BASS. XLV. Quantifying Active Galactic Nuclei Selection Effects in the Chandra COSMOS-legacy Survey with BASS
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Date
2025
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Publisher
IOP Publishing Ltd
Abstract
Deep extragalactic X-ray surveys, such as the Chandra COSMOS-Legacy field (CCLS), are prone to be biased against active galactic nuclei (AGN) with high column densities due to their lower count rates at a given luminosity. To quantify this selection effect, we forward model nearby (z similar to 0.05) AGN from the BAT AGN Spectroscopic Survey (BASS) with well-characterized (greater than or similar to 1000 cts) broadband X-ray spectra (0.5-195 keV) to simulate the CCLS absorption distribution. We utilize the BASS low-redshift analogs with similar luminosities to the CCLS (L-2-10keV(int) similar to 10(42-45) erg s), which are much less affected by obscuration and low-count statistics, as the seed for our simulations and follow the spectral fitting of the CCLS. Our simulations reveal that Chandra would fail to detect the majority (53.3%; 563/1056) of obscured (N-H >= 10(22) cm(-2)) simulated BASS AGN given the observed redshift and luminosity distribution of the CCLS. Even for detected sources with sufficient counts (>= 30) for spectral modeling, the level of obscuration is significantly overestimated. This bias is most extreme for objects whose best fit indicates a high-column density AGN (N-H >= 10(24) cm(-2)), since the majority (66.7%; 18/27) of these are actually unobscured sources (N-H < 10(22) cm(-2)). This implies that previous studies may have significantly overestimated the increase in the obscured fraction with redshift and the fraction of luminous obscured AGN. Our findings highlight the importance of directly considering obscuration biases and forward modeling in X-ray surveys, as well as the need for higher-sensitivity X-ray missions such as the Advanced X-ray Imaging Satellite (AXIS), and the importance of multiwavelength indicators to estimate obscuration in distant supermassive black holes.