The Variability Structure Function of the Highest-Luminosity Quasars on Short Timescales

TL;DR

This study analyzes the short-timescale variability of the highest-luminosity quasars, finding that their UV variability structure function is well-described by a single-slope random walk model over 10 to 250 days, with implications for understanding quasar variability mechanisms.

Contribution

It provides the first detailed analysis of short-timescale UV variability in high-luminosity quasars, showing a simple linear structure function model fits the data well.

Findings

UV variability follows a single-slope random walk model

No strong evidence for breaks in the structure function

Variability amplitude scales with timescale from 10 to 250 days

Abstract

The stochastic photometric variability of quasars is known to follow a random-walk phenomenology on emission timescales of months to years. Some high-cadence restframe optical monitoring in the past has hinted at a suppression of variability amplitudes on shorter timescales of a few days or weeks, opening the question of what drives the suppression and how it might scale with quasar properties. Here, we study a few thousand of the highest-luminosity quasars in the sky, mostly in the luminosity range of $L_{\rm bol}=[46.4, 47.3]$ and redshift range of $z=[0.7, 2.4]$. We use a dataset from the NASA/ATLAS facility with nightly cadence, weather permitting, which has been used before to quantify strong regularity in longer-term restframe-UV variability. As we focus on a careful treatment of short timescales across the sample, we find that a linear function is sufficient to describe the UV variability structure function. Although the result can not rule out the existence of breaks in some groups completely, a simpler model is usually favoured under this circumstance. In conclusion, the data is consistent with a single-slope random walk across restframe timescales of $\Delta t=[10, 250]$ days.