Dense gas linked to star-forming regions photoionized by embedded gamma-ray bursts

Abstract The 1–100-pc region embedding long-duration gamma-ray bursts (GRBs) has been hitherto unexplored, as extremely high ionization by the GRB prevents the application of optical absorption spectroscopy at such distances. Here we show that the ionizing flux of the GRB imprints a unique time- and spatially dependent ionization structure on the gas, which can be probed by X-ray absorption. Application of this model to a selected sample of seven bright GRB X-ray afterglow spectra observed by XMM-Newton EPIC-pn enables an independent, quantitative estimation of the density (log( n ) ≈ 2–4) and distances (5–100 pc) of the ionized absorber directly from the GRB X-ray spectrum, thereby allowing us to locate the absorbing medium of this representative sample of long-duration GRBs in the region of the density–size diagram populated by star-forming regions versus other gravitationally bound objects in the Universe. Our results provide one of the most direct links between long-duration GRBs and star formation and show the potential of high-resolution X-ray spectroscopy as a powerful probe of star-forming regions that embed GRBs up to the highest redshifts.