Modeling the He ii Transverse Proximity Effect: Constraints on Quasar Lifetime and Obscuration

Abstract

The He ii transverse proximity effect -enhanced He ii Lyα transmission in a background sightline caused by the ionizing radiation of a foreground quasar -offers a unique opportunity to probe the emission properties of quasars, in particular the emission geometry (obscuration, beaming) and the quasar lifetime. Building on the foreground quasar survey published in Schmidt et al. (2017), we present a detailed model of the He ii transverse proximity effect, specifically designed to include light travel time effects, finite quasar ages, and quasar obscuration. We post-process outputs from a cosmological hydrodynamical simulation with a fluctuating He ii UV background model, plus the added effect of the radiation from a single bright foreground quasar. We vary the age t age and obscured sky fractions Ω obsc of the foreground quasar, and explore the resulting effect on the He ii transverse proximity effect signal. Fluctuations in IGM density and the UV background, as well as the unknown orientation of the foreground quasar, result in a large variance of the He ii Lyα transmission along the background sightline. We develop a fully Bayesian statistical formalism to compare far UV He ii Lyα transmission spectra of the background quasars to our models, and extract joint constraints on t age and Ω obsc for the six Schmidt et al. (2017) foreground quasars with the highest implied He ii photoionization rates. Our analysis suggests a bimodal distribution of quasar emission properties, whereby one foreground quasar, associated with a strong He ii transmission spike, is relatively old (22 Myr) and unobscured Ω obsc < 35%, whereas three others are either younger than 10 Myr or highly obscured (Ω obsc > 70%).

Time-dependent behaviour of quasar proximity zones at z ∼ 6

“…Future samples of high-quality z ∼ 6 quasar spectra with precise systemic redshift estimates from current observational programs will soon increase the number of z ∼ 6 proximity zone measurements by a factor of a few. Studying these proximity zones in the context of the non-equilibrium phenomenology developed here will allow for powerful constraints on the lightcurves of accreting supermassive black holes, especially when combined with additional IGM-based probes of quasar activity at lower redshift that are sensitive to much longer timescales (Khrykin et al 2016(Khrykin et al , 2017(Khrykin et al , 2019Schmidt et al 2017Schmidt et al , 2018b. While we have focused on R p in this work, in principle the timescale at which the proximity zone is sensitive to variability differs along the line of sight as t eq ∝ r 2 .…”

Probing the Environment of High-z Quasars Using the Proximity Effect in Projected Quasar Pairs

“…However, based on the quasar variability, Schawinski et al (2015) estimated the typical value to be 0.1 Myr (see also, Schawinski et al 2010). On the other extreme, Schmidt et al (2018) have used longitudinal He ii proximity effect and estimated quasar's lifetime to be ∼30 Myr (see also, Hogan et al 1997;Anderson et al 1999;Zheng et al 2015;Khrykin et al 2016;Schmidt et al 2017). Given the wide range of inferred values of quasar's lifetime it is difficult to draw any conclusion on the value of 1.6 Myr as we have estimated above.…”

“…While most such studies using Lyα forest were done in the longitudinal direction, the environment of a quasar can also be probed in the transverse direction using quasar pairs, commonly known as transverse proximity effect (Adelberger 2004;Schirber et al 2004;Rollinde et al 2005;Worseck et al 2007;Gonçalves et al 2008;Gallerani 2011;Schmidt et al 2018). The main principle here is to use Lyα absorption lines detected along the sight-line of a background quasar, near the redshift of the foreground quasar, to probe the ionization effect due to a foreground quasar in the transverse direction.…”

Mapping Quasar Light Echoes in 3D with Lyα Forest Tomography

“…A background sightline at transverse distance R ⊥ probes the foreground quasars emission at earlier times than the light we directly receive from the quasar (see e.g. Adelberger 2004;Kirkman & Tytler 2008;Furlanetto & Lidz 2011;Schmidt et al 2017Schmidt et al , 2018. This arises from the fact that the geometric path length from the foreground quasar to a location along the background sightline, and from there to the observer (as probed in absorption by the background sightline) is longer compared to the direct path from the foreground quasar to Earth.…”

“…For calculating the H i density n HI we follow, like in Schmidt et al (2018), the approach described in Rahmati et al (2013). We take the total ionization rate Γ HI tot as the sum of photoionization Γ HI phot = Γ HI UVB + Γ HI QSO and collisional ionization.…”