The core-dominated highly polarized blazar 0420-014 (z=0.915) is known to be active and variable at all wavelengths. It is a ROSAT source (Brinkmann et al. 1994), and has been identified with an EGRET -ray source (Fichtel et al. 1994). At radio frequencies, the morphology of the source shows a typical core-jet structure from milliarcsecond (Krichbaum et al. 1994; Wagner et al. 1995; Shen et al. 1997) to arcsecond scale (Antonucci & Ulvestad 1985). Simultaneous optical and -ray flares in 1992 were found (Wagner et al. 1995). There is an outburst at millimeter wavelengths in 1992 (Tornikoski et al. 1996).
Our 5 GHz image of 0420-014 (Fig. 1) shows a bent morphology from
position angle (PA) to . The jet
structure is in agreement with
previous results (Krichbaum et al. 1994; Wagner et al. 1995). The data can
be fitted with three components. The model fitting parameters
we obtained are listed in
Table 3. Comparison of our results with those relative to epoch 1992
(Wagner et al. 1996) shows that
components 2 and 3 moved away from the bright component with an
angular velocity of
0.06 0.02 and mas yr-1 respectively, corresponding to apparent
transversal speeds 0.6 h-1 and .
Superluminal motion in the jet component 2 was reported, with a
speed = 3.9 h-1, at 8.4 GHz between epochs 1990 and 1992
(Wagner et al. 1995) and at 5 GHz during the period 1986.50 to 1992.88
(Shen et al. 1997).
|Figure 1: Image peak: 1.47 Jy/beam. Contours: 1.5 mJy/beam (-3, 3, 6, 12, 24, 48, 96, 192, 384, 768). Beam FWHM: 1.11 0.722 (mas) at , rms = 1.5 mJy/beam|
|Figure 2: Image peak: 3.81 Jy/beam. Contours: 6 mJy/beam (-3, 3, 6, 12, 24, 48, 96, 192, 384). Beam FWHM: 1.38 0.719 (mas) at , rms = 6 mJy/beam|
|Figure 3: Image peak: 2.8 Jy/beam. Contours: 6 mJy/beam (-3, 3, 6, 12, 24, 48, 96, 192, 384). Beam FWHM: 1.11 0.765 (mas) at , rms = 6 mJy/beam|
|Figure 4: Image peak: 1.75 Jy/beam. Contours: 2 mJy/beam (-3, 3, 6, 12, 24, 48, 96, 192, 384, 768). Beam FWHM: 1.17 0.723 (mas) at , rms = 2 mJy/beam|
DA193 (z=2.365) is one of the most compact radio sources known to date. It is identified with a 18th magnitude quasar. The source has been observed with VLBI at several frequencies. At 2.3 GHz, DA193 was barely resolved with a slight extension in PA = (Charlot 1990). At 5 GHz, it showed a core-jet structure consisting of two components with a separation of 0.37 mas at PA = at epoch 1981.61 (Spangler et al. 1983). At 8.4 GHz, the VLBI image showed an extension at PA = with a separation of 0.52 mas at epoch 1985.37 (Charlot 1990), and at PA = with a separation of 0.7 mas at epoch 1994.52 (Fey et al. 1996). At 10.7 GHz, the source showed a core halo structure with the major axis of the halo at PA = (Schilizzi et al. 1981).
DA193 was also observed at 5 GHz using a complete track to test VLBA for high dynamic range image. The source is resolved at a resolution of 1.8 mas 1.5 mas, and can be fitted by a two elliptical Gaussian component model (Briggs et al. 1994).
Our 5 GHz image of the source is shown in Fig. 2. It can be fitted by two components: a compact strong core component of 4.14 Jy and a jet component of 1.24 Jy with a separation of 0.75 mas in PA = . Charlot (1990) estimated a proper motion of 0.04 mas yr-1 from the epoch 1981 at 5 GHz to epoch 1985 at 8.4 GHz, which corresponds to an apparent transversal speed = 1.7 h-1.
Our model fitting of the source shows a separation of 0.75 mas between the two components (Table 3). By comparing the result with that of Spangler et al. (1983) at the same frequency, an average apparent angular velocity of about 0.03 0.01 mas yr-1 can be estimated in the jet between epoch 1981.61 and epoch 1995.83, which is same as reported by Bajkova et al. (1996). The proper motion corresponds to a speed of 1.3 0.4 h-1 c. No obvious proper motion has been detected at 8.4 GHz by VLBA between July of 1994 and October of 1995 (Fey et al. 1996, 1997), because the time scale is too short to detect it.
1334-127 (z=0.539) is a highly polarized quasar (Impey & Tapia 1990). Its VLA image at 1.4 GHz shows a curved jet extending 6.5 arcseconds to the east of the source (Perley 1982). The source was detected by ROSAT, with a flux density of 0.63 Jy at 1.3 KeV (Brinkmann et al. 1994).
VLBI observation carried out at 5 GHz in 1986.9 (Wehrle et al. 1992) indicated that the source was barely resolved. A jet component was located at 1.7 mas from the core in PA = and by comparison with the 1992.88 data, a proper motion of 0.28 mas yr-1 was estimated, which corresponds to a = 4.5 h-1 (Shen et al. 1997). VLBA images show a core-jet structure in PA at 2.3 GHz, PA at 8.4 GHz (Fey et al. 1996), and PA at 15 GHz (Kellermann et al. 1998).
Our image is shown in Fig. 3. The source shows a jet structure extending to south-east (PA ). The parameters of the model fitting are listed in Table 3. The comparison of our map with the published data at similar resolution indicate that the position angle of the jet component is quite different at different epochs. The reason for such difference is unclear and needs further investigation. Assuming that the jet component at different epochs is the same one, we derived that it moves with an apparent velocity of by comparing the results of two epochs (1992.88 and 1995.83) of observation at same frequency.
2345-167 (z=0.576) is an optically violent variable and a highly polarized blazar. X-ray emission was detected by ROSAT (Brinkmann et al. 1994). It has a complex radio spectrum with a peak around 5 GHz, and it is a low frequency variabile source (McAdam and White 1983). VLA observations show a jet of 4.0 arcseconds in PA = .
Our image of 2345-167 shows a clear core-jet structure (Fig. 4).
The jet component is located 3.2 mas from the core in PA = .Comparing our result with that of epoch 1992.88 (Shen et al. 1997),
a proper motion of 0.08 0.03 mas yr-1 can be estimated,
to a . A proper motion of 0.26 mas yr-1,
corresponding to an apparent velocity of about 5 h-1 c, was reported by
Shen et al. (1997). Our result suggests that the jet decelerates as the
component moves outwards, as observed in other sources.
The pc scale structure is almost orthogonal to the arcsecond scale
structure, as seen in some radio loud quasars such as 3C 345. A flux density
of 0.65 Jy is missing from the map of 2345-167. This may indicate the source
has extended emission.
In this sub-section we estimate the physical parameters of the four superluminal sources and report them in Table 4, which contain the following information: in Col. 1 we give the source name; Col. 2 reports the X-ray flux density at 1.3 keV (Brinkmann et al. 1994). No X-ray emission has been detected for DA193; Col. 3 gives the FWHM angular size of the VLBI core, computed as , where and are respectively the major and minor axis of the VLBI core, as given in Table 3. The equipartition Doppler factor , the inverse Compton Doppler factor ,the Lorentz factor and the viewing angle between the jet axis and the line of sight are computed and listed in Cols. 4, 5, 6 and 7 respectively.
Equipartition Doppler factor
The Doppler factor of the outflow from the compact radio cores can be estimated assuming that the particles and magnetic field are in equipartition (Readhead 1994; Guijosa & Daly 1996),
where ) is given by Scott & Readhead (1977), and
(assumed to be -0.75) is the spectral index of the radio emission.
In particular, F(-0.75)=3.4. z is the redshift,
is the observed peak
in Jy, at the observed frequency in GHz.
The angular diameter (in mas) of the core component is
greater than the
observed angular diameter. Marscher (1987)
suggested a correction for this by using .Column 4 lists the equipartition Doppler factors (Eq. (1)) computes
assuming h=1, , and ,where is the frequency of our observations and is
the flux density we derived.
Inverse Compton Doppler factors
For comparison we also estimated the Doppler factor with the self-Compton emission in a uniform spherical model (Marscher 1987) by comparing the predicted flux with the observed Self-Compton flux (Ghisellini et al. 1993),
where is the observed X-ray density (in Jy) at frequency
is the frequency at which the radio peak occurs (in GHz), and
is the synchrotron high-frequency cutoff, which we assumed to be
105 GHz. Our choice is justified by the fact that
our objects are blazars. Furthermore,
; , assumed to be -0.75,
is the spectral index of the radio emission and
=1.8 .From the use of the extrapolated value of the flux density
(Marscher 1987) we can derive that the flux is about a factor of 2
larger than the observed peak flux density .Column 5 in Table 4 lists the ,computed with Eq. (2). The of 0420-014 and 1334-127
is less than its . This can be explained
assuming that should be considered
a lower limit, since part of the X-ray emission
may come from different mechanisms rather than Inverse Compton.
Lorentz factor and viewing angle
In the relativistic beaming model is related to the true velocity , and the angle to the line of sight (e.g., Pearson & Zensus 1987),
and the Doppler factor can be written as
where . On the basis of Eqs. (3) and (4), the Lorentz factor and the angle to the line of sight can be calculated with known and . Columns 6 and 7 of Table 4 list the Lorentz factors and viewing angles which were estimated by assuming h=1 and .
We can see that these four sources have properties typical of superluminal objects, i.e. high Doppler boosting and small viewing angle between the jet axis and the line of sight.
1504-166 (z=0.876) is classified as a highly polarized quasar (Impey & Tapia 1990), and is known to be a low frequency variable source (Padrielli et al. 1986). It was unresolved at 1.4 GHz by VLA (Perley 1982).
Our VLBI image shows a slightly curved core-jet structure to
the south-east (Fig. 5). Our model fitting consists of two components,
and the parameters are listed in Table 3. The jet position angle is
consistent with that derived at 1.7 GHz by Romney et al. (1984).
It seems that the jet component is bending clockwise, and closer to
the the direction of the jet component at 1.7 GHz (Padrielli et al. 1986).
The 5 GHz VLBI image obtained by Shen et al. (1997) at epoch 1992.83
consists of three components, i.e. a compact core with a flux density of
1.3 Jy and a diameter of 0.5 mas (component 1), a 0.6 Jy jet
at 1.12 mas from core in PA = (component 2) and a 0.3 Jy
component at 0.8 mas from core in PA = (component 3).
Assuming the jet component 2 in our image is the same as the second component
at epoch 1992 (Shen et al. 1997), we can estimate a proper motion of
0.08 mas yr-1, which corresponds to h-1.
Therefore we suggest that 1504-166 is a superluminal candidate.
|Figure 5: Image peak: 1.13 Jy/beam. Contours: 2 mJy/beam (-3, 3, 6, 12, 24, 48, 96, 192, 384). Beam FWHM: 1.16 0.733 (mas) at , rms = 2 mJy/beam|
|Figure 6: Image peak: 0.95 Jy/beam. Contours: 2 mJy/beam (-3, 3, 6, 12, 24, 48, 96, 192, 384). Beam FWHM: 1.12 0.76(mas) at , rms = 2 mJy/beam|
2243-123 (z=0.630) is a highly polarized quasar. Emission at X-ray and -ray energies was detected (Maisack et al. 1994, Fichtel et al. 1994). The VLA image shows an unresolved core and an extended component at 4 from the core in PA = (Morganti et al. 1993; Browne et al. 1986; Perley 1982).
Our new image (Fig. 6) shows a core-jet structure, with a jet component located at 1.07 mas to the south of the core. A VLBI image of the source at 5 GHz at epoch 1993 shows a compact central core elongated in the north-south direction (Shen et al. 1998). The total flux density monitoring at millimeter wavelength obtained by single dish observations, shows a peak at 1992.90 (Tornikoski et al. 1996). If we assume that the jet component was ejected as a consequence of that outburst, the detected proper motion of the jet would be 0.40 mas yr-1, which corresponds to = 4.2 h-1. So, 2243-123 may be considered as a new superluminal source candidate.
The source 1532+016 is identified with a 19.8 magnitude quasar (McEwan et al. 1975) with emission lines, and is located at a redshift z=1.435. X-ray and infrared emission of the source were detected (Impey 1988). VLA observations show that the source has a secondary component at 1.4 from the core in PA = (Perley 1982). A correlated flux density of 0.67 Jy at 8.4 GHz with VLBI resolution for this source was reported by Morabito et al. 1986.
We obtained the first VLBI image of 1532+016 (Fig. 7), which shows a
structure extended in PA = . The data can be fitted by two
components. The model parameters are listed in Table 3. We point out that the
milliarcsecond scale structure is orthogonal to the arcsecond scale
structure, as often seen in radio loud quasars.
2320-035 is a 19th magnitude quasar at z=1.411
VLA observations show that the source is unresolved on the arcsecond scale.
In Fig. 8 we present the first epoch 5 GHz VLBI image of the source.
The image shows a core-jet structure oriented to south-west. The data
can be fitted by three components (see Table 3).
|Figure 7: Image peak: 0.51 Jy/beam. Contours: 1 mJy/beam (-3, 3, 6, 12, 24, 48, 96, 192, 384). Beam FWHM: 1.14 0.79 (mas) at , rms = 1 mJy/beam|
|Figure 8: Image peak: 0.49 Jy/beam. Contours: 2.5 mJy/beam (-3, 3, 6, 12, 24, 48, 96, 192). Beam FWHM: 1.02 0.85 (mas) at , rms = 2.5 mJy/beam|
|Figure 9: Image peak: 0.69 Jy/beam. Contours: 1.5 mJy/beam (-3, 3, 6, 12, 24, 48, 96, 192, 384). Beam FWHM: 1.18 0.713 (mas) at , rms = 1.5 mJy/beam|
|Figure 10: Image peak: 0.91 Jy/beam. Contours: 1 mJy/beam (-3, 3, 6, 12, 24, 48, 96, 192, 384, 768). Beam FWHM: 1.26 0.72 (mas) at , rms = 1 mJy/beam|
|Figure 11: Image peak: 2.95 Jy/beam. Contours: 4 mJy/beam (-3, 3, 6, 12, 24, 48, 96, 192, 384). Beam FWHM: 1.13 0.73 (mas) at , rms = 4 mJy/beam|
The source 2351-154 (z=2.675) is associated with a 17th magnitude quasar (Hewitt & Burbridge 1993). Its X-ray emission was detected by Brinkmann et al. (1994). The VLA image of the source at 1.6 GHz shows a deformed core, elongated to northeast (Neff & Hutchings 1990), which is interpreted as a jet seen nearly end on (Punsly 1995).
Our VLBI image of the source (Fig. 9) shows a compact component. The structure can be fitted with one Gaussian component (Table 3). The flux density of the component is 0.73 Jy which is only 75% of the total flux density. This suggests that extended emission may exist.
1034-293 is a radio-selected blazar with redshift z=0.312.
High optical polarization and X-ray emission
were detected (Wills et al. 1992; Maraschi et al. 1995).
Our image of the source shows an extended emission in PA =
(Fig. 10). The data can be fitted with two components with 1.63 mas
separation. Because of the poor (u, v) coverage and the lack of
short baselines of our VLBI array, the extended structure was not so
clearly imaged as in the VLBA images (Fey et al. 1996), where the source
shows a core-jet morphology both at 2.3 and 8.5 GHz in PA = .The flux density at 5 GHz of the core decreased from
1.5 Jy (Shen et al. 1998) in 1992 to about 1 Jy in 1995.
NRAO530 (z=0.902) is a well known optically violent variable blazar, identified with a -ray source (Fichtel et al. 1994). It has been monitored at low frequency to look for variability (Ghosh et al. 1994; Bondi et al. 1994). Weak polarization was detected both at optical and radio bands. VLA observation at 1.4 GHz showed an unresolved core and a second unresolved, possibly unrelated, component at about 11 in PA = (Perley 1982).
Our image of 1730-130 shows a core-jet structure (Fig. 11). The jet is oriented in PA = . The data can be fitted with two components separated by 2.0 mas, i.e. a strong component, possibly the core of the radio emission, with flux density S = 3.4 Jy and a jet with S = 0.5 Jy. The jet orientation is roughly in agreement with the 1.7 GHz image, which shows the structure extending out to 26 mas in PA = (Romney et al. 1984), but it is obviously different from the 5 GHz VLBI image by Shen et al. (1997), where the position angle of the jet changes from PA = at 1.3 mas to PA = at 4.0 mas. The complex structural variations are difficult to be explained in terms of angular expansion. The flux density of the strongest component increased from 2.34 Jy in 1992 (Shen et al. 1997) to 3.4 Jy in 1995. This may indicate an outburst of the birth of a new jet component. More observations of the source with better (u, v) coverage are necessary to obtain the proper motion in the jet component and to study the relation of the emission mechanisms between radio and high energy radiation.
Four of the remaining ten sources in our survey, i.e. 0256+075, 0338-214, 2131-021, and 2325-150, were detected on most baselines with strong fringes, while the other six, 0008-264, 0108-079, 0528-250, 0922+005, 1012+232, and 1430-178, were barely detected on some baselines, with poor fringes. The average (over baselines) correlated flux densities of the sources are presented in Table 5. Column 3 lists the spacing of detected baselines.
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