Space Telescope and Optical Reverberation Mapping Project. Iii. Optical Continuum Emission and Broadband Time Delays in NGC 5548

Abstract

We present ground-based optical photometric monitoring data for NGC 5548, part of an extended multiwavelength reverberation mapping campaign. The light curves have nearly daily cadence from 2014 January to July in nine filters (BVRI and ugriz). Combined with ultraviolet data from the Hubble Space Telescope and Swift, we confirm significant time delays between the continuum bands as a function of wavelength, extending the wavelength coverage from 1158 Å to the z band (∼ 9160 Å). We find that the lags at wavelengths longer than the V band are equal to or greater than the lags of high-ionization-state emission lines (such as He II λ1640 and λ4686), suggesting that the continuum-emitting source is of a physical size comparable to the inner broad-line region (BLR). The trend of lag with wavelength is broadly consistent with the prediction for continuum reprocessing by an accretion disk with τ ∝ λ 4/3 . However, the lags also imply a disk radius that is 3 times larger than the prediction from standard thin-disk theory, assuming that the bolometric luminosity is 10% of the Eddington luminosity (L = 0.1L Edd ). Using optical spectra from the Large Binocular Telescope, we estimate the bias of the interband continuum lags due to BLR emission observed in the filters. We find that the bias for filters with high levels of BLR contamination (∼ 20%) can be important for the shortest continuum lags, and likely has a significant impact on the u and U bands owing to Balmer continuum emission.

Measuring the Innermost Stable Circular Orbits of Supermassive Black Holes

“…General relativistic (radiation) magnetohydrodynamical simulations indicate that the analytical equations describe accretion disks reasonably accurately (Noble et al 2011;Kulkarni et al 2011;Penna et al 2012;Sadowski 2016). However, optical/UV observations of microlensing events in quasars indicate that accretion disks are larger than predicted by thin disk theory (Morgan et al 2010), and similar discrepancies are found using measurements of continuum lags in the nearby Seyferts NGC2617 (Shappee et al 2014) and NGC5548 (Edelson et al 2015;Fausnaugh et al 2016).…”

Space Telescope and Optical Reverberation Mapping Project. V. Optical Spectroscopic Campaign and Emission-line Analysis for NGC 5548

“…However, wavelengths shorter than ∼3200Å are inaccessible from the ground, so the rest-frame optical continuum is often used as a proxy for the ionizing source in low-redshift AGNs. Although the far-UV and optical continua have been shown to vary almost simultaneously in some cases (e.g., Clavel et al 1991;Reichert et al 1994;Korista et al 1995;Wanders et al 1997), more recent high-cadence studies have found that the optical continuum can lag the UV continuum by up to a few days (Collier et al 1998;Sergeev et al 2005;McHardy et al 2014;Shappee et al 2014;Edelson et al 2015;Fausnaugh et al 2016). This can significantly affect the measured broadline lag if the BLR has a characteristic radius on the order of light-days.…”

“…This assumption implies that the disk size can be neglected when determining R BLR from RM data. However, Fausnaugh et al (2016) have shown that the optically emitting portion of the accretion disk has a lag similar to that of the inner portion of the BLR. If we assume a model in which the measured lags are purely dependent on the radial distance from the ionizing source, then the emission-line lags measured using the optical continuum may significantly underestimate the BLR characteristic radius.…”

“…This can significantly affect the measured broadline lag if the BLR has a characteristic radius on the order of light-days. The variable optical continuum has also been shown to have smoother features and smaller amplitudes than its UV counterpart (e.g., Peterson et al 1991;Dietrich et al 1993Dietrich et al , 1998Stirpe et al 1994;Santos-Lleó et al 1997;Shappee et al 2014;Fausnaugh et al 2016). These differences between the UV and optical continua suggest that the optical continuum is not fully interchangeable with the ionizing source for determining reverberation lags.…”

“…Concurrent with the HST program were 4 months of Swift observations and 6 months of ground-based photometric and spectroscopic observations. First results of the HST, Swift, and ground-based photometry programs were presented by De Rosa et al (2015), Edelson et al (2015), and Fausnaugh et al (2016) (Papers I-III, respectively). Goad et al (2016) (Paper IV) explore the anomalous behavior of the UV continuum and broad emission-line light curves observed during a portion of this campaign.…”