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

Abstract

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.

AGILE detection of intenseγ-ray activity from the blazar PKS 0537–441 in October 2008

“…Using this instrument, variability up to a factor 3 was found with a time scale of several years (Pian et al 2002). More than ten years after the EGRET era this blazar appears also in the First AGILE-GRID Catalog of High Confidence Gamma-Ray Sources (Pittori et al 2009) and in the First Catalog of Active Galactic Nuclei detected by the Fermi-Large Area Telescope (LAT) (Abdo et al 2010), confirming its γ-ray activity over timescales of at least two decades.…”

“…Subsequently it was observed in several different states (Treves et al 1993;Hartman et al 1999;Pian et al 2002) with an integrated flux above 100 MeV between (16.5±4.5) and (91.1±14.6)×10 −8 photons cm −2 s −1 . The average flux over all the EGRET observations was (25.3 ± 3.1) × 10 −8 photons cm −2 s −1 .…”

“…The average flux over all the EGRET observations was (25.3 ± 3.1) × 10 −8 photons cm −2 s −1 . However, the peak of the γ-ray emission was observed by EGRET in January 1995 at (200±50)×10 −8 photons cm −2 s −1 (Pian et al 2002), considering a temporal binning of 2 days. During its first eleven months of observations (2008 August -2009 July) Fermi-LAT observed an average flux of F E>100 MeV = (37.77 ± 1.06) × 10 −8 photons cm −2 s −1 (Abdo et al 2010).…”

“…Instead, a prominent emission feature at 5304 Å, likely associated to the Mg II λ2798 line, was observed in the optical spectrum of PKS 0537−441, which indicates a redshift of 0.894 (Peterson et al 1976;Stickel et al 1993). Furthermore strong, broad emission lines of Lyα and C IV were observed by Hubble Space Telescope and, on the basis of their measured central wavelengths, Pian et al (2002) derived z = 0.896 ± 0.001. The detection of these broad emission lines suggests a mistaken classification of this object as BL Lac.…”

“…The source is strongly variable at all wavelengths. The overall spectrum in different intensity states has been discussed in detail by Pian et al (2002). Recently, this source was the target of simultaneous long-term optical and near-infrared monitoring with the Rapid Eye Mounting (REM) telescope and UV and X-ray monitoring with the Swift satellite between 2004 December and 2005 November (Dolcini et al 2005;Pian et al 2007).…”