HYPerluminous quasars at the Epoch of ReionizatION (HYPERION). A new regime for the X-ray nuclear properties of the first quasars
Journal
Date Issued
2023
Author(s)
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Saccheo, I.
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Vignali, C.
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Volonteri, M.
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Done, C.
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Elvis, M.
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La Franca, F.
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Laurenti, M.
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Miniutti, G.
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Brusa, M.
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Civano, F.
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Carniani, S.
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Gallerani, S.
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Guainazzi, M.
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Marinucci, A.
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Middei, R.
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Puccetti, S.
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Haardt, F.
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Maiolino, R.
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Tripodi, R.
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Abstract
The existence of luminous quasars (QSO) at the Epoch of Reionization (EoR;
i.e. z>6) powered by supermassive black holes (SMBH) with masses
$\gtrsim10^9~M_\odot$ challenges models of early SMBH formation. To shed light
on the nature of these sources we started a multiwavelength programme based on
a sample of 18 HYPerluminous quasars at the Epoch of ReionizatION (HYPERION).
These are the luminous QSOs whose SMBH must have had the fastest mass growth
during the Universe first Gyr. In this paper we present the HYPERION sample and
report on the first of the 3 years planned observations of the 2.4 Ms
XMM-Newton Multi-Year Heritage program on which HYPERION is based. The goal of
this program is to accurately characterize the X-ray nuclear properties of QSOs
at the EoR. Through a joint X-ray spectral analysis of 10 sources, in the
rest-frame $\sim2-50$ keV range, we report a steep average photon index
($\Gamma\sim2.4\pm0.1$). Absorption is not required. The average $\Gamma$ is
inconsistent at $\geq4\sigma$ level with the canonical 1.8-2 value measured in
QSO at z<6. This spectral slope is also much steeper than that reported in
lower-z QSOs with similar luminosity or accretion rate, thus suggesting a
genuine redshift evolution. Alternatively, we can interpret this result as the
presence of an unusually low-energy cutoff $E_{cut}\sim20$ keV on a standard
$\Gamma=1.9$ power-law. We also report on mild indications that HYPERION QSOs
show higher soft X-ray emission at 2 keV compared to the UV one at 2500A than
expected by lower-z luminous AGN. We speculate that a redshift-dependent
coupling between the corona and accretion disc or intrinsically different
coronal properties may account for the steep spectral slopes, especially in the
presence of powerful winds. The reported slopes, if confirmed at lower
luminosities, may have an important impact on future X-ray AGN studies in the
early Universe.
Volume
678
Start page
A201
Issn Identifier
0004-6361
Rights
open.access
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