HAL: in2p3-00587827, version 1

arXiv: 1101.5905

DOI: 10.1088/0004-637X/730/2/101

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Astrophysical Journal 730 (2011) 101

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

A. A. Abdo, M. Ackermann, M. Ajello, E. Antolini, L. Baldini, J. Ballet 1, G. Barbiellini, D. Bastieri, K. Bechtol, R. Bellazzini, B. Berenji, R. D. Blandford, E. Bonamente, A. W. Borgland, J. Bregeon, A. Brez, M. Brigida, P. 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, J. 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

For the Fermi-LAT collaboration(s)

(2011-04-01)

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.

1:	Astrophysique Interactions Multi-échelles (AIM)
	CNRS : UMR7158 – INSU – Université Paris VII - Paris Diderot – CEA : DSM/IRFU
2:	Laboratoire Leprince-Ringuet (LLR)
	CNRS : UMR7638 – IN2P3 – Polytechnique - X
3:	Laboratoire de Physique Théorique et Astroparticules (LPTA)
	CNRS : UMR5207 – IN2P3 – Université Montpellier II - Sciences et techniques
4:	Centre d'Etudes Nucléaires de Bordeaux Gradignan (CENBG)
	CNRS : UMR5797 – IN2P3 – Université Sciences et Technologies - Bordeaux I
5:	Centre d'étude spatiale des rayonnements (CESR)
	CNRS : UMR5187 – Observatoire Midi-Pyrénées – INSU – Université Paul Sabatier [UPS] - Toulouse III

Subject(s)

:

Physics/Astrophysics/High Energy Astrophysical Phenomena Sciences of the Universe/Astrophysics/High Energy Astrophysical Phenomena Physics/Astrophysics/Cosmology and Extra-Galactic Astrophysics Sciences of the Universe/Astrophysics/Cosmology and Extra-Galactic Astrophysics

Keyword(s): BL Lacertae objects: general – BL Lacertae objects: individual (BL Lacertae) – galaxies: jets – gamma rays: galaxies – radiation mechanisms: non-thermal – X-rays: galaxies

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From: Virginie Mas <>

Submitted for: Benoit Lott <>

Submitted on: Thursday, 21 April 2011 15:09:24

Updated on: Friday, 22 April 2011 14:53:48