Cosmological Evolution of the Absorption of γ-Ray Burst X-Ray Afterglows

Monthly Notices of the Royal Astronomical Society

Fumagalli

2021

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.

Section: Discussion a N D C O M Pa R I S O N W I T H Ot H E R S T U D I E Ssupporting

confidence: 64%

“…The other school of thought argues that the IGM is the cause of excess absorption and redshift relation (e.g. Starling et al 2013;Arcodia, Campana & Salvaterra 2016;Rahin & Behar 2019). While we acknowledge that the GRB host may contribute to the excess absorption, it is the IGM that is the focus of this paper.…”

mentioning

confidence: 99%

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

Monthly Notices of the Royal Astronomical Society

Fumagalli

2021

“…Schady et al 2011;Watson et al 2013;Buchner, Schulze & Bauer 2017). The other school of thought argues that while the host can have an absorption contribution, the IGM contributes substantially to the excess absorption and is redshift related (e.g Starling et al 2013a;Arcodia, Campana & Salvaterra 2016;Rahin & Behar 2019;Dalton & Morris 2020;Dalton, Morris & Fumagalli 2021, hereafter D20, D21). The convention in earlier studies using AGN and GRBs was to use solar metallicity for a neutral absorber, as a device used to place all of the absorbing column density measurements on a comparable scale.…”

Section: Introductionmentioning

confidence: 99%

Probing the physical properties of the intergalactic medium using blazars

Monthly Notices of the Royal Astronomical Society

Fumagalli

et al. 2021

We use Swift blazar spectra to estimate the key intergalactic medium (IGM) properties of hydrogen column density (${N} {\rm \small {HXIGM}}$), metallicity and temperature over a redshift range of 0.03 ≤ z ≤ 4.7, using a collisional ionisation equilibrium (CIE) model for the ionised plasma. We adopted a conservative approach to the blazar continuum model given its intrinsic variability and use a range of power law models. We subjected our results to a number of tests and found that the ${N}{\rm \small {hxigm}}$ parameter was robust with respect to individual exposure data and co-added spectra for each source, and between Swift and XMM-Newton source data. We also found no relation between ${N}{\rm \small {hxigm}}$ and variations in source flux or intrinsic power laws. Though some objects may have a bulk Comptonisation component which could mimic absorption, it did not alter our overall results. The ${N}{\rm \small {hxigm}}$ from the combined blazar sample scales as (1 + z)1.8 ± 0.2. The mean hydrogen density at z = 0 is n0 = (3.2 ± 0.5) × 10−7 cm−3. The mean IGM temperature over the full redshift range is log(T/K) =6.1 ± 0.1, and the mean metallicity is [X/H] = −1.62 ± 0.04(Z ∼ 0.02). When combining with the results with a gamma-ray burst (GRB) sample, we find the results are consistent over an extended redshift range of 0.03 ≤ z ≤ 6.3. Using our model for blazars and GRBs, we conclude that the IGM contributes substantially to the total absorption seen in both blazar and GRB spectra.

“…Campana et al, 2010;Behar et al, 2011;Watson et al, 2013). A claimed strong correlation with redshift has recently been updated and confirmed with a much larger GRB sample by Rahin and Behar, (2019). It has also been reported in many papers that the neutral intrinsic hydrogen column (NHI) in GRB has no significant correlation with redshift (e.g.…”

Section: Introductionmentioning

confidence: 74%

“…Perley et al, (2016) found that GRB with measured redshifts tend to be found in brighter galaxies, which could produce such a bias. However, Rahin and Behar, (2019) compared the NHX versus redshift trend for that paper's (smaller) unbiased sample with the full Swift sample and found no notable difference. In, Section 3, where we require that GRB have both optical and X-ray spectra, this requirement can introduce a selection effect against dim or highly dust extinguished GRBs (S11).…”

Section: Methodology and Data Selectionmentioning

confidence: 92%

Using realistic host galaxy metallicities to improve the GRB X-ray equivalent total hydrogen column density and constrain the intergalactic medium density

Monthly Notices of the Royal Astronomical Society

2020

It is known that the GRB equivalent hydrogen column density (NHX) changes with redshift and that, typically, NHX is greater than the GRB host neutral hydrogen column density. We have compiled a large sample of data for GRB NHX and metallicity [X/H]. The main aims of this paper are to generate improved NHX for our sample by using actual metallicities, dust corrected where available for detections, and for the remaining GRB, a more realistic average intrinsic metallicity using a standard adjustment from solar. Then, by approximating the GRB host intrinsic hydrogen column density using the measured neutral column (NHI, IC) adjusted for the ionization fraction, we isolate a more accurate estimate for the intergalactic medium (IGM) contribution. The GRB sample mean metallicity is = −1.17 ± 0.09 rms (or 0.07 ± 0.05 Z/Zsol) from a sample of 36 GRB with a redshift 1.76 ≤ z ≤ 5.91, substantially lower than the assumption of solar metallicity used as standard for many fitted NHX. Lower GRB host mean metallicity results in increased estimated NHX with the correction scaling with redshift as Δlog (NHX cm−2) = (0.59 ± 0.04)log(1 + z) + 0.18 ± 0.02. Of the 128 GRB with data for both NHX and NHI, IC in our sample, only six have NHI, IC > NHX when revised for realistic metallicity, compared to 32 when solar metallicity is assumed. The lower envelope of the revised NHX – NHI, IC, plotted against redshift can be fit by log(NHX – NHI, IC cm−2) = 20.3 + 2.4 log(1 + z). This is taken to be an estimate for the maximum IGM hydrogen column density as a function of redshift. Using this approach, we estimate an upper limit to the hydrogen density at redshift zero (n0) to be consistent with n0 = 0.17 × 10−7cm−3.