Investigating time variability of X-ray absorption in Swift GRBs

Abstract

The existence of excess absorption in the X-ray spectra of GRBs is well known, but the primary location of the absorbing material is still uncertain. To gain more knowledge about this, we have performed a time-resolved analysis of the X-ray spectra of 199 GRBs observed by the \textit{Swift} X-ray telescope, searching for evidence of a decreasing column density ($N_{\mathrm{H},\mathrm{intr}}$) that would indicate that the GRBs are ionizing matter in their surroundings. We structured the analysis as Bayesian inference and used an absorbed power-law as our baseline model. We also explored alternative spectral models in cases where decreasing absorption was inferred. The analysis reveals seven GRBs that show signs of a decrease in $N_{\mathrm{H},\mathrm{intr}}$, but we note that alternative models for the spectral evolution cannot be ruled out. We conclude that the excess absorption in the vast majority of GRBs must originate on large scales of the host galaxies and/or in the intergalactic medium. Our results also imply that an evolving column density is unlikely to affect the spectral analysis of the early X-ray spectra of GRBs. In line with this, we show that estimating the total $N_{\mathrm{H},\mathrm{intr}}$ from early {\it Swift} data in Window Timing mode reveals the same increasing trend with redshift as previous results based on data taken at later times, but with tighter constraints.

arXiv commentAccepted for publication in ApJ

References in corpus (47)

• Planck 2013 results. XVI. Cosmological parameters
• On the Absorption of X-rays in the Interstellar Medium
• The Leiden/Argentine/Bonn (LAB) Survey of Galactic HI: Final data release of the combined LDS and IAR surveys with improved stray-radiation corrections
• Gravitational Waves and Gamma-rays from a Binary Neutron Star Merger: GW170817 and GRB 170817A
• MultiNest: an efficient and robust Bayesian inference tool for cosmology and particle physics
• The Swift X-ray Telescope
• Spectra and Light Curves of Gamma-Ray Burst Afterglows
• Discovery of the peculiar supernova 1998bw in the error box of GRB980425
• Methods and results of an automatic analysis of a complete sample of Swift-XRT observations of GRBs
• Multimodal nested sampling: an efficient and robust alternative to MCMC methods for astronomical data analysis
• X-ray spectral modelling of the AGN obscuring region in the CDFS: Bayesian model selection and catalogue
• The Physics of Gamma-Ray Bursts and Relativistic Jets
• Calibration of X-ray absorption in our Galaxy
• The Observed Offset Distribution of Gamma-Ray Bursts from Their Host Galaxies: A Robust Clue to the Nature of the Progenitors
• Bayesian Blocks, A New Method to Analyze Structure in Photon Counting Data
• Jet breaks and Energetics of Swift GRB X-ray Afterglows
• Prompt and delayed emission properties of Gamma-Ray Bursts observed with BeppoSAX
• Absorption in Gamma Ray Burst afterglows
• Swift and XMM-Newton Observations of the Extraordinary GRB 060729: An afterglow with a more than 100 days X-ray light curve
• X-ray Hardness Evolution in GRB Afterglows and Flares: Late Time GRB Activity Without N_H Variations
• The X-ray absorbing column densities of Swift Gamma-ray bursts
• Time-Dependent Photoionization in a Dusty Medium I: Code Description and General Results
• Identifying the Environment and Redshift of GRB Afterglows from the Time-Dependence of Their Absorption Spectra
• The X-ray absorbing column density of a complete sample of bright Swift Gamma-Ray Bursts
• Short GRB Host Galaxies. II. A Legacy Sample of Redshifts, Stellar Population Properties, and Implications for their Neutron Star Merger Origins
• Gas and dust properties in the afterglow spectra of GRB 050730
• The fraction of ionizing radiation from massive stars that escapes to the intergalactic medium
• X-ray absorption evolution in Gamma-Ray Bursts: intergalactic medium or evolutionary signature of their host galaxies?
• A metal rich molecular cloud surrounds GRB 050904 at redshift 6.3
• Determining the location of Gamma-Ray Bursts through the evolution of their soft X-ray absorption
• The missing gas problem in GRB host galaxies: evidence for a highly ionised component
• Can the soft X-ray opacity towards high redshift sources probe the missing baryons?
• Helium in natal HII regions: the origin of the X-ray absorption in gamma-ray burst afterglows
• Missing cosmic metals revealed by X-ray absorption towards distant sources
• Dust extinction bias in the column density distribution of gamma-ray bursts; high column density, low redshift GRBs are more heavily obscured
• The variable absorption in the X-ray spectrum of GRB 190114C
• Cosmological evolution of the absorption of $\gamma$-ray burst X-ray afterglows
• Non-parametric Morphologies of Mergers in the Illustris Simulation
• Using realistic host galaxy metallicities to improve the GRB X-ray equivalent total hydrogen column density and constrain the intergalactic medium density
• The metallicity and star formation activity of long gamma-ray burst hosts for z$<$3: insights from the Illustris simulation
• Investigating subphotospheric dissipation in gamma-ray bursts using joint Fermi-Swift observations
• Effective absorbing column density in the gamma-ray burst afterglow X-ray spectra
• Searching for narrow absorption and emission lines in XMM-Newton spectra of gamma-ray bursts
• Probing the physical properties of the intergalactic medium using gamma-ray bursts
• Thermal components in the early X-ray afterglows of GRBs: likely cocoon emission and constraints on the progenitors
• A comprehensive view of blackbody components in the X-ray spectra of GRBs
• Soft X-ray absorption excess in gamma-ray burst afterglow spectra: Absorption by turbulent ISM

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