Stellar transits in active galactic nuclei

TL;DR

This paper explores the potential for detecting stellar transits in active galactic nuclei using optical, UV, and X-ray surveys, which could reveal properties of black holes and their surrounding stellar environments.

Contribution

It models AGN transit lightcurves with relativistic effects and assesses the feasibility of detecting such transits in current and future surveys.

Findings

Red giant transits are most common around 10^6 solar mass SMBHs.

Detection requires repeated observations with high photometric accuracy.

Transit lightcurves can help image accretion disks and measure SMBH parameters.

Abstract

Supermassive black holes (SMBH) are typically surrounded by a dense stellar population in galactic nuclei. Stars crossing the line of site in active galactic nuclei (AGN) produce a characteristic transit lightcurve, just like extrasolar planets do when they transit their host star. We examine the possibility of finding such AGN transits in deep optical, UV, and X-ray surveys. We calculate transit lightcurves using the Novikov--Thorne thin accretion disk model, including general relatistic effects. Based on the expected properties of stellar cusps, we find that around 10^6 solar mass SMBHs, transits of red giants are most common for stars on close orbits with transit durations of a few weeks and orbital periods of a few years. We find that detecting AGN transits requires repeated observations of thousands of low mass AGNs to 1% photometric accuracy in optical, or ~ 10% in UV bands or soft X-ray. It may be possible to identify stellar transits in the Pan-STARRS and LSST optical and the eROSITA X-ray surveys. Such observations could be used to constrain black hole mass, spin, inclination and accretion rate. Transit rates and durations could give valuable information on the circumnuclear stellar clusters as well. Transit lightcurves could be used to image accretion disks with unprecedented resolution, allowing to resolve the SMBH silhouette in distant AGNs.