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

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.

Synchrotron emission from the blazar PG 1553+113. An analysis of its flux and polarization variability

“…We adopted a log-parabolic model (Landau et al 1986;Massaro et al 2004), with reddening applied to the OM data and absorption to the EPIC data (redden*tbabs*logpar). Galactic reddening was set to E(B − V ) = 0.05 (see Raiteri et al 2015).…”

“…De-reddened flux densities were obtained from UVOT data by following the prescriptions of Raiteri et al (2015).…”

“…In 2013 the Whole Earth Blazar Telescope 1 (WEBT) coordinated a multiwavelength campaign on PG 1553+113 in support of MAGIC observations. The results of the WEBT radio-to-optical monitoring were presented in Raiteri et al (2015) and used together with observations by Swift and XMM-Newton to analyse the source synchrotron emission. The picture that emerged was enigmatic, because the connection between the UV and X-ray spectra seemed to require a double curvature.…”

“…Nicastro et al 2013, and references therein). At the same time, this offered an extraordinary opportunity to study in detail the emission of the blazar itself and, in particular, the problem of the spectral connection between the UV and X-ray regions discussed by Raiteri et al (2015). To maximize the scientific output, the WEBT organized a new multifrequency campaign involving optical, near-infrared, and radio observing facilities.…”

A search for cyclical sources of γ-ray emission on the period range from days to years in the Fermi-LAT sky

“…Periodic γ-ray emission from blazars is expected if supermassive binary black holes are present at their centres (e.g., Rieger 2007). The geometrical origin of periodicity in γ-ray blazars, involving jet precession, intrinsic jet rotation, or helical structures in jets, is also possible (e.g., Rieger 2004;Mohan & Mangalam 2015;Raiteri et al 2015;Cavaliere et al 2017;Sobacchi et al 2017). Quasi-periodic fluctuations in the energy outflow efficiency in the magnetically arrested accretion regime (Tchekhovskoy et al 2011) suggest another potential mechanism for cyclic variability.…”