A Recollimation Shock in a Stationary Jet Feature with Limb-brightening in the Gamma-Ray-emitting Narrow-line Seyfert 1 Galaxy 1H 0323+342

Pc-scale jet structures were modeled using a series of discrete components. Figure 1(a) shows a stacked image made from images from all eight epochs. Figure 1(b) shows the jet components regarding the positions and deconvolved sizes, overlaid with the stacked image. There is an emission gap in the jet at a radial distance of z ∼ 3–5 mas from the radio core; the intensity subsequently rises in the downstream at z ∼ 7 mas. This panel indicates that the evolution of component sizes experiences a local minimum near the local maximum of intensity in the jet.

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Figure 2(a) illustrates the component size profiles along jet axis. The jet initially expands in a parabolic shape after launched from the core. The jet becomes narrowed at a downstream. The local minimum is located at z = 7.40 ± 0.17 mas from the core (d = 0.39 ± 0.13 mas in size), which is the average and standard deviation of measurements for one or two local minimums at each epoch. We call this location the "convergence site." The jet appears to expand again at the downstream of the convergence site.

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The jet-width profile over a wide distance range for 1H 0323+342 is separately reported by Hada et al. (2018), which also confirms a locally narrowed jet based on multi-frequency (1.4–43 GHz).

Figure 2(b) exhibits slice profiles of intensity along the jet axis of PA = 1250, on the basis of images regenerated with a common restored beam of 0.75 mas, which is the geometric mean of the major and minor axes of original beam sizes. We found that the intensity maximum in the jet is placed at z = 6.97 ± 0.17 mas from the core, which is the average and standard deviation of the peak positions at all epochs. The location of the convergence site is slightly shifted from that of the intensity peak in the jet feature.

It has been pointed out that this jet feature is stationary for a corresponding component; Wajima et al. (2014) reported a proper motion of μ_app = −0.115 ± 0.083 mas yr⁻¹ during ∼15 year. Figure 3 shows the historical locations, including the measurements in our analyses for an additional eight epochs in MOJAVE data. We derived a proper motion of μ_app =0.00 ± 0.01 mas yr⁻¹ and a historical position of 7.0 ±0.1 mas for ∼18 year. We found some degree of wobble in distance position during the MOJAVE period. Hence, we call this feature "quasi-stationary feature S."

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We recognize the existence of another potential stationary component very close to the core, z ∼ 0.3 mas, as previously pointed out by Wajima et al. (2014) and Fuhrmann et al. (2016).

Figure 4(b) is the plot for the positions of components in distance from the core. The identifications of a series of components and proper motions are similar to those in previous studies using the same MOJAVE data (Fuhrmann et al. 2016; Lister et al. 2016). The component C4 has been evidently identified as a superluminal component with the highest apparent speed in this source. A model with a constant apparent speed fits the measurement points quite well, with χ²/ndf = 5.112/6 and a probability of p = 0.53, where ndf is the number of degrees of freedom.

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Figure 2(c) shows the evolutions of C4 regarding its flux density and angular size. This component initially expanded as it became dark, then turned to flare up as it approached the convergence site along the intensity profile of the quasi-stationary feature S. The other components also showed similar evolution (not shown). This behavior indicates how the part of the jet is maintained as a quasi-stationary feature.

Figure 5 shows the CLEAN image on the epoch of 2011 July 15, which was convolved with a circular restoring beam with 0.56 mas that was equivalent to the minor-axis size of the original synthesized beam. We found limb-brightening intensity profiles at both the upstream and downstream with respect to the intensity peak and the convergence site in the quasi-stationary feature S. We performed a pixel-based analysis by slicing in transverse directions of the jet at z = 6 and z = 9 on the image; a double-peaked slice profile is evident in both cases. Similar structures were also apparent at upstream at four epochs and/or downstream at six epochs among the eight epochs.

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These characteristic jet structures in 1H 0323+342 are reminiscent of the M87 jets in pc scales and the HST-1 complex at a deprojected distance of 120 pc. The inner radio jet of M87 showing a limb-brightened structure (e.g., Kovalev et al. 2007). Asada & Nakamura (2012) discovered a jet structure locally convergent at the location of HST-1, which is the transitional boundary of a jet acceleration region with a parabolically expanding streamline and a deceleration region with a conical expansion (e.g., Nakamura & Asada 2013; Asada et al. 2014). 1H 0323+342 also shows a parabolic shape in the inner jet and a hyperbolic/conical shape at larger scales (Hada et al. 2018). The location of the upstream end in the HST-1 complex in M87 has been stable to within ∼2 mas at 860 mas from the core (Cheung et al. 2007). Although there has been no report on the limb-brightened structure near the HST-1 complex, superluminal ejections from HST-1 showed a variation in the position angle (Giroletti et al. 2012), which may be because the downstream of HST-1 has a wide opening angle. It is proposed that HST-1 is the result of a recollimation shock, a stationary shocked region resulting from overcollimation in the jet, which explains the broadband flaring activity including TeV γ-ray emission (Stawarz et al. 2006; Cheung et al. 2007).

We propose that the combination of the observed limb-brightened structure and the convergence site in the quasi-stationary feature in the 1H 0323+342 jet can be explained in the framework of the recollimation shock. We consider that the limb-brightening at the upstream of the convergence site is possibly a radiation layer of shocked materials flowing between a contact discontinuity that separates from the external medium and the recollimation shock front. The low-brightness inner layer is considered to be an unshocked spine flow. Such a picture has been demonstrated by Bromberg & Levinson (2009) and Bodo & Tavecchio (2018) to explain the activity in a small region located at a considerable distance from the central engine in several blazars and M87. The shape of the post-shocked dissipation layer after the recollimation shock is very similar to the limb-brightening at the upstream of the convergence site in 1H 0323+342. Bromberg & Levinson (2009) and Bodo & Tavecchio (2018) also revealed that significant pressure loss caused by radiative cooling can lead to the jet being focused in a tiny cross-sectional radius by external pressure. This picture explain well the fact that the intensity peak and convergence site are adjacent to one another, as observed in 1H 0323+342. The limb-brightening at the downstream in the 1H 0323+342 jet can be caused by emission from the combination of a shocked outer layer with a relatively high Γ and inner layer strongly decelerated through a reflected shock, as shown in Bodo & Tavecchio (2018).

We also discuss possible correlations between γ-ray activities and passing events of the superluminal component through the intensity peak of the quasi-stationary feature S and the convergence site. This is the possible third finding in the present study (Figure 4(b)). The most striking event is that the component C4 reached the intensity peak of S during the known GeV γ-ray active phase F2 (MJD = 56470–56500; Paliya et al. 2014). Furthermore, the subsequent γ-ray active phase F3 (MJD = 56531–56535) coincides with the component C4's arrival at the convergence site. The estimated dates of passing through the two critical sites are listed in Table 1.

Table 1 also presents corresponding dates for components C3 and C5. The date of C3 crossing the convergence site was coincident with the previous γ-ray active phase F1 (MJD = 56,262–56,310). The proper motion of C5 is expected to cross the two critical sites at MJD ∼ 56900–57200. At this period the Fermi LAT daily and weekly quick-look light curves¹⁰ (not shown) show γ-ray recurrence in the duration of MJD ∼ 57000–57300. As far as these data are concerned, there are apparent associations between γ-ray emission and passage events of jet components. The recollimation site in the quasi-stationary feature is a possible candidate for observed γ-ray emission in 1H 0323+342.

X-ray spectral modeling suggested a dominance by nonthermal jetted (inverse-Compton) emission during both F2 and F3, whereas in the quiescent (and F1) phase there is a significant dominance by thermal corona (Paliya et al. 2014). Given the hour-scale light curve in F3 and its estimated γ-ray luminosity, a good fraction of the total kinetic energy of the jet was converted into radiation (Paliya et al. 2014). They proposed that the dissipation sites for all of these γ-ray phases are located ∼100R_S (∼0.002 pc) from the nucleus, with region sizes of ∼0.0005 pc based on the one-zone broadband spectral modeling. On the other hand, the convergence site has a cross-sectional diameter of ∼0.4 mas (∼0.3 pc). Some compact substructures confined to the narrowing cross section are required to reconcile the γ-ray productions at the quasi-stationary feature.

C4 showed a progressively increasing flux density by a factor of three (Figure 2(c)). This indicates how the quasi-stationary feature keeps stationary, i.e., as a result of the aggregation of passing components that brighten as they approach the convergence site. However, these flux densities were, at most, at the mJy level. On the other hand, radio flares seen in the F-GAMMA light curves based on single-dish monitoring were on the order of 1 Jy (Angelakis et al. 2015). We examined the γ-ray/radio correlation on spatially resolved light curves using the MOJAVE images (Figure 4(b), a similar plot was also presented by Fuhrmann et al. 2016). However, we found no clear evidence of γ-ray/radio correlation, either in the core region or the quasi-stationary feature. These inconclusive results may be due to the MOJAVE's sparse sampling (≥50 days) compared to the flare's timescales (≲30 days) in the total radio flux (Angelakis et al. 2015). Searching for component emergence events for both the core and the stationary feature helps to identify the γ-ray emitting site (A. Doi et al. 2018, in preparation). The superluminal ejections from the stationary location in the HST-1 complex were observed during the 2005 TeV γ-ray event in M87 (Cheung et al. 2007; Giroletti et al. 2012).