Variability of the blazar 4C 38.41 (B3 1633+382) from GHz frequencies to GeV energies^⋆

¹ INAF, Osservatorio Astronomico di Torino, Italy
e-mail: villata@oato.inaf.it
² Steward Observatory, University of Arizona, Tucson, AZ, USA
³ Astron. Inst., St.-Petersburg State Univ., Russia
⁴ Pulkovo Observatory, St.-Petersburg, Russia
⁵ Isaac Newton Institute of Chile, St.-Petersburg Branch, Russia
⁶ Instituto de Astrofisica de Canarias (IAC), La Laguna, Tenerife, Spain
⁷ Departamento de Astrofisica, Universidad de La Laguna, La Laguna, Tenerife, Spain
⁸ Department of Astronomy, University of Michigan, MI, USA
⁹ Dip. di Fisica, Università degli Studi di Perugia, Perugia, Italy
¹⁰ Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, USA
¹¹ Institute for Astrophysical Research, Boston University, MA, USA
¹² Abastumani Observatory, Mt. Kanobili, Abastumani, Georgia
¹³ Astrophysikalisches Institut Potsdam, An der Sternwarte 16, Potsdam, Germany
¹⁴ Landessternwarte Heidelberg-Königstuhl, Heidelberg, Germany
¹⁵ Engelhardt Astronomical Observatory, Kazan Federal Univ., Tatarstan, Russia
¹⁶ Aalto University Metsähovi Radio Observatory, Kylmälä, Finland
¹⁷ Maidanak Observatory of the Ulugh Beg Astronomical Institute, Uzbekistan
¹⁸ Instituto de Astrofísica de Andalucía, CSIC, Granada, Spain
¹⁹ Max-Planck-Institut für Radioastronomie, Bonn, Germany
²⁰ Instituto de Astronomía, Universidad Nacional Autónoma de México, México DF, México
²¹ Tuorla Observatory, Dept. of Physics and Astronomy, Univ. of Turku, Piikkiö, Finland
²² INAF, Osservatorio Astrofisico di Catania, Italy
²³ EPT Observatories, Tijarafe, La Palma, Spain
²⁴ INAF, TNG Fundación Galileo Galilei, La Palma, Spain
²⁵ Graduate Inst. of Astronomy, National Central Univ., Jhongli, Taiwan
²⁶ INAF, Osservatorio Astronomico di Roma, Italy
²⁷ INAF, Osservatorio Astronomico di Collurania Teramo, Italy
²⁸ Instituto de Astronoma, Universidad Nacional Autónoma de México, Ensenada, México
²⁹ Department of Physics, National Taiwan University, Taipei, Taiwan
³⁰ Dept. of Physics and Astronomy, Univ. of Southampton, Southampton, UK
³¹ Instituto Nacional de Astrofisica, Óptica y Electrónica, Puebla, México
³² Finnish Centre for Astronomy with ESO (FINCA), University of Turku, Piikkiö, Finland
³³ Department of Astronomy, University of Sofia Sofia, Bulgaria
³⁴ European Space Astronomy Centre (INSA-ESAC), Villanueva de la Caada, Madrid, Spain
³⁵ Agrupació Astronòmica de Sabadell, Spain
³⁶ CRESST and Astroparticle Physics Laboratory NASA/GSFC, Greenbelt, MD, USA
³⁷ Dept. of Physics, Univ. of Maryland, Baltimore County, Baltimore, MD, USA
³⁸ Galaxy View Observatory, Sequim, Washington, USA
³⁹ Cork Institute of Technology, Cork, Ireland
⁴⁰ Lowell Observatory, Flagstaff, AZ, USA
⁴¹ Institut de Radio Astronomie Millimétrique, St. Martin d’ Hères, France
⁴² Space Science Institute, Boulder, CO, USA

Received: 27 April 2012
Accepted: 18 June 2012

Abstract

Context. After years of modest optical activity, the quasar-type blazar 4C 38.41 (B3 1633+382) experienced a large outburst in 2011, which was detected throughout the entire electromagnetic spectrum, renewing interest in this source.

Aims. We present the results of low-energy multifrequency monitoring by the GLAST-AGILE Support Program (GASP) of the Whole Earth Blazar Telescope (WEBT) consortium and collaborators, as well as those of spectropolarimetric/spectrophotometric monitoring at the Steward Observatory. We also analyse high-energy observations of the Swift and Fermi satellites. This combined study aims to provide insights into the source broad-band emission and variability properties.

Methods. We assemble optical, near-infrared, millimetre, and radio light curves and investigate their features and correlations. In the optical, we also analyse the spectroscopic and polarimetric properties of the source. We then compare the low-energy emission behaviour with that at high energies.

Results. In the optical-UV band, several results indicate that there is a contribution from a quasi-stellar-object (QSO) like emission component, in addition to both variable and polarised jet emission. In the optical, the source is redder-when-brighter, at least for R ≳ 16. The optical spectra display broad emission lines, whose flux is constant in time. The observed degree of polarisation increases with flux and is higher in the red than the blue. The spectral energy distribution reveals a bump peaking around the U band. The unpolarised emission component is likely thermal radiation from the accretion disc that dilutes the jet polarisation. We estimate its brightness to be R_QSO ~ 17.85–18 and derive the intrinsic jet polarisation degree. We find no clear correlation between the optical and radio light curves, while the correlation between the optical and γ-ray flux apparently fades in time, likely because of an increasing optical to γ-ray flux ratio.

Conclusions. As suggested for other blazars, the long-term variability of 4C 38.41 can be interpreted in terms of an inhomogeneous bent jet, where different emitting regions can change their alignment with respect to the line of sight, leading to variations in the Doppler factor δ. Under the hypothesis that in the period 2008–2011 all the γ-ray and optical variability on a one-week timescale were due to changes in δ, this would range between ~7 and ~21. If the variability were caused by changes in the viewing angle θ only, then θ would go from ~2.6° to ~5°. Variations in the viewing angle would also account for the dependence of the polarisation degree on the source brightness in the framework of a shock-in-jet model.