Accretion disk reverberation with Hubble Space Telescope observations of NGC 4593: evidence for diffuse continuum lags

Understand

The Seyfert 1 galaxy NGC 4593 was monitored spectroscopically with the Hubble Space Telescope as part of a reverberation mapping campaign that also included Swift, Kepler and ground-based photometric monitoring.

• During 2016 July 12 - August 6, we obtained 26 spectra across a nearly continuous wavelength range of ~1150 - 10,000A.
• These were combined with Swift data to produce a UV/optical "lag spectrum", which shows the interband lag relative to the Swift UVW2 band as a function of wavelength.
• The broad shape of the lag spectrum appears to follow the $\tau \propto \lambda^{4/3}$ relation seen previously in photometric interband lag measurements of other active galactic nuclei (AGN).