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The Spectroscopic Variability of GRB 021004

By Thomas Matheson, Peter M. Garnavich, Craig Foltz, S. West, G. Williams, Michael L. Calkins, Francisco J. Cast, Eric Gawiser, Saurabh Jha, David Bersier and Krzysztof Z. Stanek


We present spectra of the optical transient (OT) associated with GRB 021004. The spectra show a blue continuum with superposed absorption features and one emission line. There are two intervening metal-line systems at z = 1.380 and z = 1.602 and one very strong absorption system at a redshift of z = 2.323. Lyα emission is also seen at this redshift. While the spectrum of the OT overall cannot be simply described with a power law, the spectral index over the range 5500 − 8850 ˚A is steep, Fν ∝ ν −0.96±0.03. Comparison of spectra from multiple epochs shows a distinct color evolution with the OT becoming redder with time, implying a B − V increase of ∼ 0.2 − 0.3 mag over the first three days. This is the first clear example of color change in an OT detected spectroscopically. Subject headings: galaxies: distances and redshifts—galaxies: ISM—gamma rays: bursts—ultraviolet: galaxies

Year: 2002
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