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The past four years have seen a scientific revolution through the birth of a new field: gravitational-wave astronomy. The first detection of gravitational waves---recognised by the 2017 Nobel Prize in Physics---provided unprecedented tests of general relativity while unveiling a previously unknown class of massive black holes, thirty times more massive than the Sun. The subsequent detection of gravitational waves from a merging binary neutron star confirmed the hypothesised connection between binary neutron stars and short gamma-ray bursts while providing an independent measurement of the expansion of the Universe. The discovery enabled precision measurement of the speed of gravity while shedding light on the origin of heavy elements. At the time of writing, the Laser Interferometer Gravitational-wave Observatory (LIGO) and its European partner, Virgo, have published the detection of eleven gravitational-wave events. New, not-yet-published detections are announced on a nearly weekly basis. This fast-growing catalogue of gravitational-wave transients is expected to yield insights into a number of topics, from the equation of state of matter at supra-nuclear densities to the fate of massive stars. The science potential of 3G observatories is enormous, enabling measurements of gravitational waves from the edge of the Universe and precise determination of the neutron star equation of state. Australia is well-positioned to help develop the required technology. The Mid-term Review for the Decadal plan for Australian astronomy 2016-2025 should consider investment in a scoping study for an Australian Gravitational-Wave Pathfinder that develops and validates core technologies required for the global 3G detector network., Comment: 8 pages, 7 figures

We investigate the prospects of detecting radio afterglows from long Gamma-Ray Bursts (GRBs) from Population III (Pop III) progenitors using the SKA precursor instruments WMA (Murchison Widefield Array) and ASKAP (Australian SKA Pathfinder). We derive a realistic model of GRB afterglows that encompasses the widest range of plausible physical parameters and observation angles. We define the best case scenario of Pop III GRB energy and redshift distributions. Using probability distribution functions fitted to the observed microphysical parameters of long GRBs, we simulate a large number of Pop III GRB afterglows to find the global probability of detection. We find that ASKAP may be able to detect 35% of Pop III GRB afterglows in the optimistic case, and 27% in the pessimistic case. A negligible number will be detectable by MWA in either case. Detections per image for ASKAP, found by incorporating intrinsic rates with detectable timescales, are as high as $\sim$ 6000 and as low as $\sim$ 11, which shows the optimistic case is unrealistic. We track how the afterglow flux density changes over various time intervals and find that, because of their very slow variability, the cadence for blind searches of these afterglows should be as long as possible. We also find Pop III GRBs at high redshift have radio afterglow lightcurves that are indistinguishable from those of regular long GRBs in the more local universe., Comment: Accepted for publication in Monthly Notices of the Royal Astronomical Society

Gamma ray burst (GRB) optical and X-ray afterglow luminosity is expected to correlate with the GRB isotropic equivalent kinetic energy of the outflow in the standard synchrotron model for GRB afterglows. Previous studies, using prompt GRB isotropic equivalent energy ($E_{\rm iso}$) as a proxy for isotropic equivalent kinetic energy, have generally confirmed a correlation between X-ray and optical afterglow luminosities. Assuming that GRB afterglow luminosity does not evolve strongly with redshift, we identify a strong Malmquist bias in GRB optical and X-ray afterglow luminosity data. We show that selection effects dominate the observed E$_{\rm iso}$ -- L$_{\rm opt,X}$ correlations, and have likely been underestimated in other studies. The bias is strongest for a subset of optically faint bursts $m>24$ at 24 hr with $z>2$. After removing this optical selection bias, the E$_{\rm iso}$ -- L$_{\rm opt,X}$ correlation for long GRBs is not statistically significant, but combining both long and short GRB luminosity data the correlation is significant. Using the median of the $E_{\rm iso}$ and $L_{\rm opt,X}$ distributions, we apply the synchrotron model assuming the same power law index for short and long GRBs, but different microphysical parameter distributions. Comparing the ratio of optical and X-ray luminosities, we find tentative evidence that the fraction of post-shock energy in magnetic fields, $\epsilon_B$, could be systematically higher in SGRBs compared to LGRBs., Comment: 6 pages, 3 figures, MNRAS Letter accepted, supplementary data to appear online

Assuming neutron star (NS) or neutron star/stellar-mass black hole (BH) mergers as progenitors of the short gamma ray bursts, we derive and demonstrate a simple analysis tool for modelling the efficiency of recovering on-axis optical afterglows triggered by a candidate gravitational wave event detected by the Advanced LIGO and Virgo network. The coincident detection efficiency has been evaluated for different classes of operating telescopes using observations of gamma ray bursts. We show how the efficiency depends on the luminosity distribution of the optical afterglows, the telescope features, and the sky localisation of gravitational wave triggers. We estimate a plausible optical afterglow and gravitational wave coincidence rate of 1 yr$^{-1}$ (0.1 yr$^{-1}$) for NS-NS (NS-BH), and how this rate is scaled down in detection efficiency by the time it takes to image the gravitational wave sky localization and the limiting magnitude of the telescopes. For NS-NS (NS-BH) we find maximum detection efficiencies of $>80%$ when the total imaging time is less than 200 min (80 min) and the limiting magnitude fainter than 20 (21). We show that relatively small telescopes $(m<18)$ can achieve similar detection efficiencies to meter class facilities $(m<20)$ with similar fields of view, only if the less sensitive instruments can respond to the trigger and image the field within 10-15 min. The inclusion of LIGO India into the gravitational wave observatory network will significantly reduce imaging time for telescopes with limiting magnitudes $\sim20$ but with modest fields of view. An optimal coincidence search requires a global network of sensitive and fast response wide field instruments that could effectively image relatively large gravitational-wave sky localisations and produce transient candidates for further photometric and spectroscopic follow-up., Comment: 6 pages, 2 figures, version 2, reference added typo correction, Accepted by MNRAS

The favoured progenitor model for short $\gamma$-ray bursts (SGRBs) is the merger of two neutron stars that triggers an explosion with a burst of collimated $\gamma$-rays. Following the initial prompt emission, some SGRBs exhibit a plateau phase in their $X$-ray light curves that indicates additional energy injection from a central engine, believed to be a rapidly rotating, highly magnetised neutron star. The collapse of this `protomagnetar' to a black hole is likely to be responsible for a steep decay in $X$-ray flux observed at the end of the plateau. In this letter, we show that these observations can be used to effectively constrain the equation of state of dense matter. In particular, we show that the known distribution of masses in binary neutron star systems, together with fits to the $X$-ray light curves, provide constraints that exclude the softest and stiffest plausible equations of state. We further illustrate how a future gravitational wave observation with Advanced LIGO/Virgo can place tight constraints on the equation of state, by adding into the picture a measurement of the chirp mass of the SGRB progenitor., Comment: accepted for publication in Phys. Rev. D

The Australian Square Kilometre Array Pathfinder (ASKAP) will give us an unprecedented opportunity to investigate the transient sky at radio wavelengths. In this paper we present VAST, an ASKAP survey for Variables and Slow Transients. VAST will exploit the wide-field survey capabilities of ASKAP to enable the discovery and investigation of variable and transient phenomena from the local to the cosmological, including flare stars, intermittent pulsars, X-ray binaries, magnetars, extreme scattering events, interstellar scintillation, radio supernovae and orphan afterglows of gamma ray bursts. In addition, it will allow us to probe unexplored regions of parameter space where new classes of transient sources may be detected. In this paper we review the known radio transient and variable populations and the current results from blind radio surveys. We outline a comprehensive program based on a multi-tiered survey strategy to characterise the radio transient sky through detection and monitoring of transient and variable sources on the ASKAP imaging timescales of five seconds and greater. We also present an analysis of the expected source populations that we will be able to detect with VAST., Comment: 29 pages, 8 figures. Submitted for publication in Pub. Astron. Soc. Australia

Presently only 30% of short gamma ray bursts (SGRBs) have accurate redshifts, and this sample is highly biased by the limited sensitivity of {\it Swift} to detect SGRBs. We account for the dominant biases to calculate a realistic SGRB rate density out to $z = 0.5$ using the {\it Swift} sample of peak fluxes, redshifts, and those SGRBs with a beaming angle constraint from X-ray/optical observations. Assuming a significant fraction of binary neutron star mergers produce SGRBs, we calculate lower and upper detection rate limits of (1-180) per Yr by an advanced LIGO and Virgo coincidence search. Our detection rate is compatible with extrapolations using Galactic pulsar observations and population synthesis., Comment: 6 pages, 1 figure, to appear in: Proceedings of Science, Gamma-Ray Bursts 2012 Conference - GRB2012, May 07-11, 2012, Munich Germany

Short gamma-ray bursts (SGRBs) observed by {\it Swift} are potentially revealing the first insight into cataclysmic compact object mergers. To ultimately acquire a fundamental understanding of these events requires pan-spectral observations and knowledge of their spatial distribution to differentiate between proposed progenitor populations. Up to April 2012 there are only some 30% of SGRBs with reasonably firm redshifts, and this sample is highly biased by the limited sensitivity of {\it Swift} to detect SGRBs. We account for the dominant biases to calculate a realistic SGRB rate density out to $z\approx0.5$ using the {\it Swift} sample of peak fluxes, redshifts, and those SGRBs with a beaming angle constraint from X-ray/optical observations. We find an SGRB lower rate density of $8^{+5}_{-3}$ $\mathrm{Gpc}^{-3}\mathrm{yr}^{-1}$ (assuming isotropic emission), and a beaming corrected upper limit of $1100^{+700}_{-470} $ $\mathrm{Gpc}^{-3}\mathrm{yr}^{-1}$. Assuming a significant fraction of binary neutron star mergers produce SGRBs, we calculate lower and upper detection rate limits of $(1-180)$ yr$^{-1}$ by an aLIGO and Virgo coincidence search. Our detection rate is similar to the lower and realistic rates inferred from extrapolations using Galactic pulsar observations and population synthesis., Comment: version 2: 7 pages, 1 figure, updated table & references, submitted to MNRAS