The SDSS-V Black Hole Mapper Reverberation Mapping Project: Light Echoes of the Coronal Line Region in a Luminous Quasar

TL;DR

This work applies reverberation mapping to the coronal line [Ne V] 3427 in the luminous quasar COS168 to localize the coronal-line region with a rest-frame lag of $281.7^{+12}_{-16}$ days, placing it beyond the broad-line region and near the dusty torus. Using RM with PyROA and cross-checks from JAVELIN against a ZTF-driven continuum, the authors find that the coronal-line region is compact (pc-scale) yet larger than the BLR, and its velocity structure is broadly virialized, yielding virial products consistent with the broad-line gas. The results imply that coronal lines can serve as BH mass indicators and motivate expanding RM to a broader AGN sample to test universality and calibrate the virial factor $f$ for coronal gas. Overall, the paper demonstrates the feasibility and value of using coronal-line RM to probe inner AGN dynamics and black hole masses.

Abstract

We present a reverberation mapping analysis of the coronal line [Ne V]$λ$3427 emitting region of the quasar COS168 (SDSS J095910.30+020732.2). [Ne V]$λ$3427 is known as one of the "coronal lines," which are a species of emission lines present in AGN spectra with high ionization potentials ($\geq$ 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 reverberation mapping analysis of [Ne V]$λ$3427, we measure a nominal `optimal emission radius' for [Ne V]$λ$3427 of $381.1^{+16}_{-22}$ light days (observed-frame). We place the coronal line region in context with other AGN regions by comparing it with the characteristic radius of H$α$, 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 ($v^2 R/G$) of both H$α$ and [Ne V]$λ$3427 is consistent within the uncertainties, implying that the coronal line region, as probed by the [Ne V]$λ$3427 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.

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