3. Comments on individual sources
Our VLBI image shows structure extending up to about 10 mas from the core to the SE direction (Fig. 1a). The position angle of the beam is not well suited to resolve the fine details of this jet-like extension. The source is unresolved with the VLA D-array in our experiment with resolution.
The source has a flat radio spectrum between 1.4 and 4.85 GHz (, White & Becker 1992). Our VLBI image shows a dominant central component and a prominent secondary component separated by 3.8 mas from the core in the NE direction (Fig. 1b).
The source is reported to be unresolved at 5 GHz with the VLA B-array ( resolution), no extended emission has been found within about from the core (Lawrence et al. 1986). The spectral index of the source is (White & Becker 1992). On the VLBI scale, it has two bright components near the core that perhaps delineate a slightly curved jet extending up to 15 mas (Fig. 1d). Our VLA D-array map shows two faint components about from the core to the SE and NW which resembles a classical double lobe structure (Fig. 2a). However, it is not clear from our VLA image whether these sources are physically related to 0830+101 or they are chance coincidences. The latter seems to be unlikely, but could not be ruled out based on our data.
Spectral indices of and are given by White & Becker (1992). If the flux density of the source did not change between the epochs of measurements this indicates that the source may be a Gigahertz Peaked Spectrum (GPS) quasar. This object has been identified as a ROSAT X-ray source (RXJ0909.2+0354). Its flux in the 0.1-2.4 keV range is erg cm-2 s-1 (Brinkmann et al. 1995). The source is unresolved with the VLA D-array at 5 GHz. On VLBI scales, the core of 0906+041 is resolved with an extension to the NE (Fig. 1e). A secondary compact component is separated by about 10 mas from the core.
This source is identified as a quasar by Beaver et al. (1976) and has a very steep optical continuum more typical of BL Lac objects (Baldwin et al. 1976). The radio continuum peaks near 1 GHz ( and , White & Becker 1992). Neff & Hutchings (1990) found radio emission with the VLA at 1.4 GHz on both sides of the radio core extending to and from the centre. The source was studied in high energy bands, however, only upper limits are available for X-ray and -ray luminosities (Zamorani et al. 1981; Fichtel et al. 1994). The source is resolved by our observations and shows an extension of about 5 mas to the East (Fig. 1f). It is unresolved with the VLA in our experiment.
This flat spectrum source (, White & Becker 1992) is a candidate IERS radio reference frame object and serves as a link to the HIPPARCOS stellar reference frame (Ma et al. 1997). It is being monitored by geodetic VLBI networks at 2.3 and 8.4 GHz. In our 5 GHz imaging experiment the source appears to be resolved and shows a double structure elongated in the S-SW direction with the angular separation of 3.65 mas (Fig. 1g). The component position angle and separation are in very good agreement with a recent 8.4 GHz global VLBI image by Bouchy et al. (1998). Due to the lower resolution of our image we can not decide whether their component "c" is present between the two dominant components seen in our image.
The radio spectral indices of the quasar 1428+423 - also known as GB1428+4217 (Fabian et al. 1997; Hook & McMahon 1998) and B3 1428+422 (Véron-Cetty & Véron 1998) - are and (White & Becker 1992) which are typical for GPS sources. It is the third highest redshift quasar known to date (Hook & McMahon 1998, ) and the most distant known radio loud quasar. The quasar was detected in X-rays with the ROSAT High Resolution Imager in the (observed) 0.1-2.4 keV band (Fabian et al. 1997) and with various ASCA detectors in the (observed) band of 0.5-10 keV (Fabian et al. 1998). Both observations are in agreement and indicate that the SED of this source is strongly dominated by X- and -ray emission. The X-ray spectrum is remarkably flat. The quasar might be the most luminous steady source in the Universe, with an apparent luminosity in excess of erg s-1. The extreme X-ray luminosity of the quasar 1428+423 suggests that the emission is highly beamed toward us (Fabian et al. 1997, 1998).
Our VLBI image (Fig. 1h) is in qualitative agreement with the relativistic beaming model of the source. The quasar appears to be almost unresolved with the VLA at 5 GHz (Laurent-Muehleisen et al. 1997) as well as by our VLBI observations up to 170 M, which corresponds to an angular resolution of 2.01.4 mas. The other two quasars imaged with VLBI also appear unresolved (1251-407 and 1508+572, Frey et al. 1997), which suggests that the high z quasars may be systematically more compact then their less distant counterparts. Alternatively, as suggested by Fabian et al. (1997), the highly beamed emission might be responsible for a selection effect resulting in detection of an otherwise weaker population of extremely high redshift quasars.
This source was detected as a mJy source at 240 GHz by McMahon et al. (1994) corresponding to . The spectral index between 1.4 and 4.85 GHz is (White & Becker 1992). The source is unresolved with the VLA B-array ( resolution, Lawrence et al. 1986).
This quasar is an IERS Celestial Reference Frame candidate source (Ma et al. 1997). It was also detected with the Parkes-Tidbinbilla interferometer at 2.3 GHz (Duncan et al. 1993) and found to be compact with a total flux density of 376 mJy. Our VLBI observations show that the source is resolved but featureless (Fig. 1j). There is no extended feature found down to 0.5% of the peak brightness.
© European Southern Observatory (ESO) 1999
Online publication: March 10, 1999