Probing the physical properties of the intergalactic medium using gamma-ray bursts

Abstract

We use Gamma-ray burst (GRB) spectra total continuum absorption to estimate the key intergalactic medium (IGM) properties of hydrogen column density ($\mathit {N}_{\mathrm{HXIGM}}$), metallicity, temperature and ionisation parameter over a redshift range of 1.6 ≤ z ≤ 6.3, using photo-ionisation (PIE) and collisional ionisation equilibrium (CIE) models for the ionised plasma. We use more realistic host metallicity, dust corrected where available, in generating the host absorption model, assuming that the host intrinsic hydrogen column density is equal to the measured ionisation corrected intrinsic neutral column from UV spectra (${\it N}_{\mathrm{H}\, \rm \small {I,IC}}$). We find that the IGM property results are similar, regardless of whether the model assumes all PIE or CIE. The $\mathit {N}_{\mathrm{HXIGM}}$ scales as (1 + z)1.0 − 1.9, with equivalent hydrogen mean density at z = 0 of $n_0 = 1.8^{+1.5}_{-1.2} \times 10^{-7}$ cm−3. The metallicity ranges from ∼0.1 Z⊙ at z ∼ 2 to ∼0.001 Z⊙ at redshift z > 4. The PIE model implies a less rapid decline in average metallicity with redshift compared to CIE. Under CIE, the temperature ranges between 5.0 < log (T/K) < 7.1. For PIE the ionisation parameter ranges between 0.1 < log (ξ) < 2.9. Using our model, we conclude that the IGM contributes substantially to the total absorption seen in GRB spectra and that this contribution rises with redshift, explaining why the hydrogen column density inferred from X-rays is substantially in excess of the intrinsic host contribution measured in UV.

Investigating time variability of X-ray absorption in Swift GRBs

“…the total N H,intr is dominated by the IGM, but also with an origin in the GRB host galaxies themselves, as long as most of the absorbing material is not in the close vicinity of the GRBs. Models considering IGM absorption predict that is should start dominating over the host contribution at z ∼ 2 − 3 (e.g., Behar et al 2011;Starling et al 2013;Dalton et al 2021). This is consistent with the fact that we do not identify any GRBs with decreasing N H,intr at high z, though it should be noted that the redshift of the spectrum also makes the detection of absorption at high z more challenging.…”

“…Some authors argue that the GRB hosts can account for the absorption and redshift evolution (Schady et al 2011;Watson & Jakobsson 2012;Watson et al 2013). Others argue that a significant part of the absorption is due to the full integrated LOS, which includes diffuse intergalactic medium (IGM) and intervening objects (Behar et al 2011;Campana et al 2012;Starling et al 2013;Campana et al 2015;Dalton et al 2021).…”

Dissecting the interstellar medium of az= 6.3 galaxy

“…Such discrepancy is often found in GRBs (see i.e., Watson et al 2007;Schady et al 2011). However, considering the high redshift, large N H can also be due to the amount of metals present along the line of sight (Behar et al 2011;Campana et al 2015;Dalton et al 2021).…”

Investigating Time Variability of X-Ray Absorption in Swift GRBs

“…Some authors argue that the GRB hosts can account for the absorption and redshift evolution (Schady et al 2011;Watson & Jakobsson 2012;Watson et al 2013). Others argue that a significant part of the absorption is due to the fully integrated LOS, which includes diffuse intergalactic medium (IGM) and intervening objects (Behar et al 2011;Campana et al 2012Campana et al , 2015Starling et al 2013;Dalton et al 2021).…”