X-ray Spectral Evolution of AGN
Presented by: George Chartas (Penn State)
In collaboration with: Cristian Saez(Penn State), Xinyu Dai(OSU), Michael Eracleous(Penn State),
Niel Brandt(Penn State), Bret Lehmer(Penn State), Franz Bauer(Columbia), Gordon Garmire (Penn State)
General Assembly of IAU, Symposium #238
Black Holes: From Stars to Galaxies
Aug 22, 2006, Prague, Czech Republic
Evolution of AGN
• Commonly used methods of studying the
evolution of AGN include :
(a) Determining the evolution of the
optical and X-ray luminosity functions and
optical and X-ray space densities of AGN.
(b) Determining the evolution of the host
galaxies.
(c) Determining the evolution of the spectra
of the AGN ( vs z,ox vs z).
Evolution of Space Density of type-I AGN
The space density of type-I AGN changes
significantly with redshift and luminosity.
The redshift at which the space density peaks
changes with luminosity from
z ~ 0.5-0.7 for logLx = 42-43 ergs s-1 to
z~2
for logLx = 45-46 ergs s-1.
The amount of change in the space density is
also strongly dependent on luminosity.
 ~ 10 for logLx = 42-43 ergs s-1
 ~ 100 for logLx = 45-46 ergs s-1
The space density of low luminosity AGN is found
to decline at high redshift.
Hasinger et al (2005)
Evolution of Host Galaxy
Barger et al. 2005
The absolute rest-frame 5000 A luminosities of the host galaxies vs. redshift for sources in
the ACS GOODS-North region of the CDF-N.
Triangles : LX > 1044 ergs s-1
Diamonds: LX = 1043 - 1044 ergs s-1
Squares:
LX = 1042 - 1043 ergs s-1
Evolution of Quasars
• One might expect to detect a change in
the X-ray emission and accretion properties
of quasars to accompany the dramatic
change in the number density of quasars
between z=1 and z=2 (Fan et al. 2001).
•
Many X-ray surveys have attempted to
find such a change by constraining  and
the optical-to-X-ray spectral index, ox
•
The evolution of  with z is still debatable
(eg., Bechtold et al. 2003, Vignali et al. 2003,
Grupe et al. 2005)
There is no indication that  correlates
with luminosity for low z quasars (George et
•
al. 2000, Reeves & Turner 2000)
Evolution of quasar comoving
number density as a function of z
(Fan et al. 2001)
Dependence of aox of AGN with UV luminosity and z
ox dependence on the 2500 A monochromatic luminosity.
The main sample is given by filled circles, the high-z
sample by open squares, and the Sy 1 sample by open
Triangles. Strateva et al. (2005)
Correlation of  ox with z, only 1 sigma significant if the lUV
dependence is taken into account. Strateva et al. (2005)
X-ray Spectra of Radio-Quiet Quasars at z > 4
χ2 contours from joint fit for
entire and common energy ranges
Shemmer et al. (2005) performed an investigation of moderate-tohigh quality X-ray spectra of 10 quasars (z = 4 - 6.28).
• They do not find any significant difference between the spectra of
these high z quasars compared to ones at lower z.
• If quasars have been evolving constantly over time observations
of the most distant ones may provided the most ``leverage'' for
constraining any changes in the X-ray spectra over cosmic time.
 = 1.97 +/- 0.05, NH < 3 X 1021 cm-2 (mean values)
Fe Kα EW < 190 eV and R < 1.2
X-ray Spectra of Radio-Quiet Quasars at z > 4
Shemmer et al. (2005) find significant scatter of  but no systematic trend of  with absolute B
magnitude and redshift.
|d/dz| < 0.04
Gravitational lensing as a tool to study AGN evolution
•
Employing the lensing magnification effect to observe high redshift quasars allows
us to probe the luminosity range of 1043-45 ergs s-1. (This luminosity range is
practically inaccessible by most Chandra observations of unlensed quasars of
similarly high redshift.)
•
The lensing magnification (from a few to ~ 100) allows us to obtain moderate to
high S/N spectra
•
The main scientific goal of our survey of quasars was to study the evolution of
spectroscopic properties of high redshift RQQs by searching for a possible
correlation between photon index  and luminosity for high redshift quasars
Evolution of Radio Quiet AGN
- LX diagram from our recent analyses of high redshift (z > 1.5) radio quiet
AGN. Significant correlations are found between  and the 0.2-2keV (210keV) luminosities. The correlations are significant at the 99.9997% (98.6%)
confidence levels, respectively. (Dai, Chartas, Eracleous & Garmire 2004)
Evolution of Radio Quiet Quasars
• Photon index vs. 2-10 keV luminosity for low redshift
(z < 0.1 mostly)
AGN. No significant correlation is found (George et al. 2000)
Evolution of Radio Quiet AGN
To confirm the previously observed correlation between  and luminosity we have:
• Observed additional high z lensed AGN as part of the Chandra GTO program
• Have analyzed moderate-to-high redshift radio quiet AGN observed in the deep field
observations performed with Chandra
The larger sample allowed us to:
• Place tighter constraints on the correlation
• Test the correlation in narrower redshift bands and thus better constrain the epochs
at which possible changes in the average emission properties of AGN occurred.
Evolution of Radio Quiet AGN
Recent lensed high redshift AGN observed with Chandra and added to our sample
Object
zs
ms
Exposure (ks)
Q 0142-100
2.72
I=16.47
15
BRI 0952-0115
4.50
I=18.3
20
Q 1017-207
2.55
I=16.78
15
SBS 1520+530
1.59
I=17.61
20
R=19.56
20
100
N
90
E
80
70
A
60
50
B
40
30
20
10
0
0
10
20
N
30
40
50
60
X-axis (0.1 arcsec per bi n
70
90
80
100
)
30
E
25
A
20
15
10
5
B
0
0
5
10
15
20
X-axis (0.15 arcsec per bin
25
30
)
50
N
45
40
E
35
A
30
B
25
20
X
15
10
5
0
0
5 10 15 20 25 30 35
X-axi s (0.1 arcsec per bin)
40
45
50
A
N
60
E
50
40
A
30
20
B
10
0
0
20
30
40
10
X-axis (0.125 arcsec per bin)
50
60
SDSS 0903+5028 3.605
Evolution of Radio Quiet AGN
Using Chandra Deep Field Observations to Study AGN Evolution
Number of Sources > N Counts
Using Chandra Deep Field Observations to Study AGN Evolution
1000.0
CDF - N
CDF - S
CDF - N, z > 1.5
CDF - S, z > 1.5
100.0
10.0
1.0
0
500
1000
N Counts
1500
2000
Using Chandra Deep Field Observations to Study AGN Evolution
Source Selection


Selected the radio-quiet AGN from the CDF surveys with
Nph (0.5-8 keV) > 200 cnts (~130 sources with z > 0.5)
Radio loud objects were filtered out using R = f5GHz/f4400A > 10
Afonso et al. (2006), Richards (2000)
(~22/152 RLQs, ~14%).




Spectral Analysis
200 < Nph < 600 Cash statistic
Nph > 600 2 statistic
Model : Absorbed power-law
Fitting range: (a) 0.5-7keV observed frame (b) 2-10keV rest frame
Using Chandra Deep Field Observations to Study AGN Evolution
Using Chandra Deep Field Observations to Study AGN Evolution
Using Chandra Deep Field Observations to Study AGN Evolution
<> = 1.64 +/- 0.34
<> ~ 2.6 x 1022 cm-2
Histograms of  and NH
Using Chandra Deep Field Observations to Study AGN Evolution
Correlation Results:
 - L(2-10 keV) & 1.6 < z < 3.3
Spearman:
rc = 0.57
P(r > rc) = 7.1 x 10-4
Pearson:
r = 0.55
P(r > rc) = 1.1 x 10-3
 - L(2-5 keV) & 1.6 < z < 3.3
Spearman:
rc = 0.59
P(r > rc) = 4.3 x 10-4
Pearson:
re = 0.61
P(r > re) = 2.3 x 10-4
All spectral fits performed in the
0.5-7 keV observed frame
Using Chandra Deep Field Observations to Study AGN Evolution
Correlation Results:
 - L(2-10 keV) & 1.6 < z < 3.3
Spearman:
rc = 0.43
P(r > rc) = 2.4 x 10-2
Pearson:
rc = 0.49
P(r > rc) = 7.6 x 10-3
 - L(2-5 keV) & 1.6 < z < 3.3
Spearman:
rc = 0.54
P(r > rc) = 2.9 x 10-3
Pearson:
rc = 0.61
P(r > rc) = 5.8 x 10-4
All spectral fits performed in the
2-10 keV rest-frame
Using Chandra Deep Field Observations to Study AGN Evolution
Using Chandra Deep Field Observations to Study AGN Evolution
Correlation Results:
 - L(2-10 keV) & 1.6 < z < 3.3
Spearman (1e43 - 5e45erg/s):
rc = 0.6
P(r > rc) = 5 x 10-7
Pearson (1e43 - 2e45erg/s):
rc = 0.51
P(r > rc) = 1.4 x 10-4

Using Chandra Deep Field Observations to Study AGN Evolution
Possible Interpretations of the LX -  Correlation
Ý
Ý
M
M
LX  MÝc  M ( ) M ( Ý )  MmÝ
M
M Edd
2
First Interpretation
• Narrow range of M at high z
• Large range of accretion rate
•
X
Ý
L m
Second Interpretation
• Narrow range of accretion rate at high z
• Large range in M
•
X
L M
Using Chandra Deep Field Observations to Study Quasar Evolution
Physical Interpretations of LX - 
•
Hot corona model by Haardt et al. 1997
predicts that
 increases with  of the corona
 decreases with T of the corona
•
If the corona is dominated by
electron-positron pairs this model also
predicts that
 Log Lx
Conclusions
• We confirm the Lx -  correlation for radio quiet AGN at high z based on the spectral
analysis of the CDF surveys.
• We find that the strength of Lx -  correlation is z dependent and peaks at z ~ 2.2
• The Hot Corona model predicts the Lx -  correlation
• The redshift dependence of the correlation suggests that quasars near the peak of
their comoving number density are accreting near Eddington and have different
accretion properties than their low-z counterparts
Evolution of Radio Quiet Quasars
Under the assumptions:
(a)
that high-z quasars emit near Eddington
(b)
that the optical depth  of the corona is
dominated by electron-positron pairs.
(c)
The observed range in luminosity is due to a
range in BH masses (~ 2-3 orders of
magnitude)
the hot corona model of Haardt & Maraschi 1993
predicts :
 log[L(2-10keV)]
The redshift dependence of the correlation
implies that quasars near the peak of their
comoving number density are accreting
near Eddington and have different accretion
properties than their low-z counterparts
Possible Interpretation of -Lx is based on
the hot corona model (Haardt & Maraschi
1993, Haardt, Maraschi, & Ghisellini 1997)
Conclusions
(a)
The spectral slope of the 1.4 < z < 4 radio-loud quasars appears not to vary
significantly over 4 orders of magnitude in 2-10 keV luminosity. We do not find a
significant correlation between the spectral slope  and X-ray luminosity as found in
our 1.5 < z < 4 radio-quiet quasar sample.
(b)
The spectral slopes of the radio-loud quasars of the sample are significantly flatter
than those of the radio-quiet sample possibly due to contamination from jet emission.
(c)
The limited number of quasars in the present sample combined with the medium S/N
of several of the observations may have led to an unaccounted for systematic effect.
Additional observations of z ~ 2 lensed radio-loud quasars with better S/N will allow
us to obtain tighter constraints on a possible correlation between  and X-ray
luminosity.
(d)
The X-ray variability of the high redshifts radio-loud quasars of our sample is
consistent with the known correlation between excess variance and luminosity
observed in NLS1s when extrapolated to the larger luminosities of the present
sample.
CREDITS
Director
George Chartas
Actors
Xinyu Dai
Michael Eracleous
Digital Camera Personnel
Gordon Garmire
Model Predictions
Haardt, Maraschi, & Ghisellini (1997) predicted:
 increases with , the optical depth of the Compton scattering.
 decreases with T, the temperature of the corona.


Optical Depth of IC Scattering
Temperature of Corona
In a “Compact” Corona
• Haardt,
Ghisellini
predicted:
Maraschi, &
(1997)
also
In COMPACT CORONA,
where the pair production
dominates,  Log Lx
• This is similar to what we
have observed.
Two Possible Interpretations of the Correlation
Ý
Ý
M
M
LX  MÝc  M ( ) M ( Ý )  MmÝ
M
M Edd
2
First Interpretation

•
Narrow range (of order a few) of M at high redshift.
•
Large range of
m .
LX  m    
Second Interpretation
 range is narrow, close to Eddington
Opposite. The m
limits, and M range is large.
•
LX  M lc   
•
The lc is the “compactness” of the corona.
•
Haardt & Maraschi (1993) predicted that M  lc, 
increases as lc increases.

• Consistent with semianlytical model of Hauffmann &
Haehnelt (2000) for the
cosmological evolution of
super massive black hole and
their fueling rates.

lc (Coronal Compactness)
Evolution of Radio Quiet Quasars
•
We recently presented results from a survey of relatively high redshift (1.5<z<4)
gravitationally lensed radio-quiet quasars (RQQs) observed with the Chandra and
XMM-Newton (Dai et al. 2004).
Evolution of Quasars
• Using gravitational lensing as a tool to study the evolution of distant quasars
• Gravitationally Lensed High-z Radio Quiet Quasars
Near Eddington Luminosites at redshifts above z~1.5
• High-z Radio Quiet Quasars from the Chandra Deep Field Surveys
• Conclusions
Gravitational lensing as a tool to study AGN evolution
Conceptual diagram of the gravitational deflection of light in a quad GL system.
Understanding the Evolution of Quasars
IC scattering
Black
Hole
Corona
Soft photons
Accretion Disc
Corona
Using Chandra Deep Field Observations to Study AGN Evolution
Histograms of Lx and z