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

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6. The SED

In Fig. 6 we show the overall spectral energy distributions of PKS 0528+134 at different epochs, as indicated by the labels. Data have been dereddened assuming [FORMULA] cm-2, corresponding to [FORMULA]. We have reported the minimum and maximum values of the radio and optical data obtained during our campaign.

[FIGURE] Fig. 6. Overall spectral energy distribution of PKS 0528+134. Simultaneous observations are indicated by the different marks, as labeled. Other non-simultaneous data are taken from the literature (see text). The solid line is a fit to the 1997 spectrum made with a homogeneous, one-zone model, corresponding to a source with a size [FORMULA] cm, a magnetic field [FORMULA] Gauss, in which beaming corresponds to a Doppler factor [FORMULA]. Relativistic electrons are injected continuously throughout the source, at a rate corresponding to an intrinsic luminosity [FORMULA] erg s-1, with a power law spectrum [FORMULA] between [FORMULA] and [FORMULA]. In this case the EC component dominates the entire high energy band. The fit to the 1995 spectrum is obtained adopting the same R, [FORMULA] and B, but with a larger injected power of [FORMULA] erg s-1 and with a flatter injected electron spectrum ([FORMULA]) between [FORMULA] and [FORMULA]. In this case both the SSC and the EC components contribute to the low energy X-ray spectrum, while only the EC component contributes above a few tens of keV. Note that the shown PDS points lie above the extrapolation from the spectrum in the LECS and MECS band. Given the possibility of contaminating sources (see text), we do not consider these points when applying the model.

The not simultaneous data shown in Fig. 6 have been taken from Wall & Peacock 1985; Bloom et al. 1994; Edelson 1987; Reuter et al. 1997; Rieke et al. 1982; Condon et al. 1977; Collmar et al. 1997; McNaron-Brown et al., 1995 and Mukherjee et al. 1996. We also show some simultaneous sets of data: the 1991 Comptel and EGRET spectrum (Collmar et al. 1997); the 1994 and 1995 X-ray (ASCA) and EGRET data (we also have an optical point during the 1994 campaign; data for these two campaigns are presented in Sambruna et al. 1997 and references therein); our 1997 IR, optical, X-ray data together with the EGRET point from Mukherjee et al. 1997b and the radio data from the GBI archive (see Fig. 1).

As all other [FORMULA]-ray bright blazars, also the SED of PKS 0528+134 is characterized by two peaks, one between the far IR and optical spectral bands, and the other at MeV energies. As discussed in Sambruna et al. (1997), the dereddened (using [FORMULA]) optical spectrum is inverted ([FORMULA]), and thus indicative of the presence of a `blue bump' component. This is not easy to reconcile with the very large and rapid optical variability. We also note that due to the presence of molecular clouds along the line of sight the gas to dust ratio may be anomalous and the value of [FORMULA] may be larger than the one adopted above. Further spectral observations in the IR optical range could help clarifying this issue.

As can be seen, the MeV-GeV emission dominates the bolometric output by a large amount, reaching an (isotropic) luminosity in excess of 1049 erg s-1 (with [FORMULA] km s-1 Mpc-1 and [FORMULA]).

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© European Southern Observatory (ESO) 1999

Online publication: July 16, 1999