Discovery of the Asymmetric Effect in the Response of Photoionization Gas

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.