BL Lacertae: Complex spectral variability and rapid synchrotron flare detected with BeppoSAX

¹ Osservatorio Astronomico di Brera, Via Bianchi 46, 23807 Merate, Italy
² ASI Science Data Center, Via Galileo Galilei, 00044 Frascati, Italy
³ Dipartimento di Fisica, Universitá La Sapienza, P.le Aldo Moro 2, 00185 Roma, Italy
⁴ Istituto di Fisica Cosmica G.Occhialini, CNR, Via Bassini 15, 20133 Milano, Italy
⁵ SISSA/ISAS, via Beirut 2-4, 34014 Trieste, Italy
⁶ Osservatorio Astronomico di Brera, Via Brera, 28, 20121 Milano, Italy
⁷ Osservatorio Astronomico, Università di Perugia, Via A. Pascoli, 06100 Perugia, Italy
⁸ Dipartimento di Scienze, Università dell'Insubria, Via Valleggio 11, 22100 Como, Italy
⁹ Foggy Bottom Observatory, Colgate University, 13 Oak Drive 13346, Hamilton NY, USA
¹⁰ Dept. of Physics and Astronomy, Western Kentucky University, 1 Big Red Way, Bowling Green, KY 42102-3576, USA
¹¹ Dept. of Astronomy, Kyoto University, Kitashirakawa-Oiwake-cho, Sakio-ku, Kyoto 606-8502, Japan
¹² Abastumani Astrophysical Observatory, 383762, Abastumani, Republic of Georgia
¹³ Dept. of Astronomy, Boston University, 725 Commonwealth Avenue, Boston MA 02215, USA
¹⁴ Osservatorio Astronomico di Torino, Strada Osservatorio 20, 10025 Pino Torinese, Italy

Corresponding author: G. Tagliaferri, tagliaferri@merate.mi.astro.it

Received: 8 October 2001
Accepted: 7 December 2001

Abstract

We report on two BeppoSAX observations of BL Lac (2200+420) performed respectively in June and December 1999, as part of a ToO program to monitor blazars in high states of activity. During both runs the source has been detected up to 100 keV, but it showed quite different spectra: in June it was concave with a very hard component above 5–6 keV (; ); in December it was well fitted by a single power law (). During the first BeppoSAX observation BL Lac showed an astonishing variability episode: the 0.3–2 keV flux doubled in ~ min, while the flux above 4 keV was almost constant. This frequency–dependent event is one of the shortest ever recorded for BL Lac objects and places lower limits on the dimension and magnetic field of the emitting region and on the energy of the synchrotron radiating electrons. A similar but less extreme behaviour is detected also in optical light curves, that display non-simultaneous, smaller fluctuations of ∼ in 20 min. We fit the spectral energy distributions with a homogeneous, one-zone model to constrain the emission region in a very simple but effective SSC + external Compton scenario, highlighting the importance of the location of the emitting region with respect to the Broad Line Region and the relative spectral shape dependence. We compare our data with historical radio to γ-ray Spectral Energy Distributions.