Space Telescope and Optical Reverberation Mapping Project. II. Swift and HST Reverberation Mapping of the Accretion Disk of NGC 5548

TL;DR

This study presents the densest UV/optical monitoring of NGC 5548, revealing accretion disk sizes larger than predicted and providing insights into the relationship between UV/optical and X-ray variability in AGNs.

Contribution

It provides the first detailed measurement of interband lags in NGC 5548 using intensive Swift and HST data, confirming the a a wavelength dependence and revealing larger-than-expected disk sizes.

Findings

UV/optical lags follow a a wavelength dependence

Accretion disk size estimated at 0.35 a 0.05 lt-day at 1367 A

Weak correlation between UV/optical and X-ray variability

Abstract

Recent intensive Swift monitoring of the Seyfert 1 galaxy NGC 5548 yielded 282 usable epochs over 125 days across six UV/optical bands and the X-rays. This is the densest extended AGN UV/optical continuum sampling ever obtained, with a mean sampling rate <0.5 day. Approximately daily HST UV sampling was also obtained. The UV/optical light curves show strong correlations (r_max = 0.57 - 0.90) and the clearest measurement to date of interband lags. These lags are well-fit by a \tau propto \lambda^4/3 wavelength dependence, with a normalization that indicates an unexpectedly large disk radius of 0.35 +/- 0.05 lt-day at 1367 A, assuming a simple face-on model. The U-band shows a marginally larger lag than expected from the fit and surrounding bands, which could be due to Balmer continuum emission from the broad-line region as suggested by Korista and Goad. The UV/X-ray correlation is weaker (r_max < 0.45) and less consistent over time. This indicates that while Swift is beginning to measure UV/optical lags in general agreement with accretion disk theory (although the derived size is larger than predicted), the relationship with X-ray variability is less well understood. Combining this accretion disk size estimate with those from quasar microlensing studies suggests that AGN disk sizes scale approximately linearly with central black hole mass over a wide range of masses.