The final chapter in the long-standing mystery of the gamma-ray bursts (GRBs) centres on the origin of the short-hard class of bursts, which are suspected on theoretical grounds to result from the coalescence of neutron-star or black-hole binary systems. Numerous searches for the afterglows of short-hard bursts have been made, galvanized by the revolution in our understanding of long-duration GRBs that followed the discovery in 1997 of their broadband (X-ray, optical and radio) afterglow emission. Here we present the discovery of the X-ray afterglow of a short-hard burst, GRB 050709, whose accurate position allows us to associate it unambiguously with a star-forming galaxy at redshift z = 0.160, and whose optical lightcurve definitively excludes a supernova association. Together with results from three other recent short-hard bursts, this suggests that short-hard bursts release much less energy than the long-duration GRBs. Models requiring young stellar populations, such as magnetars and collapsars, are ruled out, while coalescing degenerate binaries remain the most promising progenitor candidates.

The gamma-ray burst (GRB) 050904 at z = 6.3 provides the first opportunity of probing the intergalactic medium (IGM) by GRBs at the epoch of the reionization. Here we present a spectral modeling analysis of the optical afterglow spectrum taken by the Subaru Telescope, aiming to constrain the reionization history. The spectrum shows a clear damping wing at wavelengths redward of the Lyman break, and the wing shape can be fit either by a damped Lyα system with a column density of log(N HI /cm −2 ) ∼ 21.6 at a redshift close to the detected metal absorption lines (z metal = 6.295), or by almost neutral IGM extending to a slightly higher redshift of z IGM,u ∼ 6.36. In the latter case, the difference from z metal may be explained by acceleration of metal absorbing shells by the activities of the GRB or its progenitor. However, we exclude this possibility by using the light transmission feature around the Lyβ resonance, leading to a firm upper limit of z IGM,u ≤ 6.314. We then show an evidence that the IGM was largely ionized already at z = 6.3, with the best-fit neutral fraction of IGM, x HI = 0.00, and upper limits of x HI < 0.17 and 0.60 at 68 and 95% C.L., respectively. This is the first direct and quantitative upper limit on x HI at z > 6. Various systematic uncertainties are examined, but none of them appears large enough to change this conclusion. To get further information on the reionization, it is important to increase the sample size of z > ∼ 6 GRBs, in order to find GRBs with low column densities (log N HI < ∼ 20) within their host galaxies, and for statistical studies of Lyα line emission from host galaxies.

Core-collapse supernovae (CC-SNe) are the explosions that announce the death of massive stars. Some CC-SNe are linked to long-duration gamma-ray bursts (GRBs) and are highly aspherical. One important question is to what extent asphericity is common to all CC-SNe. Here we present late-time spectra for a number of CC-SNe from stripped-envelope stars and use them to explore any asphericity generated in the inner part of the exploding star, near the site of collapse. A range of oxygen emission-line profiles is observed, including a high incidence of double-peaked profiles, a distinct signature of an aspherical explosion. Our results suggest that all CC-SNe from stripped-envelope stars are aspherical explosions and that SNe accompanied by GRBs exhibit the highest degree of asphericity.