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Soft X-ray absorption in excess of Galactic is observed in the afterglows ofmost gamma-ray bursts (GRBs), but the correct solution to its origin has notbeen arrived at after more than a decade of work, preventing its use as apowerful diagnostic tool. We resolve this long-standing problem and find thatHe in the GRB's host HII region is responsible for most of the absorption. Weshow that the X-ray absorbing column density (N_Hx) is correlated with both theneutral gas column density and with the optical afterglow extinction (Av). Thiscorrelation explains the connection between dark bursts and bursts with highN_Hx values. From these correlations we exclude an origin of the X-rayabsorption which is not related to the host galaxy, i.e. the intergalacticmedium or intervening absorbers are not responsible. We find that thecorrelation with the dust column has a strong redshift evolution, whereas thecorrelation with the neutral gas does not. From this we conclude that thecolumn density of the X-ray absorption is correlated with the total gas columndensity in the host galaxy rather than the metal column density, in spite ofthe fact that X-ray absorption is typically dominated by metals. The strongredshift evolution of N_Hx/Av is thus a reflection of the cosmic metallicityevolution of star-forming galaxies. We conclude that the absorption of X-raysin GRB afterglows is caused by He in the HII region hosting the GRB. While dustis destroyed and metals are stripped of all of their electrons by the GRB togreat distances, the abundance of He saturates the He-ionising UV continuummuch closer to the GRB, allowing it to remain in the neutral or singly-ionisedstate. Helium X-ray absorption explains the correlation with total gas, thelack of strong evolution with redshift as well as the absence of dust, metal orhydrogen absorption features in the optical-UV spectra.

Helium in natal HII regions: the origin of the X-ray absorption in gamma-ray burst afterglows