Broad-band continuum and line emission of the γ-ray blazar PKS 0537–441

¹ Osservatorio Astronomico di Trieste, Via G.B. Tiepolo 11, 34131 Trieste, Italy
² Osservatorio Astronomico di Padova, via dell'Osservatorio 5, 35122 Padova, Italy
³ NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
⁴ Osservatorio Astronomico di Brera, via Brera 28, 20121 Milano, Italy
⁵ Metsahovi Radio Observatory, Metsahovintie 114, 02540 Kylmala, Finland
⁶ Dipartimento di Scienze, University of Insubria, Via Valleggio 11, 22100 Como, Italy
⁷ Department of Physics, Yale University, PO Box 208121, New Haven, CT 06520-8121, USA
⁸ Department of Physics and Astronomy, Barnard College, Columbia University, New York, NY 10027, USA
⁹ European Southern Observatory, 3107 Alonso de Cordova, Santiago, Chile
¹⁰ Eureka Scientific and George Mason University, 4400 University Drive, Fairfax, Virginia, VA 22030, USA

Corresponding author: E. Pian, pian@ts.astro.it

Received: 19 February 2002
Accepted: 10 June 2002

Abstract

PKS 0537–441, a bright γ-ray emitting blazar, was observed at radio, optical, UV and X-ray frequencies during various EGRET pointings, often quasi-simultaneously. In 1995 the object was found in an intense emission state at all wavelengths. BeppoSAX observations made in 1998, non-simultaneously with exposures at other frequencies, allow us to characterize precisely the spectral shape of the high energy blazar component, which we attribute to inverse Compton scattering. The optical-to-γ-ray spectral energy distributions at the different epochs show that the γ-ray luminosity dominates the bolometric output. This, together with the presence of optical and UV line emission, suggests that, besides the synchrotron self-Compton mechanism, the Compton upscattering of photons external to the jet (e.g., in the broad line region) may have a significant role in the production of high energy radiation. The multiwavelength variability can be reproduced by changes of the plasma bulk Lorentz factor. The spectrum secured by IUE in 1995 appears to be partially absorbed shortward of ~1700 Å. However, this signature is not detected in the HST spectrum taken during a lower state of the source. The presence of intervening absorbers is not supported by optical imaging and spectroscopy of the field.