Optical variability of ICRF3 quasars in the Pan-STARRS 3Pi survey with functional principal components analysis

TL;DR

This study analyzes optical variability in ICRF3 quasars using Pan-STARRS data and a novel functional principal components analysis method to understand their brightness fluctuations across different wavelengths.

Contribution

It introduces a new method based on functional principal component analysis for analyzing sparse, irregularly sampled quasar light curves across multiple bands.

Findings

Variability amplitudes are generally small, peaking at ~0.1 mag.

Variability is lowest in the r band and highest in the y band.

A decline in variability with redshift is observed, consistent with time dilation effects.

Abstract

We make use of individual (epoch) detection data from the Pan-STARRS 3Pi survey for 2863 optical ICRF3 counterparts in the five wavelength bands g, r, i, z, and y, published as part of the Data Release 2. A dedicated method based on the Functional Principal Component Analysis is developed for these sparse and irregularly sampled data. With certain regularization and normalization constraints, it allows us to obtain uniform and compatible estimates of the variability amplitudes and average magnitudes between the passbands and objects. We find that the starting assumption of affinity of the light curves for a given object at different wavelengths is violated for several percent of the sample. The distributions of root-mean-square variability amplitudes are strongly skewed toward small values, peaking at ~0.1 mag with tails stretching to 2 mag. Statistically, the lowest variability is found for the r band and the largest for the reddest y band. A small "brighter-redder" effect is present, with amplitudes in y greater than amplitudes in g in 57% of the sample. The variability versus redshift dependence shows a strong decline with z toward redshift 3, which we interpret as the time dilation of the dominant time frequencies. The colors of radio-loud ICRF3 quasars are correlated with redshift in a complicated, wavy pattern governed by the emergence of brightest emission lines within the five passbands.