The redshift distribution of the short-duration GRBs is a crucial, but currently fragmentary, clue to the nature of their progenitors. Here we present optical observations of nine short GRBs obtained with Gemini, Magellan, and the Hubble Space Telescope. We detect the afterglows and host galaxies of two short bursts, and host galaxies for two additional bursts with known optical afterglow positions, and five with X-ray positions ( ∼ < 6 ′′ radius). In eight of the nine cases we find that the most probable host galaxies are faint, R ≈ 23 − 26.5 mag, and are therefore starkly different from the first few short GRB hosts with R ≈ 17 − 22 mag and z ∼ < 0.5. Indeed, we measure spectroscopic redshifts of z ≈ 0.4 − 1.1 for the four brightest hosts. A comparison to large field galaxy samples, as well as the hosts of long GRBs and previous short GRBs, indicates that the fainter hosts likely reside at z ∼ > 1. Our most conservative limit is that at least half of the five hosts without a known redshift reside at z > 0.7 (97% confidence level), suggesting that about 1/3 − 2/3 of all short GRBs originate at higher redshifts than previously determined. This has two important implications: (i) We constrain the acceptable age distributions to a wide lognormal (σ ∼ > 1) with τ * ∼ 4 − 8 Gyr, or to a power law, P (τ ) ∝ τ n , with −1 ∼ < n ∼ < 0; and (ii) the inferred isotropic energies, E γ,iso ∼ 10 50 − 10 52 erg, are significantly larger than ∼ 10 48 − 10 49 erg for the low redshift short GRBs, indicating a large spread in energy release or jet opening angles. Finally, we re-iterate the importance of short GRBs as potential gravitational wave sources and find a conservative Advanced LIGO detection rate of ∼ 2 − 6 yr −1 .

We present multiwavelength observations of Cygnus X-3 during an extended outburst in 1994 February -March. Intensive radio monitoring at 13.3, 3.6 & 2.0 cm is complemented by observations at (sub)millimetre and infrared wavelengths, which find Cyg X-3 to be unusually bright and variable, and include the first reported detection of the source at 0.45 mm. We report the first confirmation of quenched radio emission prior to radio flaring independent of observations at Green Bank. The observations reveal evidence for wavelength-dependent radiation losses and gradually decreasing opacity in the environment of the radio jet. We find that the radiation losses are likely to be predominantly inverse Compton losses experienced by the radio-emitting electrons in the strong radiation field of a luminous companion to the compact object. We interpret the decreasing opacity during the flare sequence as resulting from a decreasing proportion of thermal electrons entrained in the jet, reflecting a decreasing density in the region of jet formation. We present, drawing in part on the work of other authors, a model based upon mass-transfer rate instability predicting γ-ray, X-ray, infrared and radio trends during a radio flaring sequence.

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We observed the energetic binary Cygnus X-3 in both quiescent and flaring states between 4 and 16 microns using the ISO satellite. We find that the quiescent source shows the thermal free-free spectrum typical of a hot, fast stellar wind, such as from a massive helium star. The quiescent mass-loss rate due to a spherically symmetric, non-accelerating wind is found to be in the range 0.4-2.9 x 10E-4 solar masses per year, consistent with other infrared and radio observations, but considerably larger than the 10E-5 solar masses per year deduced from both the orbital change and the X-ray column density. There is rapid, large amplitude flaring at 4.5 and 11.5 microns at the same time as enhanced radio and X-ray activity, with the infrared spectrum apparently becoming flatter in the flaring state. We believe non-thermal processes are operating, perhaps along with enhanced thermal emission.Comment: Accepted for publication in MNRAS, 11 pages, 6 figure

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