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Issue A&A Volume 408, Number 2, September III 2003 Page(s) 479 - 491 Section Galactic structure and dynamics DOI 10.1051/0004-6361:20031015

A&A 408, 479-491 (2003)
DOI: 10.1051/0004-6361:20031015

BeppoSAX and multiwavelength observations of BL Lacertae in 2000

M. Ravasio¹, G. Tagliaferri¹, G. Ghisellini¹, F. Tavecchio¹, M. Böttcher² and M. Sikora<sup>3

1</sup>  Osservatorio Astronomico di Brera, Via Bianchi 46, 23807 Merate, Italy
²  Department of Physics and Astronomy, Clippinger 339, Ohio University, Athens, OH 45701, USA
³  Nicolaus Copernicus Astronomical Center, Bartycka 18, 00-716 Warsaw, Poland

(Received 14 May 2003 / Accepted 18 June 2003 )

Abstract
We present two BeppoSAX observations of BL Lac (2200+420) as part of a multiwavelength campaign performed in 2000. The source was in different states of activity: in June, the X-ray spectrum was faint and hard ( ), with positive residuals towards low energies. In October we detected the highest [2-10] keV flux ever measured for the source. During this observation, the spectrum was soft ( ) up to 10 keV, while above this energy a hard component was dominating. The BeppoSAX data are confirmed by simultaneous RXTE short observations. During the first observation the soft X-ray flux was variable on timescales of a few hours, while the hard X-ray flux was almost constant. During the second observation, instead, the soft spectrum displayed an erratic behaviour with large variations (up to factors 3-4) on timescales smaller than 2 hrs. The analysis of the multiwavelength SED of October showed an intriguing feature: the optical and X-ray sections of the SED are misaligned, while in the prevailing standard picture, they are both thought to be produced via synchrotron emission. We suggested four scenarios to account for this discrepancy: a higher than galactic dust-to-gas ratio towards the source, the first detection of bulk Compton emission in the X-ray band, the presence of two synchrotron emitting regions located at different distances from the nucleus, the detection of a Klein-Nishina effect on the synchrotron spectrum. We discuss the favorable and critical points of each scenario, but, at present, we cannot discriminate between them.

Key words: BL Lacertae objects: general -- X-rays: galaxies -- BL Lacertae objects: individual: BL Lacertae (2200+420)

Offprint request: M. Ravasio, ravasio@merate.mi.astro.it

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