The WEBT campaign on the BL Lac object PG 1553+113 in 2013. An analysis of the enigmatic synchrotron emission
Date Issued
2015
Author(s)
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Larionov, V. M.
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Acosta-Pulido, J. A.
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Arévalo, M. J.
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Arkharov, A. A.
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Bachev, R.
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Benítez, E.
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Bozhilov, V. V.
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Borman, G. A.
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Calcidese, P.
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Carosati, D.
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Chigladze, R. A.
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Damljanovic, G.
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Doroshenko, V. T.
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Efimova, N. V.
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Ehgamberdiev, Sh. A.
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González-Morales, P. A.
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Grinon-Marin, A. B.
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Grishina, T. S.
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Hiriart, D.
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Ibryamov, S.
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Klimanov, S. A.
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Kopatskaya, E. N.
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Kurtanidze, O. M.
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Kurtanidze, S. O.
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Kurtenkov, A. A.
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Larionova, L. V.
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Larionova, E. G.
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Lázaro, C.
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Lähteenmäki, A.
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Markovic, G.
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Mirzaqulov, D. O.
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Mokrushina, A. A.
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Morozova, D. A.
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Mújica, R.
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Nazarov, S. V.
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Nikolashvili, M. G.
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Ohlert, J. M.
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Ovcharov, E. P.
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Pastor Yabar, A.
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Ramakrishnan, V.
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Sadun, A. C.
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Semkov, E.
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Sigua, L. A.
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Strigachev, A.
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Tammi, J.
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Tornikoski, M.
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Troitskaya, Yu. V.
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Troitsky, I. S.
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Velasco, S.
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Vince, O.
Description
The data collected by the WEBT collaboration are stored in the WEBT archive; for questions regarding their availability, please contact the WEBT President Massimo Villata ( villata@oato.inaf.it ). 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. 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. Based (partly) on data obtained with the STELLA robotic telescopes in Tenerife, an AIP facility jointly operated by AIP and IAC. This research was (partially) funded by the Italian Ministry for Research and Scuola Normale Superiore. St Petersburg University team acknowledges support from Russian RFBR grant 15-02-00949 and St Petersburg University research grant 6.38.335.2015. AZT-24 observations are made within an agreement between Pulkovo, Rome, and Teramo observatories. The Abastumani team acknowledges financial support of the project FR/638/6-320/12 by the Shota Rustaveli National Science Foundation under contract 31/77. Data at Rozhen NAO and Plana SAO were obtained with support by BG051 PO001-3.3.06-0057 project within Human Resources Development Operational Programme 2007–2013. This research was partially supported by Scientific Research Fund of the Bulgarian Ministry of Education and Sciences under grant DO 02-137 (BIn-13/09). The Skinakas Observatory is a collaborative project of the University of Crete, the Foundation for Research and Technology – Hellas, and the Max-Planck-Institut für Extraterrestrische Physik. Uzbekistan's authors acknowledge the support from Committee for coordination of science and technology development (Project N. F2-FA-F027). GD and OV gratefully acknowledge the observing grant support from the Institute of Astronomy and Rozhen National Astronomical Observatory, Bulgaria Academy of Sciences. This work is a part of the Projects No 176011 (Dynamics and kinematics of celestial bodies and systems), No. 176004 (Stellar physics) and No. 176021 (Visible and invisible matter in nearby galaxies: theory and observations) supported by the Ministry of Education, Science and Technological Development of the Republic of Serbia. SPM observatory data were obtained through support given by PAPIIT grant 111514. The Metsähovi team acknowledges the support from the Academy of Finland to our observing projects (numbers 212656, 210338, 121148, and others). Based on observations with the Medicina and Noto telescopes operated by INAF – Istituto di Radioastronomia.
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 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 Hubble Space Telescope/Cosmic Origins Spectrograph and International Ultraviolet Explorer 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.
Volume
454
Issue
1
Start page
353
Issn Identifier
0035-8711
Ads BibCode
2015MNRAS.454..353R
Rights
open.access
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