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Department of Physics

# Staff profile

## Publication details for Prof David Alexander

Koss, Michael J., Assef, R., Baloković, M., Stern, D., Gandhi, P., Lamperti, I., Alexander, D. M., Ballantyne, D. R., Bauer, F. E., Berney, S., Brandt, W. N., Comastri, A., Gehrels, N., Harrison, F. A., Lansbury, G., Markwardt, C., Ricci, C., Rivers, E., Schawinski, K., Trakhtenbrot, B., Treister, E. & Urry, C. Megan (2016). A New Population of Compton-Thick AGN Identified Using the Spectral Curvature Above 10 keV. The Astrophysical Journal 825(2): 85.

### Abstract

We present a new metric that uses the spectral curvature (SC) above 10 keV to identify Compton-thick active galactic nuclei (AGNs) in low-quality Swift/Burst Alert Telescope (BAT) X-ray data. Using NuSTAR, we observe nine high SC-selected AGNs. We find that high-sensitivity spectra show that the majority are Compton-thick (78% or 7/9) and the remaining two are nearly Compton-thick (N H sime (5–8) × 1023 cm−2 ). We find that the SC BAT and SC NuSTAR measurements are consistent, suggesting that this technique can be applied to future telescopes. We tested the SC method on well-known Compton-thick AGNs and found that it is much more effective than broadband ratios (e.g., 100% using SC versus 20% using 8–24 keV/3–8 keV). Our results suggest that using the >10 keV emission may be the only way to identify this population since only two sources show Compton-thick levels of excess in the Balmer decrement corrected [O iii] to observed X-ray emission ratio (${F}_{[{\rm{O}}{\rm{III}}]}/{F}_{2\mbox{--}10\;\;\mathrm{keV}}^{\mathrm{obs}}\gt 1$) and WISE colors do not identify most of them as AGNs. Based on this small sample, we find that a higher fraction of these AGNs are in the final merger stage (<10 kpc) than typical BAT AGNs. Additionally, these nine obscured AGNs have, on average, ≈4× higher accretion rates than other BAT-detected AGNs ($\langle {\lambda }_{\mathrm{Edd}}\rangle \;=\;0.068\pm 0.023$ compared to $\langle {\lambda }_{\mathrm{Edd}}\rangle \;=\;0.016\pm 0.004$). The robustness of SC at identifying Compton-thick AGNs implies that a higher fraction of nearby AGNs may be Compton-thick (≈22%) and the sum of black hole growth in Compton-thick AGNs (Eddington ratio times population percentage) is nearly as large as mildly obscured and unobscured AGNs.