Modeling multiband SEDs and light curves of BL Lacertae using a time-dependent shock-in-jet model

TL;DR

This paper models the spectral energy distribution and light curves of BL Lacertae using a time-dependent shock-in-jet model, explaining its rapid multi-band variability observed during a 2021 multiwavelength campaign.

Contribution

It introduces a novel physical model that accurately reproduces the multi-band temporal variability and hour-scale fluctuations of BL Lacertae.

Findings

Gamma-ray variability is the highest, with timescales of a few hours.

X-ray variability occurs on the order of a few kiloseconds.

The model successfully reproduces observed multi-band light curves and spectral features.

Abstract

The origin of fast flux variability in blazars is a long-standing problem, with many theoretical models proposed to explain it. In this study, we focus on BL Lacertae to model its spectral energy distribution (SED) and broadband light curves using a diffusive shock acceleration process involving multiple mildly relativistic shocks, coupled with a time-dependent radiation transfer code. BL Lacertae was the target of a comprehensive multiwavelength monitoring campaign in early July 2021. We present a detailed investigation of the source's broadband spectral and light curve features using simultaneous observations at optical-UV frequencies with Swift-UVOT, in X-rays with Swift-XRT and AstroSat-SXT/LAXPC, and in gamma-rays with Fermi-LAT, covering the period from July to August 2021 (MJD 59400 to 59450). A fractional variability analysis shows that the source is most variable in gamma-rays, followed by X-rays, UV, and optical. This allowed us to determine the fastest variability time in gamma-rays to be on the order of a few hours. The AstroSat-SXT and LAXPC light curves indicate X-ray variability on the order of a few kiloseconds. Modeling simultaneously the SEDs of low and high flux states of the source and the multiband light curves provided insights into the particle acceleration mechanisms at play. This is the first instance of a physical model that accurately captures the multi-band temporal variability of BL Lacertae, including the hour-scale fluctuations observed during the flare.