Volume 545, September 2012
|Number of page(s)||18|
|Published online||04 September 2012|
Variability of the blazar 4C 38.41 (B3 1633+382) from GHz frequencies to GeV energies⋆
INAF, Osservatorio Astronomico di Torino,
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
Accepted: 18 June 2012
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 RQSO ~ 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.
Key words: galaxies: active / quasars: general / quasars: individual: 4C 38.41 / galaxies: jets
© ESO, 2012
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