The WEBT campaign on the BL Lac object PG 1553+113 in 2013. An analysis of the enigmatic synchrotron emission

TL;DR

This study presents a detailed multiwavelength analysis of the BL Lac object PG 1553+113 during 2013, revealing insights into its synchrotron emission, spectral energy distribution, and potential emission mechanisms.

Contribution

It provides the first comprehensive multiwavelength campaign on PG 1553+113, analyzing optical, IR, radio, UV, and X-ray data to characterize its synchrotron emission and spectral features.

Findings

Synchrotron peak likely in 4-30 eV range.

Optical spectrum shows a bluer-when-brighter trend.

X-ray spectrum remains stable but hardens with increased flux.

Abstract

A multifrequency campaign on the BL Lac object PG 1553+113 was organized by the Whole Earth Blazar Telescope (WEBT) in 2013 April-August, involving 19 optical, two near-IR, and three radio telescopes. The aim was to study the source behaviour at low energies during and around the high-energy observations by the Major Atmospheric Gamma-ray Imaging Cherenkov (MAGIC) telescopes in April-July. We also analyse the UV and X-ray data acquired by the Swift and XMM-Newton satellites in the same period. The WEBT and satellite observations allow us to detail the synchrotron emission bump in the source spectral energy distribution (SED). In the optical we found a general bluer-when-brighter trend. The X-ray spectrum remained stable during 2013, but a comparison with previous observations suggests that it becomes harder when the X-ray flux increases. The long XMM-Newton exposure reveals a curved X-ray spectrum. In the SED, the XMM-Newton data show a hard near-UV spectrum, while Swift data display a softer shape that is confirmed by previous HST-COS and IUE observations. Polynomial fits to the optical-X-ray SED show that the synchrotron peak likely lies in the 4-30 eV energy range, with a general shift towards higher frequencies for increasing X-ray brightness. However, the UV and X-ray spectra do not connect smoothly. Possible interpretations include: i) orientation effects, ii) additional absorption, iii) multiple emission components, and iv) a peculiar energy distribution of relativistic electrons. We discuss the first possibility in terms of an inhomogeneous helical jet model.