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

Abstract

Soft X-ray absorption in excess of Galactic is observed in the afterglows of most gamma-ray bursts (GRBs), but the correct solution to its origin has not been arrived at after more than a decade of work, preventing its use as a powerful diagnostic tool. We resolve this long-standing problem and find that He in the GRB's host HII region is responsible for most of the absorption. We show that the 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. From these correlations we exclude an origin of the X-ray absorption which is not related to the host galaxy, i.e. the intergalactic medium or intervening absorbers are not responsible. We find that the correlation with the dust column has a strong redshift evolution, whereas the correlation with the neutral gas does not. From this we conclude that the 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 evolution of N_Hx/Av is thus a reflection of the cosmic metallicity evolution of star-forming galaxies. We conclude that the 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 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.

10 pages, 4 figures, submitted to ApJ

References in corpus (19)

• A giant outburst two years before the core-collapse of a massive star
• Measured Metallicities at the Sites of Nearby Broad-Lined Type Ic Supernovae and Implications for the SN-GRB Connection
• The optically unbiased GRB host (TOUGH) survey. I. Survey design and catalogs
• The First Positive Detection of Molecular Gas in a GRB Host Galaxy
• The line-of-sight towards GRB 030429 at z = 2.66: Probing the matter at stellar, galactic and intergalactic scales
• Very different X-ray to optical column density ratios in gamma-ray burst afterglows: ionisation in GRB environments
• X-ray Hardness Evolution in GRB Afterglows and Flares: Late Time GRB Activity Without N_H Variations
• GRB 100219A with X-shooter - abundances in a galaxy at z = 4.7
• High-Ion Absorption in Seven GRB Host Galaxies at z=2-4: Evidence for both Circumburst Plasma and Outflowing Interstellar Gas
• Radio and X-ray observations of SN 2006jd: Another strongly interacting Type IIn supernova
• The distribution of equivalent widths in long GRB afterglow spectra
• The optically unbiased GRB host (TOUGH) survey. V. VLT/X-shooter emission-line redshifts for Swift GRBs at z~2
• The prompt, high resolution spectroscopic view of the "naked-eye" GRB080319B
• A metal rich molecular cloud surrounds GRB 050904 at redshift 6.3
• Did we observe the supernova shock breakout in GRB 060218?
• Time-dependent excitation and ionization modelling of absorption-line variability due to GRB 080310
• The Molecular Hydrogen Deficit in Gamma-Ray Burst Afterglows
• Differential X-ray Absorption and Dust-To-Gas Ratios of the Lens Galaxies SBS0909+523, FBQS0951+2635, and B1152+199
• Dust extinction bias in the column density distribution of gamma-ray bursts; high column density, low redshift GRBs are more heavily obscured

Cited by in corpus (40)

• A complete sample of bright Swift short Gamma-Ray Bursts
• The X-shooter GRB afterglow legacy sample (XS-GRB)
• Molecular Hydrogen in the Damped Lyman-alpha System towards GRB 120815A at z=2.36
• The fraction of ionizing radiation from massive stars that escapes to the intergalactic medium
• The metals-to-dust ratio to very low metallicities using GRB and QSO absorbers; extremely rapid dust formation
• Gamma-ray burst optical light-curve zoo: comparison with X-ray observations
• VLT/X-shooter spectroscopy of the GRB 120327A afterglow
• Little red dots as young supermassive black holes in dense ionized cocoons
• Galaxy gas as obscurer: I. GRBs x-ray galaxies and find a N_H ~ M* relation
• The mysterious optical afterglow spectrum of GRB140506A at z=0.889
• Spectrophotometric analysis of GRB afterglow extinction curves with X-shooter
• The host galaxy of the short GRB111117A at $z=2.211$: impact on the short GRB redshift distribution and progenitor channels
• Highly-ionized metals as probes of the circumburst gas in the natal regions of gamma-ray bursts
• A detailed study of the optical attenuation of gamma-ray bursts in the Swift era
• Dissecting the interstellar medium of a z=6.3 galaxy: X-shooter spectroscopy and HST imaging of the afterglow and environment of the Swift GRB 210905A
• VLT/X-shooter GRBs: Individual extinction curves of star-forming regions
• Long-Duration Gamma-Ray Burst Host Galaxies in Emission and Absorption
• The Host Galaxies of Long-Duration Gamma-Ray Bursts
• X-shooting GRBs at high redshift: probing dust production history
• X-ray Absorption of High Redshift Quasars
• An Explanation for the Different X-ray to Optical Column Densities in the Environments of Gamma Ray Bursts: A Progenitor Embedded in a Dense Medium
• A blast from the infant Universe: the very high-z GRB 210905A
• The dependence of gamma-ray burst X-ray column densities on the model for Galactic hydrogen
• The variable absorption in the X-ray spectrum of GRB 190114C
• Using realistic host galaxy metallicities to improve the GRB X-ray equivalent total hydrogen column density and constrain the intergalactic medium density
• Detection capabilities of the Athena X-IFU for the warm-hot intergalactic medium using gamma-ray burst X-ray afterglows
• Searching for narrow absorption and emission lines in XMM-Newton spectra of gamma-ray bursts
• Investigating time variability of X-ray absorption in Swift GRBs
• Probing the physical properties of the intergalactic medium using gamma-ray bursts
• Probing the physical properties of the intergalactic medium using blazars
• Evidence of Contribution of Intervening Clouds to GRB's X-ray Column Density
• GRB 141221A: gone is the wind
• Soft X-ray absorption excess in gamma-ray burst afterglow spectra: Absorption by turbulent ISM
• A search for lines in the bright X-ray afterglow of GRB120711A
• Probing the physical properties of the IGM using SRG/eROSITA spectra from blazars
• Modelling of long gamma-ray burst host galaxies at cosmic noon from damped Lyman-α absorption statistics
• Passage of a Gamma-Ray Burst Through a Molecular Cloud: Cloud Ionization Structure
• GRBs as Probes of the IGM
• Dense gas linked to star-forming regions photoionised by embedded gamma-ray bursts
• Passage of a Gamma-Ray Burst Through a Molecular Cloud: The Absorption of its Afterglow in the X-ray Wavelength Range