On the Incidence of C IV Absorbers Along the Sight Lines to Gamma-Ray Bursts

We report on the statistics of strong (Wr > 0.15 A) 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, dN/dz|GRB(z ~ 2.5) = 2.3 and dN/dz|GRB(z ~ 3.5) = 1.1. 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 II 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.

[1]  C. Steidel.  A high-redshift extension of the survey for C IV absorption in the spectra of QSOs - The redshift evolution of the heavy element absorbers , 1990 .

[2]  Jason X. Prochaska,et al.  Echelle Spectroscopy of a Gamma-Ray Burst Afterglow at z = 3.969: A New Probe of the Interstellar and Intergalactic Media in the Young Universe , 2005 .

[3]  Jason X. Prochaska,et al.  Accepted for Publication in the Astrophysical Journal Preprint typeset using L ATEX style emulateapj v. 20/04/00 ON THE ABSENCE OF WIND SIGNATURES IN GRB AFTERGLOW SPECTRA: CONSTRAINTS ON THE WOLF-RAYET WINDS OF GRB PROGENITORS , 2007 .

[4]  A. Moorwood,et al.  Instrument Design and Performance for Optical/Infrared Ground-based Telescopes, , 2003 .

[5]  Simon J. Lilly,et al.  Strong Mg II Systems in Quasar and Gamma-Ray Burst Spectra , 2007 .

[6]  Bernard Delabre,et al.  Design, construction, and performance of UVES, the echelle spectrograph for the UT2 Kueyen Telescope at the ESO Paranal Observatory , 2000, Astronomical Telescopes and Instrumentation.

[7]  A. Boksenberg,et al.  C IV Absorption in a New Sample of 55 QSOs: Evolution and Clustering of the Heavy-Element Absorption Redshifts , 1988 .

[8]  M. C. Bentz,et al.  Strongly variable z = 1.48 Fe II and Mg II absorption in the spectra of z = 4.05 GRB 060206 , 2006 .

[9]  U. Michigan,et al.  The Interstellar Medium of Gamma-Ray Burst Host Galaxies. I. Echelle Spectra of Swift GRB Afterglows , 2006, astro-ph/0611092.

[10]  B. Ménard,et al.  Probing Galaxies through Quasar Absorption Lines , 2005 .

[11]  Caltech,et al.  First Comparison of Ionization and Metallicity in Two Lines of Sight toward HE 1104-1805 AB at z=1.66 , 1998, astro-ph/9806143.

[12]  N. Gehrels.  Confidence limits for small numbers of events in astrophysical data , 1986 .

[13]  M. Couture,et al.  HIRES: the high-resolution echelle spectrometer on the Keck 10-m Telescope , 1994, Astronomical Telescopes and Instrumentation.

[14]  Stephen A. Shectman,et al.  MIKE: A Double Echelle Spectrograph for the Magellan Telescopes at Las Campanas Observatory , 2003, SPIE Astronomical Telescopes + Instrumentation.

[15]  S. Covino,et al.  A Flash in the Dark: UVES Very Large Telescope High-Resolution Spectroscopy of Gamma-Ray Burst Afterglows , 2004, astro-ph/0409717.

[16]  New perspectives on strong z ≃ 0.5 Mg II absorbers: Are halo mass and equivalent width anticorrelated? , 2006, astro-ph/0606328.

[17]  K. Lanzetta,et al.  An Absorption-Line Survey of 32 QSOs at Red Wavelengths: Properties of the MG II Absorbers , 1987 .

[18]  B. Krauskopf,et al.  Proc of SPIE , 2003 .

[19]  IoA,et al.  On the Incidence of Strong Mg II Absorbers along Gamma-Ray Burst Sight Lines , 2006 .

[20]  J. Prochaska,et al.  On the Incidence and Kinematics of Strong Mg II Absorbers , 2004, astro-ph/0411776.