1212.4492
Helium in natal HII regions: the origin of the X-ray absorption in gamma-ray burst afterglows
Watson et al
As the title says. Soft X-ray absorption in excess of Galactic observed in GRBs were a mystery prior to this study. Find He in GRB's host HII region is responsible for most of the absorption. Show X-ray absorbing column density (N_Hx) is correlated with both the neutral gas column density and with the optical afterglow extinction (Av). This correlation explains the connection between dark bursts and bursts with high N_Hx values. Find that the correlation with the dust column has a strong z evolution, whereas the correlation with the neutral gas does not. Column density of the X-ray absorption is correlated with the total gas column density in the host galaxy rather than the metal column density, in spite of the fact that X-ray absorption is typically dominated by metals. The strong redshift absorption of X-rays in GRB afterglows is caused by He in the HII region hosting the GRB. While dust is destroyed and metals are stripped of all of their electrons by the GRB to great distances, the abundance of He saturates the He-ionising UV continuum much closer to the GRB, allowing it to remain in the neutral or singly-ionised state. Helium X-ray absorption explains the correlation with the total gas, the lack of strong evolution with redshift as well as the absence of dust, metal or hydrogen absorption features in the optical-UV spectra.
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