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Astrophysics and Astronomy

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Published in the journal Astronomy & Astrophysics in the section Extragalactic Astronomy.

Raiteri, C.M. et al., A&A, 545., A48, 2012, reproduced with permission, © ESO

We thank the referee, Robert Hartman, for his useful comments and suggestions. We are grateful to Stefano Vercellone for information about the AGILE detection of 4C 38.41. We acknowledge financial contribution from the agreement ASI-INAF I/009/10/0. Partly based on observations with the Medicina and Noto telescopes operated by INAF – Istituto di Radioastronomia. The work at U. Michigan was supported by NSF grant AST-0607523 and NASA/Fermi GI grants NNX09AU16G, NNX10AP16G and NNX11AO13G. The Submillimeter Array is a joint project between the Smithsonian Astrophysical Observatory and the Academia Sinica Institute of Astronomy and Astrophysics and is funded by the Smithsonian Institution and the Academia Sinica. This paper is partly based on observations carried out at the German-Spanish Calar Alto Observatory, which is jointly operated by the MPIA and the IAA-CSIC. This paper is partly based on observations carried out at the IRAM-30 m Telescope, which is supported by INSU/CNRS (France), MPG (Germany), and IGN (Spain). Acquisition of the MAPCAT, POLAMI, and MAPI data is partly supported by CEIC (Andalucía) grant P09-FQM-4784 and by MINECO (Spain) grant and AYA2010-14844. This work is partly supported by the Georgian National Science foundation grant GNSF/ST09/4-521. This article is partly based on observations made with the telescopes IAC80 and TCS operated by the Instituto de Astrofisica de Canarias in the Spanish Observatorio del Teide on the island of Tenerife. Most of the observations were taken under the rutinary observation programme. The IAC team acknowledges the support from the group of support astronomers and telescope operators of the Observatorio del Teide. The Steward Observatory spectropolarimetric monitoring project is supported by the Fermi Guest Investigator grants NNX08AW56G and NNX09AU10G. Data at SPM observatory were obtained through the support given by PAPIIT grant IN116211. The Metsähovi team acknowledges the support from the Academy of Finland to our observing projects (numbers 212656, 210338, 121148, and others). The BU group acknowledges support by NASA grants NNX08AV61G, NNX10AU15G, and NNX11AQ03G. The PRISM camera at Lowell Observatory was developed by K. Janes et al. at BU and Lowell Observatory, with funding from the NSF, BU, and Lowell Observatory. The Liverpool Telescope is operated on the island of La Palma by Liverpool John Moores University in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias, with funding from the UK Science and Technology Facilities Council. The St. Petersburg University team acknowledges support from Russian RFBR foundation via grant 12-02-00452. AZT-24 observations are made within an agreement between Pulkovo, Rome, and Teramo observatories. Data at NAO Rozhen were obtained through the support given by the BSF grant DO02 340/08. We acknowledge the use of public data from the Swift data archive. This research has made of the XRT Data Analysis Software (XRTDAS) developed under the responsibility of the ASI Science Data Center (ASDC), Italy. This research has made use of the SAO/NASA’s Astrophysics Data System (ADS) and of the NASA/IPAC Extragalactic Database (NED), which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. This research has made use of data obtained through the High Energy Astrophysics Science Archive Research Center Online Service, provided by the NASA/Goddard Space Flight Center.


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.