Probing the accretion disk in the Seyfert 1 NGC 4593
Abstract
More than forty years since nearby AGN were suggested to be powered by accretion onto supermassive black holes we still do not have strong observational constraints on the structure of AGN accretion disks. However, wavelength-dependent continuum reverberation provides a unique probe of the disk. Hot, inner parts of the accretion disk respond to irradiating, variable, X-ray flux before the cooler, outer parts, simply due to light crossing time. Hence, time lags between UV (hot) and optical (cool) lightcurves are observed. But, for the best-studied source, NGC 5548, the measured lags appear anomalously large for its mass and accretion rate. Either our understanding of AGN accretion disks is wrong, or, the continuum is significantly contaminated by more slowly varying components arising at larger radii. Thus, further high quality lag measurements are needed in order to address this. The bright, nearby AGN NGC 4593 will be in the Kepler field in July - September 2016, providing a remarkably high-cadence (every 30 min) and high S/N (0.1% photometry) broadband continuum lightcurve. Here, we are proposing a 1-month, daily monitoring campaign of NGC 4593 with HST/STIS spectroscopy to measure wavelength-dependent lags. We will utilize the unprecedented quality of the Kepler lightcurve as a reference band to obtain high-fidelity lag measurements in order to probe the accretion disk in this object. As a secondary goal we will also obtain lightcurves from strong optical and UV emission lines to probe the stratification of the broad-line region.
- Publication:
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HST Proposal
- Pub Date:
- October 2015
- Bibcode:
- 2015hst..prop14121C