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Astron. Astrophys. 329, 399-408 (1998)

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5. Results on radio-loud quasars

The three quasars show a sharp steepening of their synchrotron spectra between rest-frame radio and submillimetre wavelengths. It is important to recall that variability may affect the flux densities of radio-loud quasars obtained at different epochs. In order to estimate the magnitude of the spectral break in the millimetre region, we evaluate the expected synchrotron flux density at [FORMULA] =230 GHz, deriving the spectral index from the available data and extrapolating the 5-GHz radio flux densities to 1.3 mm. If two flux densities are available at the same frequency, we adopt the average value. For PKS 1251-407, we use only the quasi-simultaneous 1.4-15-GHz data of Shaver et al. (1996). For MRC 1043-291, we find that [FORMULA], and that the expected flux density at 230 GHz is [FORMULA] mJy. For PKS 1251-407 and PKS 1354-107, we find [FORMULA], [FORMULA], with [FORMULA] mJy and [FORMULA] mJy, respectively. If we compare the expected flux densities with the observed upper limits, we find that [FORMULA], i.e. the observed flux density can be more than an order of magnitude lower than expected. We cannot know what fraction of this cut-off may be due to variability; however, the typical maximum amplitude of the variability of flat-spectrum radio sources at [FORMULA] 1 mm-11 cm, over timescales of months-years, is 40-60% (Jones et al. 1981 and references therein). It therefore seems unlikely that variability alone can explain the sharp cut-offs observed in the three quasars in our sample.

Moreover, high-frequency spectral turnovers have been often observed in radio-loud quasars at lower redshifts (Chini et al. 1989; Antonucci, Barvainis & Alloin 1990; Klein et al. 1996). In particular, the spectral break occurs in radio-loud quasars with either flat or steep radio spectra, whereas in BL Lac objects the submillimetre flux densities lie on an extrapolation of the radio spectrum (Knapp & Patten 1991). Our observations show a turnover at log [FORMULA], but the lack of further spectral information does not allow us to infer whether the steepening of the synchrotron-sub-mm spectra is related to thermal emission from dust peaking at the higher frequencies. It is important to recall that the question of dust in high-redshift, radio-loud quasars is very relevant to unification models (see Baker & Hunstead 1995 for a discussion of dust in radio-loud quasars) and to test the hypothesis that many quasars may be missed in optical surveys (see Webster et al. 1995). Future observations with ISO and SCUBA will extend the SED coverage to higher frequencies for a large number of objects.

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© European Southern Observatory (ESO) 1998

Online publication: December 8, 1997