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Astron. Astrophys. 348, 63-70 (1999)

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1. Introduction

Blazars received an unexpected boost of interest since the discovery, driven by EGRET (and confirmed by OSSE and COMPTEL) that they emit the bulk of their radiative power at [FORMULA]-ray energies. We are now finding spectral differences among blazars which have already been detected by EGRET. BL Lac objects seem to be characterized by a [FORMULA]-ray luminosity, [FORMULA], which is comparable to the luminosity emitted in the rest of the spectrum, while [FORMULA] tends to dominate in blazars with visible broad emission lines. At the same time, the [FORMULA]-ray dominance correlates with the overall spectral energy distribution (SED) (Comastri et al. 1997, Fossati et al. 1998). All blazar SED are in fact characterized by two broad emission peaks, believed to be produced by the synchrotron and the inverse Compton processes. The location of these peaks (their frequency and relative flux) is a strong diagnostic tool to discriminate among theoretical models and to find the intrinsic physical parameters of the emitting region (see e.g. Ghisellini et al. 1996; Sikora et al. 1997; Dermer et al. 1997). Examining the SED of [FORMULA]ray detected blazars, one finds that both peaks are located at lower frequencies in more powerful, and more [FORMULA]-ray dominated, sources (Fossati et al. 1998; Ghisellini et al. 1998).

PKS 0528+134, a very powerful blazar, is no exception: its SED shows that the high energy peak is located in the 10-100 MeV range, and the low energy peak between the far IR and the optical bands. For this source, the X-ray band falls near the minimum between the two peaks. Previous X-ray observations by the ASCA satellite showed a flat ([FORMULA], with [FORMULA]) spectrum, possibly connecting with the flux in the MeV-GeV band (Sambruna et al. 1997). The uncertainty comes from the paucity of simultaneous observations in both bands, and from the relatively narrow energy windows of the used X-ray detectors. Indeed, if one could join the X-ray and the [FORMULA]-ray emission with a smooth curve and, more importantly, simultaneous variability were detected, then it would be possible to confidently argue that the emission in both bands is produced by the same mechanism and in the same region.

Furthermore, the fact that the X-ray emission of this source is in a minimum of the SED is of particular interest. In fact, the possible reprocessing of high energy photons, via the creation of electron-positron pairs, leads inevitably to the production of radiation, mainly in the X-ray band. It is therefore crucial to determine how deep this minimum is: a deep minimum would be a signature that this process does not occur (Ghisellini & Madau 1996).

For these reasons we have undertaken a program to observe PKS 0528+134 with the X-ray broad band Beppo SAX satellite simultaneously with CGRO (in particular with the EGRET instrument) and other, ground-based, telescopes. In this paper we mainly present the Beppo SAX observations, compare them with previous X-ray observations, and construct a simultaneous SED with the optical and EGRET data, briefly discussing our findings.

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© European Southern Observatory (ESO) 1999

Online publication: July 16, 1999