The Spectroscopic Variability of GRB 021004
  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. We confirm 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. Ly_alpha 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, F_nu ~ nu^(-0.96 +/- 0.03). Comparison of spectra from multiple epochs shows a distinct color evolution with the OT becoming redder with time over the first three days. This is the first clear example of color change in an OT detected spectroscopically. 
T. Matheson, P. M. Garnavich, C. Foltz, S. West, G. Williams, E.   Falco, M. L. Calkins, F. J. Castander, E. Gawiser, S. Jha...