Echo mapping of the black hole accretion flow in NGC 7469

Abstract

Reverberation mapping (RM) can measure black hole accretion disc sizes and radial structure through observed time lags that should increase with wavelength as $\tau \propto {\lambda }^{4/3}$. Our 250-day RM campaign on NGC 7469 combines sub-day cadence 7-band photometry from the Las Cumbres Observatory robotic telescopes and weekly X-ray and UVOT data from Swift. By fitting these light curves, we measure the spectral energy distribution of the variable accretion disc and inter-band lags of just 1.5 days across the UV to the optical range. The disc SED is close to the expected $f_{\nu }\propto {\nu }^{1/3}$, and the lags are consistent with $\tau \propto {\lambda }^{4/3}$, but three times larger than expected. We consider several possible modifications to standard disc assumptions. First, for a $9\times 10^6$ M${}_{\odot }$ black hole and 2 possible spins $a^{\star }=(0,1)$, we fit the X-ray-UV-optical SED with a compact relativistic corona at height $H_x=(46,27)R_g$ irradiating a flat disc with accretion rate ${\dot{m}}_{Edd}\sim (0.23,0.24)$ inclined to the line of sight by $i<20^{\circ }$. To fit the lags as well as the SED, this model requires a low spin $a^{\star }=0$ and boosts disc color temperatures by a factor $f_{col}=1.8$, which shifts reprocessed light to shorter wavelengths. Our Bowl model with $f_{col}=1$ neglects relativity near the black hole but fits the UV-optical lags and SEDs using a flat disc with ${\dot{m}}_{Edd}<0.06$ and a steep outer rim at $R_{out}/c\sim 5-10$ days with H/R<1%. This rim occurs near the $10^3$K dust sublimation temperature in the disc atmosphere, supporting models that invoke dust opacity to thicken the disc and launch failed radiatively-driven dusty outflows at the inner edge of the broad line region (BLR). Finally, the disc lags and SEDs exhibit a significant excess in the $u$ and $r$ bands, suggesting the Balmer continuum and H$\alpha$ emission, respectively, from the BLR.

Accepted for publication in MNRAS