We identify sources with extremely hard X-ray spectra (i.e., with photon indices of G<=0.6) in the 13 deg2 NuSTAR serendipitous survey, to search for the most highly obscured active galactic nuclei (AGNs) detected at >10 keV. Eight extreme NuSTAR sources are identified, and we use the NuSTAR data in combination with lower-energy X-ray observations (from Chandra, Swift XRT, and XMM-Newton) to characterize the broadband (0.5-24 keV) X-ray spectra. We find that all of the extreme sources are highly obscured AGNs, including three robust Compton-thick (CT; NH>1.5 1E24 cm-2) AGNs at low redshift (z<=0.1) and a likely CT AGN at higher redshift (z = 0.16). Most of the extreme sources would not have been identified as highly obscured based on the low-energy (<10 keV) X-ray coverage alone. The multiwavelength properties (e.g., optical spectra and X-ray-mid-IR luminosity ratios) provide further support for the eight sources being significantly obscured. Correcting for absorption, the intrinsic rest-frame 10-40 keV luminosities of the extreme sources cover a broad range, from 5E42 to 1045 erg s-1. The estimated number counts of CT AGNs in the NuSTAR serendipitous survey are in broad agreement with model expectations based on previous X-ray surveys, except for the lowest redshifts (z<0.07), where we measure a high CT fraction of fCT obs = 30% -12+16 . For the small sample of CT AGNs, we find a high fraction of galaxy major mergers (50%+-33%) compared to control samples of "normal" AGNs.
The NuSTAR Serendipitous Survey: Hunting for the Most Extreme Obscured AGN at >10 keV / Lansbury G.B.; Alexander D.M.; Aird J.; Gandhi P.; Stern D.; Koss M.; Lamperti I.; Ajello M.; Annuar A.; Assef R.J.; Ballantyne D.R.; Balokovic M.; Bauer F.E.; Brandt W.N.; Brightman M.; Chen C.-T.J.; Civano F.; Comastri A.; Moro A.D.; Fuentes C.; Harrison F.A.; Marchesi S.; Masini A.; Mullaney J.R.; Ricci C.; Saez C.; Tomsick J.A.; Treister E.; Walton D.J.; Zappacosta L.. - In: THE ASTROPHYSICAL JOURNAL. - ISSN 0004-637X. - ELETTRONICO. - 846:1(2017), pp. 20.1-20.19. [10.3847/1538-4357/aa8176]
The NuSTAR Serendipitous Survey: Hunting for the Most Extreme Obscured AGN at >10 keV
Marchesi S.Writing – Review & Editing
;
2017
Abstract
We identify sources with extremely hard X-ray spectra (i.e., with photon indices of G<=0.6) in the 13 deg2 NuSTAR serendipitous survey, to search for the most highly obscured active galactic nuclei (AGNs) detected at >10 keV. Eight extreme NuSTAR sources are identified, and we use the NuSTAR data in combination with lower-energy X-ray observations (from Chandra, Swift XRT, and XMM-Newton) to characterize the broadband (0.5-24 keV) X-ray spectra. We find that all of the extreme sources are highly obscured AGNs, including three robust Compton-thick (CT; NH>1.5 1E24 cm-2) AGNs at low redshift (z<=0.1) and a likely CT AGN at higher redshift (z = 0.16). Most of the extreme sources would not have been identified as highly obscured based on the low-energy (<10 keV) X-ray coverage alone. The multiwavelength properties (e.g., optical spectra and X-ray-mid-IR luminosity ratios) provide further support for the eight sources being significantly obscured. Correcting for absorption, the intrinsic rest-frame 10-40 keV luminosities of the extreme sources cover a broad range, from 5E42 to 1045 erg s-1. The estimated number counts of CT AGNs in the NuSTAR serendipitous survey are in broad agreement with model expectations based on previous X-ray surveys, except for the lowest redshifts (z<0.07), where we measure a high CT fraction of fCT obs = 30% -12+16 . For the small sample of CT AGNs, we find a high fraction of galaxy major mergers (50%+-33%) compared to control samples of "normal" AGNs.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
