UV/optical active galactic nuclei reverberation mapping: using a SmallSat to trade time resolution for spatial resolution to study accretion disks around supermassive black holes at the centers of galaxies

The energy generated by the accretion disks (ADs) around supermassive black holes (SMBHs) at the centers of galaxies plays a critical role in the evolution of their host galaxy. Recent studies have shown that theoretical models for AD sizes do not match indirect observations of disks. Unfortunately, direct observation of AD around SMBHs is not possible, but we can exchange time resolution for spatial resolution to study the radius-to-temperature relation for these disks and derive their true sizes. Reverberation mapping (RM) provides the best close-in probe of the structure of the SMBH environment. RM measures the time delay among fluctuations in the short wavelength continuum arising from close to the SMBH (likely from its corona) and the subsequent longer wavelength response of the AD continuum and gas in the broad line region (BLR). However, RM is a telescope-intensive process requiring regular observations over a long period, but due to the scarcity of observing time on ultraviolet (UV)-capable facilities, very few active galactic nuclei (AGNs) have had this type of intensive multiwavelength monitoring. To solve this problem, we have proposed a space mission dedicated to AGN RM: the Black Hole Accretion and Growth Experiment with Reverberation Analysis, which is equipped with a 20-cm telescope coupled to a far UV/optical (1400 to 6000 Å) low-resolution spectrograph. By monitoring changes from the far UV to the optical, we trace the changes from close to the SMBH all the way out to the optical emitting region, and we can distinguish between continuum emission from the disk and continuum emission from elsewhere (e.g., potentially from the BLR). By observing 10 nearby AGNs, we can sample the diversity of AGN AD properties over a range of SMBH masses and accretion rates to gain insight into the fundamental physics of accretion disks and how they ultimately connect to their host galaxies.