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The most luminous blue quasars at 3.0<z<3.3. II. CIV/X-ray emission and accretion disc physics

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 نشر من قبل Elisabeta Lusso Dr
 تاريخ النشر 2021
  مجال البحث فيزياء
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We analyse the properties of the CIV broad emission line in connection with the X-ray emission of 30 bright SDSS quasars at z~3.0-3.3 with pointed XMM-Newton observations, which were selected to test the suitability of AGN as cosmological tools. In our previous work, we found that a large fraction (~25%) of the quasars in this sample are X-ray underluminous by factors of >3-10. As absorbing columns of >10$^{23}$ cm$^{-2}$ can be safely ruled out, their weakness is most likely intrinsic. Here we explore possible correlations between the UV and X-ray features of these sources to investigate the origin of X-ray weakness. We fit their UV SDSS spectra and analyse their CIV properties (e.g., equivalent width, EW; line peak velocity, $upsilon_{rm peak}$) as a function of the X-ray photon index and 2-10 keV flux. We confirm the trends of CIV $upsilon_{rm peak}$ and EW with UV luminosity at 2500 angstrom for both X-ray weak and X-ray normal quasars, as well as the correlation between X-ray weakness and CIV EW. In contrast to some recent work, we do not observe any clear relation between the 2-10 keV luminosity and $upsilon_{rm peak}$. We find a correlation between the hard X-ray flux and the integrated CIV flux for X-ray normal quasars, whilst X-ray weak quasars deviate from the main trend by more than 0.5 dex. We argue that X-ray weakness might be interpreted in a starved X-ray corona picture associated with an ongoing disc-wind phase. If the wind is ejected in the vicinity of the black hole, the extreme-UV radiation that reaches the corona will be depleted, depriving the corona of seeds photons and generating an X-ray weak quasar. Yet, at the largest UV luminosities (>10$^{47}$ erg s$^{-1}$), there will still be an ample reservoir of ionising photons that can explain the excess CIV emission observed in the X-ray weak quasars with respect to normal sources of similar X-ray luminosities.



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(abridged) We present the X-ray analysis of a sample of 30 luminous quasars at $zsimeq3.0-3.3$ with deep XMM-Newton observations, selected from the SDSS-DR7 to be representative of the most luminous, intrinsically blue quasar population. By construct ion, the sample boasts a unique degree of homogeneity in terms of optical/UV properties. In the X-rays, only four sources are too faint for a detailed spectral analysis. Neglecting a radio-loud object, the other 25 quasars are, as a whole, the most X-ray luminous ever observed, with rest-frame 2-10 keV luminosities of $0.5-7times10^{45}$ erg/s. The continuum photon index distribution, centred at $Gammasim1.85$, is in excellent agreement with those in place at lower redshift, luminosity and black-hole mass, confirming the universal nature of the X-ray emission mechanism in quasars. Even so, when compared against the well-known $L_{rm X}-L_{rm UV}$ correlation, our quasars unexpectedly split into two distinct subsets. About 2/3 of the sources are clustered around the relation with a minimal scatter of 0.1 dex, while the remaining 1/3 appear to be X-ray underluminous by factors of $>3-10$. Such a large incidence ($approx25%$) of X-ray weakness has never been reported in radio-quiet, non-BAL quasar samples. Several factors could contribute to enhance the X-ray weakness fraction among our $zsimeq3$ blue quasars. However, the X-ray weak objects also have, on average, flatter spectra, with no clear evidence of absorption. Indeed, column densities in excess of a few $times10^{22}$ cm$^{-2}$ can be ruled out for most of the sample. We suggest that, at least in some of our X-ray weak quasars, the corona might experience a radiatively inefficient phase due to the presence of a powerful accretion-disc wind, which substantially reduces the accretion rate through the inner disc and so the availability of seed photons for Compton up-scattering.
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