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

Tony Dalton,Simon L Morris,Michele Fumagalli

doi:10.1093/mnras/stab335

Abstract

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 ionization parameter over a redshift range of 1.6 ≤ z ≤ 6.3, using photoionization equilibrium (PIE) and collisional ionization equilibrium (CIE) models for the ionized 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 ionization corrected intrinsic neutral column from UV spectra (${\it N}_{\mathrm{H}\,{\small I,IC}}$). 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 $\sim 0.1\, \mathrm{Z}_{\odot }$ at redshift z ∼ 2 to $\sim 0.001\, \mathrm{Z}_{\odot }$ at redshift z &gt; 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 &lt; log (T/K) &lt; 7.1. For PIE the ionization parameter ranges between 0.1 &lt; log (ξ) &lt; 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.

Highlights

The main objective of this paper is to estimate the key intergalactic medium (IGM) parameters of column density, metallicity, temperature, and ionization, using the latest models for ionized absorbers on the line of sight (LOS) to gamma-ray bursts (GRBs)
We started by fitting a subsample of GRB from our full sample to examine the impact of adding additional model components leading to the full model including the ionized IGM
It is notable that the lowest redshift GRB140430A z = 1.6 in our sample has a best-fitting NHXIGM again considerably higher than the mean density curve, similar to the collisional ionization equilibrium (CIE) model

Summary

Introduction

The main objective of this paper is to estimate the key intergalactic medium (IGM) parameters of column density, metallicity, temperature, and ionization, using the latest models for ionized absorbers on the line of sight (LOS) to gamma-ray bursts (GRBs). Recent simulations predict that up to 50 per cent of the baryons by mass have been shockheated into a warm-hot phase (WHIM) at low redshift z < 2 with T = 105–107 K and nb = 10−6–10−4 cm−3, where nb is the baryon density Martizzi et al (2019, hereafter M19), using the IllustrisTNG simulations (Piattella 2018), estimated that the cool diffuse IGM constitutes ∼ 39 per cent and the WHIM ∼ 46 per cent of the baryons at redshift z = 0. Observations of the cool diffuse IGM and WHIM are essential for effective tracing of matter across time and to validate the simulations (Danforth et al 2016). We adopt the common temperature naming convention for IGM plasma: cool is log (T/K) < 5 and warm-hot is log(T/K) ∼ 5–7 (M19). Though we concentrate on the very low density IGM, where relevant we use the common names for systems of different column densities – strong Ly α forest systems (SLFSs): 15 < log NH I < 16.22; partial Lyman limit systems (pLLSs): 16.2 < logNH I < 17.2; Lyman limit systems (LLSs): 17.2 < logNH I < 19 ; super-LLSs (sLLSs): 19.0 < logNH I < 20.3; and damped Ly α systems (DLAs): logNH I >20.3 (Fumagalli 2014, hereafter F14)

Objectives

Results

Discussion

Conclusion