We report phase-referencing VLBA observations of H 2 O masers near the starforming region W3(OH) to measure their parallax and absolute proper motions. The measured annual parallax is 0.489 ± 0.017 milli-arcseconds (2.04 ± 0.07 kpc), where the error is dominated by a systematic atmospheric contribution. This distance is consistent with photometric distances from previous observations and with the distance determined from CH 3 OH maser astrometry presented in a related paper. We also find that the source driving the H 2 O outflow, the "TW-object", moves with a 3-dimensional velocity of > 7 km s −1 relative to the ultracompact H II region W3(OH).

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 γ-ray photons 2 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 (IC 35) detected by IceCube. Based on an analysis of the full sample of γ-ray blazars in the IC 35 field and assuming a photo-hadronic emission model, we show that the long-term average γ-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.The neutrino excess detected by IceCube comprises 37 events with energies between 30 TeV and 2 PeV, rejecting a purely atmospheric origin at a significance of 5.7 standard deviations [1][2][3] .These events show a broad distribution across both hemispheres of the sky consistent with an extragalactic source population. Due to the very steep background of atmospheric neutrinos, events at PeV energies are best suited for attempting to establish associations with individual blazars. In the first two years of observations, IceCube detected two events with about 1 PeV of deposited energy 1, 2 (IC 14, and IC 20; dubbed 'Bert' and 'Ernie') the diffuse neutrino flux due to the integrated emission of AGN in a given large field at a given time, as well as the maximum possible neutrino flux associated with an individual object of the sample.Blazars are radio-loud AGN with jets oriented close to the line of sight. This substantially increases the apparent brightness of these objects owing to the Doppler boosting of the emission from the relativistically moving emission zones. A direct association of a PeV-neutrino with an * A different analysis of IceCube muon neutrinos finds an excess signal also from the northern sky 14 . † http://pulsar.sternwarte.uni-erlangen.de/tanami 4 individual γ-ray blazar would have the important implication that a sizeable fraction of their observed γ-ray emission must be due to hadronic decays, and that blazar jets are also sources of ultra-high-energy cos...

Context. A number of theoretical models vie to explain the γ-ray emission from active galactic nuclei (AGN). This was a key discovery of EGRET. With its broader energy coverage, higher resolution, wider field of view and greater sensitivity, the Large Area Telescope (LAT) of the Fermi Gamma-ray Space Telescope is dramatically increasing our knowledge of AGN γ-ray emission. However, discriminating between competing theoretical models requires quasi-simultaneous observations across the electromagnetic spectrum. By resolving the powerful parsecscale relativistic outflows in extragalactic jets and thereby allowing us to measure critical physical properties, Very Long Baseline Interferometry observations are crucial to understanding the physics of extragalactic γ-ray objects. Results. We present first epoch images for 43 sources, some observed for the first time at milliarcsecond resolution. Parameters of these images as well as physical parameters derived from them are also presented and discussed. These and subsequent images from the TANAMI survey are available at http://pulsar.sternwarte.uni-erlangen.de/tanami/. Conclusions. We obtain reliable, high dynamic range images of the southern hemisphere AGN. All the quasars and BL Lac objects in the sample have a single-sided radio morphology. Galaxies are either double-sided, single-sided or irregular. About 28% of the TANAMI sample has been detected by LAT during its first three months of operations. Initial analysis suggests that when galaxies are excluded, sources detected by LAT have larger opening angles than those not detected by LAT. Brightness temperatures of LAT detections and non-detections seem to have similar distributions. The redshift distributions of the TANAMI sample and sub-samples are similar to those seen for the bright γ-ray AGN seen by LAT and EGRET but none of the sources with a redshift above 1.8 have been detected by LAT.

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