Browsing by Author "Wijers, RAMJ"

The association of a supernova with GRB030329(1,2) strongly supports the 'collapsar' model(3) of gamma-ray bursts, where a relativistic jet(4) forms after the progenitor star collapses. Such jets cannot be spatially resolved because gamma-ray bursts lie at cosmological distances; their existence is instead inferred from 'breaks' in the light curves of the afterglows, and from the theoretical desire to reduce the estimated total energy of the burst by proposing that most of it comes out in narrow beams. Temporal evolution of the polarization of the afterglows(5-7) may provide independent evidence for the jet structure of the relativistic outflow. Small-level polarization (similar to1-3 per cent)(8-17) has been reported for a few bursts, but its temporal evolution has yet to be established. Here we report polarimetric observations of the afterglow of GRB030329. We establish the polarization light curve, detect sustained polarization at the per cent level, and find significant variability. The data imply that the afterglow magnetic field has a small coherence length and is mostly random, probably generated by turbulence, in contrast with the picture arising from the high polarization detected in the prompt gamma-rays from GRB021206 (ref. 18).

We report optical and near-infrared follow-up observations of GRB 011121 collected predominantly at ESO telescopes in Chile. We discover a break in the afterglow light curve after 1.3 days, which implies an initial jet opening angle of about 9degrees. The jet origin of this break is supported by the fact that the spectral energy distribution is achromatic during the first 4 days. During later phases, GRB 011121 shows significant excess emission above the flux predicted by a power law, which we interpret as additional light from an underlying supernova. In particular, the spectral energy distribution of the optical transient approximately 2 weeks after the burst is clearly not of power-law type but can be presented by a blackbody with a temperature of similar to6000 K. The deduced parameters for the decay slope and the spectral index favor a wind scenario, i.e., an outflow into a circumburst environment shaped by the stellar wind of a massive gamma-ray burst (GRB) progenitor. Because of its low redshift of z = 0: 36, GRB 011121 has been the best example for the GRB-supernova connection until GRB 030329 and provides compelling evidence for a circumburster wind region expected to exist if the progenitor was a massive star.

We present time-resolved optical spectroscopy of the afterglows of the gamma-ray bursts GRB 990510 and GRB 990712. Through the identification of several absorption lines in the first-epoch GRB 990510 spectrum, we determine the redshift for this burst at z greater than or equal to 1.619. No clear emission lines are detected. The strength of the Mg I feature is indicative of a dense environment, most likely the host galaxy of GRB 990510. Although the host is extremely faint the GRB afterglow allows us to probe its inter-stellar medium and-in principle-to measure its metallicity. The optical spectrum of GRB 990712 (whose host galaxy is the brightest of the known GRB hosts at cosmological redshifts) shows clear features both in emission and absorption, at a redshift of z = 0.4331 +/- 0.0004. On the basis of several line emission diagnostic diagrams, we conclude that the host galaxy of GRB 990712 is most likely an H II galaxy. We derive an unreddened [O II] star formation rate of 2.7 +/- 0.8 M-. yr(-1). Correcting for the measured extinction intrinsic to the host galaxy (A(v) = 3.4(-1.7)(+2.4)), this value increases to 35(-25)(+178) M-. yr(-1). The [O II] equivalent width, compared to that of field galaxies at z less than or equal to1, also suggests that the host of GRB 990712 is vigorously forming stars. We employ the oxygen and H beta emission-line intensities to estimate the global oxygen abundance for the host of GRB 990712: log (O/H) = 3.7 +/- 0.4, which is slightly below the lowest metallicity one finds in nearby spiral galaxies. For both GRBs we study the time evolution of the adsorption lines, whose equivalent width might be expected to change with time if the burst resides in a dense compact medium. We find no evidence for a significant change in the Mg II width.