rise to a 15-mJy radio flare and generates the observed radio polarization. This fades and becomes resolved out after 4 days, after which a second component appears to the north, moving in the opposite direction. From the timing of the appearance of the knots relative to the X-ray state transition, a 90\degree\ swing of the inferred magnetic field orientation, the asymmetric appearance of the knots, their complex and evolving morphology, and their low speeds, we interpret the knots as working surfaces where the jets impact the surrounding medium. This would imply a substantially denser environment surrounding XTE J1908+094 than has been inferred to exist around the microquasar sources GRS 1915+105 and GRO J1655-40., Comment: 17 pages, 9 figures, accepted in MNRAS

A small fraction of Tidal Disruption Events (TDE) produce relativistic jets, evidenced by their non-thermal X-ray spectra and transient radio emission. Here we present milliarcsecond-resolution imaging results on TDE J1644+5734 with the European VLBI Network (EVN). These provide a strong astrometric constraint on the average apparent jet velocity <0.27, that constrains the intrinsic jet velocity for a given viewing angle., Comment: 4 pages, 2 figures. Accepted for publication in the Proceedings of the IAU Symposium No. 324, New Frontiers in Black Hole Astrophysics. v2 is identical to v1; corrected a typo only in the metadata

Blazars are active galactic nuclei (AGN) whose relativistic jets point nearly to the line of sight. Their compact radio structure can be imaged with very long baseline interferometry (VLBI) on parsec scales. Blazars at extremely high redshifts provide a unique insight into the AGN phenomena in the early Universe. We observed four radio sources at redshift $z>4$ with the European VLBI Network (EVN) at 1.7 and 5 GHz. These objects were previously classified as blazar candidates based on X-ray observations. One of them, J2134$-$0419 is firmly confirmed as a blazar with our VLBI observations, due to its relativistically beamed radio emission. Its radio jet extended to $\sim$10 milli-arcsec scale makes this source a promising target for follow-up VLBI observations to reveal any apparent proper motion. Another target, J0839+5112 shows a compact radio structure typical of quasars. There is evidence for flux density variability and its radio "core" has a flat spectrum. However, the EVN data suggest that its emission is not Doppler-boosted. The remaining two blazar candidates (J1420+1205 and J2220+0025) show radio properties totally unexpected from radio AGN with small-inclination jet. Their emission extends to arcsec scales and the Doppler factors of the central components are well below 1. Their structures resemble that of double-lobed radio AGN with large inclination to the line of sight. This is in contrast with the blazar-type modeling of their multi-band spectral energy distributions. Our work underlines the importance of high-resolution VLBI imaging in confirming the blazar nature of high-redshift radio sources., Comment: 11 pages, 4 figures, 5 tables, MNRAS accepted

In hierarchical structure formation scenarios, merging galaxies are expected to be seen in different phases of their coalescence. Simulations suggest that simultaneous activity of the supermassive black holes (SMBHs) in the centres of the merging galaxies may be expected at kpc-scale separations. Currently, there are no direct observational methods which allow the selection of a large number of such dual active galactic nuclei (AGN) candidates. SDSS J142507.32+323137.4 was reported as a promising candidate source based on its optical spectrum. Here we report on our sensitive e-MERLIN observations performed at $1.6$ and at $5$ GHz, which show that the optical spectrum of the source can be more straightforwardly explained with jet-cloud interactions instead of the dual AGN scenario., Comment: 4 pages, 1 figure. Accepted for publication in the Proceedings of the IAU Symposium No. 324, New Frontiers in Black Hole Astrophysics

Fast radio bursts are astronomical radio flashes of unknown physical nature with durations of milliseconds. Their dispersive arrival times suggest an extragalactic origin and imply radio luminosities orders of magnitude larger than any other kind of known short-duration radio transient. Thus far, all FRBs have been detected with large single-dish telescopes with arcminute localizations, and attempts to identify their counterparts (source or host galaxy) have relied on contemporaneous variability of field sources or the presence of peculiar field stars or galaxies. These attempts have not resulted in an unambiguous association with a host or multi-wavelength counterpart. Here we report the sub-arcsecond localization of FRB 121102, the only known repeating burst source, using high-time-resolution radio interferometric observations that directly image the bursts themselves. Our precise localization reveals that FRB 121102 originates within 100 mas of a faint 180 uJy persistent radio source with a continuum spectrum that is consistent with non-thermal emission, and a faint (25th magnitude) optical counterpart. The flux density of the persistent radio source varies by tens of percent on day timescales, and very long baseline radio interferometry yields an angular size less than 1.7 mas. Our observations are inconsistent with the fast radio burst having a Galactic origin or its source being located within a prominent star-forming galaxy. Instead, the source appears to be co-located with a low-luminosity active galactic nucleus or a previously unknown type of extragalactic source. [Truncated] If other fast radio bursts have similarly faint radio and optical counterparts, our findings imply that direct sub-arcsecond localizations of FRBs may be the only way to provide reliable associations., Comment: Nature, published online on 4 Jan 2017, DOI: 10.1038/nature20797