Studying the dust distribution around accreting black holes with reverberation mapping using PRIMA

Variability studies are a powerful tool for studying the structures of unresolved sources. One such type of variability study, called reverberation mapping (RM), established that the dominant source of infrared radiation from an active galactic nucleus (AGN) was from dust absorption and re-emission, which demonstrated that the optical brightening and fading of light from the accretion disk (AD) around a supermassive black hole was followed by a corresponding (delayed) variation at infrared (IR) wavelengths from the surrounding dust distribution. Since that time, a great deal more has been learned about the dust distribution around ADs, both near the AD in the form of a potential torus, as well as extended emission in the form of polar dust outflows. Understanding the dust distribution is vital to understanding how AGNs affect their host galaxy as well as the overall energetics of AGNs as ∼50% of energy from an AGN comes out in the IR. Dust RM has been done exclusively in the near-IR (1 to 5 μm), which traces the inner edge of the dust near the dust sublimation radius. Hence, extending RM to the mid-IR, especially to the peak of the dust emission between 25 and 30 μm, allows for an examination of the dust distribution around ADs and potentially traces the source of the polar outflow. RM with the proposed PRobe far-Infrared Mission for Astrophysics (PRIMA) can focus on variability-monitoring of a sample of low- to high-luminosity AGNs to trace the 25 to 30 μm emission that is reverberated from the UV/optical AD emission, which will be monitored by other space and ground-based observatories and made available to PRIMA users. Then, by detailed modeling of the response of the dust emission to shorter wavelengths, the distribution of the dust around an AGN will be revealed and can be linked to the accretion disk luminosities.