Near infrared measurements were obtained of 30 quasars originally found serendipitously as X-ray sources in fields of other objects. The observations show that the infrared characteristics of these quasars do not differ significantly from those of quasars selected by other criteria. Because this X-ray selected sample is subject to different selection biases than previous radio and optical surveys, this conclusion is useful in validating previous inferences regarding the infrared colors of 'typical' quasars

Chanan, G. A.

Margon, B.

Matthews, K.

Neugebauer, G.

Soifer, B. T.

English

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Infrared Properties of Serendipitous X-ray Quasars
r'" G. Neugebauer, B. T, Soifer, K. Matthews
Palomar Observatory, California Institute of Technology
B. Margon*
Astronomy Department, University of Washington
G. A. Chanan
Columbia Astrophysics laboratory, Columbia University
ABSTRACT
Near infrared measurements have been obtained of 30 quasars
originally found se; endfpitously as X-ray  sources in fields of other
objects. The observations show that the infrared characteristics of
these quasars do not differ significantly from those of quasars
selected by other criteria. Because this X-ray  selected sample is
subject to different selection biases than previous radio and optical
surveys, this conclusion is useful in validating previous inferences
regarding the infrared colors of 'typical' quasars.
Introduction
Chanan, Margon and Downes ( 1981, hereafter CMD) have published optical
identifications of 19 active galactic nuclei found by serendipitous observations in
Einstein Observatory X-ray  fields. Further identifications of similar serendipitous
observations have since been obtained which more than double the sample of X-
ray  selected quasars. Complete details on the entire sample are given by Mar-
gon, Downes and Chanan ( 1982). Margon, Chanan and Downes ( 1982) have found
0A1fred P. Sloan Foundation research Fellow
(N AS A —CR-169454) INFRARED PROPERTIES OF
	 N83-10995
SERENDIPITOUS X —RAY QUASARS (Palomar
Observatory) 18 p HC AU2/MF A01
	 CSCL 03A
Unclas
G3/89 00556
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s
v
s
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I	 1
a significantly lower optical luminosity for this sample than for quasars found in
previous surveys with similar visual thresholds but different selection tech-
niques,
The sample of quasars selected serendipitously as X-ray sources potentially
can serve as a basis for the ,study of properties ,ich would be masked in radio
or visually selected samples. In this paper a iwar infrared study of 30 such u
serendipitously X-ray  selected quasars is presented.
Observations and Sample
In this study, 30 objects were measured at the f/70 focus of the 5-m Hale
Telescope at Palomar Mountain within the three photometric bands J (1.27 µm),
AX = 0.24 µm; H (1.65 Am), AX = 0.32 µm; and K (2.23 µm), AX = 0.40 µm. The
detector was an InSb photovoltaic diode cooled to solid nitrogen temperature.
The photometric calibration was with respect o standard stars listed by Elias, at
al. (1982). The flux densities in the near infrared were derived from the following
calibrations for the flux density at 0 magnitude: f„(-t=0 mag) 1580 Jy, f„(H=0
mag) - 1040 Jy, f„(K=0 mag) = 650 Jy. Observations in, the photometric bands
were obtained sequentially in a single night. The H and K observations of nine of
the quasars were repeated on two nights separated in time by more than nine
months; J magnitudes were generally only obtained on the latter of these nights.
The 30 objects studied in this sample include 16 objects presented by CMD
as well as 12 others selected from fields which satisfy the criteria outlined by
CMD, Briefly, the objects included in the present sample were selected on the
basis of their X-ray  emission and were subsequently found to exhibit visual emis-
sion lines characteristic of quasars. The optical search was complete to •18,5
mag; some of the identified objects were subsequently found to be fainter than
this limit. The finite visual threshold implies that very faint red objects such as
r{	 ,
-3-
those found by Rieke, Lebofsky and Kinman ( 1979) would not be identified. If
there were qun-drs which were visually faint relative to their X-ray  brightness,
these would also be discriminated against. The finite X-ray threshold would simi-
larly exclude objects of highly unusual optical to X-ray
 flux ratios.
The quasar 2251-178, discovered by Ricker at al. (1979) previous to EV70tein
observations, was also included in the current sample since it was selected by its
X-ray  emission. This object was also observed in the 10 µm photometric band at
the 5-m telescope with a result [ 10 µm.1 = 6.7 t 0.' mag corresponding to a flux
density of 80 f 20 m +.Ty (Soifer, Nougebauer and Matthews, 1979). The X-ray
Huxes reported here are from Binste6i observations.
Reanits and Discussion
The near infrared magnitudes of the 30 objects observed are listed in Table
1 and various features of the sample are shown in the figures. Figures 1 - 4 show
various relations between the infrared and visual magnitudes. For the nine qua-
ears which were measured twice, average values are given in Table 1 although
about half of these showed evidence of variability. The infrared colors of these
quasars which showed variability are, however, derived from observations taken
on a single night. The maximum amplitude of variability observed, - 0.3 mag, is
	
s
	 consistent with the variability
 seen in other quasars (Neugebauer, Soifer, and
i
Matthews 1982) over comparable time periods,
r
f The visual ma,,VAtudes were obtained either from image diameters on the
Palomar sky survey prints ( 19 objects) or from photoelectric photometry (11
objec^ly). The visual magnitudes thus refer to the fluxes at times quite different
	
-	 from those when the infrared data were obtained.
The X-ray  properties of the sample are summarized in Figures 5 and 6. The
d
X-ray
 fluxes are from Margon, Downes and Chanan (1982). Since complete
t
t
-4-
energy distributions are not available for these objects, all photometric colors
are in the observed frame. On each figure where color indices are given, power
law slopes a are 9so presented; the dux density is assumed to follow a law of the
form f,, a 0. The Hubble constant and deceleration parameters in these figures
and throughout this paper have been taken as 11, = 50 km s' 1 Mpe -1 and a = 0.
The main result of this paper is that there is no strong difference between
the near infrared properties of quasars in this sample and of quasars discovered
by other mean,,,. This is illustrated by the figures in which the results of the X-
ray selected sample are compared with similar results for quasars selected via
other criteria. In Figures 1 - 4 the second sample, selected by Green on, the
basis of ultraviolet excess, is from the Palomar bright quasar survey (Schmidt
and Green 1981). The comparison object in Figure 6 are not from a well de tned
sample, but were chosen because X-ray fluxes were available in either Ku, Hel-
fand and Lucy (1880) or in Zamorani at at. (1881) and infrared measurements
were available at Caltech. This sample represents a variety of radio and visually
selected objects. In all cases, the infrared observations of the second samples
were obtained at the 5-m telescope,
The data of Figure 1 indicate a good correlation between the H-K and J-K
colon indices as is expected from measurements at such close wavelengths. More
importantly, however, both the mean slopes and the intercepts of the X-ray
selected sample and the optically selected sample are the same within small
uncertainties. Figure 1 represents observations each taken nearly simultane-
ously and under good photometric conditions. The similarity of the distributions
is thus a prominent feature of these data.
Figure 2 shows an apparent difference in slope of V K versus J-K between
the X-ray selected and ultraviolet-excess selected quaroars. Caution is needed in
interpreting this difference, however, as most of the X-ray  sample have
x	 i
-5-
photographically determined V magnitudes of low accuracy (x 0.5 mag'^; while
t
the. size of the sample with accurate visual photoelectric photometry (s 0.05
7 mag accuracy) is so small that it poorly detrirmines the slope compared with the
determination from the much more extensive Schmidt and Green (1981) sample.
Both the slope and intercept of a linear fit to the observations of x-ray selected
quasars with photoelectrically determined V magnitudes differ by 2 sigma from
those of the ultraviolet selected quasars. Furthermore, visual magnitudes of the
ultraviolet sample were obtained closely in time with the infrared data, while, as
stressad, previously, the infrared and visual photometry for the X-ray  sample was
separated in many cases by > 1 year. Thus any Aux variability of the x-ray  sam
ple, even if it is color-independent, will introduce further spurious scatter into
Figure 2. Although there is a marginally significant difference between the two
samples displayed in figure 2, we feel the similarities shown in figure 1 are based
on more controlled observations and thus we cannot assert that there is definite
evidence that these very different selection procedures are sampling different
f
{ parent populations, Similarly, Figure 3 shorts an apparent difference in the (V
lid widths of the X-ray  and ultraviolet-selected samples, but the aborts considera-
tions again lead us not to attach strong significance to this effect.
Figures 4 and 5 indicate that the dependence of infrared color on either
visual or X-ray  luminosity is independent of the sample. Both figures emphasize
the fact that quasars selected as serendipitous X-ray sources are less optically
luminous than those found by other selection techniques, as has been noted pre-
viously (Margon, Chanan and Downes 1982). Here we present the result that this
first infrared study shows no overwhelming evidence for a prominent infrared
color,
 effect in this sample which can be attributed to the X-ray  selection pro-
f
6i
	 cess. Conversely, because this X-ray  selected sample of quasars is subject to
different Wasps than previous samples of optically and radio-selected quasars,
F
-6-
this commonality of infrared colors between samples is on important validation
of previous inferences (Neugebauer at al. 1979, Hyland and Allen 1982) regarding
the infrared colors of the "typical" quasar,
Figure 6 shows that the range of both infrared and visual flux densities is
ti
comparable to the range of the X-ray fluxes, although there is no strong correla-
tion between the strength of the X-ray  flux and either the visual or infrared (2.2
µm) fluxes. The effect of the X-ray flux limited sampling is plainly seen at I, N
10 counts s' t . The power law slopes between visual and X-ray  frequencies (ate
are compared with the slopes within the near infrared, (ajo in Figure 7. The
colors show a statistically marginal correlation in the sense that quasars with
the steeper infrared slopes exhibit shallower visual to X-ray  slopes.
In each of the figures where a comparison is made, the quasars initially
selected by their X-ray emission are indistinguishable from those selected by
other techniques. Although the quasars of this sample have a lower luminosity
than others, this decreased luminosity is not accompanied by any marked
characteristic in the slopes or strength of the continua. Recent studies of the
visual and near infrared continua of quasars have failed to short strong correla-
tions between these continua which might have elucidated the nature of the
emission mechanisms producing the visual and. near infrare^ ,. fluxes in the qua-
mars. Indeed, there is growing evidence that the mechanisms producing the
radiation at infrared and shorter wavelengths are remarkably ubiquitous
although the individual parameters defining the quantitative appearance vary
significantly. The present observations show that the selection of quasars on the
basis of their X-ray  emission has apparently also found objects which have simi-
lar emission mechanisms at the visible and near infrared wavelengths.
This work has been supported by NSF Grants AST 77-27745 and AST 80-
17335, NASA Contracts NAS 8-30753 and NGL 05-002-207, and the Alfred P. Sloan
-7-
Foundation. We thank R.A. Downes and S.F. Anderson for aid in reducing the
data. The data on the sample of quasars found from their ultraviolet excess are
from a forthcoming paper with R. Green and M. Schmidt who kindly gave permis-
sion to quote results prior to publication,
-8-
l[WEMCES
Chanan, G.A., Mergon, H, and Downes, R.A. 1981, Ap. J, (Letters), 243, L5.
Elias, J., Frogel, J., Matthews, K, and Neugebauer, G, 1982 A.J., in press
Hyland, A.R. and Allen, D.A. 1982, M. N. R. A. S., 199, 943.
Ku, W., Helfand, D. and Lucy, 1r. 1980, Nature, 288., 328,
Margon, H„ Downes, R.A. and Chanan. G.A. 1982, Ap, J. Sutppl., in preparation
Margon„ H., Chanan, G.A. and Downes, R.A. 1982, Ap. J. (Letters), 2S9, L7.
Neugebauer, G., Oke, J.H., Hecklin, E.E. and Matthews, K,, 1979, Ap, J., 230. ?9.
Neugebauer, G., Soifer, H. T, and Matthews K., 1982, in preparation.
Ricker, G.R., Clark, G.W., Doxsey, R.E., Dower, R.G., Jernigan, ,4.G., Delvaille, J,P.,
Macalpine, G.M. and Nellming, R.M. 1978, Nature, 271, 35.
Rieke, G, H., Lebofsky, M, J. and Mnman, T. D. 1979, Ap. J: (Letters), 232, L151..
Schmidt, M. and Green, R. 1981, Vatican Study Week on Cosmology and Funda-
mental Physics, Ed. G. Coyne, in press.
Soifer, H.T., Neugebauer, G. and Mattbpws, K. 1979, Nature, 27M 231.
Zamorani, G., Henry, J,P., Maccacaro, T., Tannanbaum, H., Soltan,A., Avni, Y.,
Liebert, J., Stocks, J., Strittmatter, P.A., Weymann, R.J., Smith, M.G, and
Condon, J.J. 1981, Ap. Jr., 246, 357.
.
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0 
11404, ?NU­
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 1 
- 9 -	 QF 1p	
QUA
Table 1
Observations of Serendipitous X-ray Quasars6
abject
	
redshift	 V	 Is	 J	 H	 K
z	 mar etS / 100(r
	 mar	 mar	 mag
0031-076 0,291 17.9 33 16,43±.24 15.46±,08 14.69±.10
0031-077 0.388 18.5 13 17,33±.10 16.33±,08 15.37±.08
002-073 0.752 18.0 15 17,75±.13 16.94±.10 18.07±.12
0037+061 0.063 17.03 37 15,20±,11 14.27±.07 13.73±.07
0100+020 0,392 16.3143' 21 16.44±,08 15,55±.08 14.50±,07
0120+092 0,176 ie.2 11 15,72±.08 15.01±,07 14.19±.07
6121+034 0,336 18.5 10 16,68±.07 15.82±.07 14.87±.07
0240+007 6.569 16.52 10 15.81±,11 14.94±.10 14.22±,10
0302-223 1.409 16.40 19 15,90:L.40 14.64±.13 14.35±.14
0318-196 0.104 14.86 12 14.54±,07 13.76±.07 13.18±.07
0351+026 0.036 16.20 100 14.30±.09 13.37±.08 12.54±.08
0844+377 0.451 17.7 20 16.95±.09 16.34±.08 15.60±.12
0919+515 0.161 17.9 41 15.55±.14 14.821.08 13.82±.08
1059+730 0,089 16.40 19 14.59±,09 13,86±.09 13.28±.07
1339+053 0,266 16.8 51 15.54±.14 14.69±.09 13.63±.178
1403+546 0.082 16.8 32 15.43±.10 14.71±.10 14.28±.09
1519+279 0.230 18.2 20 ' 16.58±.08 15.77±.07 14.47±.07
1526+286 0.450 16.39 49 15.24±.11 14.81±.08 14.05±.08
1557+272 0.065 16.33 22 14.28±.09 13.541:.CIB 12.97±.09
1640+396 _ 0.540 18.3 30 17.35±.23 16.30±,11 15.68±.13
1640+401 0.986 i 7,1 11 16.65±.08 16.40±.CI9 15.89±.10
1641+3996 0,594 19.3 13 17.65±.12 16.84±.10 15.87±.14
1701+610 0,164 17.0 14 15.44±.12 14.78±. CIB 14.10±.08
1704+607 01.080 17.73 7 15.87±.16 15.28±.CI9 14.74±.11
1726+499 0.815. 19.3 12 76.60±.12 16.49±.CI9 15.54±.09
1847+335 0.509 17.7 36 16.85±.21 16.03±.10 15,04±.10
2215-037 0.242 17.20 41 15.18±.10 14.25±49 13.10±.07
2216-043 0.243 18.5 15 15.58±.11 14.58±.11 13.51±.09
2251-178 0.064 14.0 606 13.13±.08 12.30±.08 11.42±.08
2344+184 0.138 15.9 13 15.64±,11 14.78±.09 14.13±.09
a) Redshitt. V and Is from Nargon, Downes, and Chan- (1982). V magnitudes canted to 0.1
mag were derived from  image diameters on the Palomar Schmidt Sky Survey Plates; V ma&
nitudes quoted to 0.01 mag were obtained photoelectrically. ,
b) Near to but distinct from 9CM5.
u
- 10 -
Figure Captions
Figure 1:
The infrared colors of the serendipitous X-ray  quasars (top) and of the
Palomar Green-Schmidt sample selected for their ultraviolet excess (bottom).
The slopes a jK and allK are defined assuming a power law dependence of the flux
C;
density ^`„ a 0.
Figure 2:
The infrared colors versus the visual-2,2µm color of the serendipitous X-ray
quasars (top) and of the Green-Schmidt sample (bottom), The V magnitudes
were found from the image diameters on the Palomar Sky survey prints (open
circles), from photoelectric measurements (crosses) or Prom numerical Integra-
tions of multi-channel data (filled circles).
Figure 9;
Histograms of the V K color for X-ray  selected quasars (top) and for quasars
selected for their ultraviolet excess (bottom). The quasars whose visual magni-
tudes were measured photoelectrically are indicated by the striped area.
Figure 4:
The infrared color as a function of the absolute visual magnitude for the X-
ray selected sample (open circles) and the sample selected on the basis of its
ultraviolet excess (crosses).
11
Figure 5.
The infrared color as a function of the X-ray luminosity for the X-ray
selected sample (crosses) and for other quasars measured at X'^-ray wavelengths
(filled circles); see text.
fture	
ii
8.	 }I
The X-ray intensity as a function of the apparent visual magnitude (V') and
of the apparent 2.2µm magnitude (K). Open, circles represent observations with
photoelectrically measured magnitudes. 	 X
7:
The power law slope (cux) between 1.25 and 2.2µm versus ayx, the power
law slope between the visual and X-ray frequencies, for the X-ray selectee. sam-
ple.
	
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Figure 7


Infrared properties of serendipitous X-ray quasars

Near-infrared measurements have been obtained of 30 quasars originally found serendipitously as x-ray sources in fields of other objects. The observations show that the infrared characteristics of these quasars do not differ significantly from those of quasars selected by other criteria. Because this x-ray-selected sample is subject to different selection biases than previous radio and optical surveys, this conclusion is useful in validating previous inferences regarding the infrared colors of “typical” quasars

Caltech Authors - Main

19
82
AJ
 
 
87
.1
63
9N
 
THE ASTRONOMICAL JOURNAL VOLUME 87, NUMBER 12 DECEMBER 1982 
INFRARED PROPERTIES OF SERENDIPITOUS X-RAY QUASARS 
G. Neugebauer, B. T. Soifer, and K. Matthews 
Palomar Observatory, California Institute of Technology, Pasadena, California 91125 
B. MARGONa) 
Astronomy Department, University of Washington, Seattle, Washington 98195 
G. A. Chanan 
Columbia Astrophysics Laboratory, Columbia University, New York, New York 10027 
Received 26 July 1982; revised 8 September 1982 
ABSTRACT 
Near-infrared measurements have been obtained of 30 quasars originally found serendipitously 
as x-ray sources in fields of other objects. The observations show that the infrared characteris- 
tics of these quasars do not differ significantly from those of quasars selected by other criteria. 
Because this x-ray-selected sample is subject to different selection biases than previous radio 
and optical surveys, this conclusion is useful in validating previous inferences regarding the 
infrared colors of “typical” quasars. 
I. INTRODUCTION 
Chanan, Margon, and Downes (1981, hereafter re- 
ferred to as CMD) have published optical identifications 
of 19 active galactic nuclei found by serendipitous obser- 
vations in Einstein Observatory x-ray fields. Further 
identifications of similar serendipitous observations 
have since been obtained which more than double the 
sample of x-ray-selected quasars. Complete details on 
the entire sample are given by Margon, Downes, and 
Chanan (1982). Margon, Chanan, and Downes (1982) 
have found a significantly lower optical luminosity for 
this sample than for quasars found in previous surveys 
with similar visual thresholds but different selection 
techniques. 
The sample of quasars selected serendipitously as x- 
ray sources potentially can serve as a basis for the study 
of properties which would be masked in radio or visually 
selected samples. In this paper a near-infrared study of 
30 such serendipitously x-ray-selected quasars is pre- 
sented. 
il observations and sample 
In this study 30 objects were measured at the //70 
focus of the 5-m Hale Telescope at Palomar Mountain 
within the three photometric bands /(l.27 //m), 
AÀ = 0.24/2m;/7(1.65//m),di/l = 0.32/zm; and/: (2.23 
//m), AÀ = 0.40 //m. The detector was an InSb photo- 
voltaic diode cooled to solid nitrogen temperature. The 
photometric calibration was with respect to standard 
stars listed by Elias et al. (1982). The flux densities in the 
near infrared were derived from the following calibra- 
tions for the flux density at 0 mag :/v(/= 0 mag) = 1580 
Jy, fv{H = 0 mag) = 1040 Jy,/v(/: = 0 mag) = 650 Jy. 
a)
 Alfred P. Sloan Foundation Research Fellow. 
1639 Astron. J. 87(12), December 1982 
Observations in the photometric bands were obtained 
sequentially in a single night. The H and K observations 
of nine of the quasars were repeated on two nights sepa- 
rated in time by more than nine months; J magnitudes 
were generally only obtained on the latter of these 
nights. 
The 30 objects studied in this sample include 18 ob- 
jects presented by CMD as well as 12 others selected 
from fields which satisfy the criteria outlined by CMD. 
Briefly, the objects included in the present sample were 
selected on the basis of their x-ray emission and were 
subsequently found to exhibit visual emission lines char- 
acteristic of quasars. The optical search was complete to 
18.5 mag; some of the identified objects were subse- 
quently found to be fainter than this limit. The finite 
visual threshold implies that very faint red objects such 
as those found by Rieke, Lebofsky, and Kinman (1979) 
would not be identified. If there were quasars which 
were visually faint relative to their x-ray brightness, 
these would also be discriminated against. The finite x- 
ray threshold would similarly exclude objects of highly 
unusual optical to x-ray flux ratios. 
The quasar 2251 — 178, discovered by Ricker et al. 
(1978) previous to Einstein observations, was also in- 
cluded in the current sample since it was selected by its 
x-ray emission. This object was also observed in the 10- 
/¿m photometric band at the 5-m telescope with a result 
[10 ¡im] = 6.7 ± 0.3 mag corresponding to a flux den- 
sity of 80 ± 20 mJy (Soifer, Neugebauer, and Matthews, 
1979). The x-ray fluxes reported here are from Einstein 
observations. 
III. RESULTS AND DISCUSSION 
The near-infrared magnitudes of the 30 objects ob- 
served are listed in Table I and various features of the 
sample are shown in the figures. Figures 1-4 show var- 
íe) 1982 Am. Astron. Soc. 1639 0004-6256/82/121639-09$00.90 
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1640 NEUGEBAUER ETAL. : INFRARED PROPERTIES OF QUASARS 1640 
Table I. Observations of serendipitous x-ray quasars.3 
object redshift 
z 
V 
mag 
I, 
cts/1000» 
J 
JB.ag- 
H 
mag 
K 
-mag.. 
0031-076 
0031- 077 
0032- 73 
0037+061 
0100+020 
0120+092 
0121+034 
0240+007 
0302-223 
0318-196 
0351+026 
0844+377 
0919+515 
1059+730 
1339+053 
1403+546 
1519+279 
1526+286 
1557+272 
1640+396 
1640+401 
1641+399b 
1701+610 
1704+607 
1726+499 
1847+335 
2215-037 
223.6-043 
2251-178 
2344+184 
0.291 
0.388 
0.752 
0.063 
0.392 
0.176 
0.336 
0.569 
1.409 
0.104 
0.036 
0.451 
0.161 
0.089 
0.266 
0.082 
0.230 
0.450 
0.065 
0.540 
0.986 
0.594 
0.164 
0.080 
0.815 
0.509 
0.242 
0.243 
0.064 
0.138 
17.9 
18.5 
18.0 
17.03 
16.39 
18.2 
18.5 
16.52 
16.40 
14.86 
16.20 
17.7 
17.9 
16.40 
16.8 
16.8 
18.2 
16.39 
16.33 
18.3 
17.1 
19.3 
17.0 
17.73 
19.3 
17.7 
17.20 
18.5 
14.0 
15.9 
33 
13 
15 
37 
21 
11 
10 
10 
19 
12 
100 
20 
41 
19 
51 
32 
20 
49 
22 
30 
11 
13 
14 
7 
12 
36 
41 
15 
606 
13 
16.43i.24 
17.33±. 10 
17.751.1  
15.201.11 
16.441.08 
15.721.08 
16.6Bi.07 
15.811.11 
15.90i.40 
14.541.07 
14.301.09 
16.951.09 
15.551.14 
14.591.09 
15.541.14 
15.431.10 
16.581.08 
15.241.11 
14.281.09 
17.35i.23 
16.651.08 
17.651.12 
15.441.12 
15.87i.16 
16.601.12 
16.85i.21 
15.181.10 
15.5Bi.ll 
13.131.08 
15.641.11 
15.461.08 
16.331.08 
16.941.10 
14.271.07 
15.551.08 
15.011.07 
15.821.07 
14.941.10 
14.64i.13 
13.761.07 
13.371.08 
16.341.08 
14.821.08 
13.861.09 
14.691.09 
14.711.10 
15.771.07 
14.811.08 
13.541.08 
16.301.11 
16.401.09 
16.841.10 
14.781.08 
15.281.09 
16.491.09 
16.031.10 
14.25i.09 
14.581.11 
12.301.08 
14.781.09 
14.691.10 
15.37i.08 
16.071.12 
13.731.07 
14.501.07 
14.191.07 
14. B7i.07 
14.221.10 
14.35i.14 
13.181.07 
12.541.08 
15.601.12 
13.821.08 
13.281.07 
13.631.08 
14.281.09 
14.471.07 
14.051.08 
12.971.09 
15.681.13 
15. B9i.10 
15.871.14 
14.101.08 
14.741.11 
15.541.09 
15.041.10 
13.101.07 
13.51i.09 
11.421.08 
14.131.09 
a Redshift, V and I* from Margon, Downes, and Chanan (1962). V magnitudes quoted to 0.1 
wag were derived from image diameters on the Palomar Schmidt Sky Survey Plates; V mag- 
nitudes quoted to 0.01 mag were obtained photoelectrically. 
b Near to but distinct from 3CS45. 
ious relations between the infrared and visual magni- 
tudes. For the nine quasars which were measured twice, 
average values are given in Table I, although about half 
of these showed evidence of variability. The infrared col- 
ors of those quasars which showed variability are, how- 
ever, derived from observations taken on a single night. 
The maximum amplitude of variability observed, ~0.3 
mag, is consistent with the variability seen in other qua- 
sars (Neugebauer, Soifer, and Matthews 1982) over 
comparable time periods. 
The visual magnitudes were obtained either from im- 
age diameters on the Palomar sky survey prints (19 ob- 
jects) or from photoelectric photometry ( 11 objects). The 
visual magnitudes thus refer to the fluxes at times quite 
different from those when the infrared data were ob- 
tained. 
The x-ray properties of the sample are summarized in 
Figs. 5 and 6. The x-ray fluxes are from Margon, 
Downes, and Chanan (1982). Since complete energy dis- 
tributions are not available for these objects, all photo- 
metric colors are in the observed frame. On each figure 
where color indices are given, power-law slopes a are 
also presented; the flux density is assumed to follow a 
law of the form/v <x Va. The Hubble constant and decel- 
© American Astronomical Society • Provided by the NASA Astrophysics Data System 
K
( 
M
A
G
) 
1641 NEUGEBAUER ETAL. : INFRARED PROPERTIES OF QUASARS 1641 
(S\ CO KO 
r" 
oo 
1.0 
aUK 
0 -1.0 
SERENDIPITOUS 
.5 - 
1.0 - 
0.5 - 
• • $*• 
•• 
• • • • •• 
% • 
0 
1.5 
UV EXCESS 
— 4.0 
2.0 
-l.o 
0 
1.0 
x 
Ö 
— 4.0 
1.0 - 
0.5 - 
0 
0.0 
• • 
• • • 
% • + * 
• v • 
• 4% 4«• 
% • 
2.0 
• • 
• • 
- 0 
- 1.0 
0.5 1.0 1.5 2.0 
J-K(MAG) 
Fig. 1. The infrared colors of the serendipitous x-ray quasars (top) and of the Palomar Green-Schmidt sample selected for their ultraviolet excess 
(bottom). The slopes aJK and affK are defined assuming a power-law dependence of the flux density /v ce Va. 
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1642 NEUGEBAUER ETAL. : INFRARED PROPERTIES OF QUASARS 1642 
a 
VK 
3 - 
2 - 
0 
1 1 r 
SERENDIPITOUS 
r\ X * Oq * 
x o 
,8 o° 
X 
o X X 
X o X 
C9 
< 
I 
~D 
0 
UV EXCESS 
2- 
c£*' 
.•V 
A • • 
tf •• 
• • 
0 
0 
1 
2 3 4 
V-K (MAG) 
- I 
-2 
0 
“3 
Ö 
0 
Fig. 2. The infrared colors vs the visual-2.2-/¿m color of the serendipitous x-ray quasars (top) and of the Green-Schmidt sample (bottom). The V 
magnitudes were found from the image diameters on the Palomar Sky survey prints (open circles), from photoelectric measurements (crosses), or 
from numerical integrations of multichannel data (closed circles). 
© American Astronomical Society • Provided by the NASA Astrophysics Data System 
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U
M
B
E
R
 
1643 NEUGEBAUER ETAL. : INFRARED PROPERTIES OF QUASARS 1643 
s (S\ 
en KO 
r" 
oo 
h) 
CM 00 
0.5 0 -0.5 -1.0 -1.5 -2.0 
Fig. 3. Histograms of the F — A' col- 
or for x-ray-selected quasars (top) 
and for quasars selected for their ul- 
traviolet excess (bottom). The qua- 
sars whose visual magnitudes were 
measured photoelectrically are indi- 
cated by the striped area. 
© American Astronomical Society Provided by the NASA Astrophysics Data System 
1644 NEUGEBAUER ETAL. : INFRARED PROPERTIES OF QUASARS 1644 
x
 U V Excess Sample 
° X-ray Sample 
3- 
e? 
< 
i 2 
XXX X 
Î ^ x 
X o x x x9c
xx 
X X 
< 
o 
X 
X>?0^ Xx< U V ■> 
x ° x x o 
o o 
X 
¿ à 
XX x Xx 
X x^ 
X X 
x
x
x 
-L ±. -L 
-20 -21 -22 -23 -24 -25 -26 -27 -28 -29 -30 -31 -32 
MV(MAG) 
Fig. 4. The infrared color as a function of the absolute visual magnitude for the x-ray-selected sample (open circles) and the sample selected on 
the basis of its ultraviolet excess (crosses). 
eration parameters in these figures and throughout this 
paper have been taken as = 50 km s~1 Mpc~1 and 
?o = 0. 
The main result of this paper is that there is no strong 
difference between the near-infrared properties of qua- 
sars in this sample and of quasars discovered by other 
means. This is illustrated by the figures in which the 
results of the x-ray selected sample are compared with 
similar results for quasars selected via other criteria. In 
Figs. 1-4 the second sample, selected by Green on the 
basis of ultraviolet excess, is from the Palomar bright 
quasar survey (Schmidt and Green 1981). The compari- 
son objects in Fig. 5 are not from a well-defined sample, 
but were chosen because x-ray fluxes were available in 
either Ku, Helfand, and Lucy (1980) or in Zamdrani et 
al. (1981) and infrared measurements were available at 
Caltech. This sample represents a variety of radio and 
visually selected objects. In all cases the infrared obser- 
vations of the second samples were obtained at the 5-m 
telescope. 
The data of Fig. 1 indicate a good correlation between 
the H — K and J — K color indices as is expected from 
measurements at such close wavelengths. More impor- 
tantly, however, both the mean slopes and the intercepts 
of the x-ray-selected sample and the optically selected 
sample are the same within small uncertainties. Figure 1 
represents observations each taken nearly simulta- 
neously and under good photometric conditions. The 
similarity of the distributions is thus a prominent fea- 
ture of these data. 
Figure 2 shows an apparent difference in slope of 
V-K vs J — K between the x-ray-selected and ultra- 
violet-excess selected quasars. Caution is needed in in- 
terpreting this difference, however, as most of the x-ray 
sample has photographically determined V magnitudes 
of low accuracy ( ^ 0.5 mag), while the size of the sample 
with accurate visual photoelectric photometry (<0.05- 
mag accuracy) is so small that it poorly determines the 
slope compared with the determination from the much 
more extensive Schmidt and Green (1981) sample. Both 
the slope and intercept of a linear fit to the observations 
of x-ray-selected quasars with photoelectrically deter- 
mined V magnitudes differ by 2a from those of the ultra- 
violet selected quasars. Furthermore, visual magnitudes 
of the ultraviolet sample were obtained closely in time 
with the infrared data, while, as stressed previously, the 
© American Astronomical Society • Provided by the NASA Astrophysics Data System 
1645 NEUGEBAUER ETAL. : INFRARED PROPERTIES OF QUASARS 1645 
(S\ CO KO 
r" 
oo 
CM 00 
2 - 
O 
< 
I 
T 
x SERENDIPITOUS 
• OTHER 
X X 
X 
X X 
X
 X* 
I - 
0 
35 
X 
X $ X 
X 
• • 
X* 
X • 
X 
36 37 38 39 40 41 
Fig. 5. The infrared color as a 
function of the x-ray luminosity 
for the x-ray-selected sample 
(crosses) and for other quasars 
measured at x-ray wavelengths 
(closed circles); see text. 
LOG Lx(W) 
Although the quasars of this sample have a lower lumi- 
nosity than others, this decreased luminosity is not ac- 
infrared and visual photometry for the x-ray sample was 
separated in many cases by > 1 yr. Thus any flux vari- 
ability of the x-ray sample, even if it is color indepen- 
dent, will introduce further spurious scatter into Fig. 2. 
Although there is a marginally significant difference 
between the two samples displayed in Fig. 2, we feel the 
similarities shown in Fig. 1 are based on more controlled 
observations and thus we cannot assert that there is defi- 
nite evidence that these very different selection proce- 
dures are sampling different parent populations. Simi- 
larly, Fig. 3 shows an apparent difference in the V — K 
widths of the x-ray- and ultraviolet-selected samples, 
but the above considerations again lead us not to attach 
strong significance to this effect. 
Figures 4 and 5 indicate that the dependence of in- 
frared color on either visual or x-ray luminosity is inde- 
pendent of the sample. Both figures emphasize the fact 
that quasars selected as serendipitous x-ray sources are 
less optically luminous than those found by other selec- 
tion techniques, as has been noted previously (Margon, 
Chanan, and Downes 1982). Here we present the result 
that this first infrared study shows no overwhelming 
evidence for a prominent infrared color effect in this 
sample which can be attributed to the x-ray selection 
process. Conversely, because this x-ray-selected sample 
of quasars is subject to different biases than previous 
samples of optically and radio-selected quasars, this 
commonality of infrared colors between samples is an 
important validation of previous inferences (Neuge- 
bauer et al 1979; Hyland and Allen 1982) regarding the 
infrared colors of the “typical” quasar. 
Figure 6 shows that the range of both infrared and 
visual flux densities is comparable to the range of the x- 
ray fluxes, although there is no strong correlation 
between the strength of the x-ray flux and either the 
visual or infrared (2.2 /zm) fluxes. The effect of the x-ray 
flux-limited sampling is plainly seen at /x^10 
counts s~1. The power-law slopes between visual and x- 
ray frequencies (avx ) are compared with the slopes with- 
in the near infrared (aJK) in Fig. 7. The colors show a 
statistically marginal correlation in the sense that qua- 
sars with the steeper infrared slopes exhibit shallower 
visual to x-ray slopes. 
In each of the figures where a comparison is made, the 
quasars initially selected by their x-ray emission are in- 
distinguishable from those selected by other techniques. 
© American Astronomical Society • Provided by the NASA Astrophysics Data System 
K
 
(M
A
G
) 
V
 
(M
A
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1646 NEUGEBAUER ETAL. : INFRARED PROPERTIES OF QUASARS 1646 
s O'! 00 UD 
r" 
oo 
CM 00 
LOG Ix (cnts/IOOOs) 
Fig. 6. The x-ray intensity as a function 
of the apparent visual magnitude ( V) and 
of the apparent 2.2-fim magnitude {K ). 
Open circles represent observations with 
photoelectrically measured magnitudes. 
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1647 NEUGEBAUER ET AL. : INFRARED PROPERTIES OF QUASARS 1647 
Fig. 7. The power-law slope {aJK) 
between 1.25 and 2.2 //m vs avx, the 
power-law slope between the visual and 
x-ray frequencies, for the x-ray-selected 
sample. 
companied by any marked characteristic in the slopes or 
strength of the continua. Recent studies of the visual 
and near-infrared continua of quasars have failed to 
show strong correlations between these continua which 
might have elucidated the nature of the emission 
mechanisms producing the visual and near-infrared 
fluxes in the quasars. Indeed, there is growing evidence 
that the mechanisms producing the radiation at infrared 
and shorter wavelengths are remarkably ubiquitous, al- 
though the individual parameters defining the quantita- 
tive appearance vary significantly. The present observa- 
tions show that the selection of quasars on the basis of 
their x-ray emission has apparently also found objects 
which have similar emission mechanisms at the visible 
and near-infrared wavelengths. 
This work has been supported by NSF Grants No. 
AST 77-27745 and No. AST 80-17335, NASA Con- 
tracts No. NAS 8-30753 and No. NGL 05-002-207, and 
the Alfred P. Sloan Foundation. We thank R. A. 
Downes and S. F. Anderson for aid in reducing the data. 
The data on the sample of quasars found from their ul- 
traviolet excess are from a forthcoming paper with R. 
Green and M. Schmidt who kindly gave permission to 
quote results prior to publication. 
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Chanan, G. A., Margon, B., and Downes, R. A. (1981). Astrophys. J. 
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Elias, J., Frogel, J., Matthews, K., and Neugebauer, G. (1982). Astron. 
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Hyland, A. R., and Allen, D. A. (1982). Mon. Not. R. Astron. Soc. 
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Ku, W., Helfand, D., and Lucy, L. (1980). Nature, 288, 323. 
Margon, B., Downes, R. A., and Chanan, G. A. (1982). Astrophys. J. 
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Margon, B., Chanan, G. A., and Downes, R. A. (1982). Astrophys. J. 
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© American Astronomical Society • Provided by the NASA Astrophysics Data System 


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Infrared properties of serendipitous X-ray quasars

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