THE FIRST<i>FERMI</i>MULTIFREQUENCY CAMPAIGN ON BL LACERTAE: CHARACTERIZING THE LOW-ACTIVITY STATE OF THE EPONYMOUS BLAZAR
A. A. AbdoNational Research Council Research Associate, National Academy of Sciences, Washington, DC 20001, USAM. AckermannW. 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, USAM. AjelloW. 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, USAE. AntoliniDipartimento di Fisica, Università degli Studi di Perugia, I-06123 Perugia, Italy; [email protected]L. BaldiniIstituto Nazionale di Fisica Nucleare, Sezione di Pisa, I-56127 Pisa, ItalyJ. BalletLaboratoire AIM, CEA-IRFU/CNRS/Université Paris Diderot, Service d'Astrophysique, CEA Saclay, 91191 Gif sur Yvette, FranceG. BarbielliniDipartimento di Fisica, Università di Trieste, I-34127 Trieste, ItalyD. BastieriDipartimento di Fisica “G. Galilei,” Università di Padova, I-35131 Padova, ItalyK. BechtolW. 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, USAR. BellazziniIstituto Nazionale di Fisica Nucleare, Sezione di Pisa, I-56127 Pisa, ItalyB. BerenjiW. 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, USAR. D. BlandfordW. 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, USAE. BonamenteDipartimento di Fisica, Università degli Studi di Perugia, I-06123 Perugia, Italy; [email protected]A. W. BorglandW. 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, USAJ. BregeonIstituto Nazionale di Fisica Nucleare, Sezione di Pisa, I-56127 Pisa, ItalyA. BrezIstituto Nazionale di Fisica Nucleare, Sezione di Pisa, I-56127 Pisa, ItalyM. BrigidaDipartimento di Fisica “M. Merlin” dell'Università e del Politecnico di Bari, I-70126 Bari, ItalyP. BruelLaboratoire Leprince-Ringuet, École polytechnique, CNRS/IN2P3, Palaiseau, FranceR. BuehlerW. 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, USAS. BusonDipartimento di Fisica “G. Galilei,” Università di Padova, I-35131 Padova, ItalyG. A. CaliandroInstitut de Ciencies de l'Espai (IEEC-CSIC), Campus UAB, 08193 Barcelona, SpainR. A. CameronW. 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, USAA. CannonNASA Goddard Space Flight Center, Greenbelt, MD 20771, USAP. A. CaraveoINAF-Istituto di Astrofisica Spaziale e Fisica Cosmica, I-20133 Milano, ItalyS. CarriganDipartimento di Fisica “G. Galilei,” Università di Padova, I-35131 Padova, ItalyJ. M. CasandjianLaboratoire AIM, CEA-IRFU/CNRS/Université Paris Diderot, Service d'Astrophysique, CEA Saclay, 91191 Gif sur Yvette, FranceC. CecchiDipartimento di Fisica, Università degli Studi di Perugia, I-06123 Perugia, Italy; [email protected]Ö. ÇelikCenter for Research and Exploration in Space Science and Technology (CRESST) and NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA; [email protected]E. CharlesW. 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, USAA. ChekhtmanGeorge Mason University, Fairfax, VA 22030, USAC. C. CheungNational Research Council Research Associate, National Academy of Sciences, Washington, DC 20001, USAJ. ChiangW. 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, USAS. CipriniDipartimento di Fisica, Università degli Studi di Perugia, I-06123 Perugia, Italy; [email protected]R. ClausW. 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, USAJ. Cohen-TanugiLaboratoire de Physique Théorique et Astroparticules, Université Montpellier 2, CNRS/IN2P3, Montpellier, FranceJ. M. ConradDepartment of Physics, Stockholm University, AlbaNova, SE-106 91 Stockholm, SwedenL. CostamanteW. 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, USAS. CutiniAgenzia Spaziale Italiana (ASI) Science Data Center, I-00044 Frascati (Roma), ItalyC. D. DermerSpace Science Division, Naval Research Laboratory, Washington, DC 20375, USAF. de PalmaDipartimento di Fisica “M. Merlin” dell'Università e del Politecnico di Bari, I-70126 Bari, ItalyD. DonatoCenter for Research and Exploration in Space Science and Technology (CRESST) and NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA; [email protected]E. do Couto e SilvaW. 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, USAP. S. DrellW. 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, USAR. DuboisW. 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, USAL. EscandeCentre d'Études Nucléaires de Bordeaux Gradignan, Université Bordeaux 1, CNRS/IN2p3, 33175 Gradignan, FranceC. FavuzziDipartimento di Fisica “M. Merlin” dell'Università e del Politecnico di Bari, I-70126 Bari, ItalyS. FeganLaboratoire Leprince-Ringuet, École polytechnique, CNRS/IN2P3, Palaiseau, FranceJ. FinkeNational Research Council Research Associate, National Academy of Sciences, Washington, DC 20001, USAW. B. FockeW. 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, USAP. FortinLaboratoire Leprince-Ringuet, École polytechnique, CNRS/IN2P3, Palaiseau, FranceM. FrailisDipartimento di Fisica, Università di Udine and Istituto Nazionale di Fisica Nucleare, Sezione di Trieste, Gruppo Collegato di Udine, I-33100 Udine, ItalyY. FukazawaDepartment of Physical Sciences, Hiroshima University, Higashi-Hiroshima, Hiroshima 739-8526, JapanS. FunkW. 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, USAP. FuscoDipartimento di Fisica “M. Merlin” dell'Università e del Politecnico di Bari, I-70126 Bari, ItalyF. GarganoIstituto Nazionale di Fisica Nucleare, Sezione di Bari, 70126 Bari, ItalyD. GasparriniAgenzia Spaziale Italiana (ASI) Science Data Center, I-00044 Frascati (Roma), ItalyN. GehrelsNASA Goddard Space Flight Center, Greenbelt, MD 20771, USAS. GermaniDipartimento di Fisica, Università degli Studi di Perugia, I-06123 Perugia, Italy; [email protected]N. GigliettoDipartimento di Fisica “M. Merlin” dell'Università e del Politecnico di Bari, I-70126 Bari, ItalyF. GiordanoDipartimento di Fisica “M. Merlin” dell'Università e del Politecnico di Bari, I-70126 Bari, ItalyM. GirolettiINAF Istituto di Radioastronomia, 40129 Bologna, ItalyT. GlanzmanW. 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, USAG. GodfreyW. 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, USAI. A. GrenierLaboratoire AIM, CEA-IRFU/CNRS/Université Paris Diderot, Service d'Astrophysique, CEA Saclay, 91191 Gif sur Yvette, FranceS. GuiriecCenter for Space Plasma and Aeronomic Research (CSPAR), University of Alabama in Huntsville, Huntsville, AL 35899, USAD. HadaschInstitut de Ciencies de l'Espai (IEEC-CSIC), Campus UAB, 08193 Barcelona, SpainM. HayashidaW. 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, USAE. HaysNASA Goddard Space Flight Center, Greenbelt, MD 20771, USAR. E. HughesDepartment of Physics, Center for Cosmology and Astro-Particle Physics, The Ohio State University, Columbus, OH 43210, USAR. ItohDepartment of Physical Sciences, Hiroshima University, Higashi-Hiroshima, Hiroshima 739-8526, JapanG. JóhannessonScience Institute, University of Iceland, IS-107 Reykjavik, IcelandA. S. JohnsonStanford UniversityW. N. JohnsonSpace Science Division, Naval Research Laboratory, Washington, DC 20375, USAT. KamaeW. 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, USAH. KatagiriDepartment of Physical Sciences, Hiroshima University, Higashi-Hiroshima, Hiroshima 739-8526, JapanJ. KataokaResearch Institute for Science and Engineering, Waseda University, 3-4-1, Okubo, Shinjuku, Tokyo 169-8555, JapanJ. KnödlsederCentre d'Étude Spatiale des Rayonnements, CNRS/UPS, BP 44346, F-30128 Toulouse Cedex 4, FranceM. KussIstituto Nazionale di Fisica Nucleare, Sezione di Pisa, I-56127 Pisa, ItalyJ. LandéW. 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, USAS. LarssonDepartment of Astronomy, Stockholm University, SE-106 91 Stockholm, SwedenL. LatronicoIstituto Nazionale di Fisica Nucleare, Sezione di Pisa, I-56127 Pisa, ItalyS.-H. LeeW. 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, USAM. Llena GardeDepartment of Physics, Stockholm University, AlbaNova, SE-106 91 Stockholm, SwedenF. LongoDipartimento di Fisica, Università di Trieste, I-34127 Trieste, ItalyF. LoparcoDipartimento di Fisica “M. Merlin” dell'Università e del Politecnico di Bari, I-70126 Bari, ItalyB. LottCentre d'Études Nucléaires de Bordeaux Gradignan, Université Bordeaux 1, CNRS/IN2p3, 33175 Gradignan, FranceM. N. LovelletteSpace Science Division, Naval Research Laboratory, Washington, DC 20375, USAP. LubranoDipartimento di Fisica, Università degli Studi di Perugia, I-06123 Perugia, Italy; [email protected]A. MakeevGeorge Mason University, Fairfax, VA 22030, USAM. N. MazziottaIstituto Nazionale di Fisica Nucleare, Sezione di Bari, 70126 Bari, ItalyJ. E. McEneryDepartment of Physics and Department of Astronomy, University of Maryland, College Park, MD 20742, USAJ. MéhaultLaboratoire de Physique Théorique et Astroparticules, Université Montpellier 2, CNRS/IN2P3, Montpellier, FranceP. F. MichelsonW. 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, USATsunefumi MizunoDepartment of Physical Sciences, Hiroshima University, Higashi-Hiroshima, Hiroshima 739-8526, JapanC. MonteDipartimento di Fisica “M. Merlin” dell'Università e del Politecnico di Bari, I-70126 Bari, ItalyM. E. MonzaniW. 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, USAA. MorselliIstituto Nazionale di Fisica Nucleare, Sezione di Roma “Tor Vergata,” I-00133 Roma, ItalyI. V. MoskalenkoW. 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, USAS. MurgiaStanford UniversityTakeshi NakamoriResearch Institute for Science and Engineering, Waseda University, 3-4-1, Okubo, Shinjuku, Tokyo 169-8555, Japan
2011en
ABI
Аннотация
We report on observations of BL Lacertae during the first 18 months of Fermi-LAT science operations and present results from a 48-day multifrequency coordinated campaign from 2008 August 19 to 2008 October 7. The radio to gamma-ray behavior of BL Lac is unveiled during a low activity state thanks to the coordinated observations of radio-band (Metsahovi and VLBA), near-IR/optical (Tuorla, Steward, OAGH and MDM) and X-ray (RXTE and Swift) observatories. No variability was resolved in gamma-rays during the campaign, and the brightness level was 15 times lower than the level of the 1997 EGRET outburst. Moderate and uncorrelated variability has been detected in UV and X-rays. The X-ray spectrum is found to be concave indicating the transition region between the low and high energy component of the spectral energy distribution (SED). VLBA observation detected a synchrotron spectrum self-absorption turnover in the innermost part of the radio jet appearing to be elongated and inhomogeneous, and constrained the average magnetic field there to be less than 3 G. Over the following months BL Lac appeared variable in gamma-rays, showing flares (in 2009 April and 2010 January). There is no evidence for correlation of the gamma-rays with the optical flux monitored from the ground in 18 months. The SED may be described by a single zone or two zone synchrotron self-Compton (SSC) model, but a hybrid SSC plus external radiation Compton (ERC) model seems preferred based on the observed variability and the fact that it provides a fit closest to equipartition.
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