Einstein Probe’s scientific opportunities in the field of active galactic nuclei

Active galactic nuclei (AGNs) are accreting supermassive black holes (SMBHs) that reside at the centers of galaxies and emit excessive electromagnetic radiation, whose ultimate energy source is the release of the gravitational potential energy of the accreted matter in the form of thermal and radiation energy. Multi-timescale and multi-band variability is a characteristic observational feature of AGNs. In particular, X-ray variability is most worth noting because of being intense and rapid, as well as carrying rich physical information of the innermost region of the accretion disk. Therefore, AGN X-ray variability has long been used as a probe for studying SMBHs. However, at present, we still lack a very fundamental understanding of AGN X-ray variability, whose origin and mechanisms remain largely unclear. The Einstein Probe (EP) satellite has an unprecedented soft X-ray survey capability, with a grasp (i.e., the product of detector effective area and field of view) being one to two orders of magnitude larger than previous similar satellites. EP is able to monitor hundreds of bright AGNs in the whole sky with sampling intervals ranging from 100 s to years, and will thereby build an exceptional AGN variability database. Accordingly, we propose to carry out AGN investigations mainly in the following four aspects: comprehensive measurement of the soft X-ray power spectral density for a large sample of bright AGNs; systematic monitoring and study of the rare large-amplitude AGN X-ray variability and flaring phenomenon; examination of long-term spectral variability for a large sample of AGNs and monitoring of their outbursts; and AGN/quasar survey. These unprecedented data will facilitate our further understanding of many scientific questions, such as the intense AGN X-ray variability and flaring phenomenon and its mechanisms, the physical conditions, structures, dynamics, and radiation processes of the AGN accretion disk, jet, and corona, as well as the AGN cosmological evolution. Furthermore, given EP’s capability of exploring a brand-new discovery space (large sky-area, long-timescale, and systematic soft X-ray monitoring), we are likely to discover some black hole accretion phenomena never seen before—the complexity and diversity of the Universe are always beyond human imagination.