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The role played by the intergalactic medium (IGM) in the X-ray absorptiontowards high-redshift sources has recently drawn more attention in spectralanalysis studies. Here, we study the X-ray absorption towards 15 flat-spectrumradio quasars at $z>2$, relying on high counting statistic ($\gtrsim10\,000$photons) provided by XMM-Newton, with additional NuSTAR (and simultaneousSwift-XRT) observations when available. Blazars can be confidently consideredto have negligible X-ray absorption along the line of sight within the hostgalaxy, likely swept by the kpc-scale relativistic jet. This makes our sourcesideal for testing the absorption component along the IGM. Our new approach isto revisit the origin of the soft X-ray spectral hardening observed in high-zblazars in terms of X-ray absorption occurring along the IGM, with the help ofa low-z sample used as comparison. We verify that the presence of absorption inexcess of the Galactic value is the preferred explanation to explain theobserved hardening, while intrinsic energy breaks, predicted by blazars'emission models, can easily occur out of the observing energy band in mostsources. First, we perform an indirect analysis comparing the inferred amountof absorption in excess of the Galactic value with a simulated IGM absorptioncontribution, that increases with redshift and includes both a minimumcomponent from diffuse IGM metals, and the additional contribution of discretedenser intervening regions. Then, we directly investigate the warm-hot IGM witha spectral model on the best candidates of our sample, obtaining an average IGMdensity of $n_0=1.01^{+0.53}_{-0.72}\times10^{-7}$ cm$^{-3}$ and temperature of$\log(T/\text{K})=6.45^{+0.51}_{-2.12}$. A more dedicated study is currentlybeyond reach, but our results can be used as a stepping stone for future moreaccurate analysis, involving Athena.

X-ray absorption towards high-redshift sources: probing the intergalactic medium with blazars