Discovery of the Asymmetric Effect in the Response of Photoionization Gas

TL;DR

This work addresses why broad absorption line (BAL) gas exhibits an asymmetric photoionization response to varying quasar radiation and how this can constrain gas density and spatial scale. By deriving analytical expressions for the response timescale $t_i^*$ and validating them with time-dependent photoionization simulations (focusing on CIV) and damped random walk quasar light curves, the authors show that high-ionization states respond faster than low-ionization states, producing predominantly negative responses. Applying these results to SDSS BAL statistics, they infer that at least $>40\%$ of BAL gas has $n_H<10^6$ cm$^{-3}$, contrasting with typical accretion-disk wind densities $>10^8$ cm$^{-3}$, suggesting BAL outflows originate from larger-scale regions or evolve in density as they propagate. The findings provide a new diagnostic framework for constraining the geometry and origin of quasar outflows and motivate cross-ion and time-domain studies to test the universality of the asymmetry across ionization species.

Abstract

The variability of quasar radiation provides a powerful probe of the photoionization response of ionized gas, which plays a central role in tracing cosmic evolution and plasma physics under extreme conditions. In this work, we investigate the physical origin of the asymmetric response observed in broad absorption line (BAL) systems and constrain the gas density and spatial scale of quasar outflows. Using time-dependent photoionization simulations and analytical estimates focused on CIV, we quantify the response timescales across different ionization states. Our results show that over 70\% of BAL gas exhibits a negative response to quasar dimming, indicating a strong asymmetry in ionization behavior. This asymmetry is driven by systematically shorter response timescales in higher ionization states. Given typical observational cadences longer than one day, the observed response pattern requires at least 40% of the BAL gas to have a density below $n_{\rm H} = 10^6\ \cc$, consistent with most measured BAL densities but significantly lower than typical accretion disk winds ($n_{\rm H} > 10^8\ \cc$). These findings suggest that BAL outflows either undergo substantial density evolution as they propagate or originate from larger-scale regions such as the dusty torus. The asymmetric response thus provides new constraints on the physical structure and origin of quasar outflows.

Figures (8)

• Figure 1: Schematic diagram of the variation of ion column density with ionization luminosity. The column density of a specific ion in the plasma initially increases to a peak and then decreases with the rise in ionization level. The vertical dashed line represents the peak position. Hence, based on the peak position, it can be classified into two stages: the low ionization state and the high ionization state.
• Figure 2: Schematic diagram of the asymmetric effect of ion response timescale. The vertical dashed line is the boundary between the low ionized state and the high ionized state. In a sufficiently low ionization state, i.e., $n_{i+1}/n_{i} \ll \alpha_{i-1}/\alpha_{i}$ or $I_{i} \ll n_e\alpha_{i-1}$, the response timescale is $t_i^*=1/(fn_e\alpha_{i-1})$. On the contrary, in a sufficiently high ionization state, i.e., $n_{i+1}/n_{i} \gg \alpha_{i-1}/\alpha_{i}$ or $I_{i} \gg n_e\alpha_{i-1}$, the response timescale is $t_i^*=-1/(f\alpha_{i} n_e \frac{n_{i+1}}{n_{i}})=-1/(fI_i)$. In highly ionized state, the response timescale is shorter than that of low ionized state, leading to the detection of more negative responses.
• Figure 3: Non-Monotonic Dependence of Carbon Ion Column Densities on Ionization Parameter. The column densities of carbon ions increase first and then decrease with ionization parameters. Based on the peak position of each ion, we can divide the parameter space into corresponding low- and high-ionization intervals.
• Figure 4: The recombination coefficients and ion concentrations predicted by equilibrium-state photoionization simulations. The recombination coefficients of C iii and C iv, as well as the population density ratio $n_{\tiny \rm \hbox{C,{\sc v}}}/n_{\tiny \rm \hbox{C,{\sc iv}}}$ under different ionization parameters, were obtained from equilibrium photoionization simulations using Cloudy version c17 ferland2017.
• Figure 5: The response timescale of C iv under different ionization parameters. The black and red curves correspond to gas densities of $10^4\ \hbox{cm$^{-3}$}$ and $10^6\ \hbox{cm$^{-3}$}$, respectively. The timescale exhibits strong asymmetry: the response timescale of highly ionized states is significantly shorter than that of low-ionized states. If the observational time interval falls within 1--1000 days, this asymmetry would not be detectable for gas at a density of $10^6\ \hbox{cm$^{-3}$}$, but it would be observable for gas with a density of $10^4\ \hbox{cm$^{-3}$}$.