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ChaMP and the High Redshift Quasars in X­rays
S. Mathur 1
Harvard Smithsonian Center for Astrophysics Cambridge, MA 02138
H. Marshall 1
MIT, Cambridge, MA 02139
N. Evans 1 , P. Green 1 and B. Wilkes 1
Harvard Smithsonian Center for Astrophysics Cambridge, MA 02138
Abstract. Chandra X­ray Observatory, (formerly known as AXAF),
will observe down to the flux limit of 2\Theta10 \Gamma16 erg s \Gamma1 cm \Gamma2 . In its first
year of operation Chandra's CCD detectors will observe over 1500 quasars
serendipitously in the soft (0.5--3.5 keV) band. Over 200 quasars will be
detected in X­rays in the redshift range 3 ! z ! 4 and over 400 quasars
in 2 ! z ! 3. This will enable us to determine the high redshift X­ray
luminosity function. This is the contribution by unabsorbed sources only.
The total numbers would be larger by ¸ 60%.
1. Introduction
NASA's Chandra X­ray Observatory was launched on July 23, 1999. The Chan­
dra Multiwavelength Project (ChaMP) will combine radio to X­ray observations
of serendipitous Chandra sources, with emphasis on optical identification. The
ChaMP is superior to previous X­ray surveys because of (1) unprecedented X­ray
positional accuracy (¸ 1 00 ), (2) X­ray flux limits 20 times deeper than current
wide area surveys (down to f(0:5 \Gamma 3:5keV ) ¸ 2 \Theta 10 \Gamma16 erg s \Gamma1 cm \Gamma2 ), (3)
larger sky coverage (¸ 8 deg 2 ) per year than current deep surveys.
2. Prediction of Redshift Distribution of Quasars in ChaMP Fields
The X­ray Luminosity Function at z = 0 is described as
\Phi(L X ) = \Phi ?
1
L \Gammafl 1
44
for L ! L ? (0)
1 The Chandra Multiwavelength Project (ChaMP) is an independent scientific collaboration for
followup studies of serendipitous X­ray sources in Chandra X­ray images. The ChaMP Web
site is http://hea­www.harvard.edu/CHAMP.
1

\Phi(L X ) = \Phi ?
2
L \Gammafl 2
44
for L ? L ? (0)
where L 44
is the X­ray luminosity in 10 44 erg s \Gamma1 . The redshift evolution of the
luminosity function is characterized by
LX (z) = LX (0)(1 + z) k
Continuity of the luminosity function at the break luminosity requires that
\Phi ?
1
= \Phi ?
2
L ?
44
fl1\Gammafl2
The total number N of quasars in the sample is obtained by integrating the
luminosity function over luminosity and volume, i.e.,
N =
Z Z
\Phi(L X ;
z)\Omega\Gamma LX ; z)dV (z)dL X
Here\Omega\Gamma LX ; z) is the solid angle covered by the survey as a function of
redshift and luminosity. The parameters of the X­ray luminosity function de­
termined by Boyle et al. (1993) are as follows: fl1 = 1:7 \Sigma 0:2, fl2 = 3:4 \Sigma 0:1,
log L ? (0) = 43:84, \Phi ?
1
= 5:7 \Theta 10 \Gamma7 Mpc \Gamma3 (10 44 erg s \Gamma1 ) fl1\Gamma1 . Following Comastri
et al. (1995), we have used k=2.6 and increased the the normalization \Phi ?
1
by
20%.
The X­ray logN­logS Curve: Using the above luminosity function we
derived the number density of quasars as a function of observed flux. The
luminosity function was integrated over the luminosity range 10 42 ! LX ! 10 48
erg s \Gamma1 and the redshift range 0 ! z ! 4. H 0 = 50 and q 0 = 0 were assumed
throughout. The predicted logN­logS curve is shown in figure 1.
0.1
1
10
100
1000
Figure 1. The predicted number counts in the soft band for unab­
sorbed quasars.
Since the unabsorbed sources dominate at the faint end in the soft X­ray
range, and since they are likely to be observed at high redshift, in the present
analysis we will concentrate on unabsorbed sources only. The absorbed sources
2

would contribute an additional ¸ 60% (Comastri et al. 1995), making the total
number consistent with the extrapolation of the empirical determination of logN­
logS (Hasinger et al. 1993). The flux of unabsorbed quasars is given by f / E \Gammaff
and in the soft X­ray band, ff is typically 1.3.
The ChaMP Sky Coverage: The ChaMP Cycle 1 consists of 85 extra­
galactic fields, ---b---? 20 o . From all the Chandra cycle 1 fields we have excluded
(1) deep fields of PI survey observations, (2) fields with extended sources & plan­
etary targets, (3) ACIS sub­arrays and continuous clocking modes. See figure 2
for ChaMP sky coverage as a function of flux limit.
Figure 2. The total sky coverage of ChaMP fields as a function of
flux limit in the soft band.
Cumulative Number Distribution in ChaMP: Integrating the pre­
dicted logN­logS over the ChaMP sky coverage, we obtained the cumulative
number distribution of quasars in the ChaMP fields (figure 3). The total num­
ber in soft band is expected to be over 1500 for unabsorbed sources and over
2500 total.
0
1000
2000
Figure 3. Expected cumulative source counts. Unabsorbed sources only.
Predicted Redshift Distribution: The histogram (figure 4) shows the
predicted number distribution of quasars in ChaMP fields. Over 200 quasars will
be detected in the redshift range 3 ! z ! 4 and over 400 quasars in 2 ! z ! 3.
3

0 1 2 3 4
0
100
200
300
400
500
600
700
z
Figure 4. Redshift distribution of the unabsorbed sources expected
to be detected in ChaMP fields.
3. Comparison with Previous X­ray Surveys
Survey Total number of Sources Quasars at z ? 2
EMSS 835 ! 5
(Gioia et al.)
ROSAT Deep 661 12
(Hasinger et al.)
ROSAT 89 ! 10
(Boyle et al.)
ChaMP ? 1500 ? 600
(soft band, unabsorbed)
We will be able to determine the X­ray luminosity function and its redshift evo­
lution with unprecedented accuracy.
References
Boyle, B., Griffiths, R., Shanks, T., Stewart, G., & Georgantopoulus, I. 1993,
MNRAS, 260, 49
Comstri, A., Setti, G., Zamorani, G., & Hasinger, G. 1995, A&A, 296, 1
Gioia et al. 1990, ApJS, 72, 567
Hasinger, G. et al. 1993, A&A, 275, 1
It's my pleasure (SM) to thank A. Comastri for useful discussions. This
work is supported in parts by NASA grant NAG5­3249 (LTSA).
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