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Title:
Casting Light on the "Anomalous" Statistics of Mg II Absorbers Toward Gamma-ray Burst Afterglows: The Incidence of Weak Systems
Authors:
Tejos, Nicolas; Lopez, Sebastian; Prochaska, J. Xavier; Bloom, Joshua S.; Chen, Hsiao-Wen; Dessauges-Zavadsky, Miroslava; Maureira, Maria J.
Affiliation:
AA(Departamento de Astronomía, Universidad de Chile, Casilla 36-D, Santiago, Chile ), AB(Departamento de Astronomía, Universidad de Chile, Casilla 36-D, Santiago, Chile ), AC(Department of Astronomy, UCO/Lick Observatory, University of California, 1156 High Street, Santa Cruz, CA 95064, USA ), AD(Department of Astronomy, 601 Campbell Hall, University of California, Berkeley, CA 94720-3411, USA ), AE(Department of Astronomy, University of Chicago, 5640 S. Ellis Ave., Chicago, IL 60637, USA ), AF(Geneva Observatory, University of Geneva, 51, Ch. des Maillettes, 1290 Sauverny, Switzerland), AG(Departamento de Astronomía, Universidad de Chile, Casilla 36-D, Santiago, Chile )
Publication:
The Astrophysical Journal, Volume 706, Issue 2, pp. 1309-1315 (2009). (ApJ Homepage)
Publication Date:
12/2009
Origin:
IOP
ApJ Keywords:
gamma rays: bursts, gamma rays: observations, gravitational lensing, intergalactic medium, quasars: absorption lines, techniques: spectroscopic
DOI:
10.1088/0004-637X/706/2/1309
Bibliographic Code:
2009ApJ...706.1309T

Abstract

We revisit echelle spectra (spectral resolution R ≈ 40,000) of eight gamma-ray burst (GRB) afterglows to obtain the incidence (dN/dz) of weak intervening Mg II systems at a mean redshift of langzrang = 1.5. We show that dN/dz of systems having rest-frame equivalent widths 0.07 Å \le W_r^{Mg\,\mathsc{ii}} < 1 Å toward GRBs is statistically consistent with the incidence toward quasi-stellar objects (QSOs). Our result is in contrast to the results for Mg II systems having Wr >= 1 Å, where dN/dz toward GRBs has been found to be larger than toward QSOs by a factor of ≈4. We confirm the overdensity albeit at a factor of ≈3 only. This suggests that any explanation for the GRB/QSO discrepancy, be it intrinsic to the absorbers or a selection effect, should be inherent only to the galaxies that host strong absorbers in the line of sight to GRBs. We argue that, of all scenarios that have been proposed, lensing amplification is the one that could explain the strong Mg II enhancement while allowing for no significant enhancement in the weak absorbers.
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