Speaker
Description
The core of active galactic nuclei (AGN) may be surrounded by numerous clouds in diverse physical states. These clumpy clouds can randomly obscure the central X-ray source, leading to eclipse events. Recent studies indicated that such events are likely common in AGN. However, the complete eclipse events are rarely discovered, because they require extensive X-ray monitoring spanning years. In this study, we conduct a systematic search of eclipse events in 40 AGNs with a total of 11486 observations by Swift X-ray telescope (XRT). Our selection is based on unusual variation of X-ray flux and hardness ratio. We discover 3 eclipse events in 3 sources, as well as 7 candidate events in 5 sources, all of which are identified in type I AGN. Compared to pervious studies, 5 events are newly discovered. The ionization parameter (logξ) derived by fitting the average spectrum during the eclipse time ranging from -0.547 to 2.443 with the column densities spanning 0.16 - 31.15 x 10^22 cm^-2. The timescales of these events vary from a few days to years. We estimate the distance of these events to the central black hole, ranging from 0.31 - 107.79 x 10^4 Rg. 6 out of 10 events have cloud locations consistent with the dust sublimation zone (DSZ) while 2 events have clouds outside the DSZ, and the remaining 2 events resided at broad line regions (BLR). The X-ray all-sky monitoring of Einstein Probe (EP) may provide a much stronger constraints on the rate of such events in the local universe.
