**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.
References
Bassett, L.C., et al. 2004, AJ, 128, 523
Brandt, W.N., et al. 2002, ApJ, 569, L5
Kaspi, S., Brandt, W.N., & Schneider, D.P. 2000, AJ, 119, 2031
Lopez, L.A., et al. 2006, AJ, 131, 1914
Piconcelli, E., et al. 2005, A&A, 432, 15
Schneider, D.P., et al. 1998, AJ, 115, 1230
Shemmer, O., et al. 2005, ApJ, 630, 729
Shemmer, O., et al. 2006a, ApJ, 644, 86
Shemmer, O., Brandt, W., Netzer, H., Maiolino, R., & Kaspi, S. 2006b, ApJ, 646, L29
Silverman, J.D., et al. 2002, ApJ, 569, L1
Steffen, A.T., et al. 2006, AJ, 131, 2826
Vignali, C., et al. 2001, AJ, 122, 2143
Vignali, C., et al. 2002, ApJ, 580, L105
Vignali, C., Brandt, W.N., Schneider, D.P., & Kaspi, S. 2005, AJ, 129, 2519

  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.

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.
References
Bassett, L.C., et al. 2004, AJ, 128, 523
Brandt, W.N., et al. 2002, ApJ, 569, L5
Kaspi, S., Brandt, W.N., & Schneider, D.P. 2000, AJ, 119, 2031
Lopez, L.A., et al. 2006, AJ, 131, 1914
Piconcelli, E., et al. 2005, A&A, 432, 15
Schneider, D.P., et al. 1998, AJ, 115, 1230
Shemmer, O., et al. 2005, ApJ, 630, 729
Shemmer, O., et al. 2006a, ApJ, 644, 86
Shemmer, O., Brandt, W., Netzer, H., Maiolino, R., & Kaspi, S. 2006b, ApJ, 646, L29
Silverman, J.D., et al. 2002, ApJ, 569, L1
Steffen, A.T., et al. 2006, AJ, 131, 2826
Vignali, C., et al. 2001, AJ, 122, 2143
Vignali, C., et al. 2002, ApJ, 580, L105
Vignali, C., Brandt, W.N., Schneider, D.P., & Kaspi, S. 2005, AJ, 129, 2519
