Ultraviolet flux and spectral variability study of blazars observed with UVIT/AstroSat

TL;DR

This study presents the first intraday ultraviolet flux variability analysis of 10 blazars using AstroSat, revealing rapid flux changes and a bluer-when-brighter trend, indicating jet synchrotron dominance.

Contribution

It provides the first detailed UV flux variability study on intraday timescales for a sample of blazars, expanding understanding of their UV emission behavior.

Findings

UV flux variability detected in 9 out of 10 blazars within hours.

Bluer-when-brighter spectral trend observed across the sample.

UV emission primarily driven by jet synchrotron processes.

Abstract

Blazars, the peculiar class of active galactic nuclei (AGN), are known to show flux variations across the accessible electromagnetic spectrum. Though they have been studied extensively for their flux variability characteristics across wavelengths, information on their ultraviolet (UV) flux variations on time scales of hours is very limited. Here, we present the first UV flux variability study on intraday time scales of a sample of 10 blazars comprising 2 flat spectrum radio quasars (FSRQs) and 8 BL Lacertae objects (BL Lacs). These objects, spanning a redshift (z) range of 0.034 <= z <= 1.003, were observed in the far-UV (FUV: 1300 - 1800 \text{\AA}) and near-UV (NUV: 2000 - 3000 \text{\AA}) wavebands using the ultraviolet imaging telescope on board AstroSat. UV flux variations on time scales of hours were detected in 9 sources out of the observed 10 blazars. The spectral variability analysis showed a bluer-when-brighter trend with no difference in the UV spectral variability behavior between the studied sample of FSRQs and BL Lacs. The observed UV flux and spectral variability in our sample of both FSRQs and BL Lacs revealed that the observed UV emission in them is dominated by jet synchrotron process.