Simulating intergalactic quasar scintillation

Abstract

Intergalactic scintillation of distant quasars is sensitive to free electrons and therefore complements Ly$\alpha$ absorption line experiments probing the neutral intergalactic medium (IGM). We present a new scheme to compute IGM refractive scintillation effects on distant sources in combination with Adaptive Mesh Refinement cosmological simulations. First we validate our model by reproducing the well-known interstellar scintillation (ISS) of Galactic sources. The simulated cosmic density field is then used to infer the statistical properties of intergalactic scintillation. Contrary to previous claims, we find that the scattering measure of the simulated IGM at $z<2$ is $\langle \mbox{SM}_{\equ}\rangle=3.879$, i.e. almost 40 times larger than for the usually assumed smooth IGM. This yield an average modulation index ranging from 0.01 ($\nu_s=5$ GHz) up to 0.2 ($\nu_s=50$ GHz); above $\nu_{s}\gsim30$ GHz the IGM contribution dominates over ISS modulation. We compare our model with data from a $0.3\leq z\leq 2$ quasar sample observed at $\nu_{\obs}=8.4$ GHz. For this high frequency ($10.92\leq \nu_s \leq 25.2$), high galactic latitude sample ISS is negligible, and IGM scintillation can reproduce the observed modulation with a 4% accuracy, without invoking intrinsic source variability. We conclude by discussing the possibility of using IGM scintillation as a tool to pinpoint the presence of intervening high-$z$ groups/clusters along the line of sight, thus making it a probe suitably complementing Sunyaev-Zeldovich data recently obtained by \textit{Planck}.