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Gravitational waves have been detected from a binary neutron star merger event, GW170817. The detection of electromagnetic radiation from the same source has shown that the merger occurred in the outskirts of the galaxy NGC 4993, at a distance of 40 megaparsecs from Earth. We report the detection of a counterpart radio source that appears 16 days after the event, allowing us to diagnose the energetics and environment of the merger. The observed radio emission can be explained by either a collimated ultrarelativistic jet, viewed off-axis, or a cocoon of mildly relativistic ejecta. Within 100 days of the merger, the radio light curves will enable observers to distinguish between these models, and the angular velocity and geometry of the debris will be directly measurable by very long baseline interferometry.

The VLA Galactic Plane Survey (VGPS) is a survey of H i and 21 cm continuum emission in the Galactic plane between longitude 18 and 67 with latitude coverage from jbj < 1N3 to jbj < 2N3. The survey area was observed with the Very Large Array in 990 pointings. Short-spacing information for the H i line emission was obtained by additional observations with the Green Bank Telescope. H i spectral line images are presented with a resolution of 1 0 ; 1 0 ; 1:56 km s À1 ( FWHM ) and an rms noise of 2 K per 0:824 km s À1 channel. Continuum images made from channels without H i line emission have 1 0 (FWHM ) resolution. The VGPS images reveal structures of atomic hydrogen and 21 cm continuum as large as several degrees with unprecedented resolution in this part of the Galaxy. With the completion of the VGPS, it is now possible for the first time to assess the consistency between arcminute-resolution surveys of Galactic H i emission. VGPS images are compared with images from the Canadian Galactic Plane Survey (CGPS) and the Southern Galactic Plane Survey (SGPS). In general, the agreement between these surveys is impressive, considering the differences in instrumentation and image-processing techniques used for each survey. The differences between VGPS and CGPS images are small, P6 K (rms) in channels in which the mean H i brightness temperature in the field exceeds 80 K. A similar degree of consistency is found between the VGPS and SGPS. The agreement we find between arcminute-resolution surveys of the Galactic plane is a crucial step toward combining these surveys into a single uniform data set that covers 90% of the Galactic disk: the International Galactic Plane Survey. The VGPS data will be made available on the World Wide Web through the Canadian Astronomy Data Centre.

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Green Bank Telescope (GBT) λ21cm observations have revealed a faint, yet extensive H I cloud population surrounding the Andromeda Galaxy (M31). The newfound objects are likely analogs to the high-velocity H I clouds (HVCs) seen around the Milky Way. At least 20 discrete features are detected within 50 kpc of the M31 disk, with radial velocities that are comparable to those of outer disk rotation. In addition, a filamentary "halo" component of at least 30 kpc extent is concentrated at the M31 systemic velocity. Some of the discrete features are organized into elongated systems with velocity continuity, suggestive of tidal streams. The discrete population can be characterized by a steep power-law distribution of number versus H I mass in the range between 10 5 and 10 7 M ⊙ . The velocity line-width of discrete clouds is correlated with the cloud H I mass: such that if the clouds are gravitationally bound this implies a dark-to H I mass ratio of ∼ 100:1. Possible origins for the discrete and "halo" M31 features include: a Local Group "cooling flow", tidal debris from recent mergers or interactions, and the gaseous counterparts of low mass dark-matter halos.

Context. Measurement of the Galactic neutral atomic hydrogen (H i) column density, N H i , and brightness temperatures, T B , is of high scientific value for a broad range of astrophysical disciplines. In the past two decades, one of the most-used legacy H i datasets has been the Leiden/Argentine/Bonn Survey (LAB). Aims. We release the H i 4π survey (HI4PI), an all-sky database of Galactic H i, which supersedes the LAB survey. Methods. The HI4PI survey is based on data from the recently completed first coverage of the Effelsberg-Bonn H i Survey (EBHIS) and from the third revision of the Galactic All-Sky Survey (GASS). EBHIS and GASS share similar angular resolution and match well in sensitivity. Combined, they are ideally suited to be a successor to LAB. Results. The new HI4PI survey outperforms the LAB in angular resolution (ϑ FWHM = 16 .2) and sensitivity (σ rms = 43 mK). Moreover, it has full spatial sampling and thus overcomes a major drawback of LAB, which severely undersamples the sky. We publish all-sky column density maps of the neutral atomic hydrogen in the Milky Way, along with full spectroscopic data, in several map projections including HEALPix.

We have measured integrated Galactic 21 cm column densities toward ~ 174 quasars and active galactic nuclei using the NRAO 140 ft telescope at Green Bank. These data have been corrected for stray radiation with the technique of Lockman et al. (1986). The 21 arcmin beam size of the 140 ft is small enough to minimize the uncertainty in N n due to angular variations in the H i of the Galaxy at high latitudes. The resulting column densities are accurate to ~1X10 19 atoms cm-2 or-5%, whichever error is larger. Opacity uncertainties dominate the errors above iV H ~4X 10 20 atoms cm-2 .