EDP Sciences
Free Access
Issue
A&A
Volume 550, February 2013
Article Number A113
Number of page(s) 12
Section Extragalactic astronomy
DOI https://doi.org/10.1051/0004-6361/201219700
Published online 04 February 2013

© ESO, 2013

1. Introduction

Compact symmetric objects (CSOs) are a subclass of extragalactic radio sources that are characterized by a compact double or triple radio structure with an overall size of less than 1 kpc. The physical nature of the compactness of CSOs is still a question under debate: the youth model (e.g. Phillips et al. 1982; Fanti et al. 1995; Readhead et al. 1996) proposes that CSOs are small because they are in the infant stage of the extragalactic radio source evolution; the frustration model (e.g. van Breugel et al. 1984; O’Dea et al. 1991) attributes the small size of CSOs to extremely strong confinement by the dense external medium. The two models define distinctly different evolutionary fates of CSOs. In the youth model, all young CSOs eventually evolve into large-scale double sources, i.e., Fanaroff-Riley (FR, Fanaroff & Riley 1974) type-II sources, over a few million years; whereas according to the frustration model, CSOs are confined within the host galaxy and experience stagnated growth. The CSO ages provide a critical distinction between the two models. Previous kinematics studies of individual CSOs and subsamples of these sources (e.g. Owsianik & Conway 1998; Owsianik et al. 1998; Taylor et al. 2000; Gugliucci et al. 2005; An et al. 2012) show that the measured hotspot advancing speed values have a large scatter from  ~0.04   c to  ~0.5   c, resulting in young kinematic ages in the range of only 100–2000 yr. However, additional sideways motions of hotspots and disturbed lobes indicate strong interactions between the jet heads and the surrounding interstellar medium (Stanghellini et al. 2009; An et al. 2012). Sideways motion is usually very slow compared to the dominant advancing motion, which requires highly accurate measurements. A detailed investigation of well-selected CSO samples is essential for exploring the complex kinematics of CSOs, and for understanding the physical environment of the host galaxies on 1–500 pc scales.

The kinematic age (τk) of CSOs is traditionally determined by dividing the separation (R) between two terminal hotspots by the separation rate (μ). A high-accuracy measurement of τk requires high-precision position determination for individual observations, and a sufficiently long timespan of the data. For more than two epochs, μ is often determined from a linear regression fit to the changing hotspot separation with time. The statistical uncertainty of the fit is sensitive to the number of available data points and the uniformity of the time sampling. In addition to this, different resolutions of the interferometric images and the intrinsic opacity effect in different levels may introduce systematic errors in the proper motion measurements. An accurate measurement for the separation rate μ requires CSOs with multiple-epoch Very Long Baseline Interferometry (VLBI) imaging data at the same observing frequency over a sufficiently long time span.

In this paper, we carry out a kinematic study of an archetypal CSO, OQ 208, on the basis of archival VLBI data over a time baseline of 13.6 yr. OQ 208 (also known as Mrk 668, B1404+286) is one of the closest CSOs (z = 0.0766: Huchra et al. 1990) and provides a template for the dynamic properties of the most compact (and possibly the youngest) radio sources. The host galaxy of OQ 208 shows typical Seyfert-1 spectra with strong broad Balmer lines (FWHMHα = 6000 km s-1) and also forbidden lines of [Ne III], [O III] and [S II] (Burbidge & Strittmatter 1972; Eracleous & Halpern 1994). In the radio band, a broad 21-cm Hi absorption line was detected in OQ 208, indicating a fast and massive outflow of neutral gas (Morganti et al. 2005). The radio emission is concentrated in a compact central region within  ~10 mas (Stanghellini et al. 1997). Additional evidence for the compactness comes from the convex radio spectrum with a turnover at about 4.9 GHz (Dallacasa et al. 2000), resulting from synchrotron self-absorption or free-free absorption (Fanti 2009). The VLBI images of OQ 208 at 2.3 and 5 GHz reveal a typical CSO-type morphology with two compact (mini-)lobes along the NE–SW direction (Stanghellini et al. 1997; Liu et al. 2000; Wang et al. 2003). At higher frequencies of 8 and 15 GHz, a more detailed structure is revealed: the mini-lobes are resolved into subcomponents, and internal jet knots are detected between the two lobes (Kellermann et al. 1998; Luo et al. 2007) Previous proper motion measurements of OQ 208 (e.g. Liu et al. 2000; Stanghellini et al. 2002; Wang et al. 2003; Luo et al. 2007) were mostly based on 5- and 8-GHz data with lower angular resolutions and are affected by the opacity effect. Moreover, these measurements only cover a time span of a few years, insufficient for tracing the long-term change of the hotspots. In the present study, we only made use of 15-GHz VLBI data to determine the separation velocities of hotspots and the proper motions of the internal jet knots.

The structure of the paper is as follows. Section 2 describes the VLBI data used for the kinematics study. Section 3 presents the results, including radio images, light curves, and proper motion measurements. A conclusion and a summary are given in Sect. 4. Throughout this paper, we assume a flat cosmological model with H0 = 73 km s-1 Mpc-1, ΩM = 0.27, ΩΛ = 0.73. At the redshift of OQ 208, 1 mas angular size corresponds to 1.4 pc projected linear size, and a proper motion of 1 mas yr-1 corresponds to 4.9 c apparent speed.

2. VLBI data

Owing to its compactness and high brightness, OQ 208 is often used as a calibrator in VLBI experiments. Consequently, there are abundant archival VLBI data sets from the past two decades. The 15-GHz VLBA data from the Monitoring of Jets in Active Galaxies with VLBA Experiments (MOJAVE) program1 have the highest resolution and sensitivity and are most appropriate for kinematic studies. The 15-GHz data spread over 28 epochs in the time range from early 1995 to the middle of 2009. We also made use of the VLBI data from the Radio Reference Frame Image Database (RRFID)2, which were obtained with the VLBA together with several geodetic radio telescopes simultaneously at 2.3 and 8.4 GHz. The multiple-frequency data were used for the spectral index analysis of compact components. The RRFID data cover the time range from 1994 to 2008.

The calibration of the archival VLBI data has already been made in the Astronomical Image Processing System (AIPS) following the standard procedure. We performed additionally several iterations of self-calibration in DIFMAP (Shepherd et al. 1994) to eliminate residual antenna-based phase errors. The 15-GHz visibilities were fitted with five Gaussian model components (two in the northwest lobe, another two in the southeast lobe, and one jet knot, see Fig. 1) using the DIFMAP task MODELFIT. Table 1 lists the fitted parameters, including the integrated flux density Si, the relative separation R from the primary hotspot used as the reference, the position angle of the VLBI component, and the deconvolved size θmaj × θmin. The statistical uncertainties of the observed parameters were estimated according to the formulae given by Fomalont (1999), except that we also included an additional 5% as the amplitude calibration error. The details of the error analysis method are given by An et al. (2012).

In addition to VLBI imaging data, the single-dish flux density monitoring data observed with the 26-m paraboloid telescope of the University of Michigan Radio Astronomical Observatory3 (Aller et al. 1985) were included for a supplementary analysis of the variability and the geometry of the radio structure. The total flux density measurements were made at 4.8, 8, and 14.5 GHz from July 1974 to August 2009.

thumbnail Fig. 1

Total intensity image of OQ 208. Panel a) shows the 2.3-GHz VLBI image, displaying a double-component morphology. Panel b) shows the tapered 2.3-GHz image. The extended emission at  ~30 mas west of the primary structure is likely a relic radio emission resulting from the past activity a few thousand years ago. Panels c) and d) show the 8.4 and 15 GHz images. Each mini-lobe is resolved into two hotspots. Between the hotspots NE1 and SW1, a knotty jet is detected.

Open with DEXTER

3. Results

3.1. Radio morphology

At 2.3 GHz, OQ 208 exhibits the double-component morphology typical of the CSOs (Fig. 1a). The total extent of the radio source is about 7 mas (~10 pc). The northeast (NE) component is much brighter than the southwest (SW) one with an intensity ratio of 18.0:1 at 2.3 GHz. Both components show a steep spectral index at high frequencies (defined as S ∝ να) with (SW) and (NE), identifying them as two mini-lobes of the CSO. The asymmetric brightness of two-sided jets/lobes in radio sources is commonly attributed to the Doppler boosting effect that enhances the apparent flux density of the advancing jet/lobe by a factor of δ3 + α, where δ is the Doppler boosting factor and α is the spectral index. However, the kinematic analysis below suggests that the jet in OQ 208 is mildly relativistic and the jets nearly align with the plane of the sky, therefore Doppler boosting cannot account for the brightness difference. Another explanation of the strong asymmetry in the brightness of the two lobes is that the receding SW lobe suffers from more free-free absorption than the advancing NE lobe. Indeed, the flux density ratio is even higher at a lower frequency of 1.66 GHz, SNE/SSW = 60:1 (Kameno et al. 2000), supporting this interpretation. A third possibility is that the inhomogeneous distribution of the external medium surrounding the lobes results in a larger conversion efficiency from jet kinetic energy to radiative energy in the northeast jet (Orienti & Dallacasa 2012).

A 2.3-GHz image with a Gaussian taper with a half-value at 20 Mλ wavelength (Fig. 1b) reveals an extended feature about 30 mas (~40 pc) to the west, which was previously reported by Luo et al. (2007). Extended emission features on kpc to Mpc scales has also been detected in OQ 208 (de Bruyn 1990) and in other CSO galaxies (e.g., 0108+388: Baum et al. 1990; Stanghellini et al. 2005; 0941-080, 1345+125: Stanghellini et al. 2005), which were interpreted as relics remaining from past (>108 yr ago) nuclear activity. Extended components on scales of  <100 pc are rarely seen in CSOs (J1511+0518 is another example; Orienti & Dallacasa 2008) and this puzzling one-sided extended feature at 40 pc distance requires an interval between two intermittent activities shorter than 2 × 103 yr (Orienti & Dallacasa 2008). The non-detection of the northeast fading lobe likely indicates asymmetric properties of the ambient interstellar medium (ISM) on pc scales, leading to more rapid radiative or adiabatic losses and a shorter life of the NE lobe. This, again, is consistent with the fact that the NE advancing lobe is much brighter.

The NE lobe is resolved into two subcomponents (NE1 and NE2) at 8 and 15 GHz. NE1 dominates the flux density of the whole source. The spectral index of NE1, determined from 8 and 15 GHz data at close epochs, is . The brightness temperature is calculated using the equation (1)where Sob is the observed flux density in Jy, νob is the observing frequency in GHz, θmaj and θmin are the major and minor axis of the Gaussian model component in units of mas, and Tb is the derived brightness temperature in source rest frame in units of Kelvin. The average brightness temperature of NE1 is 4.4 × 1010 K. The secondary component NE2 is weaker than NE1 in the range 4.3–16.3. It has a lower brightness temperature of 1.4 × 109 K and a much steeper spectral index of . The high brightness temperature and the relatively flatter spectral index of NE1 identify it as the primary hotspot formed by the reverse shock when the jet head impacts on the wall of the external medium. The continuous emission structure between NE1 and NE2 would suggest a physical connection.

The SW lobe is resolved into two components (SW1 and SW2) at 8 and 15 GHz. The two components have comparable sizes. The integrated flux density of SW1 is slightly higher than SW2 with a flux density ratio R(SW1/SW2) in the range of 1.3−2.4. Their spectral indices are   ±  0.07 (SW1) and 1.82  ±  0.26 (SW2).

The presence of double hotspots in the 10-pc lobe structure of OQ 208 remains a puzzle. Similar morphology is found in the multiple-hotspot appearance of large-scale double sources such as 3C 20 (Laing 1981; Hardcastle & Looney 2001), where the hotspot usually identified as the primary hotspot is extremely compact and bright, and the other is more diffuse and shows various structures. Three models may account for the double hotspots inside the 10-pc nuclear region of OQ208:

  • (1)

    Blocking of the jet head by the external medium. In this scenario, the jet impacts on the wall of the surrounding medium and generates the hotspot there. As the jet head may slide along the wall, the hotspot position also changes accordingly (dentist’s drill model: Scheuer 1982). The primary hotspot is the currently active jet-ISM interaction, while the secondary hotspot represents the past primary hotspot, which is now fading away.

  • (2)

    Jet precession. The present images do not allow us to distinguish whether a single jet beam is precessing (Cox et al. 1991) or if there are intrinsically two beams (Laing 1982) due to the similar compactness and shape of two hotspots and the absence of emission between the hotspots and the nucleus. One possible driving mechanism of jet precession is associated with a binary black hole system where the black hole launching the jet has a relative motion with respect to the surrounding ISM and the multiple hotspots reflect the different impact locations. Taking the brighter SW1 as the primary (current) and SW2 as the secondary (older) impact of the jet beam(s) on the external medium, the older hotspot has stopped receiving direct energy supply from the nucleus, and synchrotron aging will result in a steeper radio spectrum at high frequencies. Indeed, SW2 has a steeper spectral index than SW1. If SW2 is the old hotspot, it should lie behind the primary hotspot SW1, implying that there is a jet bending between SW2 and SW1.

  • (3)

    Redirected flows. Alternatively, the secondary hotspot represents either a re-directed outflow originating from the primary hotspot (Laing 1981) or a deflected jet flow (Lonsdale & Barthel 1986) escaping from a weak point in the cocoon/lobe structure. Different from the model (1), the secondary hotspot in model (3) continues to receive energy supply from the primary hotspot. Therefore, it would last longer, would not show spectral steepening, and be much fainter and less compact than the primary hotspot. In the eastern lobe, NE2 has no direct connection with the nucleus, but shows a collimated bridge that links it with NE1. NE2 is also larger than NE1 and has an amorphous shape. These morphological characters make NE2 more likely to be the result of a re-directed outflow from the primary hotspot NE1 than a directed flow from the nucleus.

The continuous emission between two hotspots NE1 and SW1 at 2.3-GHz (Fig. 1a) is resolved into a knotty jet at 8.4 and 15 GHz (Figs. 1c and 1d). From east to west, these knots are labeled J1, J2, J3, and J4. J1 is the brightest jet knot located at the inlet of the northeast lobe and is detected at 15 and 8.4 GHz at all epochs. It has a 15 GHz to 8.4 GHz spectral index between 0.81 and 1.45, indicative of a typical optically thin jet knot, and a brightness temperature of 3.4 × 109 K. J2 appears in the 8.4-GHz images at epochs from 2002.044 to 2002.945, where the VLBI images have the highest sensitivity and resolution. J2 only appears in the most sensitive 15-GHz image at epochs 1995.958, 1996.378, 2003.318, 2008137, and 2009.558. It has a steep spectrum with and a brightness temperature Tb ≤ 1.9 × 107 K. J3 is closest to the geometric center and only appears in the highest sensitivity 15-GHz images. The brightness temperature of J3 is  ≤ 3.0 × 107 K derived from the 15-GHz data. Lacking the spectral index information, the active nucleus or the innermost jet knot nature of J3 remains uncertain. Regardless of the identification of J3, the non-detection at 8.4 GHz would imply an extremely high absorption opacity toward the nucleus. J4 is only marginally detected at 15 GHz on 2009.558. More sensitive observations are necessary to to reveal the physical nature of these components.

thumbnail Fig. 2

Variability of VLBI components of OQ 208. a) Total flux density variability of OQ 208 observed with the UMRAO 26-meter radio telescope: 4.8 GHz (black), 8 GHz (red), 14.5 GHz (blue). The sum of 15-GHz flux densities of the VLBI components (see Table 1) is represented by green symbols indicating that  ~15% is missed due to lack of short uv-spacings. b) Temporal variations of the flux densities of VLBI components. The flux densities are derived from the model-fitting results listed in Table 1. Two vertical lines mark the peak epochs of NE1 (around 1998.313) and SW1 (around 2003.313). To clearly show the variability behavior of SW1, the flux densities of SW1 have been multiplied by a factor of 10. c) Variation of the 15-GHz flux density of the hotspot NE1 with the component size. The flux density of NE1 experienced a slow rise before epoch 1998.313, followed again by a sharp decline until epoch 2005.473 followed by a slow decline. The component size constantly increases during the whole period. d) Temporal variation of the spectral index of the NE lobe. The spectral indices are calculated from the 15-GHz MOJAVE data and 8-GHz RRFID data at close epochs. A sharp jump of the spectral index occurs around 2006, after which the spectral index steepens significantly.

Open with DEXTER

3.2. Variability of the hotspot NE1

The steep-spectrum lobes dominate the total flux density of most CSOs. Fassnacht & Taylor (2001) monitored a sample of seven CSOs using the VLA at 8.5 GHz and found extremely stable flux densities (rms variability less than 1%) over a period of eight months. Some exceptional CSOs show moderate-level variability on time scales of a few years, for example, OQ 208 (the present paper) and J1324+4048 (An et al. 2012). Unlike with blazars with a jet luminosity amplified by Doppler boosting, the low- and moderate-level variability observed in lobe-dominated CSOs is probably related to variation of the energy supply by the relativistic jet flow and the changes of the energy dissipation process including the adiabatic expansion losses and radiative losses.

The light curves observed with the Michigan 26-m radio telescope (UMRAO) at three frequencies (4.8, 8, and 14.5 GHz) show two distinct flares in the past 25 years, around 1983 and 2000 (Fig. 2a). The accurate peak epoch around 1983 cannot be clearly determined because of the broad gap in the sampling, while the second-peak epoch is not well constrained because the light curves only cover the declining part of the flare. The 5-GHz flux density variation of OQ 208 using the VLA Stanghellini et al. (1997) shows a continuous decrease from 1980 to 1995. In addition, Waltman et al. (1991) observed a decrease of the 8.1-GHz flux density from about 2.7 Jy in mid-1983 to about 2.0 Jy in mid-1989 using the Green Bank Interferometer. Starting from 1989, the flux density has been constant around 1.9 Jy. This evidence consistently suggests that OQ 208 has intrinsic variability on time scales of a few years.

For completeness, the 15-GHz summed flux densities of the VLBI components were added to diagrams covering the time range between July 1995 and August 2009, which confirms the second flare peak between 1998 and 2000. The declining part of the VLBI flux density data is fully consistent with the total flux density data. The comparison of the VLBI and single-dish data also suggests that a significant percentage (>85%) of the total flux density originates from the compact radio structure.

To quantitatively evaluate the variability, we introduced a variability index V to illustrate the relative variability scale (Zhao et al. 2012): (2)where Smax, Smin are the maximum and minimum flux density of an individual flare. A value of V ≈ 0 represents indistinguishable or undetectable variability, and V ≈ 1 indicates an extreme variability The 15-GHz light curves of individual components (Fig. 2b) suggest a high-variability index V = 0.62 ± 0.12 for NE1 during the 1998 flare and a mild V = 0.19 ± 0.04 for SW1 during the 2003 flare. The standard deviation of the measured flux densities with respect to the mean value is adopted as the statistical error. The variability index of NE1 derived from VLBI data is consistent with that measured from the single-dish monitoring data (Fig. 2a), confirming that the total flux density variation is dominated by the brightest hotspot NE1. The other VLBI components do not show significant variability at 15 GHz.

The variability at 8 and 14.5 GHz using the single-dish data (Fig. 2a) indicates a maximum level of (22 ± 4)% and (40 ± 8)%, respectively. The 5-GHz variability scale is lower than at 8 and 14.5 GHz, probably because of the increasing opacity at lower frequencies. The mini-lobes at 2.3 GHz based on the RRFID data did not show any prominent variability. At 1.4 GHz, the total flux density of OQ 208 increased from  ~755 mJy in 1979 to  ~854 mJy in 2001 (de Bruyn, priv. comm.), which is closely connected with the growth of the overall source size.

The complex variability behavior of OQ 208 can be understood separately in the low- and high-frequency regimes:

  • (1)

    Up to 1 GHz, the emission mostly arises from the extended structure surrounding the compact jets and lobes, which are strongly absorbed. The flux density of an opaque source can be simply expressed as Sν ∝ Tb × A, where Sν is the flux density at an optically thick frequency ν, Tb is the brightness temperature at frequency ν, and A is the surface area of the extended structure. The continuous increase of the flux density from 1.4 GHz to 335 MHz results from the (slow) growth of the overall source size as a result of the expansion of the radio jets and lobes.

  • (2)

    Above 1.4 GHz, the synchrotron radiation from the extended structure drops abruptly and the flux density is dominated by the compact hotspot, the mini-lobe, and the jet components. Arguments can be made that radiative losses play a dominant role in the energy balance of young and compact radio sources in the presence of strong (milliGauss; mG) magnetic fields (Stanghellini et al. 1997; Orienti et al. 2006; Orienti & Dallacasa 2012). The adiabatic expansion would then only lead to a decrease in the optically thin section of the spectrum. However, the source does not show prominent variability at 2.3-GHz, which could mean that the source is still opaque at this frequency and that any low-amplitude flux density variation is smeared out by the opacity effect.

  • (3)

    At frequencies above 5 GHz, the spectrum becomes optically thin and the observed variability is governed at the hotspots, i.e., the flux density from the most active jet-ISM interaction interface varies with the balance between the feeding and acceleration of fresh relativistic electrons and the radiative losses. The variation of the 15-GHz flux density with the component size (Fig. 2c) shows and increase of the flux density increases with the increasing size before the flare peak at epoch 1998.313, indicating the injection of particles, energy, and magnetic fields into the lobes. During this stage, the hotspot is still optically thick. After 1998, the intermittent feeding through the jet decreases and the hotspot continues to expand adiabatically and to become more optically thin. The strong synchrotron radiation results in a sharp decrease of the flux density from  ~900 mJy at 1998.313 to  ~400 mJy at 2005.473. Figure 2d shows the variation of the spectral index with time, using the 15-GHz MOJAVE data and 8.4-GHz RRFID data at close epochs. After the clear division around epoch 2006, the spectrum is significantly steepened. This supports the idea described above that the flux density variation of hotspot NE1 is regulated by intermittent nuclear feeding. When there are no or less fresh relativistic electrons inserted into the hotspots, the old electrons suffer from synchrotron aging, and the spectral index in the optically-thin part steepens.

Stanghellini et al. (1997) estimated the lifetime of the electrons radiating at 5 GHz assuming equipartition magnetic fields and a homogeneous spherical geometry for the hotspot. The resulting magnetic field in hotspot NE1 (component A in Stanghellini et al. 1997) is 90 mG, and an electron lifetime at 5 GHz of 25 yr. The synchrotron aging time scales with the observing frequency and the brightness temperature Tb as . Using the same arguments, the lifetime of electrons radiating at 15 GHz is about 4 yr, in agreement with the observed variability time scale of  ~15 yr (from 1983 to 1998).

3.3. Geometry of the radio structure

According to Fig. 2b, the light curve of the northeast hotspot NE1 peaks around 1998.313. The peak of the light curve of SW1 is not very prominent and falls in a broad time range between 2002 and 2005. A mediate epoch of is chosen as an estimate of the peak epoch, in which the uncertainty is represented by the scattering of the peak epoch. If the radio flux density enhancement in both hotspots is associated with the same episodic nuclear activity, the observation that the flare of NE1 is leading that of SW1 by  yr suggests that NE1 is the advancing lobe and SW1 is the receding one. The light travel time along the line of sight between the two hotspots corresponds to a radial difference of light years, or  pc. Assuming that the two lobes are symmetric relative to the core (separation  ~9.5 pc) and that both jets are ejected at the same velocity, an inclination angle of the radio structure of may be derived between the jet and the line of sight. This calculation suggests that the jet axis of OQ 208 is very close to the plane of the sky. It is supported by the observation that the jet emission at the nucleus is not Doppler boosted (Sects. 3.1 and 3.4).

However, this implication appears to be conflict with the optical spectroscopic identification of OQ 208 as a Seyfert 1 galaxy with broad Balmer emission lines (Burbidge & Strittmatter 1972). In the standard unification model for AGNs (Antonucci 1993), the Type-I AGNs should have an inclination angle less than 70° (Zhang 2005; Liu et al. 2007). Such an angle of 70° would predict a time delay between NE1 and SW1 of about 10.97 yr, for which there is no evidence in the light curves. The inconsistency between these inclination angles may suggest that the radio jet does not align with the optical axis of symmetry.

If the brightness asymmetry of the two lobes is caused by differential free-free absorption (Sect. 3.1), the physical properties of the ISM can be constrained. The spectral fit of free-free absorption gives a differential opacity Δτff = 5.3 (Kameno et al. 2000). The electron density is about 2700 cm-3 along a 1.53 pc path, assuming that the ISM surrounding the northeast and southwest lobes is homogeneous and the electron temperature is Te = 104 K. This electron density is typical for the narrow-line regions (NLR). An inhomogeneous ISM would require a higher density (in a clumpy medium) to account for the free-free absorption discrepancy.

thumbnail Fig. 3

Relative proper motions of VLBI components with respect to the brightest hotspot NE1. The proper motions of the VLBI components are determined by linear fits to the separations as a function of time. The angular separation rate is calculated as μ(J1) = −0.034 ± 0.001 mas yr-1, μ(SW1) = 0.027 ± 0.002 mas yr-1, and μ(SW2) = 0.022 ± 0.002 mas yr-1. The hotspot NE2 does not show significant proper motion.

Open with DEXTER

3.4. Kinematics and jet properties – a young radio source

Existing kinematics studies of CSOs show that the hotspot advancing speed is typically around 0.2c, and that they are young with kinematic ages of only between 100 and 2000 yr (Owsianik & Conway 1998; Owsianik et al. 1998; Polatidis & Conway 2003; Taylor et al. 2000; Gugliucci et al. 2005; An et al. 2012). In the present work, the slow proper motion of OQ 208 components are determined from making use of the long time-baseline 15-GHz VLBA data to eliminate systematic errors induced by different resolutions, different uv coverage, and different frequencies. Since the identification of the central core of OQ 208 is not certain yet, the brightest hotspot NE1 is used as the reference, and relative proper motions of other VLBI components are measured.

Figure 3 shows the variation of the distance of the VLBI components using 28 individual measurements over a time span is 13.6 yr. Linear regression was used to determine the separation rate. The positional uncertainty (σ ≈ 0.02 has) of each component at each epoch was used for weighting the individual data points (1/σ2). The fits give the separation rates 0.027 ± 0.002 mas yr-1 (0.134 ± 0.009   c, SW1–NE1), 0.022 ± 0.002 mas yr-1(0.108 ± 0.012   c, SW2–NE1) and −0.034 ± 0.001 mas yr-1(0.168 ± 0.005   c, J1–NE1). Positive velocities of SW1 and SW2 signify an advancing motion of both hotspots. The negative velocity of J1 indicates that the internal jet moves toward the terminal hotspot NE1. The separation of J1 at the first three epochs, 1995.266, 1995.395, and 1996.375, shows a strong deviation from the general trend of the data points beyond 1997. During the fitting process, double uncertainties were assigned to these data points in order to reduce their weight and thus to avoid a bias induced by these large deviations. The separation of NE2–NE1 is about 1.2 mas, and the linear regression fit did not give a significant proper motion, because NE2 shows a significant transverse motion rather than a radial motion (Fig. 4).

Liu et al. (2000) reported a first determination of the NE1−SW1 angular separation rate of 0.058 ± 0.038 mas yr-1, derived from 8.4-GHz data with only six epochs between 1994 and 1997. Subsequently, Luo et al. (2007) improved this value to 0.031 ± 0.006 mas/yr (5σ), on the basis of a longer time span (11 yr) and nine epochs. Compared to the previous 8.4-GHz work, the current proper motion measurements greatly improve the accuracy in the following ways: (1) the 15 GHz data provide a higher resolution (typically 0.8 × 0.6 mas2), while the previous measurements at 8.4 GHz had a resolution (1.3 × 1.0 mas2) that is comparable with the separation between SW1 and SW2. At 8.4-GHz the positions of SW1 and SW2 are affected by the flux density variation of the two components (mainly SW1), which is also true for the northeast lobe where the positions of NE2 and J1 are affected by the flux density and structure variations of NE1. At 15 GHz, SW1 is clearly separated from SW2. (2) The present work covers a time span of 13.6 yr, four times longer than in the earliest work by Liu et al. (2000), and  ~2.3 yr longer than that in Luo et al. (2007). (3) The data from 28 individual epochs give a better time sampling compared to the sparse sampling in previous works. (4) The 15-GHz VLBA data provide better and more uniform u,v coverage than the 8.4-GHz data.

According to the new measurement of μ = 0.027 ± 0.002 mas yr-1 for SW1–NE1 14σ detection), the kinematic age of OQ 208 is calculated as 255 ± 17 yr in the source rest frame. This age classifies OQ 208 as one of the youngest CSOs (Polatidis & Conway 2003; Gugliucci et al. 2005; Giroletti & Polatidis 2009; An et al. 2012).

Assuming equal advancing velocities for NE1 and SW1, the internal jet J1 is found to move 3.5 times faster (vJ1 = 0.23 c) than the terminal hotspots. A similar phenomenon has also been observed in other CSOs (e.g., B0710+439 and B2352+495: Taylor et al. 2000; J0132+5620: An et al. 2012). While the radio lobes sweep the ambient ISM, a jet knot moves in an excavated channel with a relatively lower ISM density.

The kinematic parameters of the OQ 208 jet derived above can be used to calculate the jet flow properties using the following equations: \arraycolsep1.95pte ear,derived above can b in 0j+495:), the kinemati ourcA simiotionsing (1(in a evalobing A simiotionsing (T<.014;c, SW2&uen(T<.013nd has a the line opresentededuring -dominatesh culatg A simiotionty of VLBI coRinatesh oldem the de thatmajaa1dominant (2000) s optically thin anvistic ele derISM density the jet 60;yr (from 1983 to 1998).

T<996 determ;0.04ition with tuomeno ohe oSng 1982 dag velocities for NE1 and on wi8776; fttrong deSW2 sigt the irstnnectiiti s63;Bu9700" t#160;yterm strund c s28 ies o the re oheght trch was used foro s thbetwJt determinaatively love ve et urce srent Bu97 radial motion (Fig. 
. The 9700-12. in 0j+49SW2 sigt ±&#ing th, jet knlumortherimary 13, indicating th may be SW1 ity s the uource1 (ce a,with characta up>. Thetrong of the sky. It is sudeflons ofy beh7. Duri maw(slo very cl assuming ISM>An et inating separaf SW1 andused to calcove NE1. N2,with tselfvm a28 ieGNs sorigoper motiatively lower war of by the y beheads advancing velas seconot NE1nows u, wh truux VLBI data is cdeflons ofy beaa1 es of Cs a(4) The ry clset reveaa phex ofy beculaeq68.png">anne,ation, e data points in2 signr itive2 signiftudi>annehan cri inhomoordinual aso b v2.htl#F1"other VLBI cor de- dng that theob s (MSO)ng ry t #16erson deocity of J1jet-data with only six t 2.html J1cnsitcor de (IGM&#e inteaor ynsitcd by the 2012)i>2; 3  run beculas VLBI data is cAssuming eqbelmaticg ry en td inand adisubson d idomiaof the -IGM bssumar="d separately in nows iu et&morptd oglatively lower m SW2. (rks. nematics studioperties ction), the kinemat res l"h inten, e data poin.

Assuminye 5-GHz geometperti>annection), the kinemaA simiotionsing Par,derive

Te<25.0sub> =&W160;=&0;e0;0.002  VLB&#iA simiotionbetwen2 sigh is als frcales with count for, de >

it be understot ac>annec angle less th2 70° (AAersto>anneco b et variability bee data pointscavated c theound Assup hydrodye l d> is i flu witig and compj#160compon s6tlculation suggest datacharact osrr tordihin Ifmotion, becausef niftudi>ry of rend n  ~9.5e assal. the oofy beSW1, the 0;±CSSdio sts 3Cariabi4860componlar ergy balad separately i is noies of CSOion o300 amplicl flux dens identAGN angle less tha3 70° (; Kameno et4ugliucci et allity scale (Zhan 2005; 3.3s c v2.;8.4&#wittent ed with the same in whd thusaa19700-12.ht2000) reed, is als oThe flux poch 2006, th osrre="ISM data 14.5 t actived to the spa s one of the youngest CSOs (AFr 7used to calmyoung athesecreasest resnecti idohe othen rgy balag that themagnetiEvrdih d untilhe bro r 7"o beeanctM beeand adi3.tscame="

3.imare youn variability is compadata M surn of t 8.4&#indicating et core (semas m SW2. (atron o b morptd ogly is coensity balat lower freq00dnortheasdicating n g reOion o determ126res76;sh rec160;GHz, NE et¡ (1.3&#pcich are strongly ab 13o very clx rpon of theity variatiSM interaction iing time scales witructu An etdata M - 8.4&# #160;Kimare ,hai J1 aby beSW14&#n> e data povel rsancing emagnetiThetification of the cench o 1 r es. Sintsp flux denm SW2. (rks.atively lower w low originatings advancing v hotspot NE2 d affected by tsme of electrons indicating 62%hs 0;(40 ± in previoQ 208 haiftudies of Cs 0;&n of the centhes the old electthe terface varies r 1998, flux dens identepebetw hotspot cont including theatively lower mle ed a first the obin hotspoeSW1times  ~2.3&ed a fig v the obby;≈llini et.00overs a tiosight.;pc path, separamov-GHz dthe inters opticrNE1 is the acing lobml#R23">Kat andis the receding os correspond along the line of sight between the two hotspots cs the l#R23">Ka corresponds ation of >-3 b) pro thaabatic elarable witict a time delayg velwe y hots-I AGNs should h males may su derived between the this dicating et¨er res76;jet andwith respdata #160;=). Soevn balanld htrong d  nent aold electthe d with tNLR60compone inclination angy be ali derived 2 ali ear ftio jetents are 00-12-eq68 is not certain yet, pot NE1 iNE1 as the reference, andwl-eq">(2)μ NE1)0;± 0.038 dicating n d8211;NE1) and 0;± 0.038 nal hotspot 5:), t with the set, pbetween the two dicating es of CSOs show that theeparat6168 ± 0.005 pc g that theOs show tha time delaycing lobees of CSnce, antspot the kinemati our nd two lobes are sz VLBAh the optics i aration o211;NE1) a4956;density ;dicating m >3.3s cmaA simioti res e data point At 15&o b o the>σ3.3s c v2.;8.4&tiospon AA jet propertA simioti s cthesoThe flux NE1 is r>anne octiohe (sscoilradiaar t 1 i

intwo dicating extended se fluxeadyz resultph ree data poas) o NE1 is rhe same episod and thesity originates ematic errorg NE1. T lobecoil-GHz da ry prle the inttiThetnthan ae f NE1 is q">(2)σae fnsitip es of Csi derivedfre of trA simioti et£ (1.3  (~4 (1.3&#pc)at ±&#ing theis arematic errnortheaslanld h, i.e., tha0;&>3  ng ry t A simioti 16erson de adldem ts aby and n the>anne ml#F2" The hyoung ;dicating rsto>annecrsto" The ng kin sx variability beh theound Assup u, ulis r> tordihi derISM data ±icdicating pa s raction ihe radio lo>AFr 7so b nce of young aractiou, ulis r>lefa>t).

>

#0;alwi"list-style-tncy.

e >

R2 70An, T., 0-12.hBa o, W.iA12.htm, ApJ, 760, 77 > "list-style-tncy.

>

3/02/aa1 bk_">>

#0;a2wi"list-style-tncy.

e >

R3 70An, T., Ho X.-Y., Hdrdca6.le,ao>AJ., be -12.ht&,aoNRAS, 40m, 87 > "list-style-tncy.

>

3/02/aa1 bk_">>

#0;a3wi"list-style-tncy.

e >

R4 70An, T., Wu,AF., Ya J., be -12.ht2, ApJS, 1opti5 > "list-style-tncy.

>

3/02/aa1 bk_">>

#0;a4wi"list-style-tncy.

e >

R5 70An12/aa19, R#R39"3, ARA0-12.A8231, 473 > "list-style-tncy.

>

3/02/aa1 bk_">>

#0;a5wi"list-style-tncy.

e >

Rlity scBaum, S.iA1, O′Dea, C.ePc,aourphy, d. W., 0-12.han#160gn, A12G#R39"&,aA0-12.A82232, 1o > "list-style-tncy.

>

3/02/aa1 bk_">>

#0;a6wi"list-style-tncy.

e >

R class=0-12/aa1,oE.fM., 0-12.h.html#R7">BufePcoA.rbidg, ApJ, 1dg, L37 > "list-style-tncy.

>

3/02/aa1 bk_">>

#0;a7wi"list-style-tncy.

e >

R8class=Cox, C.eI., Gull, S.iF., 0-12.h.cheuBufePcoA.rG#R39"1,aoNRAS, 25g, 588 > "list-style-tncy.

>

3/02/aa1 bk_">>

#0;a8wi"list-style-tncy.

e >

R9 2005 > "list-style-tncy.

>

3/02/aa1 bk_">>

#0;a9wi"list-style-tncy.

e >

R1. 1998 > "list-style-tncy.

other VLBI F3.html" targZ3988013itlllowi"list-style-tncy.

> <12/aa19700-12-eq50.png""list-style-tncy.

>

>

#0;a1. 1"list-style-tncy.

e >

R11 2005 > "list-style-tncy.

>

3/02/aa1 bk_">>

#0;a1lwi"list-style-tncy.

e >

R12 70Fanaroff, B. L., 0-12.hRiley J.eM.rbid4,aoNRAS, 1678231 > "list-style-tncy.

>

3/02/aa1 bk_">>

#0;a12wi"list-style-tncy.

e >

R13 70Fant9, C.etha9, AN8233&,a120 > "list-style-tncy.

>

3/02/aa1 bk_">>

#0;a13wi"list-style-tncy.

e >

R14 70Fant9, C.,hFant9, R#, dallacasa, d., be -12 tim,aA0-12.A8230m, 317 > "list-style-tncy.

>

3/02/aa1 bk_">>

#0;a14wi"list-style-tncy.

e >

R15 70Fassnacht, C.eD., 0-12.h-12/aa,aGcoB.etha1,aAJ, 122, 1661 > "list-style-tncy.

>

3/02/aa1 bk_">>

#0;a15wi"list-style-tncy.

e >

Rllity scFepe, W.-X., An, T., Ho X.-Y., be -12.h0m,aA0-12.A82434, 101 > "list-style-tncy.

>

3/02/aa1 bk_">>

#0;a16wi"list-style-tncy.

e >

R1 class=Fomq00&t,oE.fB12 ti9,aineSyn=&#&Imar

intRce ofA6.375om F3.htmxmlns:mmlr>

#0;a17wi"list-style-tncy.

e >

Rl. 2005<-12/aa197,fM., 0-12.holetti &a,iA12.ha9, AN8233&,a193 > "list-style-tncy.

>

3/02/aa1 bk_">>

#0;a18wi"list-style-tncy.

e >

R19 2005<-12/aa197, NcoE., -12/aa,aGcoB., Peck,iA12B., 0-12.h-12/aa197,fM.2.h0m,aApJ, 622, 136 > "list-style-tncy.

>

3/02/aa1 bk_">>

#0;a19wi"list-style-tncy.

e >

R2. 1998AJ., 0-12.hLooney L. W.etha1,aoNRAS, 32082355 > "list-style-tncy.

>

3/02/aa1 bk_">>

#0;a2. 1"list-style-tncy.

e >

R21 2005AJ., de L in 0j+4, V., 0-12.hCorwin,aHe G#R39"&,aApJS, dg, 433 > "list-style-tncy.

>

3/02/aa1 bk_">>

#0;a2lwi"list-style-tncy.

e >

R22 70Kaieno,aC. R., 0-12.hB th, P. Ncoeen7,aoNRAS, 381,a1548 > "list-style-tncy.

>

3/02/aa1 bk_">>

#0;a22wi"list-style-tncy.

e >

R23 70KaA si, S., Horaa1hi, S., Shen,aZ.-Q., be -12.h0&,aPASJ, 5g, 20o > "list-style-tncy.

>

3/02/aa1 bk_">>

#0;a23wi"list-style-tncy.

e >

R24 2005 > "list-style-tncy.

>

3/02/aa1 bk_">>

#0;a24wi"list-style-tncy.

e >

R2n 70Kawak , Nc,0Nagai,aHe,00-12.hKino,fM.2.h09, AN8233&,a283 > "list-style-tncy.

>

3/02/aa1 bk_">>

#0;a25wi"list-style-tncy.

e >

R2lity scKellnemann,aK.eI., Vnemeulis, R#RC.,hZensua,iA12J., be -12 ti8,aAJ, 115, 1295 > "list-style-tncy.

>

3/02/aa1 bk_">>

#0;a26wi"list-style-tncy.

e >

R2 class=La RcoA.rbi81,aoNRAS, 1im,a261 > "list-style-tncy.

>

3/02/aa1 bk_">>

#0;a27wi"list-style-tncy.

e >

R28class=La RcoA.rbi82,aineProc. IAU Symp. 97, E inacnsitRce ofSyoung (Dordrecht: Reidel); ed D. S. Heeschen,a0-12.hC.eM.rWade ASP af. Ser., 77, 161 > "list-style-tncy.

other VLBI F3.html" targZ3988013itlllowi"list-style-tncy.

> <12/aa19700-12-eq50.png""list-style-tncy.

>

>

#0;a28wi"list-style-tncy.

e >

R29 2005 > "list-style-tncy.

>

3/02/aa1 bk_">>

#0;a29wi"list-style-tncy.

e >

R30 2005 > "list-style-tncy.

>

3/02/aa1 bk_">>

#0;a3. 1"list-style-tncy.

e >

R31 2005AJ., 0-12.hBar=&#l, P. D.rbi86,aAJ, 92, 12 > "list-style-tncy.

>

3/02/aa1 bk_">>

#0;a3lwi"list-style-tncy.

e >

R32 70Luo, W.iF., Ya J., Cui, L., Liu X., 0-12.h.hen,aZ.-Q.oeen7,aChJAA, 5, 611 > "list-style-tncy.

>

3/02/aa1 bk_">>

#0;a32wi"list-style-tncy.

e >

R33 70Mttpant9, R#, Tadh a ,aC>ANc,00-12.hOove loo, T.iA12.ham,aA0-12.A82444, Lo > "list-style-tncy.

>

3/02/aa1 bk_">>

#0;a33wi"list-style-tncy.

e >

R34 2005 > "list-style-tncy.

>

3/02/aa1 bk_">>

#0;a34wi"list-style-tncy.

e >

R3n 70Orhre S,fM.2.h06,a7thzNaseparat afrespondsn A60;&# G#16cnsitN witi (AGN7),sheld 23–26 May,aineMontagnana, Italy8235 > "list-style-tncy.

other VLBI F3.html" targZ3988013itlllowi"list-style-tncy.

> <12/aa19700-12-eq50.png""list-style-tncy.

>

>

#0;a35wi"list-style-tncy.

e >

R36 70Orhre S,fM., 0-12.hdallacasa, d.2.h08,aA0-12.A82487, 885 > "list-style-tncy.

>

3/02/aa1 bk_">>

#0;a36wi"list-style-tncy.

e >

R37 70Orhre S,fM., 0-12.hdallacasa, d.2.ht2, oNRAS, 424, 532 > "list-style-tncy.

>

3/02/aa1 bk_">>

#0;a37wi"list-style-tncy.

e >

R38class=Owsianik,eI., 0-12.hConway J.eE12 ti8,aA0-12.A82337, 6o > "list-style-tncy.

>

3/02/aa1 bk_">>

#0;a38wi"list-style-tncy.

e >

R39class=Owsianik,eI., Conway J.eE1, 0-12.holetti &a,iA12G12 ti8,aA0-12.A82336, L37 > "list-style-tncy.

>

3/02/aa1 bk_">>

#0;a39wi"list-style-tncy.

e >

R40 2005 > "list-style-tncy.

>

3/02/aa1 bk_">>

#0;a4. 1"list-style-tncy.

e >

R41 2005 > "list-style-tncy.

>

3/02/aa1 bk_">>

#0;a4lwi"list-style-tncy.

e >

R42 2005 > "list-style-tncy.

>

3/02/aa1 bk_">>

#0;a42wi"list-style-tncy.

e >

R43 70.cheuBufePcoA.rG#R3982,aineE inacnsitRce ofSyoung ,eProc. IAU Symp. 97, ed D. S. Heeschen,a0-12.hC.eM.rWade (Dordrecht: Reidel); ASP af. Ser. 77, 163 > "list-style-tncy.

other VLBI F3.html" targZ3988013itlllowi"list-style-tncy.

> <12/aa19700-12-eq50.png""list-style-tncy.

>

>

#0;a43wi"list-style-tncy.

e >

R44 2005 > "list-style-tncy.

>

3/02/aa1 bk_">>

#0;a44wi"list-style-tncy.

e >

R4n 70St2/ahel bi, C., Bond7,fM., dallacasa, d., be -12 ti7,aA0-12.A82318, 376 > "list-style-tncy.

>

3/02/aa1 bk_">>

#0;a45wi"list-style-tncy.

e >

R46 70St2/ahel bi, C., Liu X., dallacasa, d., 0-12.hBond7,fM.2.h02,aNewfA6.375. Rev., 46,a287 > "list-style-tncy.

>

3/02/aa1 bk_">>

#0;a46wi"list-style-tncy.

e >

R47 70St2/ahel bi, C., O’Dea, C.ePc,adallacasa, d., be -12.h0m,aA0-12.A82433, 891 > "list-style-tncy.

>

3/02/aa1 bk_">>

#0;a47wi"list-style-tncy.

e >

R48 70St2/ahel bi, C., dallacasa, d., Venturi, T., be -12.h09, AN8233&,a153 > "list-style-tncy.

>

3/02/aa1 bk_">>

#0;a48wi"list-style-tncy.

e >

R49class=-12/aa,aGcoB., Marr J.eM., Pearson, T.iJ., 0-12.hR adh ad,iA12C. S.R#R49,aApJ, 541,a112 > "list-style-tncy.

>

3/02/aa1 bk_">>

#0;a49wi"list-style-tncy.

e >

R50 2005 > "list-style-tncy.

>

3/02/aa1 bk_">>

#0;a5. 1"list-style-tncy.

e >

R51 2005 > "list-style-tncy.

>

3/02/aa1 bk_">>

#0;a5lwi"list-style-tncy.

e >

R52 2005 > "list-style-tncy.

>

3/02/aa1 bk_">>

#0;a52wi"list-style-tncy.

e >

R53 70Wilkinson, P. Nc, Tzioum&a,iA12K., Benson, J.eM., be -12 ti1,aNature8235m, 313 > "list-style-tncy.

>

3/02/aa1 bk_">>

#0;a53wi"list-style-tncy.

e >

R54 2005AJ., Pearson, T.iJ., 0-12.hR adh ad,iA12C. S.R#R44,aoNRAS, 347, 632 > "list-style-tncy.

>

3/02/aa1 bk_">>

#0;a54wi"list-style-tncy.

e >

R5n 70Zha S. Ncoeenm,aApJ, 618, L7o > "list-style-tncy.

>

3/02/aa1 bk_">>

#0;a55wi"list-style-tncy.

e >

R56 70Zhao, W., Ho X. Y., An, T., be -12.h12.aA0-12.A82529, 113 > "list-style-tncy.

>

3/02/aa1 bk_">>

#0;a56wi"list-style-tncy.

t. S10 2005 SD1 2005<

T1 2005<>

:/e="InR55">3/02/aa1_blank"><3.html" targbold">Table 1<12/aa1005<par ters. tabs 2005> :/e="InR55">3/02/aa1_blank"><3.html" targbold">Table 1<12/aa1005<par ters.

>

:/e="InR55"> figs 2005

>

:/e="InR55">3/02/aa1_blank">srcr>:/e="InR55">"

>

:/e="InR55">3/02/aa1_blank"><3.html" targbold">Fig.th.;1<12/aa1005<a) shows;= .3-GHz VLBI image,t.

display

a double-component morphology. Panel b) shows;=taperedt.

2.3-GHz image. T xtended emission at  ~30 mas west of =primary ex ucture ist.

likely>a relic rce ofemission result

from =pa6. 6cnsvity a few =&ousand yearst.

ago. Panels c) and d) show =&# 8.4 and 15 GHz images. Eacht.

mini-lobe is resolved into two hotspots. Betwee0;=hotspots NE1 and SW1, a knottyt.

j beis d tected.
:/e="InR55">

>

:/e="InR55">3/02/aa1_blank">srcr>:/e="InR55">"

>

:/e="InR55">3/02/aa1_blank"><3.html" targbold">Fig.th.;2<12/aa1005<a) Total flux d nsityt.

vbility of OQ 208 observed with =UMRAO 26- ter rce oftenR5cope: 4.8 GHzt.

(black), 8 GHz (red), 14.5 GHz (blue). Tsum of 15-GHz flux d nsities of =VLBIt.

components (see Table 1) is represented by gree0;symbols indicat

that  ~15% ist.

missed du#16o lack of short uv-2/ac b) Temporal vsepas of =&#t.

flux d nsities of VLBI components. Tflux d nsities are derived from =&#t.

model-fitt

results listed in Table 1. Two vee="Ial bes mark =peak epochs oft.

NE1 (around bi98.313) and SW1 (around tha3.313). Toml"early>show =&# vbilityt.

behavior of SW1, tflux d nsities of SW1 have bee0;mult plied by a f6cnor of letother VLBI c) Vsepa of =s-GHz flux d nsity of =hotspot NE1 with =&#t.

component size. Tflux d nsity of NE1 experhref=d a slow rise before epocht.

bi98.313, follow=d again by a sharp d c be until epochoeenm.473 follow=d by a slowt.

d c be. Tcomponent size1constantly>increases dur

the whol#1period. d)t.

Temporal vsepa of =spectral ind0xother VLBI <3.html" targimg-in be">:/e="InR55">
:/e="InR55">

>

:/e="InR55">3/02/aa1_blank">srcr>:/e="InR55">"

>

:/e="InR55">3/02/aa1_blank"><3.html" targbold">Fig.th.;3<12/aa1005< NE1. Tproper mosepas of =VLBI components are determin=d by bear fits16o =&#t.

separ sepas as a funcsepa of =ime. Tangular separ sepa rcte is Ialculated ast.

μ(J1) = −0.034 ± 0.001 masth.;yr-1<12up>,t.

μ(SW1) = 0.027 ± 0.002 masth.;yr-1<12up>, and9700-12-eq50.png" μ(SW2) = 0.022 ± 0.002 masth.;yr-1<12up>. T◤-12-eq50.png"hotspot NE2 does not>show signif"Iant1proper mosepa.
:/e="InR55">

>

:/e="InR55">3/02/aa1_blank">srcr>:/e="InR55">"

>

:/e="InR55">3/02/aa1_blank"><3.html" targbold">Fig.th.;4<12/aa1005<:/e="InR55"> t t g" la19700-12">"" Current usage metrics" About ae="InR metrics"g" " tdata-for="metrics-siq"> tdata-for="metrics-alm"> tdata-for="info"> ""

Current usage metrics>show cumuettiv#1count of Ae="InR Views (full-60xt ae="InR views includ

HTML views, PDF and ePub downloads, accord

6o =avai2able data) and Ahex s Views onmVision4Press pettform.Data correspond 6o usage o0;=petteformaafter .ht5. Tcurrent usage metrics>is avai2able 48-96"hours after on be publicat on and is updated dai2y o0;week days. <3.html" targico ico-info">002/aa19 Initial download of =metrics>may>3/k#1a while. t srcr>:tempettes/syoung/js/metrics-tab.js 200script> t other VLBI t. t t.

t
g"
:component/rsslist/?task=jyounal0-12.Itemid=263013itllloSubscrib#16o A0-12.A rss feeds"><3.ht> 002/aa1 RSS feeds<05<
© TEuropehtmSouthern1Observatory (

3/02/aa1_blank">>

:mre Spas-leganR5" >Mre Spas léganR5<05<

>

:component/e><_cont /" >Cont 5<05< t g" t <3.html" targx nsl sepas0>data- separ sor_and0>data-valu and0>002/aa19<3.html" targx nsl sepas0>data- show_short_summary0>data-valu Show short summary0>002/aa19<3.html" targx nsl sepas0>data- hide_short_summary0>data-valu Hide short summary0>002/aa19<3.html" targx nsl sepas0>data- cookie_t0xt0>data-valu By us

this website, you agree>that EDP S href= >may>store web aud spondmeasuremre cookies and, o0;som#1pagg ,ecookies from social networks.

> 002/aa19<3.html" targx nsl sepas0>data- cookie_l"ose0>data-valu Click16o l"ose this notif"Iasepa0>002/aa19<3.html" targx nsl sepas0>data- load data-valu Load

002/aa19<3.html" targx nsl sepas0>data- view_fullscreen0>data-valu Click16o view fullscreen0>002/aa19<3.html" targx nsl sepas0>data- hide_fullscreen0>data-valu Click1anywher#16o hide tfullscreen overlay0>002/aa19<3.html" targx nsl sepas0>data- play_movie0>data-valu Click16o play =movie0>002/aa19<3.html" targx nsl sepas0>data- cookie_is_mobile0>data-valu 0>002/aa19<3.html" targx nsl sepas0>data- m/k# data-valu :component/m/k# data- authors_url0>data-valu :component/ae="InR/?task=2/a_authors">002/aa19stylllodisplay:nobe"> <3.htmid="wait-progress">002/aa1 t <3.htmid="google_analy="Is0>data-hosaa1cds data-domaina1cds.aandahttp" data-t1="UA-39520934-1" data-t2="UA-5187131-2"ml" targhidden0>002/aa19<3cript1srcr>:tempettes/syoung/js/cookie.js 200script> <3cript1srcr>:tempettes/syoung/js/common.js 200script> t

thumbnail Fig. r sct)srent e-I AGNs sh NE1. The proper motio htflow ori8201;yr-1. The d oeilityrce rrespondnortheasnaignificant proper motion.

Open with DEXTER
tRt).

>

tRt).

>

tR d ommotio hiling the plane of g A simiotiex inthe sk1NaseparatBa oResAt Programistin sha (973 Program)ior ofz rauseNoRRFID data tha9CB249 only bet13CB83790velocitS href=10-12.hTechnd ogly alaxy wivea812/ahaiFID data Mun ipa208 h(06DZ2210absorptin sha-Ht par="ColltiorGNs shoud Ex mtecProgramifun radthe sky. It is Ies forseparatCoopand B auaS5"> AF.W.ith60kt NE1 JIme/ASTRON Summocdicating Scy auseProgramihe rovi">3 pita208 haiftudiJIme. S.F.h com plane of the sk1Ht pariE1old electS hre SinchResAt Fthe (OTKA K104539 #in elith60kiGnelan#160;~rovi"the uou sdicating nion o frcales withQ 208 haifty of VLBI co hiliresAt lar onente determineNASA/IPACdicating E inacnsitDks.ni-l (NEDwo componentopand of the sk1n bePropulxy wiLtiorGNohe,eCa20oviniay. It is Ie tu is Technd ogl,ior ofz ext raction iNaseparatAerparuname="S7"Spet-data withAdt lo 13, NaseparatRce ofA6.375om Oported ohegi ac;208 haiftudiNaseparaold electS href=1Fssuma#963aepand of or ofz eopand bout theAh hotspotsUni itig , Iecby these largeh time,pro the plane of or ofzNaseparatS href=1Fssuma#963a rause0807860-ASTha time delayNASA-F NEz rauseNNX08AV67Gco hiliresAt lar onente determineUni of St#160;Navraold electOported oheg(USNO)tRce ofR rrespondF

>

R1 2005AF., Lang oe,aGcoE., 0-12.hHodiffePcoE.f , ApJS, 59, 513 > "list-style-tncy.

>

3/02/aa1 bk_">>

[NASA ADS]60;aother VLBI

3/02/aa1 bk_">>

> <12/aa19700-12-eq50.png""list-style-tncy.

>

>

[NASA ADS]60;aother VLBI

3/02/aa1 bk_">>

> <12/aa19700-12-eq50.png""list-style-tncy.

>

>

[NASA ADS]60;aother VLBI

3/02/aa1 bk_">>

> <12/aa19700-12-eq50.png""list-style-tncy.

>

>

[NASA ADS]60;aother VLBI

3/02/aa1 bk_">>

> <12/aa19700-12-eq50.png""list-style-tncy.

>

>

[NASA ADS]60;aother VLBI

3/02/aa1 bk_">>

> <12/aa19700-12-eq50.png""list-style-tncy.

>

>

[NASA ADS]60;aother VLBI other VLBI F3.html" targZ3988013itlllowi"list-style-tncy.

> <12/aa19700-12-eq50.png""list-style-tncy.

>

>

[NASA ADS]60;aother VLBI >

3/02/aa1 bk_">>

> <12/aa19700-12-eq50.png""list-style-tncy.

>

>

[NASA ADS]60;aother VLBI >

3/02/aa1 bk_">>

> <12/aa19700-12-eq50.png""list-style-tncy.

>

>

[NASA ADS]60;aother VLBI other VLBI F3.html" targZ3988013itlllowi"list-style-tncy.

> <12/aa19700-12-eq50.png""list-style-tncy.

>

>

[NASA ADS]60;aother VLBI >

3/02/aa1 bk_">>

> <12/aa19700-12-eq50.png""list-style-tncy.

>

>

[NASA ADS]60;aother VLBI >

3/02/aa1 bk_">>

> <12/aa19700-12-eq50.png""list-style-tncy.

>

>

[NASA ADS]60;aother VLBI >

3/02/aa1 bk_">>

> <12/aa19700-12-eq50.png""list-style-tncy.

>

>

[NASA ADS]60;aother VLBI other VLBI F3.html" targZ3988013itlllowi"list-style-tncy.

> <12/aa19700-12-eq50.png""list-style-tncy.

>

>

[NASA ADS]60;aother VLBI >

3/02/aa1 bk_">>

> <12/aa19700-12-eq50.png""list-style-tncy.

>

>

23165">2316tml/20>[NASA ADS]60;aother VLBI >

3/02/aa1 bk_">>

23165">2316tml/20>[CrossR r]60;aother VLBI >

3/02/aa1 bk_">>

23165">2316tml/20>[EDP S href= ]60;aother VLBI other VLBI F3.html" targZ3988013itlllowi"list-style-tncy.

> <12/aa19700-12-eq50.png""list-style-tncy.

>

>

ii<12/aa1; ed GcoB.e-12/aa,aC. L. Clli, 0-12.hR#RA. Perley ASP af. Ser., 18082301 > "list-style-tncy.

other VLBI F3.html" targZ3988013itlllowi"list-style-tncy.

> <12/aa19700-12-eq50.png""list-style-tncy.

>

>

[NASA ADS]60;aother VLBI >

3/02/aa1 bk_">>

> <12/aa19700-12-eq50.png""list-style-tncy.

>

>

[NASA ADS]60;aother VLBI >

3/02/aa1 bk_">>

> <12/aa19700-12-eq50.png""list-style-tncy.

>

>

[NASA ADS]60;aother VLBI >

3/02/aa1 bk_">>

> <12/aa19700-12-eq50.png""list-style-tncy.

>

>

[NASA ADS]60;aother VLBI >

3/02/aa1 bk_">>

> <12/aa19700-12-eq50.png""list-style-tncy.

>

>

[NASA ADS]60;aother VLBI >

3/02/aa1 bk_">>

> <12/aa19700-12-eq50.png""list-style-tncy.

>

>

[NASA ADS]60;aother VLBI other VLBI F3.html" targZ3988013itlllowi"list-style-tncy.

> <12/aa19700-12-eq50.png""list-style-tncy.

>

>

[NASA ADS]60;aother VLBI >

3/02/aa1 bk_">>

> <12/aa19700-12-eq50.png""list-style-tncy.

>

>

[NASA ADS]60;aother VLBI other VLBI F3.html" targZ3988013itlllowi"list-style-tncy.

> <12/aa19700-12-eq50.png""list-style-tncy.

>

>

[NASA ADS]60;aother VLBI >

3/02/aa1 bk_">>

> <12/aa19700-12-eq50.png""list-style-tncy.

>

>

[NASA ADS]60;aother VLBI >

3/02/aa1 bk_">>

> <12/aa19700-12-eq50.png""list-style-tncy.

>

>

[NASA ADS]60;aother VLBI >

3/02/aa1 bk_">>

> <12/aa19700-12-eq50.png""list-style-tncy.

>

>

[NASA ADS]60;aother VLBI >

3/02/aa1 bk_">>

> <12/aa19700-12-eq50.png""list-style-tncy.

>

>

[NASA ADS]60;aother VLBI >

3/02/aa1 bk_">>

> <12/aa19700-12-eq50.png""list-style-tncy.

>

>

[NASA ADS]60;aother VLBI >

3/02/aa1 bk_">>

> <12/aa19700-12-eq50.png""list-style-tncy.

>

>

[NASA ADS]60;aother VLBI >

3/02/aa1 bk_">>

[CrossR r]60;aother VLBI >

3/02/aa1 bk_">>

[EDP S href= ]60;aother VLBI other VLBI F3.html" targZ3988013itlllowi"list-style-tncy.

> <12/aa19700-12-eq50.png""list-style-tncy.

>

>

[NASA ADS]60;aother VLBI >

3/02/aa1 bk_">>

> <12/aa19700-12-eq50.png""list-style-tncy.

>

>

09948-a8/">09948-a8tml/20>[NASA ADS]60;aother VLBI >

3/02/aa1 bk_">>

09948-a8/">09948-a8tml/20>[CrossR r]60;aother VLBI >

3/02/aa1 bk_">>

09948-a8/">09948-a8tml/20>[EDP S href= ]60;aother VLBI other VLBI F3.html" targZ3988013itlllowi"list-style-tncy.

> <12/aa19700-12-eq50.png""list-style-tncy.

>

>

[NASA ADS]60;aother VLBI >

3/02/aa1 bk_">>

> <12/aa19700-12-eq50.png""list-style-tncy.

>

>

[NASA ADS]60;aother VLBI other VLBI F3.html" targZ3988013itlllowi"list-style-tncy.

> <12/aa19700-12-eq50.png""list-style-tncy.

>

>

[NASA ADS]60;aother VLBI other VLBI F3.html" targZ3988013itlllowi"list-style-tncy.

> <12/aa19700-12-eq50.png""list-style-tncy.

>

>

[NASA ADS]60;aother VLBI >

3/02/aa1 bk_">>

> <12/aa19700-12-eq50.png""list-style-tncy.

>

>

[NASA ADS]60;aother VLBI other VLBI F3.html" targZ3988013itlllowi"list-style-tncy.

> <12/aa19700-12-eq50.png""list-style-tncy.

>

>

[NASA ADS]60;aother VLBI >

3/02/aa1 bk_">>

> <12/aa19700-12-eq50.png""list-style-tncy.

>

>

[NASA ADS]60;aother VLBI other VLBI F3.html" targZ3988013itlllowi"list-style-tncy.

> <12/aa19700-12-eq50.png""list-style-tncy.

>

>

[NASA ADS]60;aother VLBI other VLBI F3.html" targZ3988013itlllowi"list-style-tncy.

> <12/aa19700-12-eq50.png""list-style-tncy.

>

>

[NASA ADS]60;aother VLBI >

3/02/aa1 bk_">>

> <12/aa19700-12-eq50.png""list-style-tncy.

>

>

2226-045">2226-04tml/20>[NASA ADS]60;aother VLBI >

3/02/aa1 bk_">>

2226-045">2226-04tml/20>[CrossR r]60;aother VLBI >

3/02/aa1 bk_">>

2226-045">2226-04tml/20>[EDP S href= ]60;aother VLBI other VLBI F3.html" targZ3988013itlllowi"list-style-tncy.

> <12/aa19700-12-eq50.png""list-style-tncy.

>

>

[NASA ADS]60;aother VLBI other VLBI F3.html" targZ3988013itlllowi"list-style-tncy.

> <12/aa19700-12-eq50.png""list-style-tncy.

>

>

[NASA ADS]60;aother VLBI >

3/02/aa1 bk_">>

> <12/aa19700-12-eq50.png""list-style-tncy.

>

>

[NASA ADS]60;aother VLBI >

3/02/aa1 bk_">>

> <12/aa19700-12-eq50.png""list-style-tncy.

>

>

[NASA ADS]60;aother VLBI >

3/02/aa1 bk_">>

> <12/aa19700-12-eq50.png""list-style-tncy.

>

>

[NASA ADS]60;aother VLBI >

3/02/aa1 bk_">>

> <12/aa19700-12-eq50.png""list-style-tncy.

>

>

[NASA ADS]60;aother VLBI >

3/02/aa1 bk_">>

> <12/aa19700-12-eq50.png""list-style-tncy.

>

>

[NASA ADS]60;aother VLBI >

3/02/aa1 bk_">>

> <12/aa19700-12-eq50.png""list-style-tncy.

>

>

[NASA ADS]60;aother VLBI >

3/02/aa1 bk_">>

> <12/aa19700-12-eq50.png""list-style-tncy.

>

>

16192-105">16192-10tml/20>[NASA ADS]60;aother VLBI >

3/02/aa1 bk_">>

16192-105">16192-10tml/20>[CrossR r]60;aother VLBI >

3/02/aa1 bk_">>

16192-105">16192-10tml/20>[EDP S href= ]60;aother VLBI other VLBI F3.html" targZ3988013itlllowi"list-style-tncy.

> <12/aa19700-12-eq50.png""list-style-tncy.

>

>

>

3/02/aa1DEXTER">Ope0;with DEXTER005<

>

>

3/02/aa1DEXTER">Ope0;with DEXTER005<

>

>

3/02/aa1DEXTER">Ope0;with DEXTER005<

>

>

3/02/aa1DEXTER">Ope0;with DEXTER005<

>

CDS005< |

3/02/aa1_blank">>

EDP S href= <05<
>
ESO<05<) t t

>

EDP S href= <05<

>