First reverberation mapping of a Bowen fluorescence line

Abstract

Reverberation mapping (RM) is a powerful tool to determine the extent, structure, and kinematics of the broad-line region (BLR) of active galactic nuclei (AGN). So far, RM of the BLR has only been performed for recombination lines responding to the varying ionizing continuum. We tested whether OI 8446, attributed to Bowen fluorescence driven by Ly$\beta$ pumping, varied on short (day- to week-long) timescales during a 2016 HST/STIS campaign of NGC 4593, and examined how it relates to other emission lines and the ionizing UV continuum. We quantified the variability of OI 8446 by calculating its rms amplitude. We then extracted integrated light curves of OI 8446 and other UV and optical emission lines, and compared them with each other and with the UV continuum light curve using correlation analyses. In addition, we used archival near-infrared spectra to assess the dominant excitation mechanism of OI 8446. We detect, for the first time, variability in OI 8446 on day timescales. The fractional rms amplitude is $\sim 4$% over the 4-week campaign. The OI 8446 light curve reverberates with a delay of $\sim 2.5$ days relative to Ly$\alpha$, used as a proxy for Ly$\beta$, detected at a false-alarm probability of 0.6% (significance of $\sim 2.8\sigma$) under our adopted null hypothesis. It closely tracks H$\alpha$ with only a minor additional delay of $\sim0.3$ days, placing its emission region at essentially the same distance as the Balmer-line weighted BLR. Line ratios indicate that Ly$\beta$ pumping is the dominant excitation mechanism for O I 8446. Our results establish OI 8446 as the first Bowen-fluorescence line reverberation-mapped, responding directly to variations in Ly$\beta$ flux. We propose that in future campaigns targeting AGN with larger BLRs, O I could enable dual-driver RM using both the continuum and the pumping line as drivers.