Science with a small two-band UV-photometry mission III: Active Galactic Nuclei and nuclear transients

TL;DR

This paper reviews the potential of a small two-band UV space telescope to study active galactic nuclei, nuclear transients, and peculiar sources, emphasizing its role in understanding black hole accretion and transient phenomena.

Contribution

It proposes observational strategies and scientific goals for a small UV mission targeting SMBHs, TDEs, and peculiar AGN sources, highlighting its unique capabilities.

Findings

High-cadence UV monitoring can distinguish TDE emission scenarios.

The mission can study variability in rare transients like partial TDEs.

It enables analysis of non-standard accretion flows in AGN.

Abstract

In this review (the third in the series focused on a small two-band UV-photometry mission), we assess possibilities for a small UV two-band photometry mission in studying accreting supermassive black holes (SMBHs; mass range $\sim 10^6$-$10^{10}\,M_{\odot}$). We focus on the following observational concepts: (i) dedicated monitoring of selected type-I Active Galactic Nuclei (AGN) in order to measure the time delay between the far-UV, the near-UV, and other wavebands (X-ray and optical), (ii) nuclear transients including (partial) tidal disruption events and repetitive nuclear transients, and (iii) the study of peculiar sources, such as changing-look AGN, hollows and gaps in accretion disks, low-luminosity AGN, and candidates for Intermediate-Mass Black Holes (IMBHs; mass range $\sim 10^2$-$10^5\,M_{\odot}$) in galactic nuclei. For tidal disruption events (TDEs), high-cadence UV monitoring is crucial for distinguishing among different scenarios for the origin of the UV emission. The small two-band UV space telescope will also provide the information about the near- and far-UV continuum variability for rare transients, such as repetitive partial TDEs and jetted TDEs. We also discuss the possibilities to study and analyze sources with non-standard accretion flows, such as AGN with gappy disks, low-luminosity active galactic nuclei with intermittent accretion, and SMBH binaries potentially involving intermediate-mass black holes.