2025ApJ...995..185S

We present a reverberation mapping (RM) analysis of the coronal line [Ne V] λ3427-emitting region of the quasar COS168 (SDSS J095910.30+020732.2). [Ne V] λ3427 is known as one of the “coronal lines,” which are a species of emission lines present in active galactic nuclei (AGN) spectra with high ionization potentials (≥ 100 eV) that can serve as tracers for AGN activity. The spatial extent of the coronal line region has been studied with only spatial resolving techniques that are not sensitive to the innermost regions of AGN. Through our RM analysis of [Ne V] λ3427, we measure a nominal “optimal emission radius” for [Ne V] λ3427 of $381.{1}_{-22}^{+16}$ lt-day (observed frame). We place the coronal line region in context with other AGN regions by comparing it with the characteristic radius of Hα, the dust-sublimation radius, and the dusty torus. The coronal line region is located at a larger radius from the black hole than the characteristic radius of the dusty torus, measured using a torus-radius luminosity relationship. The virial product (v²R/G) of both Hα and [Ne V] λ3427 is consistent within the uncertainties, implying that the coronal line region, as probed by the [Ne V] λ3427 line, may be in a virialized orbit that is dominated by the gravitational potential of the black hole. This plausibly suggests that coronal lines could be an effective method for estimating black hole masses.