TY - JOUR
T1 - The X-ray properties of z > 6 quasars
T2 - No evident evolution of accretion physics in the first Gyr of the Universe
AU - Vito, F.
AU - Brandt, W. N.
AU - Bauer, F. E.
AU - Calura, F.
AU - Gilli, R.
AU - Luo, B.
AU - Shemmer, O.
AU - Vignali, C.
AU - Zamorani, G.
AU - Brusa, M.
AU - Civano, F.
AU - Comastri, A.
AU - Nanni, R.
N1 - Funding Information:
FV acknowledges financial support from CONICYT and CASSACA through the Fourth call for tenders of the CAS-CONICYT Fund, CONICYT grants Basal-CATA AFB-170002 (FV, FEB), the Ministry of Economy, Development, and Tourism's Millennium Science Initiative through grant IC120009, awarded to The Millennium Institute of Astrophysics, MAS (FEB). WNB acknowledges support from CXC grant G08-19076X and NASA ADP grant 80NSSC18K0878. BL acknowledges financial support from the National Key R&D Program of China grant 2016YFA0400702 and National Natural Science Foundation of China grant 11673010. We acknowledge financial contribution from the agreement ASI-INAF n.2017-14-H.O.
Funding Information:
Acknowledgements. We thank the anonymous referee for useful feedback. We thank L. Jiang for providing the near-IR spectrum of SDSSJ2310+1855, A. Moretti for his help in reducing Swift data, C. Willott for useful discussions, E. Picconcelli and S. Martocchia for providing bolometric luminosities of the WISSH QSOs, and F. Duras for providing their functional form of the bolometric correction curve. FV acknowledges financial support from CONICYT and CASSACA through the Fourth call for tenders of the CAS-CONICYT Fund, CONICYT grants Basal-CATA AFB-170002 (FV, FEB), the Ministry of Economy, Development, and Tourism’s Millennium Science Initiative through grant IC120009, awarded to The Millennium Institute of Astrophysics, MAS (FEB). WNB acknowledges support from CXC grant G08–19076X and NASA ADP grant 80NSSC18K0878. BL acknowledges financial support from the National Key R&D Program of China grant 2016YFA0400702 and National Natural Science Foundation of China grant 11673010. We acknowledge financial contribution from the agreement ASI-INAF n.2017-14-H.O.
Publisher Copyright:
© ESO 2019.
PY - 2019/10/1
Y1 - 2019/10/1
N2 - Context. X-ray emission from quasars (QSOs) has been used to assess supermassive black hole accretion properties up to z 6. However, at z > 6 only 15 QSOs are covered by sensitive X-ray observations, preventing a statistically significant investigation of the X-ray properties of the QSO population in the first Gyr of the Universe. Aims. We present new Chandra observations of a sample of 10 z > 6 QSOs, selected to have virial black-hole mass estimates from Mg II line spectroscopy & (log M BH M 8.5-9.6). Adding archival X-ray data for an additional 15 z > 6 QSOs, we investigate the X-ray properties of the QSO population in the first Gyr of the Universe. In particular, we focus on the LUV - LX relation, which is traced by the αox parameter, and the shape of their X-ray spectra. Methods. We performed photometric analyses to derive estimates of the X-ray luminosities of our z > 6 QSOs, and thus their αox values and bolometric corrections (Kbol = Lbol/LX). We compared the resulting αox and Kbol distributions with the results found for QSO samples at lower redshift, and ran several statistical tests to check for a possible evolution of the LUV - LX relation. Finally, we performed a basic X-ray spectral analysis of the brightest z > 6 QSOs to derive their individual photon indices, and joint spectral analysis of the whole sample to estimate the average photon index. Results. We detect seven of the new Chandra targets in at least one standard energy band, while two more are detected discarding energies E > 5 keV, where background dominates. We confirm a lack of significant evolution of αox with redshift, which extends the results from previous works up to z > 6 with a statistically significant QSO sample. Furthermore, we confirm the trend of an increasing bolometric correction with increasing luminosity found for QSOs at lower redshifts. The average power-law photon index of our sample (( Γ»= 2.20-0.34+0.39 and ( Γ»= 2.13-0.13+0.13 for sources with < 30 and > 30 net counts, respectively) is slightly steeper than, but still consistent with, typical QSOs at z = 1 - 6. Conclusions. All of these results indicate a lack of substantial evolution of the inner accretion-disk and hot-corona structure in QSOs from low redshift to z > 6. Our data hint at generally high Eddington ratios at z > 6.
AB - Context. X-ray emission from quasars (QSOs) has been used to assess supermassive black hole accretion properties up to z 6. However, at z > 6 only 15 QSOs are covered by sensitive X-ray observations, preventing a statistically significant investigation of the X-ray properties of the QSO population in the first Gyr of the Universe. Aims. We present new Chandra observations of a sample of 10 z > 6 QSOs, selected to have virial black-hole mass estimates from Mg II line spectroscopy & (log M BH M 8.5-9.6). Adding archival X-ray data for an additional 15 z > 6 QSOs, we investigate the X-ray properties of the QSO population in the first Gyr of the Universe. In particular, we focus on the LUV - LX relation, which is traced by the αox parameter, and the shape of their X-ray spectra. Methods. We performed photometric analyses to derive estimates of the X-ray luminosities of our z > 6 QSOs, and thus their αox values and bolometric corrections (Kbol = Lbol/LX). We compared the resulting αox and Kbol distributions with the results found for QSO samples at lower redshift, and ran several statistical tests to check for a possible evolution of the LUV - LX relation. Finally, we performed a basic X-ray spectral analysis of the brightest z > 6 QSOs to derive their individual photon indices, and joint spectral analysis of the whole sample to estimate the average photon index. Results. We detect seven of the new Chandra targets in at least one standard energy band, while two more are detected discarding energies E > 5 keV, where background dominates. We confirm a lack of significant evolution of αox with redshift, which extends the results from previous works up to z > 6 with a statistically significant QSO sample. Furthermore, we confirm the trend of an increasing bolometric correction with increasing luminosity found for QSOs at lower redshifts. The average power-law photon index of our sample (( Γ»= 2.20-0.34+0.39 and ( Γ»= 2.13-0.13+0.13 for sources with < 30 and > 30 net counts, respectively) is slightly steeper than, but still consistent with, typical QSOs at z = 1 - 6. Conclusions. All of these results indicate a lack of substantial evolution of the inner accretion-disk and hot-corona structure in QSOs from low redshift to z > 6. Our data hint at generally high Eddington ratios at z > 6.
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U2 - 10.1051/0004-6361/201936217
DO - 10.1051/0004-6361/201936217
M3 - Article
AN - SCOPUS:85073252518
VL - 630
JO - Astronomy and Astrophysics
JF - Astronomy and Astrophysics
SN - 0004-6361
M1 - A118
ER -