What controls the UV-to-X-ray continuum shape in quasars?

Timlin, John; Brandt, W. N.; Laor, Ari

doi:10.1093/mnras/stab1217

Cited by 20 publications

(9 citation statements)

References 91 publications

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“…A more suitable line in this sense is the He ii λ1640 line. The He ii EW values of the z 3 quasars are in the range ∼0.04−0.90 Å, consistent with the expectations from the Baldwin effect of the He ii line at the UV luminosities of our sample (see the recent analysis by Timlin et al 2021;Rankine et al 2020). We were not able to achieve a good fit to the He ii line for 7 quasars (4 N, 2 W and 1 w) as the line is significantly blended with the C iv red wing and the Fe ii UV complex.…”

Section: Discussionsupporting

confidence: 90%

“…In X-ray-weak sources, the integrated C iv flux presumably suffers from the inefficient line excitation due to the scarcity of hot electrons, rather than the limited production of C iv due to the lack of ionising photons (see also Timlin et al 2021). In normal quasars of comparable X-ray luminosity, but lower L UV , the flux of ionising photons is smaller, hence the C iv emission line is fainter.…”

Section: Discussionmentioning

confidence: 99%

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The most luminous blue quasars at 3.0 < z < 3.3

Lusso

Nardini

Bisogni

et al. 2021

A&A

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We analyse the properties of the high-ionisation C IVλ1549 broad emission line in connection with the X-ray emission of 30 bright, optically selected quasars at z ≃ 3.0−3.3 with pointed XMM-Newton observations, which were selected to test the suitability of active galactic nuclei as cosmological tools. In our previous work, we found that a large fraction (≈25%) of the quasars in this sample are X-ray under-luminous by factors of > 3−10. As absorbing columns of ≳1023 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 with respect to X-ray-normal quasars at similar redshifts. We fit the UV spectra from the Sloan Digital Sky Survey of the quasars in our sample and analyse their C IV properties – for example equivalent width (EW) and line peak velocity (υpeak) – as a function of the X-ray photon index and 2−10 keV flux. We confirm the statistically significant trends of C IVυpeak and EW with UV luminosity at 2500 Å for both X-ray-weak and X-ray-normal quasars, as well as the correlation between X-ray weakness (parametrised through Δαox) and C IV EW. In contrast to some recent work, we do not observe any clear relation between the 2−10 keV luminosity and υpeak. We find a statistically significant correlation between the hard X-ray flux and the integrated C IV flux for X-ray-normal quasars, which extends across more than three (two) decades in C IV (X-ray) luminosity, 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 seed photons and generating an X-ray-weak quasar. Nonetheless, at the largest UV luminosities (> 1047 erg s−1) there will still be an ample reservoir of ionising photons that can explain the ‘excess’ C IV emission observed in the X-ray-weak quasars with respect to normal sources of similar X-ray luminosities.

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Section: Discussionsupporting

confidence: 90%

Section: Discussionmentioning

confidence: 99%

The most luminous blue quasars at 3.0 < z < 3.3

Lusso

Nardini

Bisogni

et al. 2021

A&A

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“…Comparison with Luo et al (2015) shows the same α ox values as those calculated in this work, and the exclusion of these sources significantly improves the correlation to r s = 0.41 and p = 4.3 × 10 −3 . The α ox -EW(C IV) correlation also seems to hold in almost any subsample (see Table 3), notwithstanding effects due to changing sample size, which supports the results of Timlin et al (2021) that α ox is expected to be correlated with EW(C IV) as an indicator of the shape of the spectral energy distribution. To see if the L/L Edd parameter contributes to this correlation, Figure 4 (c) shows larger symbols that correspond to larger values of L/L Edd ; however, no trend with L/L Edd has been found.…”

Section: Resultssupporting

confidence: 81%

Can X-Ray Observations Improve Optical-UV-based Accretion-rate Estimates for Quasars?

Marlar

Shemmer

Brotherton

et al. 2022

ApJ

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Current estimates of the normalized accretion rates of quasars (L/L Edd) rely on measuring the velocity widths of broad optical-UV emission lines (e.g., Hβ and Mg ii λ2800). However, such lines tend to be weak or inaccessible in the most distant quasars, leading to increasing uncertainty in L/L Edd estimates at z > 6. Utilizing a carefully selected sample of 53 radio-quiet quasars that have Hβ and C iv λ1549 spectroscopy as well as Chandra coverage, we searched for a robust accretion-rate indicator for quasars, particularly at the highest-accessible redshifts (z ∼ 6–7). Our analysis explored relationships between the Hβ-based L/L Edd, the equivalent width (EW) of C iv, and the optical-to-X-ray spectral slope (α ox). Our results show that EW(C iv) is the strongest indicator of the Hβ-based L/L Edd parameter, consistent with previous studies, although significant scatter persists particularly for sources with weak C iv lines. We do not find evidence for the α ox parameter improving this relation, and we do not find a significant correlation between α ox and Hβ-based L/L Edd. This absence of an improved relationship may reveal a limitation of our sample. X-ray observations of additional luminous sources, found at z ≳ 1, may allow us to mitigate the biases inherent in our archival sample and test whether X-ray data could improve L/L Edd estimates. Furthermore, deeper X-ray observations of our sources may provide accurate measurements of the hard-X-ray power-law photon index (Γ), which is considered an unbiased L/L Edd indicator. Correlations between EW(C iv) and α ox with a Γ-based L/L Edd may yield a more robust prediction of a quasar normalized accretion rate.

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“…Comparison with Luo et al (2015) shows the same α ox values as those calculated in this work, and exclusion of these sources significantly improves the correlation to r s = 0.41 and p = 4.3 × 10 −3 . The α ox -EW(C IV) correlation also seems to hold in almost any subsample (see Table 3), notwithstanding effects due to changing sample size, which supports the results of Timlin et al (2021) that α ox is expected to be correlated with EW(C IV) as an indicator of the shape of the spectral energy distribution. To see if the L/L Edd parameter contributes to this correlation, Figure 4 (c) shows larger symbols that correspond to larger values of L/L Edd ; however no trend with L/L Edd has been found.…”

Section: Resultssupporting

confidence: 81%

Can X-ray Observations Improve Optical-UV-based Accretion-Rate Estimates for Quasars?

Marlar,

Shemmer,

Brotherton

et al. 2022

Preprint

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Current estimates of the normalized accretion rates of quasars (L/L Edd ), rely on measuring the velocity widths of broad optical-UV emission lines (e.g., Hβ and Mg II λ 2800). However, such lines tend to be weak or inaccessible in the most distant quasars, leading to increasing uncertainty in L/L Edd estimates at z > 6. Utilizing a carefully selected sample of 53 radio-quiet quasars that have Hβ and C IV λ 1549 spectroscopy as well as Chandra coverage, we searched for a robust accretion-rate indicator for quasars, particularly at the highest-accessible redshifts (z ∼ 6 − 7). Our analysis explored relationships between the Hβ -based L/L Edd , the equivalent width (EW) of C IV, and the optical-to-X-ray spectral slope (α ox ). Our results show that EW(C IV) is the strongest indicator of the Hβ -based L/L Edd parameter, consistent with previous studies, although significant scatter persists particularly for sources with weak C IV lines. We do not find evidence for the α ox parameter improving this relation, and we do not find a significant correlation between α ox and Hβ -based L/L Edd . This absence of an improved relationship may reveal a limitation in our sample. X-ray observations of additional luminous sources, found at z 1, may allow us to mitigate the biases inherent in our archival sample and test whether X-ray data could improve L/L Edd estimates. Furthermore, deeper X-ray observations of our sources may provide accurate measurements of the hard-X-ray power-law photon index (Γ), which is considered an unbiased L/L Edd indicator. Correlations between EW(C IV) and α ox with Γ-based L/L Edd may yield a more robust prediction of a quasar normalized accretion rate.

show abstract

What controls the UV-to-X-ray continuum shape in quasars?

Cited by 20 publications

References 91 publications

The most luminous blue quasars at 3.0 < z < 3.3

The most luminous blue quasars at 3.0 < z < 3.3

Can X-Ray Observations Improve Optical-UV-based Accretion-rate Estimates for Quasars?

Can X-ray Observations Improve Optical-UV-based Accretion-Rate Estimates for Quasars?

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