The Realm of the First Quasars in the Universe: the X-ray View

TL;DR

This paper reviews X-ray observations of high-redshift quasars, highlighting their similar properties to local quasars and suggesting that their small-scale X-ray emission regions are unaffected by cosmic evolution.

Contribution

It provides a comprehensive review of X-ray properties of z>4 quasars, emphasizing the consistency across cosmic time and the implications for AGN emission regions.

Findings

X-ray properties of high-redshift quasars are similar to local ones

Small-scale X-ray emission regions are insensitive to cosmic evolution

Supports the universality of AGN X-ray emission mechanisms

Abstract

We review the X-ray studies of the highest redshift quasars, focusing on the results obtained with Chandra and XMM-Newton. Overall, the X-ray and broad-band properties of z>4 quasars and local quasars are similar, suggesting that the small-scale X-ray emission regions of AGN are insensitive to the significant changes occurring at z=0-6.

Figures

a), the number of X-ray detected AGN at $z>4$ has increased to more than 110 (Fig. 1b), mostly thanks to exploratory observations with *Chandra* (e.g., Vignali et al. 2001, 2005; Brandt et al. 2002; Bassett et al. 2004; Lopez et al. 2006; Shemmer et al. 2006a) and longer exposures with XMM-*Newton* (e.g., Shemmer et al. 2005). At the very faint X-ray fluxes, X-ray surveys have provided detection of several $z>4$ AGN and quasars (e.g., Schneider et al. 1998; Silverman et al. 2002; Vignali et al. 2002).

a), the number of X-ray detected AGN at $z>4$ has increased to more than 110 (Fig. 1b), mostly thanks to exploratory observations with *Chandra* (e.g., Vignali et al. 2001, 2005; Brandt et al. 2002; Bassett et al. 2004; Lopez et al. 2006; Shemmer et al. 2006a) and longer exposures with XMM-*Newton* (e.g., Shemmer et al. 2005). At the very faint X-ray fluxes, X-ray surveys have provided detection of several $z>4$ AGN and quasars (e.g., Schneider et al. 1998; Silverman et al. 2002; Vignali et al. 2002).

Full text

(1) Dipartimento di Astronomia, Università degli Studi di Bologna, Italy; (2) Department of Astronomy & Astrophysics, Pennsylvania State University, University Park, USA; (3) Wise Observatory, Tel Aviv University, Israel; (4) Physics Department, Technion, Haifa, Israel.

1. Introduction

In recent years, optical surveys (e.g, the Sloan Digital Sky Survey and the Digital Palomar Sky Survey) have discovered a large number ($\approx$ 1000) of quasars at $z>4$. From the pioneering study of Kaspi et al. (2000; see Fig. 1a), the number of X-ray detected AGN at $z>4$ has increased to more than 110 (Fig. 1b), mostly thanks to exploratory observations with Chandra (e.g., Vignali et al. 2001, 2005; Brandt et al. 2002; Bassett et al. 2004; Lopez et al. 2006; Shemmer et al. 2006a) and longer exposures with XMM-Newton (e.g., Shemmer et al. 2005). At the very faint X-ray fluxes, X-ray surveys have provided detection of several $z>4$ AGN and quasars (e.g., Schneider et al. 1998; Silverman et al. 2002; Vignali et al. 2002).

Here we provide a summary of some of the main recent results:

$\bullet$ X-ray emission is a universal property of AGN. The X-ray properties of high-redshift AGN and quasars (derived from either stacked or individual X-ray spectra) are similar to those of local quasars, with no evidence for widespread absorption. For radio-quiet quasars (RQQs), a photon index of $\Gamma\approx$1.9–2.0 is obtained (e.g., Vignali et al. 2005; Shemmer et al. 2005), also at $z>5$ (Shemmer et al. 2006a), while for “moderate” radio-loud quasars (RLQs) and blazars, $\Gamma\approx$1.7 and $\Gamma\approx$1.5 are obtained (Lopez et al. 2006), respectively.

$\bullet$ The comparison with the lower redshift (luminosity) Palomar-Green quasars observed by XMM-Newton (Piconcelli et al. 2005) indicates that the photon index does not vary significantly with redshift and luminosity, but seems to depend primarily on the accretion rate (i.e., steeper X-ray slopes are associated with higher Eddington ratio sources; Shemmer et al. 2006b).

$\bullet$ Following X-ray studies of early ’80 and ’90, the relation between X-ray and longer wavelength emission has been investigated by means of the point-to-point spectral slope between 2500 Å and 2 keV in the source rest frame ($\alpha_{\rm ox}$). Any changes in the accretion rate over cosmic time might lead to changes in the fraction of total power emitted as X-rays. Using 333 AGN at z$$\approx0–6.3 (88% X-ray detections), Steffen et al. (2006) confirmed that $log~{}L_{2500~{}\mbox{\scriptsize\rm\AA}}$ correlates with $log~{}L_{\rm 2~{}keV}$ with an index $<1$, and $\alpha_{\rm ox}$ depends upon $log~{}L_{2500~{}\mbox{\scriptsize\rm\AA}}$ (with the slope perhaps depending on $L_{2500~{}\mbox{\scriptsize\rm\AA}}$).

The research field related to $z>4$ AGN still offers plenty of opportunities. In particular, the detection of X-ray variability in some $z>4$ quasars over time scales of month-year (Shemmer et al. 2005) needs further investigations to check the possibility that quasars are more variable in the early Universe. Furthermore, detailed X-ray spectra of $z>4$ RLQs filling the observational gap between “moderate” RLQs and blazars are still needed, as well as studies of “peculiar” quasars and faint AGN population at the highest redshifts.