An Independent Measurement of the Incidence of MgII Absorbers along Gamma-Ray Burst Sightlines: the End of the Mystery?

Abstract

In 2006, Prochter et al. reported a statistically significant enhancement of very strong Mg II absorption systems intervening the sightlines to gamma-ray bursts (GRBs) relative to the in- cidence of such absorption along quasar sightlines. This counterintuitive result, has inspired a diverse set of astrophysical explanations (e.g. dust, gravitational lensing) but none of these has obviously resolved the puzzle. Using the largest set of GRB afterglow spectra available, we reexamine the purported enhancement. In an independent sample of GRB spectra with a survey path 3 times larger than Prochter et al., we measure the incidence per unit redshift of $\ge 1$Årest-frame equivalent width Mg II absorbers at $z\approx 1$ to be l(z)= 0.18 $\pm$ 0.06. This is fully consistent with current estimates for the incidence of such absorbers along quasar sightlines. Therefore, we do not confirm the original enhancement and suggest those results suffered from a statistical fluke. Signatures of the original result do remain in our full sample (l(z) shows an $\approx 1.5$ enhancement over l(z)QSO), but the statistical significance now lies at $\approx 90$ c.l. Restricting our analysis to the subset of high-resolution spectra of GRB afterglows (which overlaps substantially with Prochter et al.), we still reproduce a statistically significant enhancement of Mg II absorption. The reason for this excess, if real, is still unclear since there is no connection between the rapid afterglow follow-up process with echelle (or echellette) spectrographs and the detectability of strong Mg II doublets. Only a larger sample of such high-resolution data will shed some light on this matter.

42 pages, 22 figures, submitted to ApJ

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• Deciphering the unusual stellar progenitor of GRB 210704A
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• Solving the conundrum of intervening strong MgII absorbers towards GRBs and quasars
• The CGM-GRB Study. I. Uncovering The CircumGalactic Medium around GRB hosts at redshifts 2-6
• X-shooter and ALMA spectroscopy of GRB 161023A - A study of metals and molecules in the line of sight towards a luminous GRB
• Evidence of Contribution of Intervening Clouds to GRB's X-ray Column Density
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