On the incidence of C IV absorbers along the sight lines to gamma-ray bursts

We report on the statistics of strong (W_r > 0.15 Å) C IV absorbers at z = 1.5-3.5 toward high-redshift gamma-ray bursts (GRBs). In contrast with a recent survey for strong Mg II absorption systems at z < 2, we find that the number of C IV absorbers per unit redshift dN/dz does not show a significant deviation from previous surveys using quasi-stellar objects (QSOs) as background sources. We find that the number density of C IV toward GRBs is dN/dz|_GRB(z ∼ 1.5) = 2.2_-1.4^+2.8, dN/dz|_GRB(z ∼ 2.5) = 2.3_-1.1^+1.8 and dN/dz|_GRB(z ∼ 3.5) = 1.1_-0.9^+2.6. These numbers are consistent with previous C IV surveys using QSO spectra. Binning the entire data set, we set a 95% c.l. upper limit to the excess of C IV absorbers along GRB sight lines at twice the incidence observed along QSO sight lines. Furthermore, the distribution of equivalent widths of the GRB and QSO samples are consistent with being drawn from the same parent population. Although the results for Mg II and C IV absorbers along GRB sight lines appear to contradict one another, we note that the surveys are nearly disjoint: the C IV survey corresponds to higher redshift and more highly ionized gas than the Mg IV survey. Nevertheless, analysis on larger statistical samples may constrain properties of the galaxies hosting these metals (e.g., mass, dust content) and/or the coherence-length of the gas giving rise to the metal-line absorption.