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Published January 10, 2016 | Published
Journal Article Open

Erratic Flaring of BL Lac in 2012-2013: Multiwavelength Observations


BL Lac, the eponymous blazar, flared to historically high levels at millimeter, infrared, X-ray, and gamma-ray wavelengths in 2012. We present observations made with Herschel, Swift, NuSTAR, Fermi, the Submillimeter Array, CARMA, and the VLBA in 2012–2013, including three months with nearly daily sampling at several wavebands. We have also conducted an intensive campaign of 30 hr with every-orbit observations by Swift and NuSTAR, accompanied by Herschel, and Fermi observations. The source was highly variable at all bands. Time lags, correlations between bands, and the changing shapes of the spectral energy distributions can be explained by synchrotron radiation and inverse Compton emission from nonthermal seed photons originating from within the jet. The passage of four new superluminal very long baseline interferometry knots through the core and two stationary knots about 4 pc downstream accompanied the high flaring in 2012–2013. The seed photons for inverse Compton scattering may arise from the stationary knots and from a Mach disk near the core where relatively slow-moving plasma generates intense nonthermal radiation. The 95 spectral energy distributions obtained on consecutive days form the most densely sampled, broad wavelength coverage for any blazar. The observed spectral energy distributions and multi-waveband light curves are similar to simulated spectral energy distributions and light curves generated with a model in which turbulent plasma crosses a conical shock with a Mach disk.

Additional Information

© 2016 The American Astronomical Society. Received 2015 February 26; accepted 2015 August 28; published 2016 January 5. We thank the anonymous reviewer for comments that improved the paper. We thank Paul Smith (Steward Observatory) and Claudia Raiteri (Osservatorio Astrofisico di Torino) for helpful discussions. We are grateful to Göran Pilbratt (Herschel) and Neil Gehrels (Swift) for allocation of Target of Opportunity time. We thank Mark Kidger and Rosario Lorente (Herschel Science Centre) for advice in observing with Herschel, David Shupe (NASA Herschel Science Center) for assistance with data reduction, and the Swift schedulers (Swift Operations Center at Pennsylvania State University) for responsive scheduling. A. Wehrle is grateful to Penny Milbouer (Houston, TX) for helpful discussions. A. Wehrle acknowledges Guest Investigator support from NASA via Herschel RSA 1427799. The Boston University group acknowledges support by NASA under Fermi Guest Investigator grants NNX11AQ03G, NNX12AO79G, NNX13AP06G, and NNX14AQ58G, and Swift Guest Investigator grants NNX14AC59G. S. G. Jorstad acknowledges support from Russian RFBR grant 15-02-00949 and St. Petersburg University research grant 6.38.335.2015. 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. The VLBA is an instrument of the National Radio Astronomy Observatory. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. This research has made use of the XRT Data Analysis Software (XRTDAS) developed under the responsibility of the ASI Science Data Center (ASDC), Italy. Swift at PSU is supported by NASA contract NAS5-00136. M. Baloković acknowledges support from the International Fulbright Science and Technology Award and from NASA Headquarters under the NASA Earth and Space Science Fellowship Program, grant NNX14AQ07H. Part of this work was supported under NASA Contract No. NNG08FD60C, and made use of data from the NuSTAR mission, a project led by the California Institute of Technology, managed by the Jet Propulsion Laboratory, and funded by the National Aeronautics and Space Administration. We thank the NuSTAR Operations, Software and Calibration teams for support with the execution and analysis of these observations. This research has made use of the NuSTAR Data Analysis Software (NuSTARDAS) jointly developed by the ASI Science Data Center (ASDC, Italy) and the California Institute of Technology (USA). T. Hovatta acknowledges support from the Jenny and Antti Wihuri foundation and Academy of Finland project number 267324. Support for CARMA construction was derived from the Gordon and Betty Moore Foundation, the Kenneth T. and Eileen L. Norris Foundation, the James S. McDonnell Foundation, the Associates of the California Institute of Technology, the University of Chicago, the states of California, Illinois, and Maryland, and the National Science Foundation. Ongoing CARMA development and operations are supported by the National Science Foundation under a cooperative agreement, and by the CARMA partner universities. The Fermi LAT Collaboration acknowledges generous on-going support from a number of agencies and institutes that have supported both the development and the operation of the LAT as well as scientific data analysis. These include the National Aeronautics and Space Administration and the Department of Energy in the United States, the Commissariat lEnergie Atomique and the Centre National de la Recherche Scientifique/Institut National de Physique Nuclaire et de Physique des Particules in France, the Agenzia Spaziale Italiana and the Istituto Nazionale di Fisica Nucleare in Italy, the Ministry of Education, Culture, Sports, Science and Technology (MEXT), High Energy Accelerator Research Organization (KEK) and Japan Aerospace Exploration Agency (JAXA) in Japan, and the K. A. Wallenberg Foundation, the Swedish Research Council and the Swedish National Space Board in Sweden. Additional support for science analysis during the operations phase is gratefully acknowledged from the Istituto Nazionale di Astrofisica in Italy and the Centre National dtudes Spatiales in France. This research has made use 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. Facilities: Herschel - European Space Agency's Herschel space observatory, Fermi - Fermi Gamma-Ray Space Telescope (formerly GLAST), Swift - Swift Gamma-Ray Burst Mission, SMA - SubMillimeter Array, CARMA - Combined Array for Research in Millimeter-Wave Astronomy, NuSTAR - The NuSTAR (Nuclear Spectroscopic Telescope Array) mission, VLBA - Very Long Baseline Array.

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