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

TL;DR

This work addresses how GRB-driven ionisation shapes the surrounding gas at 5–100 pc from the burst, a regime difficult to study with optical spectroscopy. By applying a time-evolving photoionisation model (TEPID) to XMM-Newton X-ray afterglows of seven bright lGRBs, the authors directly constrain the absorber’s density ($\log(n) \sim 2-4$ cm$^{-3}$) and distance ($r \sim 5-100$ pc), locating the gas within star-forming regions of the hosts. The analysis demonstrates that the late-time X-ray absorption is dominated by ionised gas, with a spectral signature arising from a stratified medium that cannot be captured by equilibrium photoionisation; this resolves long-standing tensions between neutral models and observed spectra. The inferred absorber properties place the GRB environment in the star-forming region regime on the density–size plane, and the results support collapsar progenitors for these long GRBs, linking high-density circumburst gas to massive-star environments and informing our understanding of GRB feedback in star-forming regions.

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.

Figures (41)

• Figure 1: Schematic representation of the stratification of the absorbing medium along the line of sight to the GRB The flux from the GRB jet directly impinges the high-ionisation region (shaded red). This region consists of the star-forming region (denser) in which the GRB is embedded and extends up to several tens of pc from the location of the GRB. The relatively neutral ISM (more tenuous) that is further away ($\gtrsim 10$ pc) is shaded blue. As the text discusses, X-ray absorption is seen from both media, whereas optical absorption is seen only from the low ionisation ISM. The transition region connecting the high and low ionisation media is shaded purple. The black lines on the right show the photons emerging from the host galaxy.
• Figure 1: Cumulative distribution functions of GRB properties for bias checksLeft: Rest-frame $E_{iso}$ (measured in 1-10$^{4}$ keV) of a sample of GRBs from 2012MNRAS.421.1256N2016ApJ...831...28T2017ApJ...837..119A. Right: Intrinsic X-ray absorption column density of the sample of GRBs from 2010MNRAS.402.2429C. In both panels, the vertical arrows mark the value for the respective burst in our sample, the blue dashed line shows the sample median and the grey lines show the first and third quartiles.
• Figure 1: Lightcurves of XRT sample of GRBs with known redshift in flux space (0.3-10 keV). The horizontal bars at the top show the duration of the XMM observations. The light curves of our sample are clearly towards the top of the scatter at the starting time of XMM observation.
• Figure 2: Luminosity distribution of the Swift-XRT sample of lGRBs with known redshift: The luminosity light curves for the full sample are plotted in the rest-frame band of 0.3 - 10 keV as grey points. Superimposed on this distribution are the light curves of our sample GRBs, colour-coded per GRB. Our sample is distributed towards the centre of the scatter. This clearly shows no bias toward high luminosity GRBs is introduced by our selection criteria.
• Figure 2: EPIC-PN count spectrum of GRB 060729 with folded model predictions: (Top panel): The EPIC-pn spectrum is plotted in grey points and has been rebinned for plotting purposes. The y-errors represent the 1-sigma uncertainty on the count-rate per bin, and the x-errors represent the bin size. The best-fit model predictions of the neutral model and TEPID model are plotted as green and orange lines respectively. (Middle and bottom panels): Corresponding residuals for the neutral and TEPID model, respectively, colour-coded as above. The data have been heavily rebinned for plotting purposes. The residuals below 1 keV seen from the neutral model-fit are not present in the TEPID model-fit, showing the improvement in the fit.