Abstract

Contributed Talk - Splinter SMBHs

Thursday, 10 September 2026, 16:33   (MW-0250)

The X-ray-Weak Tail of the Quasar Population: Optical-variability vs X-ray selection in eFEDS

P. Arévalo(1), J. Buchner(2), M. Salvato(2), P. Sánchez-Sáez(4) and collaborators
1) U. Valparaiso, Chile, 2) MPE, 3) ESO Garching,

As we enter the era of the Vera Rubin Observatory, leveraging the time domain is essential for a complete census of supermassive black hole accretion. While X-ray surveys like eROSITA provide a very powerful window into the AGN population, detecting even weak AGN and obscured accretion, they still do not capture the full diversity of the population. In this talk, I will present a new selection of approximately 300,000 AGN candidates in the 8.000 deg2 searched, identified through a novel combination of ZTF difference-image photometry and the ALeRCE machine learning classifier (Arevalo et al. 2006, AA 705, 247). By cross-matching this variability-selected sample with the eROSITA eFEDS field and utilizing DESI spectroscopy, we demonstrate a significant synergy—and a striking divergence—between optical variability and X-ray selection. We find that 25% of spectroscopically confirmed, variability-selected AGN lack X-ray counterparts, even in deep, homogenous X-ray data. These X-ray-undetected sources are not random; they exhibit a strong redshift dependence and host over 90% of the Broad Absorption Line (BAL) quasars in our variability-selected sample. Furthermore, these sources show systematically weaker broad emission lines compared to their X-ray-detected counterparts, despite having identical narrow-line equivalent widths. These results suggest that variability- based selections are not merely a complement to X-ray surveys but are vital for capturing specific evolutionary phases and populations that X-rays might miss.