Another look at the BL Lacertae flux and spectral variability

Observations by GASP-WEBT, XMM-Newton, and Swift in 2008–2009^⋆

¹ INAF – Osservatorio Astronomico di Torino, Italy
e-mail: raiteri@oato.inaf.it
² Dipartimento di Fisica Generale, Università di Torino, Italy
³ Abastumani Observatory, Mt. Kanobili, Georgia
⁴ Astron. Inst. St.-Petersburg State Univ., Russia
⁵ Pulkovo Observatory, St. Petersburg, Russia
⁶ Isaac Newton Institute of Chile, St.-Petersburg Branch, Russia
⁷ INAF – IASF Palermo, Italy
⁸ Institute for Astrophysical Research, Boston University, MA, USA
⁹ Instituto de Astrofísica de Andalucía, CSIC, Granada, Spain
¹⁰ Department of Astronomy, University of Michigan, MI, USA
¹¹ Max-Planck-Institut für Radioastronomie, Bonn, Germany
¹² Tuorla Observatory, Dept. of Physics and Astronomy, Univ. of Turku, Finland
¹³ Department of Physics and Astronomy, Ohio Univ., OH, USA
¹⁴ INAF, Osservatorio Astrofisico di Catania, Italy
¹⁵ Osservatorio Astronomico della Regione Autonoma Valle d’Aosta, Italy
¹⁶ Armenzano Astronomical Observatory, Italy
¹⁷ Inst. de Ciències de l’Espai (CSIC-IEEC), Spain
¹⁸ Agrupació Astronòmica de Sabadell, Spain
¹⁹ Graduate Institute of Astronomy, National Central University, Taiwan
²⁰ Observatori El Vendrell, Spain
²¹ INAF, Osservatorio Astronomico di Roma, Italy
²² INAF, Osservatorio Astronomico di Collurania Teramo, Italy
²³ Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, USA
²⁴ Aalto University Metsähovi Radio Observatory, Finland
²⁵ Department of Physics, Purdue University, USA
²⁶ Lulin Observatory, National Central University, Taiwan
²⁷ School of Cosmic Physics, Dublin Institute For Advanced Studies, Ireland
²⁸ Circolo Astrofili Talmassons, Italy
²⁹ Finnish Centre for Astronomy with ESO (FINCA), University of Turku, Finland
³⁰ Dept. of Phys. Univ. of Colorado Denver, CO, USA

Received: 10 June 2010
Accepted: 2 September 2010

Abstract

Aims. In a previous study we suggested that the broad-band emission and variability properties of BL Lacertae can be accounted for by a double synchrotron emission component with relatedinverse-Compton emission from the jet, plus thermal radiation from the accretion disc. Here we investigate the matter with further data extending over a wider energy range.

Methods. The GLAST-AGILE Support Program (GASP) of the whole earth blazar telescope (WEBT) monitored BL Lacertae in 2008–2009 at radio, near-IR, and optical frequencies to follow its flux behaviour. During this period, high-energy observations were performed by XMM-Newton, Swift, and Fermi. We analyse these data with particular attention to the calibration of Swift UV data, and apply a helical jet model to interpret the source broad-band variability.

Results. The GASP-WEBT observations show an optical flare in 2008 February-March, and oscillations of several tenths of mag on a few-day time scale afterwards. The radio flux is only mildly variable. The UV data from both XMM-Newton and Swift seem to confirm a UV excess that is likely caused by thermal emission from the accretion disc. The X-ray data from XMM-Newton indicate a strongly concave spectrum, as well as moderate (~4–7%) flux variability on an hour time scale. The Swift X-ray data reveal fast (interday) flux changes, not correlated with those observed at lower energies. We compare the spectral energy distribution (SED) corresponding to the 2008 low-brightness state, which was characterised by a synchrotron dominance, to the 1997 outburst state, where the inverse-Compton emission was prevailing. A fit with an inhomogeneous helical jet model suggests that two synchrotron components are at work with their self inverse-Compton emission. Most likely, they represent the radiation from two distinct emitting regions in the jet. We show that the difference between the source SEDs in 2008 and 1997 can be explained in terms of pure geometrical variations. The outburst state occurred when the jet-emitting regions were better aligned with the line of sight, producing an increase of the Doppler beaming factor.

Conclusions. Our analysis demonstrates that the jet geometry can play an extremely important role in the BL Lacertae flux and spectral variability. Indeed, the emitting jet is probably a bent and dynamic structure, and hence changes in the emitting regions viewing angles are likely to happen, with strong consequences on the source multiwavelength behaviour.