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Register a new userDust Depletion and Extinction in a Gamma-Ray Burst Afterglow
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Sandra Savaglio INAF-Rome
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We put stringent constraints for the first time on the dust properties in the circumburst medium of a gamma-ray burst (GRB) afterglow. This is based on the optical spectrum of GRB 020813 (z=1.255), obtained with Keck I LRIS 4.65 h after the burst. From the absorption lines in the spectrum, we derive very high column densities for six heavy elements with different refractory properties. The relative abundances resemble the dust depletion patterns in the Milky Way, from which we infer a visual extinction of A_V=0.4 and A_V>0.3 at 95% confidence level. However, the high columns of metals and dust contrast with an observed UV continuum spectrum that is remarkably close to a power law of the form F_nu propto nu^{-0.9}, with no sign of curvature, or a 2200 A extinction feature, suggesting low reddening. The Milky Way or Magellanic Cloud reddenings are possible only for very low visual extinctions (A_V<0.08 or A_V<0.2, respectively at 95% confidence), inconsistent with the A_V values inferred from the depletion analysis. If we assume a GRB intrinsic spectrum and an extinction law of the forms F_lambda^i = F_V (5500/lambda)^alpha and A_lambda= A_V (5500/lambda)^gamma, we obtain (for A_V=0.4) the constraints from continuum spectrum: gamma<0.85 and alpha<1.72.
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In order to study the effect of dust extinction on the afterglow of gamma-ray bursts (GRBs), we carry out numerical calculations with high precision based on rigorous Mie theory and latest optical properties of interstellar dust grains, and analyze the different extinction curves produced by dust grains with different physical parameters. Our results indicate that the absolute extinction quantity is substantially determined by the medium density and metallicity. However, the shape of the extinction curve is mainly determined by the size distribution of the dust grains. If the dust grains aggregate to form larger ones, they will cause a flatter or grayer extinction curve with lower extinction quantity. On the contrary, if the dust grains are disassociated to smaller ones due to some uncertain processes, they will cause a steeper extinction curve with larger amount of extinction. These results might provide an important insight into understanding the origin of the optically dark GRBs.
The composition and amount of interstellar dust within gamma-ray burst (GRB) host galaxies is of key importance when addressing selection effects in the GRB redshift distribution, and when studying the properties of their host galaxies. As well as the implications for GRB research, probing the dust within the high-z hosts of GRBs also contributes to our understanding of the conditions of the interstellar medium and star-formation in the distant Universe. Nevertheless, the physical properties of dust within GRB host galaxies continues to be a highly contended issue. In this paper we explore the mean extinction properties of dust within the host galaxies of a sample of 17 GRBs with total host galaxy visual extinction Av<1 (<Av>=0.4), covering a redshift range z=0.7-3.1. We find the average host extinction curve to have an ultraviolet slope comparable to that of the LMC, but with little evidence of a 2175Angs dust extinction feature as observed along Milky Way and LMC sightlines. We cannot at present rule out the presence of a 2175Angs feature, and both the standard SMC and LMC extinction curves also provide good fits to our data. However, we can reject an extinction curve that has a UV slope as flat as the mean Milky Way extinction curve, whilst also having a 2175Angs feature as prominent as seen in the mean Milky Way extinction curve. This is in contrast to the clear detection of a 2175Angs bump and the flatter extinction curves of some more heavily extinguished GRBs (Av>1), which may be indicative of there being a dependence between dust abundance and the wavelength dependence of dust extinction, as has been previously speculated.
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