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arXiv · 2601.07919

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

Aishwarya Linesh Thakur·Luigi Piro·Alfredo Luminari·Fabrizio Nicastro·Sandra Savaglio·Yair Krongold·Bruce Gendre
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Abstract

The 1-100 pc region embedding long-duration gamma-ray bursts (lGRBs) has been hitherto unexplored, as extremely high ionisation by the GRB prevents application of optical absorption spectroscopy on such distances. We show that the GRB ionising flux imprints a unique time- and spatially-dependent ionisation structure on the gas, that can be probed by X-ray absorption. Application of this model to a selected sample of 7 bright GRB X-ray afterglow spectra observed by \textit{XMM-Newton} EPIC-pn enables an independent, quantitative estimation of the density (log(n) $\sim$ 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 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 lGRBs and star formation and open the potential of high-resolution X-ray spectroscopy as a powerful probe of star-forming regions that embed GRBs up to the highest redshifts.

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Aishwarya Linesh Thakur, Luigi Piro, Alfredo Luminari, Fabrizio Nicastro, Sandra Savaglio, Yair Krongold, Bruce Gendre. 2026-01-12. Dense gas linked to star-forming regions photoionised by embedded gamma-ray bursts. https://doi.org/10.1038/s41550-026-02786-w

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