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Solving the conundrum of intervening strong MgII absorbers towards GRBs and quasars
journal contributionposted on 2018-04-10, 15:33 authored by L. Christensen, S. D. Vergani, S. Schulze, N. Annau, J. Selsing, J. P. U. Fynbo, A. D. U. Postigo, R. Cañameras, S. Lopez, D. Passi, P. Cortés-Zuleta, S. L. Ellison, V. D'Odorico, G. Becker, T. A. M. Berg, Z. Cano, S. Covino, G. Cupani, V. D'Elia, P. Goldoni, A. Gomboc, F. Hammer, K. E. Heintz, P. Jakobsson, J. Japelj, L. Kaper, D. Malesani, P. Møller, P. Petitjean, V. Pugliese, R. Sánchez-Ramírez, N. R. Tanvir, C. C. Thöne, M. Vestergaard, K. Wiersema, G. Worseck
Previous studies have shown that the incidence rate of intervening strong Mg ii absorbers towards gamma-ray bursts (GRBs) were a factor of 2–4 higher than towards quasars. Exploring the similar sized and uniformly selected legacy data sets XQ-100 and XSGRB, each consisting of 100 quasar and 81 GRB afterglow spectra obtained with a single instrument (VLT/X-shooter), we demonstrate that there is no disagreement in the number density of strong Mg ii absorbers with rest-frame equivalent widths Wλ2796 r > 1 Å towards GRBs and quasars in the redshift range 0.1 . z . 5. With large and similar sample sizes, and path length coverages of ∆z = 57.8 and 254.4 for GRBs and quasars, respectively, the incidences of intervening absorbers are consistent within 1σ uncertainty levels at all redshifts. For absorbers at z < 2.3, the incidence towards GRBs is a factor of 1.5 ± 0.4 higher than the expected number of strong Mg ii absorbers in Sloan Digital Sky Survey (SDSS) quasar spectra, while for quasar absorbers observed with X-shooter we find an excess factor of 1.4 ± 0.2 relative to SDSS quasars. Conversely, the incidence rates agree at all redshifts with reported high-spectralresolution quasar data, and no excess is found. The only remaining discrepancy in incidences is between SDSS Mg ii catalogues and high-spectral-resolution studies. The rest-frame equivalent-width distribution also agrees to within 1σ uncertainty levels between the GRB and quasar samples. Intervening strong Mg ii absorbers towards GRBs are therefore neither unusually frequent, nor unusually strong.
CitationAstronomy and Astrophysics, 2017, 608, A84
Author affiliation/Organisation/COLLEGE OF SCIENCE AND ENGINEERING/Department of Physics and Astronomy
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