Abstract
We present near-infrared and optical observations of the afterglow to the gamma-ray burst 991216 obtained with the Fred L. Whipple Observatory 1.2 m telescope and the University of Hawaii 2.2 m telescope. The observations range from 15 hours to 3.8 days after the burst. The temporal behavior of the data is well described by a single power-law decay t-1.36±0.04, independent of wavelength. The optical spectral energy distribution, which is corrected for significant Galactic reddening of E(B-V) = 0.626, is well fitted by a single power law with ν-0.58±0.08. Combining the IR/optical observations with a Chandra X-Ray Observatory measurement gives a spectral index of -0.8 ± 0.1 in the synchrotron cooling regime. A comparison between the spectral and temporal power-law indices suggests that a jet is a better match to the observations than a simple spherical shock.
Highlights
The BeppoSAX (Boella et al 1997) and Rossi X-Ray T iming Explorer (RXT E ; Levine et al 1996) satellites have brought a new dimension to gamma-ray burst (GRB) research by providing rapid localizations of several bursts per year
We present optical and near-IR photometry of GRB 991216 from observations obtained at the Hawaii 2.2 m and the Fred L
Combining our three R-band observations with six observations from the GRB Coordinates Network (GCN) Circulars that were obtained within 4 days of the burst gives a power-law index of a \ 1.30 ^ 0.05, which is somewhat steeper than the decay rate found by Sagar et al TABLE 3 GRB 991216 R-BAND LIGHT CURVE
Summary
The BeppoSAX (Boella et al 1997) and Rossi X-Ray T iming Explorer (RXT E ; Levine et al 1996) satellites have brought a new dimension to gamma-ray burst (GRB) research by providing rapid localizations of several bursts per year. This has allowed many GRBs to be followed up at other wavelengths, ranging from the X-ray (Costa et al 1997) and optical (van Paradijs et al 1997) to the radio (Frail et al 1997). The RXT E Proportional Counter Array search for the X-ray afterglow of GRB 991216 started about 4 hours after the burst (Takeshima et al 1999) and detected a strong, decaying X-ray afterglow, providing a much improved burst position. In ° 4 we describe the broadband spectral properties of the afterglow deduced from our IR/optical data
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