摘要：Context. Radio galaxies classified as X-shaped/winged, are characterised
by two pairs of extended and misaligned lobes, which suggest a rapid realignment of the
jet axis, for which a potential cause (including binary supermassive black holes, a black
hole merger, or a Lense-Thirring precession) is still under debate.
Aims. Here we analyse the complex radio structure of 3C 293 winged
source hosted by the post-merger galaxy UGC 8782, which uniquely displays a significant
asymmetry between the sizes (and therefore the ages) of the two pairs of lobes, indicating
that an episode of jet realignment took place only very recently. This allows us to
tightly constrain the corresponding timescales, and therefore to discriminate between
different models proposed for the formation of X-shaped radio galaxies in general.
Methods. Based on all the available and carefully re-analysed radio data
for 3C 293, we have performed a detailed spectral modelling for the older and younger
lobes in the system, using the existing evolutionary DYNAGE algorithm. In this way we
derived the lobes’ ages and jet energetics, which we then compared to the accretion power
in the source.
Results. We found that the 200 kpc-scale outer lobes of 3C 293 are ~ 60 Myr old and, until very recently,
have been supplied with fresh electrons and magnetic field by the jets, i.e., jet activity
related to the formation of the outer lobes ceased within the last Myr. Meanwhile, the
inner 4 kpc-scale lobes, tilted by ~
40° with respect to the outer ones, are only about ~ 0.3 Myr old. Interestingly, the best
model fits also return identical values of the jet power supplying the outer and the inner
structures. This power, moreover, is of the order of the maximum kinetic luminosity of a
Blandford-Znajek jet for a given black hole mass and accretion rate, but only in the case
of relatively low values of a black hole spin, a ~ 0.2.
Conclusions. The derived jet energetics and timescales, along with the
presence of two optical nuclei in UGC 8782, all provide a strong support to the
Lense-Thirring precession model in which the supermassive black hole spin, and therefore
the jet axis, flips rapidly owing to the interactions with the tilted accretion disk in a
new tidal interaction episode of the merging process. We further speculate that, in
general, X-shape radio morphology forms in post-merger systems that are rich in cold
molecular gas, and only host slowly spinning supermassive black holes.
