The Realm of the First Quasars in the Universe: the X-ray View
**FULL TITLE**
ASP Conference Series, Vol. **VOLUME**, **YEAR OF PUBLICATION**
**NAMES OF EDITORS**
The Realm of the First Quasars in the Universe:
the X-ray View
C. Vignali (1), W.N. Brandt (2), O. Shemmer (2), A. Steffen (2), D.P.
Schneider (2), S. Kaspi (3,4)
(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.
Abstract. We review the X-ray studies of the highest redshift quasars, fo-
cusing 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.
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 (≈ 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:
• 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 spec-
tra) are similar to those of local quasars, with no evidence for widespread absorp-
tion. For radio-quiet quasars (RQQs), a photon index of Γ ≈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, Γ ≈1.7
and Γ ≈1.5 are obtained (Lopez et al. 2006), respectively.
• 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).
• 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 (αox). Any
http://arxiv.org/abs/0704.1299v1
2 Vignali et al.
Figure 1. Observed-frame, Galactic absorption-corrected 0.5–2 keV flux
versus AB1450(1+z) magnitude for z > 4 AGN and quasars. (a) The situ-
ation after the Kaspi et al. (2000) work using ROSAT data; (b) the up-
dated census of X-ray observations of z > 4 AGN, including the results from
moderate-depth and ultra-deep X-ray surveys.
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≈0–6.3 (88%
X-ray detections), Steffen et al. (2006) confirmed that log L
2500 Å
correlates
with log L2 keV with an index < 1, and αox depends upon log L2500 Å (with the
slope perhaps depending on L
2500 Å
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.
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