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L1
The Astrophysical Journal, 569:L1--L4, 2002 April 10
# 2002. The American Astronomical Society. All rights reserved. Printed in U.S.A.
DISCOVERY OF A z p 4.93, XíRAY--SELECTED QUASAR BY THE
CHANDRA MULTIWAVELENGTH PROJECT (ChaMP)
John D. Silverman, 1,2,3 Paul J. Green, 1 DongíWoo Kim, 1 Belinda J. Wilkes, 1 Robert A. Cameron, 1 David Morris, 1
Anil Dosaj, 1,3 Chris Smith, 4 Leopoldo Infante, 5,6 Paul S. Smith, 7 Buell T. Jannuzi, 8 and Smita Mathur 9
Received 2002 January 22; accepted 2002 February 28; published 2002 March 11
ABSTRACT
We present Xíray and optical observations of CXOMP J213945.0#234655, a highíredshift ( ) quasar
z p 4.93
discovered through the Chandra Multiwavelength Project (ChaMP). This object is the most distant Xíray--selected
quasar published, with a restíframe Xíray luminosity of ergs s #1 (measured in the 0.3--2.5 keV
44
L p 5.9 # 10
X
band and corrected for Galactic absorption). CXOMP J213945.0#234655 is a dropout object (126.2), with
g
and . The restíframe Xíray--to--optical flux ratio is similar to quasars at lower redshifts and
#
r p 22.87 i p 21.36
slightly Xíray bright relative to optically selected quasars observed with Chandra. The ChaMP is beginning
z 1 4
to acquire significant numbers of highíredshift quasars to investigate the Xíray luminosity function out to .
z # 5
Subject headings: galaxies: active --- galaxies: nuclei --- quasars: general ---
quasars: individual (CXOMP J213945.0#234655) --- Xírays: general
1. INTRODUCTION
The observed characteristics of known quasars are remarkí
ably similar over a broad range of redshift. For example, Xí
ray studies utilizing the ROSAT database (Green et al. 1995;
Kaspi, Brandt, & Schneider 2000) show little variation of the
ratio of Xíray--to--optical flux for optically selected quasars.
Also, the restíframe UV spectra of quasars, including the broad
Lya, N v, and C iv emission lines, are nearly identical for a
large range of redshift and present no evidence for subsolar
metallicities even up to a (Fan et al. 2001).
z # 6
Even though the individual properties of quasars are similar,
the comoving space density of quasars changes drastically with
redshift. At high redshift ( ), a significant dropíoff in the
z 1 4
comoving space density of quasars seen in optical (e.g., Schmidt,
Schneider, & Gunn 1995; Warren, Hewett, & Osmer 1994; Osí
mer 1982) and radio (Shaver et al. 1996) surveys hints at either
the detection of the onset of accretion onto supermassive black
holes or a missed highíredshift population, possibly due to obí
scuration. Xíray--selected quasars from ROSAT have been used
to support the latter interpretation based on evidence for constant
space densities beyond a redshift of 2 (Miyaji, Hasinger, &
Schmidt 2000). Unfortunately, the ROSAT sample size is small,
with only eight quasars beyond a redshift of 3.
Significant numbers of quasars with are being amassed
z 1 4
to investigate both their intrinsic properties and the evolutionary
1 HarvardíSmithsonian Center for Astrophysics, 60 Garden Street, Camí
bridge, MA 02138; jds@headícfa.harvard.edu.
2 Astronomy Department, University of Virginia, P.O. Box 3818, Charí
lottesville, VA 22903í0818.
3 Visiting Astronomer, Cerro Tololo InteríAmerican Observatory, National
Optical Astronomy Observatory, which is operated by the Association of Unií
versities for Research in Astronomy (AURA), Inc., under cooperative agreeí
ment with the National Science Foundation.
4 Cerro Tololo InteríAmerican Observatory, National Optical Astronomy
Observatory, Casilla 603, La Serena, Chile.
5 Departamento de AstronomÐÒa y AstrofÐÒsica, P. Universidad CatoÒlica, Casí
illa 306, Santiago, Chile.
6 Visiting Astronomer, ESO New Technology Telescope.
7 Steward Observatory, University of Arizona, 933 North Cherry Avenue,
Tucson, AZ 85721.
8 National Optical Astronomy Observatory, P.O. Box 26732, Tucson, AZ
85726í6732.
9 Astronomy Department, Ohio State University, 140 West 18th Avenue,
Columbus, OH 43210.
behavior of the quasar population. The Sloan Digital Sky Survey
(SDSS) reports 123 optically selected quasars with (Schneií
z 1 4
der et al. 2002; Anderson et al. 2001). However, optical surveys
suffer from selection effects due to intrinsic obscuration and the
intervening Lya forest. Current Xíray surveys with Chandra
and XMM do not have a strong selection effect based on redshift
and can detect emission up to 10 keV (observed frame) to reveal
hidden populations of active galactic nuclei (AGNs) including
heavily obscured quasars (Norman et al. 2001; Stern et al. 2002).
Highíz objects can be detected through a larger intrinsic abí
sorbing column of gas and dust because the observedíframe Xí
ray bandpass corresponds to higher energy, more penetrating Xí
rays at the source. 10 Therefore, optical and Xíray surveys will
complement each other, providing a fair census of mass accretion
onto black holes at high redshift.
Larger samples of Xíray observations of quasars are
z 1 4
needed since there are currently only 24 (Vignali et al. 2001),
of which only three are Xíray--selected quasars. Chandra and
XMMíNewton are beginning to probe faint flux levels for the
first time to detect the highíz quasar population. Initial Chandra
and XMMíNewton observations of optically selected quasars
have shown a systematically lower Xíray flux relative to the
optical at high redshift (Vignali et al. 2001; Brandt et al. 2001a).
In this Letter, we present the Xíray and optical properties of
a newly discovered, Xíray--selected quasar with the
z p 4.93
Chandra XíRay Observatory. This quasar is the highest redshift
object published 11 from an Xíray survey.
These results are a component of the Chandra Multiwaveí
length Project (ChaMP; Wilkes et al. 2001). A primary aim of
the ChaMP is to measure the intrinsic luminosity function of
quasars and lower luminosity AGNs out to . The survey
z # 5
will provide a mediumídepth, wideíarea sample of serendipií
tous Xíray sources from archival Chandra fields in Cycles 1
and 2 covering #14 deg 2 . The broadband sensitivity between
0.3 and 8.0 keV enables the selection to be far less affected
by absorption than previous optical, UV, or soft Xíray surveys.
Chandra's small pointíspread function (#1# resolution oníaxis)
and low background allow sources to be detected to fainter
10 The observedíframe, effective absorbing column is eff 2.6
N # N /(1 # z)
H H
(Wilman & Fabian 1999).
11 A , Xíray--selected quasar detected in the Chandra Deep Field--
z # 5.2
North was presented at the 199th AAS meeting (Brandt et al. 2001c).

L2 CHANDRA DISCOVERY OF z p 4.93 QUASAR Vol. 569
Fig. 1.---Optical ( ) and Xíray (0.3--2.5 keV) imaging. To improve the visual clarity, in this figure we have smoothed the Chandra image with a Gaussian
i
function ( ). The spatial distribution of the 17 Xíray counts at 9#.1 offíaxis is as expected from a point source. The circles show the region containing 50%
j p 1#.5
of the encircled energy ( ) of the Chandra counts. The plus signs mark the centroid of the Xíray emission in both images.
radius p 7#.3
TABLE 1
Properties of CXOMP J213945.0#234655
Parameter Value
a J2000.0
a . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 h 39 m 44 s .99
d J2000.0
a . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . #23#46#56#.6
z . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.930 # 0.004
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
#
g 126.2
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
#
r 22.87 # 0.07
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
#
i 21.36 # 0.10
Xíray counts b . . . . . . . . . . . . . . . . . . . . . . . . 16.7 # 7.5
f X (#10 #15 ergs s #1 cm #2 ) b, c . . . . . . . . 2.82 # 1.26
L X (#10 44 ergs s #1 ) c, d . . . . . . . . . . . . . . . 5.89 # 2.63
Hardness ratio e . . . . . . . . . . . . . . . . . . . . . . . !#0.54
a ox . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . #0.08
1.52 #0.05
AB 1450(1 # z)
f . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.62
a Error !0#.5.
b Observed frame; 0.3--2.5 keV.
c Based on an assumed Xíray powerílaw spectrum (S #
E
; ); Galactic absorption--corrected (
#a
E a p 1.0 N p 3.55 #
H
cm #2 ; Dickey & Lockman 1990).
20
10
d Rest frame; 0.3--2.5 keV.
e . Soft band (S): 0.3--2.5 keV; hard band (H):
(H # S)/(H # S)
2.5--8.0 keV.
f Observed monochromatic, Galactic absorption--corrected,
magnitude (Fukugita et al. 1996) emitted at 1450 A Ú
AB 1450(1#z)
in the quasar's rest frame; based on an assumed optical powerí
law spectrum ( ; ).
#a
S # n a p 0.5
n
flux levels, while the #1# Xíray astrometry greatly facilitates
unambiguous optical identification of Xíray counterparts. The
project will effectively bridge the gap between flux limits
achieved with the Chandra Deep Field observations and those
of past ROSAT surveys.
Throughout this Letter, we assume km s #1 Mpc #1
H p 50
0
and a flat cosmology with .
q p 0.5
0
2. OBSERVATIONS AND DATA ANALYSIS
2.1. XíRay
The Xíray source CXOMP J213945.0#234655 (sequence
800104) was observed on 1999 November 18 by Chandra
(Weisskopf et al. 2000) with the Advanced CCD Imaging Specí
trometer (ACISíI; Nousek et al. 1998) in the field of the Xí
ray cluster MS 2137.3#2353 (PI: M. Wise). We have used data
reprocessed (in 2001 April) at the Chandra Xíray Center
(CXC). 12 We then ran a detection algorithm, XPIPE (D.íW.
Kim et al. 2002, in preparation), which was specifically deí
signed for the ChaMP to produce a uniform and highíquality
source catalog.
CXOMP J213945.0#234655 is one of 72 sources detected
using CIAO/WAVDETECT (Freeman et al. 2002) within the
ACIS configuration (Fig. 1). The 41 ks observation yielded a
net counts within the soft bandpass (0.3--2.5 keV)
16.7 # 7.5
and no counts in the hard bandpass (2.5--8.0 keV). This correí
sponds to a Galactic absorption--corrected, observed frame Xí
ray flux of ergs cm #2
#15
f (0.3--2.5 keV) p (2.82 # 1.26) # 10
s #1 (Table 1).
The sourceínaming convention of the ChaMP (CXOMP
Jhhmmss.s#ddmmss) is given with a prefix CXOMP (Chaní
dra Xíray Observatory Multiwavelength Project) and affixed
with the truncated J2000.0 position of the Xíray source after
a mean field offset correction is applied, derived from the crossí
correlation of optical and Xíray sources in each field.
2.2. Optical Imaging and Source Matching
We obtained optical imaging of the field in three NOAO/
Cerro Tololo InteríAmerican Observatory (CTIO) SDSS filters
( , , and ; Fukugita et al. 1996) with the CTIO 4 m/MOSAIC
#
g r i
on 2000 October 29 as part of the ChaMP optical identification
program (P. J. Green et al. 2002, in preparation). Integration
time in each band ranged from 12 to 15 minutes during seeing
of 1#.3--1#.8 FWHM. Image reduction was performed with the
IRAF(v2.11)/MSCRED package. We used SExtractor (Bertin
& Arnouts 1996) to detect sources and measure (pixel) posií
tions and magnitudes. Landolt standard stars were transformed
to the SDSS photometric system (Fukugita et al. 1996) and
used to calibrate the photometric solution. Following the coní
vention of the early data release of the SDSS quasar catalog
(Schneider et al. 2002), we present the optical photometry here
as , , and since the SDSS photometry system is not yet
# # #
g r i
finalized and the CTIO filters are not a perfect match to the
SDSS filters. The limiting magnitudes for a point source are
given as the mean of 3 j detections: ,
# #
g p 26.18 r p
, .
#
25.54 i p 25.11
12 CXCDS version R4CU5UPD14.1, along with ACIS calibration data from
the Chandra CALDB 2.0b.

No. 1, 2002 SILVERMAN ET AL. L3
Fig. 2.---Optical spectroscopy of CXOMP J213945.0#234655. The top
spectrum is the discovery observation taken with the CTIO 4 m/HYDRA on
2001 October 15. The spectrum has been binned to produce a resolution of
16.4 A Ú . The bottom panel is a followíup observation with the NTT on the
next evening to detect spectral features redward of Lya (11 A Ú resolution).
Dashed lines indicate the expected positions of emission lines at a redshift of
4.93. Shaded regions mark the uncorrected telluric O 2 absorption band regions.
As evident from Figure 1, there are three optical sources
detected down to a limiting magnitude of 25.1 within the
#
i
50% encircled energy radius of the Xíray centroid. The two
primary candidates, based on optical brightness, have offsets
between the optical and Xíray positions of 1#.87 and 4#.94. To
determine whether either of these sources are the likely couní
terpart to the Xíray detection, we have determined errors así
sociated with the Xíray astrometric solution.
D.íW. Kim et al. (2002, in preparation) have carried out
extensive simulations of point sources generated using the
SAOSAC rayítrace program 13 and detected using CIAO/
WAVDETECT. For weak sources of #20 counts between 8#
and 10#, offíaxis from the aim point, the reported Xíray centroid
position is correct within 1#.8, corresponding to a 1 j confidence
contour. Therefore, the nearby optical source ( ) is the
Dr p 1#.9
likely counterpart to the Xíray detection. The (J2000.0) position
of the optical counterpart as measured from the image refí
#
r
erenced to the Guide Star Catalog II 14 is ,
h m s
a p 21 39 44.99
.
d p #23#46 56#.6
2.3. Optical Spectroscopy
We obtained a lowíresolution optical spectrum of CXOMP
J213945.0#234655 (Fig. 2) with the CTIO 4 m/HYDRA mulí
tifiber spectrograph on 2001 October 15. Spectra of 17 of 22
optical counterparts to Xíray sources with a magnitude #
r !
were acquired in a 3 hr integration within the Chandra field.
21
The spectrograph has 2# diameter fibers and was configured
with a 527 line mm #1 grating that provided #2800 A Ú of spectral
coverage with a resolution of #4 A Ú . The sky background was
measured using 81 fibers not assigned to the Chandra Xíray
detections within the 1# field spectrograph. We processed the
data using the IRAF(v2.11)/HYDRA reduction package.
An additional spectrum of the highíredshift quasar (Fig. 2)
and the optically brighter source 4#.9 west of the Chandra Xíray
position were obtained on the following evening with the ESO
New Technology Telescope (NTT) 3.5 m to verify the intriguing
Hydra spectrum and obtain greater wavelength coverage. A 300
line mm #1 grating was implemented with a wavelength coverage
of 4000 A Ú and a resolution of #11 A Ú . Because of poor weather
conditions at the end of the evening, flux calibration was done
using the standard star LTT 2415 observed the following night.
From the NTT spectrum, we classify the brighter object as an
M3 dwarf with no sign of emission lines, confirming the quasar
as the optical counterpart of the Xíray source.
We measured a mean redshift from the
z p 4.930 # 0.004
Lyb#O vi, C ii, Si iv#O iv], and C iv emission lines in the
NTT spectrum. Using this redshift, the Lya line centroid is
shifted by #4 A Ú redward from the expected rest wavelength,
probably as a result of significant H i absorption. This is similar
to the mean shift of Lya observed in a sample of 33 highí
redshift quasars by Schneider, Schmidt, & Gunn (1991).
The spectrum obtained at the NTT was used to measure
the restíframe equivalent widths of Lyb/O vi ( A Ú ),
30 # 7
Lya#N v ( A Ú ), and C iv ( A Ú ). For comparison,
73 # 5 40 # 8
we also measured Lya#N v for 10 highíredshift quasars in the
range from the SDSS spectra of Anderson et al.
4.8 ! z ! 5.1
(2001). This subsample has a similar mean redshift (4.91), but
with an average is 4.5 times more optically luminous
#
i p 19.7
than CXOMP J213945.0#234655. Nevertheless, the mean restí
13 See http://heaíwww.harvard.edu/MST.
14 The Guide Star Catalog II is a joint project of the Space Telescope Science
Institute and the Osservatorio Astronomico di Torino.
frame equivalent width of Lya#N v in the SDSS subsample is
consistent at 79 A Ú , with an rms dispersion of 27 A Ú . The poor
signalítoínoise ratio of the SDSS spectra and the strong Lya
forest prevent meaningful comparison of other line strengths.
3. RESULTS
To compare the broadband spectral energy distribution of
CXOMP J213945.0#234655 to other Xíray--detected quasars,
we have calculated a ox (Tananbaum et al. 1979), the slope of a
hypothetical power law between the Xíray and optical flux. The
restíframe, monochromatic luminosity at 2 keV corresponding
to the derived Xíray flux is ergs s #1 Hz #1 .
log l p 26.76
2 keV
Assuming for the optical continuum powerílaw slope,
a p 0.5
we derive the restíframe, monochromatic optical luminosity at
2500 A Ú from the magnitude to be 2500 A Ú p 30.73 ergs s #1
#
i log l
Hz #1 . We thus find . Table 1 lists the measured
#0.08
a p 1.52
ox #0.05
Xíray and optical properties of CXOMP J213945.0#234655.
We compare the Xíray--to--optical flux ratio of CXOMP
J213945.0#234655 to other quasars by plotting the
z 1 4
observedíframe, Galactic absorption--corrected 0.5--2.0 keV Xí
ray flux versus the magnitude (Fig. 3). The plotted
AB 1450(1#z)
lines represent the locus of points for a hypothetical quasar
with a wide range of luminosities and an a p 1.6 # 0.15
ox
(Green et al. 1995), representative of the mean for quasars
selected from the Large Bright Quasar Survey and detected
in the ROSAT AllíSky Survey. The a ox of CXOMP
J213945.0#234655 is comparable with lowíredshift quasars in
contrast to the Xíray faint Chandra detections of optically seí
lected quasars at (Vignali et al. 2001). The Xíray weakí
z 1 4
ness of the latter may be due to intrinsic absorption by large
amounts of gas in the quasars' host galaxies.
Xíray and optical observations of CXOMP J213945.0#
234655 show no direct evidence of significant obscuration. The
optical color ( # ) is consistent with optically
# #
r i p 1.51 # 0.12
selected quasars. We measured the mean color # from 15
# #
r i
SDSS quasars (Anderson et al. 2001) with to be
4.7 ! z ! 5.2
1.69 with rms dispersion of 0.30. The upper limit to the Xíray
hardness ratio (!#0.54) hints at an unobscured Xíray spectrum,

L4 CHANDRA DISCOVERY OF z p 4.93 QUASAR Vol. 569
Fig. 3.---Xíray--to--optical flux correlation for AGNs (adapted from
z 1 4
Vignali et al. 2001). The primary symbols represent the Xíray observatory
used. Squares mark Xíray--selected AGNs. The faintest source shown is a
radioíselected Seyfert galaxy at (Brandt et al. 2001b). The dashed
z p 4.424
lines display the relation for AGNs with at (Green
a p 1.6 # 0.15 z p 4.9
ox
et al. 1995).
although a moderately absorbed component, if present, would
be redshifted out of the Chandra bandpass.
Xíray--selected samples may be less biased against absorbers
(both intrinsic and line of sight) than are optically selected samí
ples, an advantage expected to be especially important at high
redshifts. From our fluxícalibrated NTT spectrum, we measure
, the flux decrement caused by the Lya
D p ( f # f )/f
A cont obs cont
forest (Oke & Korycansky 1982) relative to an extrapolated
powerílaw continuum 15 in the region between restíframe limits
1050#1170 A Ú . The value we measure of is
D p 0.79 # 0.02
A
between the measurement of 0.54 from Rauch et al. (1997)
Õ
z # 4
and the measurements of from four SDSS quasars
z # 6 D # 0.9
A
in Becker et al. (2001). While CXOMP J213945.0#234655 thus
appears consistent with the handful of bracketing measurements
of optically selected quasars (see also Riediger, Petitjean, &
15 As in Fan et al. (2001), we measure the observed flux relative to a
f obs
powerílaw continuum normalized to the observed flux in the region
#1.5
f # l
l
A Ú in the rest frame. We derive uncertainties by measuring against
1270 # 10
continua with slopes in the range # .
0.5 ! a # 1.5
MuØcket 1998), more highíredshift Xíray--selected quasars are
needed to test possible biases caused by absorption.
CXOMP J213945.0#234655 exemplifies the potential for
the ChaMP project to detect quasars with fluxes at the faint
end of the parameter space (Fig. 3). This will allow the
f íf
X opt
ChaMP to acquire significant numbers of highíredshift quasars.
From the first year of spectroscopic followíup of Chandra Xí
ray sources to , we currently have 22 newly identified
i # 21
quasars with and eight with , approximately two to
z 1 2 z 1 3
three such objects per field. Nearly 5% of ChaMP sources
identified to date are quasars.
z 1 3
4. CONCLUSION
We present the discovery of CXOMP J213945.0#234655,
at the most distant Xíray--selected object published
z p 4.93
to date. With a measured optical--to--Xíray flux ratio a p
ox
, CXOMP J213945.0#234655 is similar to lowíredshift
1.52
quasars, in contrast to several optically selected quasars
z 1 4
previously detected by Chandra.
This detection highlights the importance of wideíarea,
intermediateídepth surveys like the ChaMP for studies of the
highíredshift quasar population ( ). The ChaMP 16 has
z # 3--5
begun to amass a sample of highíredshift, Xíray--selected quaí
sars with the goal of measuring the cosmic evolution of
accretionípowered sources relatively unhampered by the abí
sorption and reddening that affects optical surveys.
We gratefully acknowledge support for this Chandra archival
research from NASA grant AR1í2003X. R. A. C., A. D.,
P. J. G., D.íW. K., D. M., and B. J. W. also acknowledge support
through NASA contract NAS8í39073 (CXC). L. I. is grateful
to ``Proyecto Puente PUC'' and the Center for Astrophysics
FONDAP for partial financial support. B. T. J. acknowledges
research support from the National Science Foundation,
through their cooperative agreement with AURA, Inc., for the
operation of the NOAO. We are thankful to Sam Barden and
Tom Ingerson (NOAO) for building and commissioning Hydra/
CTIO. We greatly appreciate the observing support from Knut
Olsen (NOAO) and constructive comments by Harvey Taní
anbaum and Dan Harris.
16 See http://heaíwww.harvard.edu/CHAMP.
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