SIMBAD references

The nuclear regions of active galactic nuclei (AGNs) likely host clumpy clouds that occasionally obscure the central X-ray source, causing eclipse events. These events offer a unique opportunity to study the properties and origins of such clouds. However, these transient events are rarely reported due to the need for extensive, long-term X-ray monitoring for years. Here, we conduct a systematic search for eclipse events in 40 AGNs well monitored by the Swift X-ray Telescope over the past 20 yr, comprising a total of ∼11,000 observations. Our selection criteria rely on significant variations in X-ray flux and spectral shape. We identify three high-confidence events in three AGNs and eight candidates in six AGNs, all in Type I AGNs. The observed clouds have column densities of N_H ∼ (0.2-31.2) × 10²² cm^–2 and ionization degrees of log ξ ∼ (-1.3-2.2). For the five events with well-constrained duration, their distances from the central black hole range from (2.4 to 179) × 10⁴R_g, with two clouds near the dust sublimation zone, two farther out. Interestingly, we find tentative correlations between the cloud properties (i.e., ionization state and column density) and the black hole mass and mass-accretion rate, implying their strong connection to the accretion process, potentially via outflows. Our study also demonstrates the potential of the new X-ray all-sky monitor, the Einstein Probe, in providing more detection and physical constraints for such events.