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Register a new userLinear and circular polarimetry observations of GRB afterglows
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Authors
K. Wiersema
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Follow-up observations of large numbers of gamma-ray burst (GRB) afterglows, facilitated by the Swift satellite, have produced a large sample of spectral energy distributions and light curves, from which the basic micro- and macrophysical parameters of afterglows may be derived. However, a number of phenomena have been observed that defy explanation by simp
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We report on a complete set of early optical afterglows of gamma-ray bursts (GRBs) obtained with the ROTSE-III telescope network from March 2005 through June 2007. This set is comprised of 12 afterglows with early optical and Swift/XRT observations, with a median ROTSE-III response time of 45 s after the start of gamma-ray emission (8 s after the GCN notice time). These afterglows span four orders of magnitude in optical luminosity, and the contemporaneous X-ray detections allow multi-wavelength spectral analysis. Excluding X-ray flares, the broadband synchrotron spectra show that the optical and X-ray emission originate in a common region, consistent with predictions of the external forward shock in the fireball model. However, the fireball model is inadequate to predict the temporal decay indices of the early afterglows, even after accounting for possible long-duration continuous energy injection. We find that the optical afterglow is a clean tracer of the forward shock, and we use the peak time of the forward shock to estimate the initial bulk Lorentz factor of the GRB outflow, and find 100<Gamma_0<1000, consistent with expectations.
We report on follow-up observations of 20 short-duration gamma-ray bursts performed in grizJHKs with the seven-channel imager GROND between mid-2007 and the end of 2010. This is one of the most comprehensive data sets on GRB afterglow observations of short bursts published so far. In three cases GROND was on target within less than 10 min after the trigger, leading to the discovery of the afterglow of GRB 081226A and its faint underlying host galaxy. In addition, GROND was able to image the optical afterglow and follow the light-curve evolution in further five cases, GRBs 090305, 090426, 090510, 090927, and 100117A. In all other cases optical/NIR upper limits can be provided on the afterglow magnitudes.
The redshift distribution of gamma-ray bursts (GRBs) is strongly biased by selection effects. We investigate, via Monte Carlo simulations, one possible selection effect that may be modifying the Swift GRB redshift distribution. We show how telescope response times to acquire a GRB redshift may, via the Malmquist effect and GRB optical afterglow brightness distribution, introduce a bias into the average of the observed redshift distribution. It is difficult to reconcile a recently reported correlated trend between telescope response time and average redshifts unless we employ a redshift-dependent optical afterglow distribution. Simulations of this selection effect suggest that GRB optical afterglows may have been either intrinsically brighter early in the Universe or suffered less local host galaxy extinction.
We report multi-frequency circular polarization measurements for the four extragalactic radio sources 0056-00, 0716+71, 3C138 and 3C161 taken at the Effelsberg 100-m radiotelescope. The data reduction is based on a new calibration procedure that allows the contemporary measurement of the four Stokes parameters at different frequencies with single-dish radiotelescopes. We are in the process of framing the observed full Stokes spectra within a theoretical model that explains that the level of measured circular polarization as Faraday conversion.
We report on a VLA survey for late-time radio emission from 59 supernovae (SNe) of Type I b/c, which have been associated with long-duration gamma-ray bursts (GRBs). An off-axis GRB burst (i.e. whose relativistic jet points away from us) is expected to have late-time radio emission even in the absence of significant prompt gamma-ray emission. From our sample, we detected only SN 2003gk with an 8.4-GHz flux density of $2260 pm 130 ,mu$Jy. Our subsequent VLBI observations of SN 2003gk, at an age of $sim$8 yr, allowed us to determine its radius to be $(2.4 pm 0.4) times 10^{17}$ cm, or $94 pm 15$ light days. This radius rules out relativistic expansion as expected for an off-axis GRB jet, and instead suggests an expansion speed of $sim 10:000$ km s$^{-1}$ typical for non-relativistic core-collapse supernovae. We attribute the late-onset radio emission to interaction of the ejecta with a dense shell caused by episodic mass-loss from the progenitor. In addition, we present new calculations for the expected radio lightcurves from GRB jets at various angles to the line of sight, and compare these to our observed limits on the flux densities of the remainder of our SN sample. From this comparison we can say that only a fraction of broadlined Type I b/c SNe have a radio-bright jet similar to those seen for GRB afterglows at cosmological distances. However, we also find that for a reasonable range of parameters, as might be representative of the actual population of GRB events rather than the detected bright ones, the radio emission from the GRB jets can be quite faint, and that at present, radio observations do not place strong constraints on off-axis GRB jets.
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