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

Creators: Mirabal, N.; Halpern, J. P.; Kulkarni, S. R.; Castro, S.; Bloom, J. S.; Djorgovski, S. G.; Galama, T. J.; Harrison, F. A.; Frail, D. A.; Price, P. A.; Reichart, D. E.; Ebeling, H.; Bunker, A.; Dawson, S.; Dey, A.; Spinrad, H.; Stern, D.

Abstract

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 H2. 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_(H I) = 10^2 cm^(-3). If depletion of metals onto dust is similar to Galactic values or less than solar abundances are present, then n_(H I) ≥ 2 × 10^4 cm^(-3) is probably ruled out in the immediate vicinity of the burst.

Additional Information

© 2002 The American Astronomical Society. Received 2002 March 4; accepted 2002 July 1. 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. We would like to thank R.-P. Kudritzki and F. Bresolin for obtaining the first set of spectra. We also acknowledge Eric Gotthelf for allowing us to use his new Alpha computer and Robert Uglesich for tips on optimization in FORTRAN. This work was supported by the National Science Foundation under grant AST 00-71108. J. S. B. is supported as a Fannie and John Hertz Fellow. The work of S.D. was supported by IGPP-LLNL University Collaborative Research Program grant 02-AP-015 and was performed under the auspices of the US Department of Energy, National Nuclear Security Administration by the University of California, Lawrence Livermore National Laboratory under contract W-7405-Eng-48. The National Optical Astronomy Observatory is operated by the Association of Universities for Research in Astronomy, Inc. (AURA) under cooperative agreement with the National Science Foundation. The work of D.S. was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA.

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