Abstract
Very early observations with the Swift satellite of γ-ray burst (GRB) afterglows reveal that the optical component is not detected in a large number of cases. This is in contrast to the bright optical flashes previously discovered in some GRBs (e.g., GRB 990123 and GRB 021211). Comparisons of the X-ray afterglow flux to the optical afterglow flux and prompt γ-ray fluence is used to quantify the seemingly deficient optical, and in some cases X-ray, light at these early epochs. This comparison reveals that some of these bursts appear to have higher than normal γ-ray efficiencies. We discuss possible mechanisms and their feasibility for explaining the apparent lack of early optical emission. The mechanisms considered include, foreground extinction, circumburst absorption, Lyα blanketing and absorption due to high-redshift, low-density environments, rapid temporal decay, and intrinsic weakness of the reverse shock. Of these, foreground extinction, circumburst absorption, and high redshift provide the best explanations for most of the nondetections in our sample. There is tentative evidence of suppression of the strong reverse shock emission. This could be because of a Poynting flux-dominated flow or a pure nonrelativistic hydrodynamic reverse shock.
Highlights
The afterglow discoveries of 1997 revealed that gamma-ray bursts (GRBs) are the brightest explosions in the universe
Between 2005 January 24 and June 30, the Swift Burst Alert Telescope (BAT; Barthelmy et al 2005) detected and localized 26 GRBs, which were thereafter observed by the Swift X-Ray Telescope (XRT; Burrows et al 2005a) and UV/Optical Telescope (UVOT)
We have compared the X-ray afterglow flux to the optical afterglow flux and prompt -ray fluence of a sample of Swift GRBs to quantify the lack of optical afterglow emission
Summary
2006), their optical afterglows often remain undetected even in deep searches (Roming & Mason 2006, hereafter RM06) Identifying and characterizing these ‘‘dark’’ bursts has been difficult due to the delays (typically hours) occurring between the detection of the GRB and the execution of the first ground-based observations. Observations of GRBs with NASA’s Swift satellite (Gehrels et al 2004) are providing prompt few-arcminute -ray localizations, rapid few-arcsecond X-ray positions, and rapid and extensive follow-up in the X-ray, UV, optical, and radio bands (e.g., Gehrels et al 2005; Cameron & Frail 2005). Thirteen of these bursts include extraordinary optical upper limits at very early epochs after the burst.
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