Probing the physical properties of the intergalactic medium using blazars

Abstract

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.

Investigating the variability of Swift-BAT blazars with NICER

“…They both calculate mean hydrogen densities of 𝑛 0 ∼ 10 −7 cm −3 (at 𝑧 = 0) in the WHIM and temperatures of log(𝑇/𝐾)∼6, which are consistent with the quantities expected of such a medium (see simulations in e.g. Cen & Ostriker 1999Davé & Oppenheimer 2007;Schaye et al 2015;Martizzi et al 2019. Dalton et al (2021 also combine their blazar data with that of gamma-ray bursts to find that their results agree over a wide range in redshift, and show that the WHIM column density is not related to variations in the flux or spectra of their sources.…”

“…As shown in Eitan & Behar (2013), high redshift blazars, most of which are FSRQs, are often absorbed. Two recent studies by Arcodia et al (2018) and Dalton et al (2021) find that absorption in excess to the Galactic absorption, as opposed to intrinsic breaks, is preferred to fully explain the hardening in their respective samples of FSRQs. They successfully describe the excess X-ray absorption as occurring due to the highly ionized "warm-hot" intergalactic medium (WHIM), which would also account for the lack of absorption seen in the optical/UV.…”

“…Many spectra show curvature in the form of either a logparabolic power law or a broken power law (e.g. Massaro et al 2004Massaro et al , 2006Paggi et al 2009;Furniss et al 2013;Arcodia et al 2018;Dalton et al 2021), with many blazar emission models predicting that spectra exhibiting either shape can result from a relativistic particle distribution that has a similar curvature (see e.g. Sikora et al 1994Sikora et al , 1997Sikora et al , 2009Tavecchio et al 2007;Tavecchio & Ghisellini 2008;Arcodia et al 2018, and references therein).…”

“…Certain studies have shown that sometimes, significant photoelectric absorption, in addition to said continuum, is required in the soft X-rays to fully describe the curved X-ray spectrum (e.g. Cappi et al 1997;Tavecchio et al 2000;Fabian et al 2001a,b;Worsley et al 2004aWorsley et al ,b, 2006Page et al 2005;Grupe et al 2004Grupe et al , 2006Sambruna et al 2007;Eitan & Behar 2013;Arcodia et al 2018;Dalton et al 2021). At first glance, this contradicts the very nature of blazars, since in general these objects are considered to have negligible X-ray absorption along the line of sight, due to their kiloparsec-scale jet likely sweeping away any potential contribution to absorption from the host galaxy.…”

“…In order to reconcile this, several studies (e.g. Fabian et al 2001a;Behar et al 2011;Campana et al 2012Campana et al , 2015Starling et al 2013;Eitan & Behar 2013;Arcodia et al 2018;Dalton et al 2021) have suggested that the absorption might be due to the highly ionized "warm-hot" intergalactic medium (WHIM), with more recent studies emphasizing that such a component from the intergalactic medium should be considered in the spectral analysis of blazars when appropriate (see e.g. Arcodia et al 2018;Dalton et al 2021).…”