Multiwavelength study of the gravitationally lensed blazar QSO B0218+357 between 2016 and 2020

TL;DR

This study presents multiwavelength observations of the gravitationally lensed blazar QSO B0218+357 from 2016 to 2020, revealing its emission characteristics, constraining its VHE gamma-ray activity, and modeling its broadband spectral energy distribution.

Contribution

It provides the first broadband low-state SED for this source, constrains its VHE duty cycle, and models the emission regions with detailed multiwavelength data.

Findings

No significant VHE gamma-ray emission detected during the study period.

The VHE duty cycle is constrained to less than 16 flares per year.

The low-state SED is constructed, with VHE flux upper limits below the 2014 flare level.

Abstract

We report multiwavelength observations of the gravitationally lensed blazar QSO B0218+357 in 2016-2020. Optical, X-ray and GeV flares were detected. The contemporaneous MAGIC observations do not show significant very-high-energy (VHE, >= 100 GeV) gamma-ray emission. The lack of enhancement in radio emission measured by OVRO indicates the multi-zone nature of the emission from this object. We constrain the VHE duty cycle of the source to be < 16 2014-like flares per year (95% confidence). For the first time for this source, a broadband low-state SED is constructed with a deep exposure up to the VHE range. A flux upper limit on the low-state VHE gamma-ray emission of an order of magnitude below that of the 2014 flare is determined. The X-ray data are used to fit the column density of (8.10 +- 0.93 stat ) x 10^21 cm^-2 of the dust in the lensing galaxy. VLBI observations show a clear radio core and jet components in both lensed images, yet no significant movement of the components is seen. The radio measurements are used to model the source-lens-observer geometry and determine the magnifications and time delays for both components. The quiescent emission is modeled with the high-energy bump explained as a combination of synchrotron-self-Compton and external Compton emission from a region located outside of the broad line region. The bulk of the low-energy emission is explained as originating from a tens-of-parsecs scale jet.