J. R. Allison

Illuminating gravitational waves: A concordant picture of photons from a neutron star merger

GROWTH observations of GW170817 The gravitational wave event GW170817 was caused by the merger of two neutron stars (see the Introduction by Smith). In three papers, teams associated with the GROWTH (Global Relay of Observatories Watching Transients Happen) project present their observations of the event at wavelengths from x-rays to radio waves. Evans et al. used space telescopes to detect GW170817 in the ultraviolet and place limits on its x-ray flux, showing that the merger generated a hot explosion known as a blue kilonova. Hallinan et al. describe radio emissions generated as the explosion slammed into the surrounding gas within the host galaxy. Kasliwal et al. present additional observations in the optical and infrared and formulate a model for the event involving a cocoon of material expanding at close to the speed of light, matching the data at all observed wavelengths. Science, this issue p. 1565, p. 1579, p. 1559; see also p. 1554 Observations of a binary neutron star merger at multiple wavelengths can be explained by an off-axis relativistic cocoon model. Merging neutron stars offer an excellent laboratory for simultaneously studying strong-field gravity and matter in extreme environments. We establish the physical association of an electromagnetic counterpart (EM170817) with gravitational waves (GW170817) detected from merging neutron stars. By synthesizing a panchromatic data set, we demonstrate that merging neutron stars are a long-sought production site forging heavy elements by r-process nucleosynthesis. The weak gamma rays seen in EM170817 are dissimilar to classical short gamma-ray bursts with ultrarelativistic jets. Instead, we suggest that breakout of a wide-angle, mildly relativistic cocoon engulfing the jet explains the low-luminosity gamma rays, the high-luminosity ultraviolet-optical-infrared, and the delayed radio and x-ray emission. We posit that all neutron star mergers may lead to a wide-angle cocoon breakout, sometimes accompanied by a successful jet and sometimes by a choked jet.

A radio counterpart to a neutron star merger

GROWTH observations of GW170817 The gravitational wave event GW170817 was caused by the merger of two neutron stars (see the Introduction by Smith). In three papers, teams associated with the GROWTH (Global Relay of Observatories Watching Transients Happen) project present their observations of the event at wavelengths from x-rays to radio waves. Evans et al. used space telescopes to detect GW170817 in the ultraviolet and place limits on its x-ray flux, showing that the merger generated a hot explosion known as a blue kilonova. Hallinan et al. describe radio emissions generated as the explosion slammed into the surrounding gas within the host galaxy. Kasliwal et al. present additional observations in the optical and infrared and formulate a model for the event involving a cocoon of material expanding at close to the speed of light, matching the data at all observed wavelengths. Science, this issue p. 1565, p. 1579, p. 1559; see also p. 1554 Radio observations constrain the energy and geometry of relativistic material ejected from a binary neutron star merger. 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.

A search for 21 cm H I absorption in AT20G compact radio galaxies

We present results from a search for 21 cm associated H I absorption in a sample of 29 radio sources selected from the Australia Telescope 20 GHz survey. Observations were conducted using the Australia Telescope Compact Array Broadband Backend, with which we can simultaneously look for 21 cm absorption in a redshift range of 0.04 . z . 0.08, with a velocity resolution of 7 km s 1 . In preparation for future largescale H I absorption surveys we test a spectral-line finding method ba sed on Bayesian inference. We use this to assign significance to our detectio ns and to determine the best-fitting number of spectral-line components. We find tha t the automated spectralline search is limited by residuals in the continuum, both fr om the band-pass calibration �

On the H i column density–radio source size anticorrelation in compact radio sources

Existing studies of the atomic hydrogen gas content in distant galaxies, through the absorption of the 21-cm line, often infer that the total column density, NHI, is anti-correlated with the linear extent of the background radio source, dem. We investigate this interpretation, by dissecting the various parameters from which NHI is derived, and find that the relationship is driven primarily by the observed optical depth, τobs, which, for a given absorber size, is anti-correlated with dem. Therefore, the inferred NHI dem anti-correlation is merely the consequence of geometry, in conjunction with the assumption of a common spin temperature/covering factor ratio for each member of the sample, an assumption for which there is scant observational justification. While geometry can expl ain the observed correlation, many radio sources comprise two radio lobes and so we model the projected area of a two component emitter intercepted by a foreground absorber. From this, the observed τobs dem relationship is best reproduced through models which approximate either of the two Fanaroff & Riley classifications, although the observed scatter in th e sample cannot be duplicated using a single deprojected radio source size. Furthermore, th e trend is best reproduced using an absorber of diameter � 100 1000 pc, which is also the range of values of dem at which the 21-cm detection rate peaks. This may indicate that this is th e characteristic linear size of the absorbing gas structure.

Infrared-correlated 31-GHz radio emission from Orion East

Lynds dark cloud LDN1622 represents one of the best examples of anomalous dust emission, possibly originating from small spinning dust grains. We present Cosmic Background Imager (CBI) 31 GHz data of LDN1621, a diffuse dark cloud to the north of LDN1622 in a region known as Orion East. A broken ring with diameter g\approx 20 arcmin of diffuse emission is detected at 31 GHz, at \approx 20-30 mJy beam

Application of a Bayesian Method to Absorption Spectral-Line Finding in Simulated ASKAP Data

^{-1}

A search for H i absorption in nearby radio galaxies using HIPASS

with an angular resolution of \approx 5 arcmin. The ring-like structure is highly correlated with Far Infra-Red emission at

H i emission and absorption in nearby, gas-rich galaxies

12-100 \mu

The Cosmic Background Imager 2

m with correlation coefficients of r \approx 0.7-0.8, significant at

Broad, weak 21 cm absorption in an early type galaxy: spectral line finding and parametrization for future surveys

\sim10\sigma