No Evidence for Variability of Intervening Absorption Lines toward GRB 060206: Implications for the Mg II Incidence Problem

We examined the variability of absorption line strength of intervening systems along the line of sight to GRB 060206 at z = 4.05, utilizing low-resolution optical spectra obtained by the Subaru telescope from six to ten hours after the burst. Strong variabilities of Fe II and Mg II lines at z = 1.48 during t = 5-8 hr have been reported for this GRB, and those have been used to support the idea of clumpy Mg it cloudlets, which was originally proposed to explain the anomalously high incidence of Mg it absorbers in the GRB spectra compared to quasars. However, our spectra with a higher signal-to-noise ratio do not show any evidence for variability in t = 6-10 hr. There is a clear discrepancy between our data and those of Hao et al. (2007, ApJ, 659, L99) in the overlapping time interval. Furthermore, the line strengths in our data are in agreement with those observed at t similar to 2 hr by Thone et al. (2008, A&A, 489, 37). We also detected Fell and Mg II absorption lines for a system at z = 2.26; these lines do not show evidence for variability either. Therefore, we conclude that there is no strong evidence for the variability in the intervening absorption lines toward GRB 060206, offering poor support for the Mg II cloudlet hypothesis by the GRB 060206 data.