Spectral Energy Distribution Modeling of BL Lacertae During a Large Submillimeter Outburst and Low X-Ray Polarization State

TL;DR

This study models the broadband spectral energy distribution of BL Lacertae during a significant submillimeter outburst, proposing a two-component synchrotron and SSC model to explain the data and low X-ray polarization.

Contribution

The paper introduces a two-component electron distribution model to better fit the broadband SED and polarization data of BL Lacertae during an outburst, improving upon single-zone models.

Findings

Two separate electron populations explain the SED features.

The model accounts for low X-ray polarization.

Fit successfully reproduces broadband observations.

Abstract

In 2023 October-November, the blazar BL Lacertae underwent a very large-amplitude submm outburst. The usual single-zone leptonic model with the lower energy peak of the spectral energy distribution (SED) fit by the synchrotron emission from one distribution of relativistic electrons in the jet and inverse-Compton (IC) scattering of lower energy photons from the synchrotron radiation in the jet itself (synchrotron self-Compton or SSC) or those from the broad line region and torus by the same distribution of electrons cannot satisfactorily fit the broadband SED with simultaneous data at submm--optical--X-ray--GeV energies. Furthermore, simultaneous observations with IXPE indicate the X-ray polarization is undetected. We consider two different synchrotron components, one for the high flux in the submm wavelengths and another for the data at the optical band, which are supposedly due to two separate distributions of electrons. In that case, the optical emission is dominated by the synchrotron radiation from one electron distribution while the X-rays are mostly due to SSC process by another, which may result in low polarization fraction due to the IC scattering. We show that such a model can fit the broadband SED satisfactorily as well as explain the low polarization fraction at the X-rays.