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Department of Physics

# Staff profile

## Publication details for Prof David Alexander

Civano, F., Hickox, R.C., Puccetti, S., Comastri, A., Mullaney, J.R., Zappacosta, L., LaMassa, S.M., Aird, J., Alexander, D.M., Ballantyne, D.R., Bauer, F.E., Brandt, W.N., Boggs, S.E., Christensen, F.E., Craig, W.W., Del-Moro, A., Elvis, M., Forster, K., Gandhi, P., Grefenstette, B.W., Hailey, C.J., Harrison, F.A., Lansbury, G.B., Luo, B., Madsen, K., Saez, C., Stern, D., Treister, E., Urry, M.C., Wik, D.R. & Zhang, W. (2015). The Nustar Extragalactic Surveys: Overview and Catalog from the COSMOS Field. The Astrophysical Journal 808(2): 185.

### Abstract

To provide the census of the sources contributing to the X-ray background peak above 10 keV, Nuclear Spectroscopic Telescope Array (NuSTAR) is performing extragalactic surveys using a three-tier "wedding cake" approach. We present the NuSTAR survey of the COSMOS field, the medium sensitivity, and medium area tier, covering 1.7 deg2 and overlapping with both Chandra and XMM-Newton data. This survey consists of 121 observations for a total exposure of ~3 Ms. To fully exploit these data, we developed a new detection strategy, carefully tested through extensive simulations. The survey sensitivity at 20% completeness is 5.9, 2.9, and 6.4 × 10−14 $\mathrm{erg}\;{\mathrm{cm}}^{-2}\;{{\rm{s}}}^{-1}$ in the 3–24, 3–8, and 8–24 keV bands, respectively. By combining detections in 3 bands, we have a sample of 91 NuSTAR sources with 1042–1045.5 $\mathrm{erg}\;{{\rm{s}}}^{-1}$ luminosities and redshift z = 0.04–2.5. Thirty-two sources are detected in the 8–24 keV band with fluxes ~100 times fainter than sources detected by Swift-BAT. Of the 91 detections, all but 4 are associated with a Chandra and/or XMM-Newton point-like counterpart. One source is associated with an extended lower energy X-ray source. We present the X-ray (hardness ratio and luminosity) and optical-to-X-ray properties. The observed fraction of candidate Compton-thick active galactic nuclei measured from the hardness ratio is between 13%–20%. We discuss the spectral properties of NuSTAR J100259+0220.6 (ID 330) at z = 0.044, with the highest hardness ratio in the entire sample. The measured column density exceeds 1024 cm−2, implying the source is Compton-thick. This source was not previously recognized as such without the >10 keV data.