The first Fermi multifrequency campaign on BL Lacertae: characterizing the low-activity state of the eponymous blazar

A. A. Abdo , Markus Ackermann , Marco Ajello , E. Antolini , Luca Baldini , Jean Ballet (1) , Guido Barbiellini , D. Bastieri , K. Bechtol , R. Bellazzini , B. Berenji , R. D. Blandford , E. Bonamente , A. W. Borgland , J. Bregeon , A. Brez , M. Brigida , Pascal Bruel (2) , R. Buehler , S. Buson , G. A. Caliandro , R. A. Cameron , A. Cannon , P. A. Caraveo , S. Carrigan , J. M. Casandjian (1) , C. Cecchi , Ö. Çelik , E. Charles , A. Chekhtman , C. C. Cheung , J. Chiang , S. Ciprini , R. Claus , J. Cohen-Tanugi (3) , J. Conrad , L. Costamante , S. Cutini , C. D. Dermer , F. de Palma , D. Donato , E. Do Couto E Silva , P. S. Drell , R. Dubois , L. Escande (4) , C. Favuzzi , S. J. Fegan (2) , J. Finke , W. B. Focke , P. Fortin (2) , M. Frailis , Y. Fukazawa , S. Funk , P. Fusco , F. Gargano , D. Gasparrini , N. Gehrels , S. Germani , N. Giglietto , F. Giordano , M. Giroletti , T. Glanzman , G. Godfrey , I. A. Grenier (1) , S. Guiriec , D. Hadasch , M. Hayashida , E. Hays , R. E. Hughes , R. Itoh , G. Jóhannesson , A. S. Johnson , W. N. Johnson , T. Kamae , H. Katagiri , J. Kataoka , Jürgen Knödlseder (5) , M. Kuss , J. Lande , S. Larsson , L. Latronico , S.-H. Lee , M. Llena Garde , F. Longo , F. Loparco , B. Lott (4) , M. N. Lovellette , P. Lubrano , A. Makeev , M. N. Mazziotta , J. E. Mcenery , J. Mehault (3) , P. F. Michelson , T. Mizuno , C. Monte , M. E. Monzani , A. Morselli , I. V. Moskalenko , S. Murgia , T. Nakamori , M. Naumann-Godo (1) , S. Nishino , P. L. Nolan , J. P. Norris , E. Nuss (3) , T. Ohsugi , A. Okumura , N. Omodei , E. Orlando , J. F. Ormes , M. Ozaki , D. Paneque , J. H. Panetta , D. Parent (4) , V. Pelassa (3) , M. Pepe , M. Pesce-Rollins , F. Piron (3) , T.A. Porter , S. Rainò , R. Rando , M. Razzano , A. Reimer , O. Reimer , S. Ritz , M. Roth , H. F.-W. Sadrozinski , D. Sanchez (2) , A. Sander , F. K. Schinzel , C. Sgrò , E. J. Siskind , P. D. Smith , K. V. Sokolovsky , G. Spandre , P. Spinelli , M. S. Strickman , D. J. Suson , H. Takahashi , T. Tanaka , J. B. Thayer , J. G. Thayer , D. J. Thompson , L. Tibaldo (1) , D. F. Torres , G. Tosti , A. Tramacere , T. Uehara , T. L. Usher , J. Vandenbroucke , V. Vasileiou , N. Vilchez (5) , V. Vitale , A. P. Waite , E. Wallace , P. Wang , B. L. Winer , K. S. Wood , Z. Yang , T. Ylinen , M. Ziegler , A. Berdyugin , M. Boettcher , A. Carramiñana , L. Carrasco , E. de La Fuente , C. Diltz , T. Hovatta , V. Kadenius , Y.Y. Kovalev , A. Lähteenmäki , E. Lindfors , A. P. Marscher , K. Nilsson , D. Pereira , R. Reinthal , P. Roustazadeh , T. Savolainen , A. Sillanpää , L. O. Takalo , M. Tornikoski

1 AIM (UMR7158 / UMR_E_9005 / UM_112) - Astrophysique Interprétation Modélisation
2 LLR - Laboratoire Leprince-Ringuet
3 LPTA - Laboratoire de Physique Théorique et Astroparticules
4 CENBG - Centre d'Etudes Nucléaires de Bordeaux Gradignan
5 CESR - Centre d'étude spatiale des rayonnements

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.

Mots clés: BL Lacertae objects: general; BL Lacertae objects: individual (BL Lacertae); galaxies: jets; gamma rays: galaxies; radiation mechanisms: non-thermal; X-rays: galaxies

Domaines: Phénomènes cosmiques de haute energie [astro-ph.HE]; Phénomènes cosmiques de haute energie [astro-ph.HE]; Cosmologie et astrophysique extra-galactique [astro-ph.CO]; Cosmologie et astrophysique extra-galactique [astro-ph.CO]

A. A. Abdo, Markus Ackermann, Marco Ajello, E. Antolini, Luca Baldini, et al.. The first Fermi multifrequency campaign on BL Lacertae: characterizing the low-activity state of the eponymous blazar. The Astrophysical Journal, 2011, 730, pp.101. ⟨10.1088/0004-637X/730/2/101⟩. ⟨in2p3-00587827⟩