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Register a new userThe Strongly Polarized Afterglow of GRB 020405
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We report polarization measurements and photometry for the optical afterglow of the gamma-ray burst GRB 020405. We measured a highly significant 9.9% polarization (in V band) 1.3 days after the burst and argue that it is intrinsic to the GRB. The light curve decay is well fitted by a $t^{-1.72}$ power-law; we do not see any evidence for a break between 1.24 and 4.3 days after the burst. We discuss these measurements in the light of several models of GRB afterglows.
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We present an optical-to-X-ray spectral analysis of the afterglow of GRB 020405. The optical spectral energy distribution not corrected for the extragalactic extinction is significantly below the X-ray extrapolation of the single powerlaw spectral model suggested by multiwavelength studies. We investigate whether considerable extinction could explain the observed spectral ``mismatch by testing several types of extinction curves. For the first time we test extinction curves computed with time-dependent numerical simulations of dust grains destruction by the burst radiation. We find that an extinction law weakly depen dent on wavelength can reconcile the unabsorbed optical and X-ray data with the expected synchrotron spectrum. A gray extinction law can be provided by a dust grain size distribution biased toward large grains.
We present the discovery of GRB 020405 made with the Inter-Planetary Network (IPN). With a duration of 60 s, the burst appears to be a typical long duration event. We observed the 75-square acrminute IPN error region with the Mount Stromlo Observatorys 50-inch robotic telescope and discovered a transient source which subsequently decayed and was also associated with a variable radio source. We identify this source as the afterglow of GRB 020405. Subsequent observations by other groups found varying polarized flux and established a redshift of 0.690 to the host galaxy. Motivated by the low redshift we triggered observations with WFPC2 on-board the Hubble Space Telescope (HST). Modeling the early ground-based data with a jet model, we find a clear red excess over the decaying optical lightcurves that is present between day 10 and day 141 (the last HST epoch). This `bump has the spectral and temporal features expected of an underlying supernova (SN). In particular, the red color of the putative SN is similar to that of the SN associated with GRB 011121, at late time. Restricting the sample of GRBs to those with z<0.7, a total of five bursts, red bumps at late times are found in GRB 970228, GRB 011121, and GRB 020405. It is possible that the simplest idea, namely that all long duration GRBs have underlying SNe with a modest dispersion in their properties (especially peak luminosity), is sufficient to explain the non detections.
We present the observations of the afterglow of gamma-ray burst GRB 090102. Optical data taken by the TAROT, REM, GROND, together with publicly available data from Palomar, IAC and NOT telescopes, and X-ray data taken by the XRT instrument on board the Swift spacecraft were used. This event features an unusual light curve. In X-rays, it presents a constant decrease with no hint of temporal break from 0.005 to 6 days after the burst. In the optical, the light curve presents a flattening after 1 ks. Before this break, the optical light curve is steeper than that of the X-ray. In the optical, no further break is observed up to 10 days after the burst. We failed to explain these observations in light of the standard fireball model. Several other models, including the cannonball model were investigated. The explanation of the broad band data by any model requires some fine tuning when taking into account both optical and X-ray bands.
We here report on the photometric, spectroscopic and polarimetric monitoring of the optical afterglow of the Gamma-Ray Burst (GRB) 030328 detected by HETE-2. We found that a smoothly broken power-law decay provides the best fit of the optical light curves, with indices alpha_1 = 0.76 +/- 0.03, alpha_2 = 1.50 +/- 0.07, and a break at t_b = 0.48 +/- 0.03 d after the GRB. Polarization is detected in the optical V-band, with P = (2.4 +/- 0.6)% and theta = (170 +/- 7) deg. Optical spectroscopy shows the presence of two absorption systems at z = 1.5216 +/- 0.0006 and at z = 1.295 +/- 0.001, the former likely associated with the GRB host galaxy. The X-ray-to-optical spectral flux distribution obtained 0.78 days after the GRB was best fitted using a broken power-law, with spectral slopes beta_opt = 0.47 +/- 0.15 and beta_X = 1.0 +/- 0.2. The discussion of these results in the context of the fireball model shows that the preferred scenario is a fixed opening angle collimated expansion in a homogeneous medium.
We present multiwavelength observations of a gamma-ray burst detected by INTEGRAL (GRB 030227) between 5.3 hours and ~1.7 days after the event. Here we report the discovery of a dim optical afterglow (OA) that would not have been detected by many previous searches due to its faintess (R~23). This OA was seen to decline following a power law decay with index Alpha_R= -0.95 +/- 0.16. The spectral index Beta_opt/NIR yielded -1.25 +/- 0.14. These values may be explained by a relativistic expansion of a fireball (with p = 2.0) in the cooling regime. We also find evidence for inverse Compton scattering in X-rays.
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