A Search for Fast X-ray Variability from Active Galactic Nuclei using Swift

TL;DR

This study analyzed Swift X-ray data of 143 active galactic nuclei, especially blazars, to search for rapid variability on timescales less than 15 minutes, finding no strong evidence of such fast changes.

Contribution

It provides the first comprehensive search for ultra-rapid X-ray variability in a large AGN sample using Swift data, setting constraints on emission region sizes.

Findings

No significant rapid ($<$15 min) X-ray variability detected.

Several low-significance potential flares observed.

Results constrain models of jet emission regions.

Abstract

Blazars are a class of active galactic nuclei (AGNs) known for their very rapid variabilty in the high energy regions of the electromagnetic spectrum. Despite this known fast variability, X-ray observations have generally not revealed variability in blazars with rate doubling or halving timescales less than approximately 15 min. Since its launch, the Swift X-ray Telescope has obtained 0.2-10 keV X-ray data on 143 AGNs, including blazars, through intense target of opportunity observations that can be analyzed in a multiwavelength context and used to model jet parameters, particularly during flare states. We have analyzed this broad Swift data set in a search for short timescale variability in blazars that could limit the size of the emission region in the blazar jet. While we do find several low-significance possible flares with potential indications of rapid variability, we find no strong evidence for rapid ($<$15 minutes) doubling or halving times in flares in the soft X-ray energy band for the AGNs analyzed.