DUST EXTINCTION BIAS IN THE COLUMN DENSITY DISTRIBUTION OF GAMMA-RAY BURSTS: HIGH COLUMN DENSITY, LOW-REDSHIFT GRBs ARE MORE HEAVILY OBSCURED

Abstract

The afterglows of gamma-ray bursts (GRBs) have more soft X-ray absorption than expected from the foreground gas column in the Galaxy. While the redshift of the absorption can in general not be constrained from current X-ray observations, it has been assumed that the absorption is due to metals in the host galaxy of the GRB. The large sample of X-ray afterglows and redshifts now available allows the construction of statistically meaningful distributions of the metal column densities. We construct such a sample and show, as found in previous studies, that the typical absorbing column density (N HX ) increases substantially with redshift, with few high column density objects found at low to moderate redshifts. We show, however, that when highly extinguished bursts are included in the sample, using redshifts from their host galaxies, high column density sources are also found at low to moderate redshift. We infer from individual objects in the sample and from observations of blazars, that the increase in column density with redshift is unlikely to be related to metals in the intergalactic medium or intervening absorbers. Instead we show that the origin of the apparent increase with redshift is primarily due to dust extinction bias: GRBs with high X-ray absorption column densities found at z 4 typically have very high dust extinction column densities, while those found at the highest redshifts do not. It is unclear how such a strongly evolving N HX /A V ratio would arise, and based on current data, remains a puzzle.

The distribution of equivalent widths in long GRB afterglow spectra

“…However, this extinction is not directly caused by hydrogen but by metals, so it is very dependent on the composition of the host galaxy. The relation between the extinction and N H is well known for the Milky Way, but the uncertainty is large in GRB host galaxies (Watson & Jakobsson 2012). Comparing the LSP with the X-ray derived N H (Fig.…”

Evidence of Contribution of Intervening Clouds to Gamma-Ray Burst's X-Ray Column Density

“…By excluding the GRBs with upper limits of N H,X , a statistical test returns a Kendall's τ = 0.409, a Z-value of 2.81, and P = 0.005, where P is the probability that there is no correlation between the two variables. A few GRBs with high column density and heavy optical extinction have been identified in the low-to-moderate redshift range (Campana et al 2012;Watson & Jakobsson 2012). The heavy extinction most likely results in a selection bias against these bursts in the current sample.…”

“…Under some conditions, the intervening absorbers can also affect the A V determination and may contribute to the observed N H,X versus A V correlation found by Watson et al (2013) and Covino et al (2013). Watson & Jakobsson (2012) argued that the intervening clouds seem to be insufficient to explain the column density excess after taking into account the metallicity evolution and unjustified overabundance of intervening clouds in the GRB's line of sight. In fact, a much weaker overabundance was recently revealed in Cucchiara et al (2013) when they re-analyzed a large sample of intervening Mg ii absorption.…”

“…The heavy extinction most likely results in a selection bias against these bursts in the current sample. Watson & Jakobsson (2012) stated that the redshifts of these bursts are mainly obtained from the observations of their host galaxies. typical dust-to-gas ratio for the Local Group in the case with solar metallicity (Welty et al 2012).…”

“…In addition, Campana et al (2012) and Watson & Jakobsson (2012) show that the bursts with high column density can occur at low-tomoderate redshift when highly extinguished events are included. There is, however, a lack of bursts with low X-ray column density at high redshift.…”