tokens: panel ). North is on the left-hand side of each plot. The core has been bypassed by both slices. Table 1. Flux densities of the components of 3C 328. Based on the outcome of the two-frequency VLA observations, the inverted-spectrum core must be self-absorbed, and hence very compact. Its VLBA images resulting from the observations mentioned in Sect. 2 are shown in Fig. 2 . At 5 GHz, the core of 3C 328 is point-like, while at 1.5 GHz it has an additional feature elongated southwards. As the same phase reference was used throughout those observations (J1606+3124), the absolute positions of the source at both frequencies were preserved. The core visible at 5 GHz is co-incident with the brightest part of the source in the 1.5 GHz image, and thus this image depicts a core-jet structure where the jet points to the southern lobe. This means that, apart from the lobe asymmetry, there is another similarity that 3C 328 shares with J1211+743, 3C 249.1, and 3C 334: All these AGNs have jets that point towards the lobe that is devoid of a hotspot. This raises the question of whether the light-travel-time-based model elaborated on in Paper II for J1211+743 and then used to interpret 3C 249.1 and 3C 334 in Paper III could also be applied to 3C 328. To find that out, one has to perform a quantitative test using that model since it imposes tight constraints on both the lower and upper limits of the length of the period of quiescence, t q . Fig. 2. VLBA images of 3C 328 at 1.5 GHz ( left-hand panel ) and at 5 GHz ( right-hand panel ). In both images, contours are at 0.1, 0.2, 0.4, 0.8, 1.6, 3.2, 6.4, and 12.8 mJy beam −1 . The projected angular far-side and near-side arm lengths of 3C 328 ( and ) respectively translate to 212 kpc and 232 kpc of the projected linear size, whereas the projected angular size of the jet () translates to 209 pc of the projected linear size. For these linear sizes, the lower () and upper () limits of t q were calculated using Eqs. (2) and (4) of Paper II for β adv = 0.35, three values of β jet (0.8, 0.9, and 1), and five values of the angle between the line of sight and the AGN axis θ . Since 3C 328 is a galaxy and not a QSO, only the values θ ≥ 45° are applicable to it ( Barthel 1989 ). The results are shown in Table 2 . Due to the small size of the jet in 3C 328, the calculated limits are almost independent from the adopted value of β adv . The definitions of β jet and β adv are provided in Paper II. Table 2. Lower and upper limits to the length of the quiescent period. 4. Conclusion Taking into account that for at least two restarted AGNs – 3C 293 ( Machalski et al. 2016 ) and J1835+6204 ( Konar et al. 2012 ) – the values of t q also stay within the limits shown in Table 2 , it is plausible that the above limits for t q may be valid for 3C 328, and as such the light-travel-time-based scenario can be used to explain its nature. It follows that the activity of 3C 328 has restarted and, given the minute size of its jet, that
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