Reverberation in the Narrow Fe K${\alpha}$ Line in the Seyfert Galaxy NGC 4151 with XRISM

Abstract

Emission lines that "echo" variations in the ionizing flux produced close to black holes are powerful probes of the central engine. In the Seyfert-1.5 galaxy NGC 4151, high-resolution X-ray spectra and time lags in low-resolution X-ray data suggest that part of the narrow Fe K$_{\alpha}$ line originates close to the optical broad line region (BLR). We report on a sequence of nine XRISM observations of NGC 4151, obtained every other day in 2024. Swift monitoring was undertaken to sample the driving flux before, during, and after the XRISM sequence. Using suitable line kernels, we measure a mean BLR component width of $\sigma = 5.36\pm 0.48\times 10^{3}~{\rm km}~ {\rm s}^{-1}$. Modeling the Swift continuum and XRISM line flux trends gives a lag of $\tau = 3.5^{+2.8}_{-1.7}$ days ($r=3.6^{+3.0}_{-1.7}\times 10^{3}~GM/c^{2}$ for $M_{BH} = 1.7\times 10^{7}~M_{\odot}$), significant at the $2\sigma$ level via Monte Carlo simulations, and consistent with prior measurements and direct spectral fits. This lag implies a black hole mass of $M_{BH}/f_{X} = 2.0^{+1.4}_{-1.0}\times 10^{7} M_{\odot}$, where $f_{X}$ is a geometrical factor. A standard optical value for this factor gives a mass that is nominally higher than typical H$\beta$ mass estimates, but formally consistent. Our results suggest that XRISM can measure lags and black hole masses in both unobscured and obscured AGN.