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The NuSTAR Extragalactic Survey: Average Broadband X-Ray Spectral Properties of the NuSTAR-detected AGNs

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journal contribution
posted on 2018-08-16, 08:28 authored by A. Del Moro, D. M. Alexander, J. A. Aird, F. E. Bauer, F. Civano, J. R. Mullaney, D. R. Ballantyne, W. N. Brandt, A. Comastri, P. Gandhi, F. A. Harrison, G. B. Lansbury, L. Lanz, B. Luo, S. Marchesi, S. Puccetti, C. Ricci, C. Saez, D. Stern, E. Treister, L. Zappacosta
We present a study of the average X-ray spectral properties of the sources detected by the NuSTAR extragalactic survey, comprising observations of the Extended Chandra Deep Field South (E-CDFS), Extended Groth Strip (EGS), and the Cosmic Evolution Survey (COSMOS). The sample includes 182 NuSTAR sources (64 detected at 8–24 keV), with 3–24 keV fluxes ranging between f(3-24 keV) ≈ 10^-14 – and 6 × 10^−13 erg cm^−2 s^−1 (f(8–24 keV) ≈ 3 x 10^-14 - 3 x 10^-13 erg cm^−2 s^−1) and redshifts in the range of z = 0.04 - 3.21. We produce composite spectra from the Chandra + NuSTAR data (E ≈ 2- 40 keV, rest frame) for all the sources with redshift identifications (95%) and investigate the intrinsic, average spectra of the sources, divided into broad-line (BL) and narrow-line (NL) active galactic nuclei (AGNs), and also in different bins of X-ray column density and luminosity. The average power-law photon index for the whole sample is Γ = 1.65^+0.03,-3.21 flatter than the Γ ≈ 1.8 typically found for AGNs. While the spectral slope of BL and X-ray unabsorbed AGNs is consistent with the typical values (Γ = 1.79^+0.01,-0.01) a significant flattening is seen in NL AGNs and heavily absorbed sources (Γ = 1.60^+0.08,-0.05 and 1.38^+0.12,-0.12, respectively), likely due to the effect of absorption and to the contribution from the Compton reflection component to the high-energy flux (E > 10 keV). We find that the typical reflection fraction in our spectra is R ≈ 0.5 (for Γ = 1.8), with a tentative indication of an increase of the reflection strength with X-ray column density. While there is no significant evidence for a dependence of the photon index on X-ray luminosity in our sample, we find that R decreases with luminosity, with relatively high levels of reflection (R ≈ 1.2) for L(10-40 keV) < 10^44 erg s^−1 and R ≈ 0.3 for L(10-40 keV) > 10^44 erg s^−1 AGNs, assuming a fixed spectral slope of Γ = 1.8.


A.D.M. thanks the financial support from the Max Plank Society and the UK Science and Technology Facilities Council (STFC, ST/ L00075X/1, A.D.M. and D.M.A.; ST/K501979/1, G.B.L.). F.E.B. and E.T. acknowledge support from CONICYT-Chile (Basal-CATA PFB-06/2007, “EMBIGGEN” Anillo ACT1101, FONDECYT Regular 1141218 [F.E.B.] and 1160999 [E.T.]); F.E.B. also thanks the Ministry of Economy, Development, and Tourism’s Millennium Science Initiative through grant IC120009, awarded to the Millennium Institute of Astrophysics (MAS). P.G. thanks the STFC for support (ST/J003697/2). W.N.B. acknowledges support from the Caltech NuSTAR subcontract 44A-1092750 and NASA ADP grant NNX10AC99G. A.C. and L.Z. acknowledge support from the ASI/INAF grant I/037/12/0 011/13. This work was supported under NASA contract no. NNG08FD60C and made use of data from the NuSTAR mission, a project led by the California Institute of Technology, managed by the Jet Propulsion Laboratory, and funded by the National Aeronautics and Space Administration. We thank the NuSTAR Operations, Software, and Calibration teams for support with the execution and analysis of these observations. This research has made use of the NuSTAR Data Analysis Software (NuSTARDAS) jointly developed by the ASI Science Data Center (ASDC, Italy) and the California Institute of Technology (USA).



Astrophysical Journal, 2017, 849, pp. 57-57

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/Organisation/COLLEGE OF SCIENCE AND ENGINEERING/Department of Physics and Astronomy


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