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Paper: Multifrequency Observations of BL Lacertae: The Quest for Accretion Disc Signatures
Volume: 427, Accretion and Ejection in AGN: a Global View
Page: 209
Authors: Raiteri, C. M.; Villata, M.; Capetti, A.; WEBT Collaboration
Abstract: BL Lacertae is the prototype of a class of active galactic nuclei whose continuum emission is dominated by non-thermal radiation from the jet. However, its optical spectrum has occasionally shown a broad and strong Hα emission line, suggesting that the broad line region is photoionized by the accretion disc radiation. The Whole Earth Blazar Telescope (WEBT) organized a huge multiwavelength campaign in 2007–2008, also involving three pointings by the XMM-Newton satellite, with the main goal to look for signatures of the accretion disc. These observations were complemented by optical spectroscopic monitoring at the 3.6 m Telescopio Nazionale Galileo (TNG). The brightness level was relatively low. However, in the optical band we observed the most noticeable inter-day variability episode ever detected by the WEBT in more than 15 years. The spectral energy distributions (SEDs) built with simultaneous radio-to-X-ray data at the epochs of the XMM-Newton observations present a prominent UV excess as well as a slight soft-X-ray excess. We interpret the UV excess as a signature of the accretion disc. Since in previous observations the X-ray spectrum was found to vary dramatically, we suggest that the X-ray emission is due to a second synchrotron component, in addition to the one responsible for the radio-to-optical emission.
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