Simultaneous Planck, Swift, and Fermi observations of X-ray and γ-ray selected blazars

¹ Aalto University Metsähovi Radio Observatory, Metsähovintie 114, 02540 Kylmälä, Finland
² Agenzia Spaziale Italiana Science Data Center, c/o ESRIN, via Galileo Galilei, Frascati, Italy
e-mail: giommi@asdc.asi.it
³ Agenzia Spaziale Italiana, Viale Liegi 26, Roma, Italy
⁴ Astronomy Department, University of Michigan, 830 Dennison Building, 500 Church Street, Ann Arbor, Michigan 48109-1042, USA
⁵ Astroparticle Physics Laboratory, NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA
⁶ Astrophysics Group, Cavendish Laboratory, University of Cambridge, J J Thomson Avenue, Cambridge CB3 0HE, UK
⁷ Australia Telescope National Facility, CSIRO, PO Box 76, Epping, NSW 1710, Australia
⁸ California Institute of Technology, Pasadena, California, USA
⁹ Departamento de Física, Universidad de Oviedo, Avda. Calvo Sotelo s/n, Oviedo, Spain
¹⁰ Department of Astronomy and Astrophysics, Pennsylvania State University, 525 Davey Lab, University Park, PA 16802, USA
¹¹ Department of Physics and Astronomy, Tufts University, Medford, MA 02155, USA
¹² Department of Physics, Gustaf Hällströmin katu 2a, University of Helsinki, Helsinki, Finland
¹³ Dipartimento di Fisica G. Galilei, Università degli Studi di Padova, viaMarzolo 8, 35131 Padova, Italy
¹⁴ Dipartimento di Fisica M. Merlin dell’Università e del Politecnico di Bari, 70126 Bari, Italy
¹⁵ Dipartimento di Fisica, Università degli Studi di Perugia, 06123 Perugia, Italy
¹⁶ Dipartimento di Fisica, Università di Ferrara, via Saragat 1, 44122 Ferrara, Italy
¹⁷ Finnish Centre for Astronomy with ESO (FINCA), University of Turku, Väisäläntie 20, 21500 Piikkiö, Finland
¹⁸ Haverford College Astronomy Department, 370 Lancaster Avenue, Haverford, Pennsylvania, USA
¹⁹ Helsinki Institute of Physics, Gustaf Hällströmin katu 2, University of Helsinki, Helsinki, Finland
²⁰ INAF – Osservatorio Astrofisico di Catania, via S. Sofia 78, Catania, Italy
²¹ INAF – Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807, Merate (LC), Italy
²² INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, Padova, Italy
²³ INAF – Osservatorio Astronomico di Roma, via di Frascati 33, Monte Porzio Catone, Italy
²⁴ INAF – Osservatorio Astronomico di Trieste, via G.B. Tiepolo 11, Trieste, Italy
²⁵ INAF Istituto di Radioastronomia, via P. Gobetti 101, 40129 Bologna, Italy
²⁶ INAF/IASF Bologna, via Gobetti 101, Bologna, Italy
²⁷ INAF/IASF, Sezione di Palermo, via Ugo La Malfa 153, 90146 Palermo, Italy
²⁸ ISDC Data Centre for Astrophysics, University of Geneva, Ch. d’Ecogia 16, Versoix, Switzerland
²⁹ Infrared Processing and Analysis Center, California Institute of Technology, Pasadena, CA 91125, USA
³⁰ Institut de Radioastronomie Millimétrique (IRAM), Avenida Divina Pastora 7, Local 20, 18012 Granada, Spain
³¹ Institute für Astro- und Teilchenphysik and Institut für Theoretische Physik, Leopold-Franzens-Universität Innsbruck, 6020 Innsbruck, Austria
³² Instituto de Física de Cantabria (CSIC-Universidad de Cantabria), Avda. de los Castros s/n, Santander, Spain
³³ Istituto Nazionale di Fisica Nucleare, Sezione di Bari, 70126 Bari, Italy
³⁴ Istituto Nazionale di Fisica Nucleare, Sezione di Padova, 35131 Padova, Italy
³⁵ Istituto Nazionale di Fisica Nucleare, Sezione di Perugia, 06123 Perugia, Italy
³⁶ Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, California, USA
³⁷ Jodrell Bank Centre for Astrophysics, Alan Turing Building, School of Physics and Astronomy, The University of Manchester, Oxford Road, Manchester, M13 9PL, UK
³⁸ Kavli Institute for Cosmology Cambridge, Madingley Road, Cambridge, CB3 0HA, UK
³⁹ Key Laboratory of Optical Astronomy, National Astronomical Observatories, Chinese Academy of Sciences, 20A Datun Road, Chaoyang District, 100012 Beijing, PR China
⁴⁰ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 85741 Garching, Germany
⁴¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁴² Owens Valley Radio Observatory, Mail code 249-17, California Institute of Technology, 1200 E California Blvd, Pasadena, CA 91125, USA
⁴³ SISSA, Astrophysics Sector, via Bonomea 265, 34136, Trieste, Italy
⁴⁴ Special Astrophysical Observatory, Russian Academy of Sciences, Nizhnij Arkhyz, Zelenchukskiy region, Karachai-Cherkessian Republic, 369167, Russia
⁴⁵ Tuorla Observatory, Department of Physics and Astronomy, University of Turku, Väisäläntie 20, 21500 Piikkiö, Finland
⁴⁶ Università degli studi di Milano-Bicocca, Dipartimento di Fisica, Piazza delle Scienze 3, 20126 Milano, Italy
⁴⁷ W. W. Hansen Experimental Physics Laboratory, Kavli Institute for Particle Astrophysics and Cosmology, Department of Physics and SLAC National Accelerator Laboratory, Stanford University, Stanford, CA 94305, USA

Received: 4 August 2011
Accepted: 31 January 2012

Abstract

We present simultaneous Planck, Swift, Fermi, and ground-based data for 105 blazars belonging to three samples with flux limits in the soft X-ray, hard X-ray, and γ-ray bands, with additional 5GHz flux-density limits to ensure a good probability of a Planck detection. We compare our results to those of a companion paper presenting simultaneous Planck and multi-frequency observations of 104 radio-loud northern active galactic nuclei selected at radio frequencies. While we confirm several previous results, our unique data set allows us to demonstrate that the selection method strongly influences the results, producing biases that cannot be ignored. Almost all the BL Lac objects have been detected by the Fermi Large AreaTelescope (LAT), whereas 30% to 40% of the flat-spectrum radio quasars (FSRQs) in the radio, soft X-ray, and hard X-ray selected samples are still below the γ-ray detection limit even after integrating 27 months of Fermi-LAT data. The radio to sub-millimetre spectral slope of blazars is quite flat, with ⟨α⟩ ~ 0 up to about 70GHz, above which it steepens to ⟨α⟩ ~ −0.65. The BL Lacs have significantly flatter spectra than FSRQs at higher frequencies. The distribution of the rest-frame synchrotron peak frequency (ν_peak^S) in the spectral energy distribution (SED) of FSRQs is the same in all the blazar samples with ⟨ν_peak^S⟩ = 10^{13.1 ± 0.1} Hz, while the mean inverse Compton peak frequency, ⟨ν_peak^IC⟩, ranges from 10²¹ to 10²² Hz. The distributions of ν_peak^S and ν_peak^IC of BL Lacs are much broader and are shifted to higher energies than those of FSRQs; their shapes strongly depend on the selection method. The Compton dominance of blazars, defined as the ratio of the inverse Compton to synchrotron peak luminosities, ranges from less than 0.2 to nearly 100, with only FSRQs reaching values larger than about 3. Its distribution is broad and depends strongly on the selection method, with γ-ray selected blazars peaking at ~7 or more, and radio-selected blazars at values close to 1, thus implying that the common assumption that the blazar power budget is largely dominated by high-energy emission is a selection effect. A comparison of our multi-frequency data with theoretical predictions shows that simple homogeneous SSC models cannot explain the simultaneous SEDs of most of the γ-ray detected blazars in all samples. The SED of the blazars that were not detected by Fermi-LAT may instead be consistent with SSC emission. Our data challenge the correlation between bolometric luminosity and ν_peak^S predicted by the blazar sequence.