Ultraviolet variability in Radio-Loud Active Galactic Nuclei observed by UVIT onboard AstroSat

TL;DR

This study systematically investigates UV flux and spectral variability in 24 radio-loud AGN using AstroSat's UVIT data, revealing significant hour-scale UV variability and a bluer-when-brighter trend in some sources, supporting jet-driven variability.

Contribution

First comprehensive analysis of UV variability in radio-loud AGN using AstroSat UVIT data, covering a large redshift range and multiple AGN types, highlighting jet-related intrinsic processes.

Findings

18 out of 24 sources showed significant UV variability on hour timescales.

Detected bluer-when-brighter trend in two sources, CTA 102 and PKS 0447-439.

UV variability supports jet-driven intrinsic processes in radio-loud AGN.

Abstract

Radio-loud active galactic nuclei (AGN) are among the most luminous objects in the Universe, emitting radiation from low-energy radio waves to high energy $\gamma$-rays. They are well known to exhibit flux variations at nearly all accessible wavelengths. However, their variability properties in the ultraviolet (UV) band remain relatively less explored compared to other wavebands. Here, we present the results of a systematic investigation of the UV flux and spectral variability characteristics of 24 radio-loud AGN spanning the redshift range 0.018 $\le$ $z$ $\le$ 2.218. The sample comprises 17 BL Lac objects, 6 flat spectrum radio quasars (FSRQs) and one radio-loud narrow line Seyfert 1 galaxy. We used observations obtained with the Ultra-Violet Imaging Telescope (UVIT) onboard AstroSat during its first ten years of operation, covering both the far-UV (FUV; 1300$-$1800 \AA) and near-UV (NUV; 2000$-$3000 \AA) bands. Of the 24 sources analysed, 18 showed significant UV variability on hour timescales. We found a bluer when brighter (BWB) spectral trend in two sources: the FSRQ CTA 102 and the BL Lac PKS 0447$-$439. The observed UV variability in our sample of radio-loud AGN, together with the BWB trend detected in these two sources, supports a scenario in which the hour timescale UV variations are driven by intrinsic processes within their relativistic jets.