Gemini Near Infrared Spectrograph$-$Distant Quasar Survey: the Chandra View

TL;DR

This study uses Chandra X-ray observations of high-redshift quasars from the GNIRS-DQS to evaluate X-ray spectral properties and their relation to accretion rates, finding C IV parameters more indicative than X-ray spectral slope.

Contribution

It demonstrates that C IV emission-line parameters are better indicators of quasar accretion rates than X-ray spectral slope at high redshifts, with implications for understanding quasar physics.

Findings

C IV parameters may better indicate accretion rates up to z~3.5.

Average X-ray photon index $\Gamma$ aligns with high Eddington ratios.

$\alpha_{ox}$ does not improve accretion-rate estimates.

Abstract

We present Chandra observations of 63 sources from the Gemini Near Infrared Spectrograph$-$Distant Quasar Survey (GNIRS-DQS) of which 54 were targeted by snapshot observations in Cycle 24. A total of 55 sources are clearly detected in at least one X-ray band, and we set stringent upper limits on the X-ray fluxes of the remaining eight sources. In combination with rest-frame ultraviolet-optical spectroscopic data for these sources, we assess whether X-rays can provide a robust accretion-rate indicator for quasars, particularly at the highest accessible redshifts. We utilize a recently modified H$\beta$-based Eddington luminosity ratio estimator, as well as the C IV $\lambda$1549 emission-line parameter space to investigate trends and correlations with the optical-X-ray spectral slope ($\alpha_{\mathrm{ox}}$) and the effective hard-X-ray power-law photon index ($\Gamma$). We find that $\alpha_{\mathrm{ox}}$ does not improve current accretion-rate estimates based on H$\beta$ or C IV. Instead, within the limitations of our sample, we confirm previous findings that the C IV parameter space may be a better indicator of the accretion rate up to $z\sim3.5$. We also find that the average $\Gamma$ values for a small subset of our sources, as well as the average $\Gamma$ value in different groupings of our sources, are consistent with their respective relatively high Eddington luminosity ratios. Deeper X-ray observations of our X-ray-detected sources are needed for measuring $\Gamma$ accurately and testing whether this parameter can serve as a robust, un-biased accretion-rate diagnostic.