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Register a new userAnomalous Optical GRB Afterglows are Common: Two z~4 Bursts, GRB 060206 and 060210
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We report on two recent z~4 gamma-ray bursts (GRBs), GRB 060206 and GRB 060210, for which we have obtained well-sampled optical light curves. Our data, combined with early optical data reported in the literature, shows unusual behavior for both afterglows. In R-band GRB 060206 (z=4.045) experienced a slow early decay, followed by a rapid increase in brightness by factor ~2.5 about 1 hour after the burst. Its afterglow then faded in a broken power-law fashion, with a smooth break at t_b=0.6 days, but with additional, less dramatic (~10%) ``bumps and wiggles, well detected in the densely sampled light curve. The R-band afterglow of GRB 060210 (z=3.91) is also unusual: the light curves was more or less flat between 60 and 300 sec after the burst, followed by ~70% increase at ~600 sec after the burst, after which the light curve declined as a t^{-1.3} power-law. Despite earlier reports to the contrary, we find that for GRB 060206 X-rays also more or less follow the optical decay, but with significant variations on short timescales. However, the X-ray afterglow is contaminated by a nearby, variable source, which especially at late times obscures the behavior of the X-ray afterglow. We argue that ``anomalous optical afterglows are likely to be the norm, and that the rapid variations often seen in Swift-XRT data would also be seen in the optical light curves, given good enough sampling. As a result, some of the often employed procedures, such as deriving the jet opening angle using a smooth broken power-law fit to the optical light curves, in many cases might have a poor statistical significance. We argue that the early increase in brighness for both bursts might be due to the turn-on of the external shock. Existence of such features could provide valuable additional information about the burst. (Abridged)
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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 use a large sample of GRB afterglow and prompt-emission data (adding further GRB afterglow observations in this work) to compare the optical afterglows (or the lack thereof) of Type I GRBs with those of Type II GRBs. In comparison to the afterglows of Type II GRBs, we find that those of Type I GRBs have a lower average luminosity and show an intrinsic spread of luminosities at least as wide. From late and deep upper limits on the optical transients, we establish limits on the maximum optical luminosity of any associated supernova, confirming older works and adding new results. We use deep upper limits on Type I GRB optical afterglows to constrain the parameter space of possible mini-SN emission associated with a compact-object merger. Using the prompt emission data, we search for correlations between the parameters of the prompt emission and the late optical afterglow luminosities. We find tentative correlations between the bolometric isotropic energy release and the optical afterglow luminosity at a fixed time after trigger (positive), and between the host offset and the luminosity (negative), but no significant correlation between the isotropic energy release and the duration of the GRBs. We also discuss three anomalous GRBs, GRB 060505, GRB 060614, and GRB 060121, in the light of their optical afterglow luminosities. (Abridged)
The CCD magnitudes in Cousins R and I photometric passbands are determined for GRB 991216 and GRB 991208 afterglows respectively about 1 and about 3 day after trigger of the corresponding gamma-ray bursts. Light curves of the afterglow emissions are obtained by combining the published data with the present measurements in R and I passbands for GRB 991208 and in R, Gunn I and J passbands for GRB 991216. They indicate that the flux decay constants of a GRB are almost the same in each passband with values about 2.2 for GRB 991208 and about 1.2 for GRB 991216 indicating very fast optical flux decay in the case of former which may be due to beaming effect. However, cause of steepening by 0.23 +/- 0.06 dex in the R light curve of GRB 991216 afterglow between 2 to 2.5 day after the burst, is presently not understood. Redshift determinations indicate that both GRBs are at cosmological distance with a value of 4.2 Gpc for GRB 991208 and 6.2 Gpc for GRB 991216. The observed fluence above 20 keV indicates, if isotropic, release of energy about 1.3 x 10^{53} erg for GRB 991208 and about 6.7 x 10^{53} erg for GRB 991216 by these bright gamma-ray flashes. The enormous amount of released energy will be reduced, if the radiation is beamed which seems to be case for GRB 991208 afterglow.The quasi-simultaneous broad-band photometric spectral energy distributions of the afterglows are determined about 8.5 day and about 35 hour after the bursts of GRB 991208 and GRB 991216 respectively.The flux decreases exponentially with frequency. The value of spectral index in the optical-near IR region is -0.75 +/- 0.03 for GRB 991208 and -1.0 +/- 0.12 for GRB 991216.
We present the time-resolved optical emission of gamma-ray bursts GRB 060904B and GRB 070420 during their prompt and early afterglow phases. We used time resolved photometry from optical data taken by the TAROT telescope and time resolved spectroscopy at high energies from the Swift spacecraft instrument. The optical emissions of both GRBs are found to increase from the end of the prompt phase, passing to a maximum of brightness at t_{peak}=9.2 min and 3.3 min for GRB 060904B and GRB 070420 respectively and then decrease. GRB 060904B presents a large optical plateau and a very large X-ray flare. We argue that the very large X-flare occurring near t_{peak} is produced by an extended internal engine activity and is only a coincidence with the optical emission. GRB 070420 observations would support this idea because there was no X-flare during the optical peak. The nature of the optical plateau of GRB 060904B is less clear and might be related to the late energy injection.
We report on the discovery of strongly variable FeII and MgII absorption lines seen at z=1.48 in the spectra of the z=4.05 GRB 060206 obtained between 4.13 to 7.63 hours (observer frame) after the burst. In particular, the FeII line equivalent width (EW) decayed rapidly from 1.72+-0.25 AA to 0.28+-0.21 AA, only to increase to 0.96+-0.21 AA in a later date spectrum. The MgII doublet shows even more complicated evolution: the weaker line of the doublet drops from 2.05+-0.25 AA to 0.92+-0.32 AA, but then more than doubles to 2.47+-0.41 AA in later data. The ratio of the EWs for the MgII doublet is also variable, being closer to 1:1 (saturated regime) when the lines are stronger and becoming closer to 2:1 (unsaturated regime) when the lines are weaker, consistent with expectations based on atomic physics. We have investigated and rejected the possibility of any instrumental or atmospheric effects causing the observed strong variations. Our discovery of clearly variable intervening FeII and MgII lines lends very strong support to their scenario, in which the characteristic size of intervening patches of MgII ``clouds is comparable to the GRB beam size, i.e, about 10^16 cm. We discuss various implications of this discovery, including the nature of the MgII absorbers, the physics of GRBs, and measurements of chemical abundances from GRB and quasar absorption lines.
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