Time-dependent Optical Spectroscopy of GRB 010222: Clues to the Gamma-Ray Burst Environment


We present sequential optical spectra of the afterglow of GRB 010222 obtained 1 day apart using the Low-Resolution Imaging Spectrometer (LRIS) and the Echellette Spectrograph and Imager (ESI) on the Keck Telescopes. Three low-ionization absorption systems are spectroscopically identified at z$_1$=1.47688, z$_2$=1.15628, and z$_3$=0.92747. The higher resolution ESI spectrum reveals two distinct components in the highest redshift system at z$_1a$=1.47590 and z$_1b$=1.47688. We interpret the z$_1b$=1.47688 system as an absorption feature of the disk of the host galaxy of GRB 010222. The best-fitted power-law optical continuum and [Zn/Cr] ratio imply low dust content or a local gray dust component near the burst site. In addition, we do not detect strong signatures of vibrationally excited states of H$_2$. If the gamma-ray burst took place in a superbubble or young stellar cluster, there are no outstanding signatures of an ionized absorber either. Analysis of the spectral time dependence at low resolution shows no significant evidence for absorption-line variability. This lack of variability is confronted with time-dependent photoionization simulations designed to apply the observed flux from GRB 010222 to a variety of assumed atomic gas densities and cloud radii. The absence of time dependence in the absorption lines implies that high- density environments are disfavored. In particular, if the GRB environment was dust free, its density was unlikely to exceed n$_HI$=10$^2$ cm$^-3$. If depletion of metals onto dust is similar to Galactic values or less than solar abundances are present, then n$_HI$>=2×10$^4$ cm$^-3$ is probably ruled out in the immediate vicinity of the burst. Based on data obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and NASA and was made possible with the generous financial support of the W. M. Keck Foundation.

Astrophysical Journal