A. C. Sadun

A MULTIWAVELENGTH VIEW OF THE TeV BLAZAR MARKARIAN 421: CORRELATED VARIABILITY, FLARING, AND SPECTRAL EVOLUTION

We report results from an intensive multiwavelength monitoring campaign on the TeV blazar Mrk 421 over the period of 2003-2004. The source was observed simultaneously at TeV energies with the Whipple 10 m telescope and at X-ray energies with the Rossi X-Ray Timing Explorer (RXTE) during each clear night within the Whipple observing windows. Supporting observations were also frequently carried out at optical and radio wavelengths to provide simultaneous or contemporaneous coverages. The large amount of simultaneous data has allowed us to examine the variability of Mrk 421 in detail, including cross-band correlation and broadband spectral variability, over a wide range of flux. The variabilities are generally correlated between the X-ray and gamma-ray bands, although the correlation appears to be fairly loose. The light curves show the presence of flares with varying amplitudes on a wide range of timescales at both X-ray and TeV energies. Of particular interest is the presence of TeV flares that have no coincident counterparts at longer wavelengths, because the phenomenon seems difficult to understand in the context of the proposed emission models for TeV blazars. We have also found that the TeV flux reached its peak days before the X-ray flux did during a giant flare (or outburst) in 2004 (with the peak flux reaching ~135 mcrab in X-rays, as seen by the RXTE ASM, and ~3 crab in gamma rays). Such a difference in the development of the flare presents a further challenge to both the leptonic and hadronic emission models. Mrk 421 varied much less at optical and radio wavelengths. Surprisingly, the normalized variability amplitude in the optical seems to be comparable to that in the radio, perhaps suggesting the presence of different populations of emitting electrons in the jet. The spectral energy distribution of Mrk 421 is seen to vary with flux, with the two characteristic peaks moving toward higher energies at higher fluxes. We have failed to fit the measured spectral energy distributions (SEDs) with a one-zone synchrotron self-Compton model; introducing additional zones greatly improves the fits. We have derived constraints on the physical properties of the X-ray/gamma-ray flaring regions from the observed variability (and SED) of the source. The implications of the results are discussed.

Coordinated Multiwavelength Observations of BL Lacertae in 2000

BL Lacertae (BL Lac) was the target of an extensive multiwavelength monitoring campaign in the second half of 2000. Simultaneous or quasi-simultaneous observations were taken at radio (University of Michigan Radio Astronomy Observatory and Metsahovi Radio Telescope) and optical (Whole Earth Blazar Telescope (WEBT) collaboration) frequencies, in X-rays (BeppoSAX and RXTE), and at very high energy gamma rays (HEGRA). The WEBT optical campaign achieved an unprecedented time coverage, virtually continuous over several 10-20 hr segments. It revealed intraday variability on timescales of � 1.5 hr and evidence for spectral hardening associated with increasing optical flux. During the campaign, BL Lac underwent a major transition from a rather quiescent state prior to 2000 September, to a flaring state for the rest of the year. ThisBL Lacertae (BL Lac) was the target of an extensive multiwavelength monitoring campaign in the second half of 2000. Simultaneous or quasi-simultaneous observations were taken at radio (University of Michigan Radio Astronomy Observatory andMetsähovi Radio Telescope) and optical (Whole Earth Blazar Telescope [WEBT] collaboration) frequencies, in X-rays (BeppoSAX and RXTE), and at very high energy gamma rays (HEGRA). The WEBT optical campaign achieved an unprecedented time coverage, virtually continuous over several 10–20 hr segments. It revealed intraday variability on timescales of 1.5 hr and evidence for spectral hardening associated with increasing optical flux. During the campaign, BL Lac underwent a major transition from a rather quiescent state prior to 2000 September, to a flaring state for the rest of the year. This 36 Department of Chemistry, Physics, and Astronomy, FrancisMarionUniversity, P.O. Box 100547, Florence, SC 29501-0547. 37 Department of Physics and Astronomy, University of SouthamptonHighfield, Southampton SO17 1BJ, UK. 38 Dipartimento di Fisica Generale, Università di Torino, Via P. Giuria 1, I-10125 Turin, Italy. 34 Department of Physics and Astronomy,Western KentuckyUniversity, 1 Big RedWay, Bowling Green, KY 42104. 35 Department of Physics and Astronomy, Georgia State University, Atlanta, GA 30303. 1 Department of Physics and Astronomy, Clippinger 339, Ohio University, Athens, OH 45701. 2 Department of Astronomy, BostonUniversity, 725 Commonwealth Avenue, Boston,MA 02215. 3 Osservatorio Astronomico di Brera, Via Bianchi 46, I-23807Merate, Italy. 4 IstitutoNazionale di Astrofisica (INAF), Osservatorio Astronomico di Torino, Via Osservatorio 20, I-10025 Pino Torinese, Italy. 5 Department of Astronomy, University ofMichigan, 810 Dennison Building, AnnArbor,MI 48109-1090. 6 Metsähovi Radio Observatory, Helsinki University of Technology,Metsähovintie 114, 02540Kylmälä, Finland. 7 Institut für Experimentelle und Angewandte Physik, Universität Kiel, Leibnitzstrasse 15–19, D-24118Kiel, Germany. 8 Max-Planck-Institut für Kernphysik, Postfach 10 39 80, D-69029Heidelberg, Germany. 9 Physics Department,WashingtonUniversity, 1 Brookings Drive, CB 1105, St. Louis, MO 63130. 10 Abastumani Observatory, 383762Abastumani, Georgia. 11 Astrophysikalisches Institute Potsdam, An der Sternwarte 16, D-14482 Potsdam, Germany. 12 Landessternwarte Heidelberg-Königstuhl, Königstuhl 12, D-69117Heidelberg, Germany. 13 Ulugh Beg Astronomical Institute, Uzbek Academy of Sciences, Astronomicheskaya 33, Tashkent 700052, Uzbekistan. 14 IsaacNewton Institute of Chile, Uzbekistan Branch. 15 Physics Department, University of Crete, 710 03Heraklion, Crete, Greece. 16 IESL, Foundation for Research and Technology-Hellas, 711 10Heraklion, Crete, Greece. 17 Astronomical Institute, OsakaKyoikuUniversity, Kashiwara-shi, Osaka 582-8582, Japan. 18 Tuorla Observatory, 21500 Piikkiö, Finland. 19 Osservatorio Astronomico, Università di Perugia, Via B. Bonfigli, I-06126 Perugia, Italy. 20 Osservatorio Astrofisico di Catania, Viale A. Doria 6, I-95125 Catania, Italy. 21 Department of Physics and Astronomy, University of Victoria, BC, Canada. 22 Institute for Astrophysical Research, BostonUniversity, 725 Commonwealth Avenue, Boston,MA 02215. 23 Center for Astrophysics, GuangzhouUniversity, Guangzhou 510400, China. 24 Astronomical Institute, St. Petersburg State University, Bibliotechnaya Pl. 2, Petrodvoretz, 198504 St. Petersburg, Russia. 25 Dipartimento di Fisica, Università La Sapienza, Piazzale A.Moro 2, I-00185Rome, Italy. 26 Department of Physics and Astronomy, University ofMissouri-St. Louis, 8001 Natural Bridge Road, St. Louis,MO 63121. 27 Jet Propulsion Laboratory, California Institute of Technology, 4800 OakGroveDrive, Pasadena, CA 91109. 28 Department of Astronomy, Faculty of Science, KyotoUniversity, Kyoto, Japan. 29 Clarke and Coyote Astrophysical Observatory, P.O. Box 930,Wilton, CA 95693. 30 Instituto de Astronomı́a, UNAM,Apartado Postal 70-264, 04510MexicoDF,Mexico. 31 Nyrölä Observatory, Jyväskylän Sirius ry, Kyllikinkatu 1, 40950 Jyväskylä, Finland. 32 GuadarramaObservatory, C/ San Pablo 5, Villalba 28409,Madrid, Spain. 33 Department of Physics, University of Colorado, P.O. Box 173364, Denver, CO 80217-3364. The Astrophysical Journal, 596:847–859, 2003 October 20 # 2003. The American Astronomical Society. All rights reserved. Printed in U.S.A. E

THE WHOLE EARTH BLAZAR TELESCOPE CAMPAIGN ON THE INTERMEDIATE BL LAC OBJECT 3C 66A IN 2007-2008lfootnote id="*" pos="info_bottom"gThe radio-to-optical data presented in this paper are stored in the WEBT archive; for questions regarding their availability,

Prompted by a high optical state in September 2007, the Whole Earth Blazar Telescope (WEBT) consortium organized an intensive optical, near-IR (JHK) and radio observing campaign on the intermediate BL Lac object 3C 66A throughout the fall and winter of 2007 -- 2008. The source remained in a high optical state throughout the observing period and exhibited several bright flares on time scales of ~ 10 days. This included an exceptional outburst around September 15 - 20, 2007, reaching a peak brightness at R ~ 13.4. Our campaign revealed microvariability with flux changes up to |dR/dt| ~ 0.02 mag/hr. Our observations do not reveal evidence for systematic spectral variability or spectral lags. We infer a value of the magnetic field in the emission region of B ~ 19 e_B^{2/7} \tau_h^{-6/7} D_1^{13/7} G. From the lack of systematic spectral variability, we can derive an upper limit on the Doppler factor, D 50, required for a one-zone SSC interpretation of some high-frequency-peaked BL Lac objects detected at TeV gamma-ray energies.

Simultaneous observations of the continuum emission of the quasar 3C 273 from radio to γ-ray energies

From June 15–28, 1991 the Compton Gamma‐Ray Observatory (CGRO) observed the radio‐loud quasar 3C 273. All four CGRO instruments detected radiation from this quasar in their relevant energy range (from 20 keV to 5 GeV). Simultaneous and quasi‐simultaneous observations by instruments sensitive at other wavelengths have also been obtained. The data from all these observations, spanning the frequency range from ∼109 Hz–∼1026 Hz, were collected and analyzed. Details of the observations and an overall energy‐density spectrum are presented. This spectrum shows two maxima of nearly equal strength. One is in the UV, while the other one is found at low‐energy γ‐rays. The implications of these simultaneous observations on some theoretical models will be discussed.