GALEX observations of quasar variability in the ultraviolet

TL;DR

This study analyzes UV variability in over 4,300 quasars using GALEX data over 5+ years, identifying hundreds of UV-variable quasars to understand emission mechanisms in active galaxies.

Contribution

First comprehensive UV variability analysis of quasars over multi-year timescales using GALEX archival data, with a new statistical method for identifying significant variability.

Findings

Identified 550 NUV variable quasars

Found 371 FUV variable quasars

Demonstrated UV variability can distinguish emission models

Abstract

Using archival observations recorded over a 5+ year timeframe with the NASA Galaxy Evolution Explorer (GALEX) satellite, we present a study of the ultraviolet (UV) variability of 4360 quasars of redshifts up to z=2.5 that have optical counterparts in the Sloan Digital Sky Survey DR5 spectroscopic catalog. The observed changes in both the far UV (FUV: 1350-1785A) and near UV (NUV: 1770-2830A) AB magnitudes as a function of time may help differentiate between models of the emission mechanisms thought to operate in these active galaxies. A list of NUV and FUV variable quasars was derived from the UV light-curves of sources with 5 or more observational visits by GALEX that spanned a time-frame greater than 3 months. By measuring the error in the derived mean UV magnitude from the series of GALEX observations for each source, quasars whose UV variability was greater than the 3-sigma variance from the mean observed value were deemed to be (intrinsically) UV variable. This conservative selection criterion (which was applied to both FUV and NUV observations) resulted in identifying 550 NUV and 371 FUV quasars as being statistically significant UV variable objects.