Kavli Affiliate: Claudio Ricci
| First 5 Authors: Yarone M. Tokayer, Michael J. Koss, C. Megan Urry, Priyamvada Natarajan, Richard Mushotzky
| Summary:
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 ($zsim0.05$) AGN from
the BAT AGN Spectroscopic Survey (BASS) with well-characterized ($gtrsim$1000
cts) broadband X-ray spectra (0.5-195 keV) to simulate the CCLS absorption
distribution. We utilize the BASS low-redshift analogs with similar
luminosities to the CCLS ($L_mathrm{2-10 keV}^mathrm{int}sim10^{42-45}
mathrm{erg} mathrm{s}^{-1}$), which are much less affected by obscuration
and low-count statistics, as the seed for our simulations, and follow the
spectral fitting of the CCLS. Our simulations reveal that Chandra would fail to
detect the majority (53.3%; 563/1056) of obscured ($N_mathrm{H}>10^{22}
mathrm{cm}^{-2}$) simulated BASS AGN given the observed redshift and
luminosity distribution of the CCLS. Even for detected sources with sufficient
counts ($geq30$) for spectral modeling, the level of obscuration is
significantly overestimated. This bias is most extreme for objects whose best
fit indicates a high-column density AGN ($N_mathrm{H}geq10^{24}
mathrm{cm}^{-2}$), since the majority (66.7%; 18/27) of these are actually
unobscured sources ($N_mathrm{H}<10^{22} mathrm{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 multi-wavelength indicators to estimate
obscuration in distant supermassive black holes.
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