Resolving the complex line-locking structure in quasar J004613+010425 with VLT/X-shooter spectroscopy

Line-locking (LL) signatures in quasar absorption-line systems provide direct observational evidence of radiatively driven outflows. We present a detailed investigation of LL phenomena in the quasar SDSS J004613.54+010425.7 (z_ ̊m em = 2.150) using medium-resolution VLT/X-shooter spectra, as part of the TOLL project. Compared with previous studies based on Sloan Digital Sky Survey spectroscopy, the X-shooter data resolve the absorption complex into 12 narrow absorption line components spanning ∼2700 ̨ms in the ̧iv, and łya transitions, a factor of ∼2.4 more components than identified from lower-resolution spectra. We detect 14 distinct LL pairs across three ionic species, which form one of the largest LL webs known to date: three pairs connecting components (2, 10), (4, 11), and (5, 12); six pairs organized into four groups; and five ̧iv pairs organized into three groups. Monte Carlo simulations show that the probability of this 12-component LL web arising by chance is $< 10^ -5 $. Photoionization modeling with ļoudy yields ionization parameters of łog U ≈ -2.4 to -0.6 and total hydrogen column densities of łog N( ̊m H ) ≈ 18.4--20.6 for the 12 components. A comparison of the properties of shadowed and shadowing clouds shows that ion column densities and ionization parameters typically agree within a factor of ∼3, broadly supporting the standard radiative acceleration picture, although several outliers suggest that multi-ion locking and cloud-cloud interactions also regulate LL stability. These results demonstrate that medium- and high-resolution spectroscopy is essential for resolving the full complexity of line-locked quasar outflows.