An X-ray spectrum of the quasi-stellar object 3C 273 in the interval 0.25 to 10 KeV was obtained by sounding rocket observations. The best-fit power law spectrum has photon index n= 1.3 with no photoelectric absorption. An upper limit on the X-ray optical depth to 3C 273 was calculated from the data, permitting upper limits to be set on the absolute abundance of helium in the intergalactic medium

Bowyer, S.

Lampton, M.

Margon, B.

English

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(NASA-CR-126310) LIMITS ON INTERGALACTIC N72-22808
HELIUM FROM THE 3C 273 X-RAY SPECTRUM B.
Margon, et al (California Univ.) Feb. 1972
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LIMITS ON 'INTERGALACTIC HELIUM
FROM ,THE 3C273 X-RAY SPEtTRUM
by
\' 'Bruce 'Mar90n, Stuart Bowyer:',
and Michael Lampton
Grant'NGR 05-003-278
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Principal Investigator: S., BO,wyer,
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" Space Scienc'es Laboratory -Series 13 Issue,2. 'J
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UNIVER,SITY OF CALIFORNIA.., ,BERKELEY
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https://ntrs.nasa.gov/search.jsp?R=19720015158 2020-03-11T18:53:25+00:00Z
LIMITS ON INTERGALACTIC HELIUM
FROM THE 3C 273 X-RAY SPECTRUM
Bruce Margon
Stuart Bowyer
Michael Lampton
February, 1972
This work was supported by NASA Grant
NGR 05-003-278 and has been accepted
for publication in The Astrophysical Journal.
Space Sciences Laboratory Series 13 Issue 2
LIMITS ON INTERGALACTIC HELIUM
FROM THE 3C 273 X-RAY SPECTRUM
Bruce Margon, Stuart Bowyer, and Michael Lampton
Department of Astronomy and Space Sciences Laboratory
University of California, Berkeley
ABSTRACT: An X-ray spectrum of the quasi-stellar object 3C 273 in the
interval 0.25 to 10 keV has been obtained by sounding rocket observations.
The best-fit power law spectrum has photon index n= 1.3 with no photo-
electric absorption. An upper limit on the X-ray optical depth to 3C 273
has been calculated from the data, permitting upper limits to be set on the
absolute abundance of helium in the intergalactic medium.
I. INTRODUCTION
Several techniques have been applied to place upper limits on the
density of the intergalactic medium (IGM). The search for red-shifted
Lyman-alpha absorption against the continuum spectrum of a quasi-stellar
object (Scheuer, 1965; Gunn and Peterson, 1965) is by far the most sensi-
tive test for neutral hydrogen and provides an upper limit of ~ 10- 13 atoms
cm-
3 (Burbidge and Burbidge, 1967) for the local density of hydrogen in
the IGM. This observation can also be used to set stringent upper limits
on the intergalactic abundance of heavy elements such as LiI, CI, 01, and
FeI (Bahcall and Salpeter, 1965). Cowsik (1971) has shown that if the break
near 40 keV in the diffuse X-ray background is due to thermal emission from
an IGM at 10 8 OK, the relative abundance of iron in this medium must be at
least a factor of 200 below that observed in the Galaxy.
The technique discussed by Bahcall and Salpeter (1965) may also be
used to limit the intergalactic abundance of HeI and Hell if these ele-
ments are in their first excited states. However, it does not seem like-
ly that this condition is satisfied. Thus no technique yet applied yields
any direct information on intergalactic helium abundance. Theoretically,
we should expect the heavy element abundance to be quite small (Wagoner,
Fowler, and Hoyle, 1967); however, the absolute abundance of heli~ may
certainly be nontrivial. Photoionization of helium may be important in
the ·thermalhistory and balance of the IGM (Arons and McCray, 1969). In
adQition, the question of the relative abundance of helium to hydrogen in
the IGM is of crucial cosmological significance.
It has been suggested (Rees, Sciama, and Setti, 1968; Rees and Setti,
2J970) that the IGM may be probed by searching for photoelectric absorp-
tion in the spectrum of the diffuse X-ray background. This is a potenti-
ally sensitive test for helium, since it has a photoelectric absorption
cross section which substantially exceeds that of hydrogen. However, pres-
ent observations of the X-ray background in the region 0.28 keV are quite
discrepant (Silk, 1970), and both the intrinsic spectrum and the absolute
intensity of the radiation are uncertain. It is therefore difficult at
this time to use this method to derive quantitative information on the
X-ray optical depth due to the IGM. An alternate technique (Rees et al.,
1968; Silk, 1971) is to search for photoelectric absorptio~ in the contin-
uous spectra of distant discrete X-ray sources. We report here the first
results derived from this technique, using an X-ray spectrum of the quasi-
stellar object 3C 273.
II. EXPERIMENTAL DATA
A complete report of the experiment detecting X-ray emission from
3C 273 has appeared previously (Bowyer et al., 1970). The source was de-
tected independently by two rocket-borne argon-methane proportional count-
ers equipped with Mylar windows. One detector, equipped with a 3 0 x 12°
FWHM fan beam collimator, observed the source for 19 sec; the other, colli-
mated with a 1.6 0 FWHM pencil beam, observed the source for approximately
10 sec. The spectrum reported here was derived solely from data from the
pencil beam detector, whose small solid angle of 7 x 10 -4 sterad causes it
to be insensitive to the diffuse X-ray background. The chief source of
background in this detector was due to unrejected cosmic-ray events; this
background level was set using 50 sec of high-latitude data which exclud-
ed l the positions of all known X-ray sources. The intensity derived for
3C 273, 0.160±O.055 keV cm- 2 sec- 1 in the interval 1 to 10 keV, agrees
well with that reported by the UHURU satellite (Kellogg et al., 1971), thus
lending further confidence to the background subtraction procedure.
In Figure 1 we present the X-ray spectrum of 3C 273, derived from
the central 8.32 sec of the transit of the source through the detector.
Each experimental point has been corrected for background, detector effi-
ciency, energy resolution, and argon escape radiation. We have fitted the
experimental data to a power law spectrum of photon index n, subjected to
photoelectric absorption with optical depth 1", through the relation
\
3
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2.0 1.6 1.2 0.8 0.4 0.0
PHOTON POWER LAW INDEX
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Figure 1. X-ray spectrum of 3C273. Error bars are ±l crstatistical
errors. Solid line: best power-law fit to the data. Inset: contour
of constant probability for the least~squares fitting procedure. The
cross marks the highest confidence model.
4where L = Ncr and N and cr are respectively the column density and the photo-
electric absorption cross section of helium. In this analysis, c, N, and
n have been permitted to vary as free parameters. The details of the least-
squares fitting procedure have appeared previously (Lampton et al., 1971).
The best-fitting spectrum, F(E) = 0.028 E- 1 . 3 photons cm- 2 sec-1 keV- 1 ,
is shown by the solid line in Figure 1. This spectrum has a confidence
of 71% and shows no evidence for photoelectric absorption ( L = 0). The
inset to Figure 1 presents a contour of constant probability for the
least-squares fits; the contour displayed is where the confidence drops
to e-O• 5 = 0.607 of the peak value, which is analagous to the standard
deviation of a Gaussian variate.
III. ANALYSIS AND DISCUSSION
Few assumptions need be made to derive an upper limit on the helium
abundance in the IGM from these data. We must aSSQme that 3C 273 is at
the cosmological distance indicated by its redshift of 0.158. This object
is then the most distant known X-ray source, at a distance of 580 Mpc,
assuming a Hubble constant of HO = 75 km sec-
1 Mpc- 1 (Sandage, 1968).
Upper limits on the absolute helium abundance in the IGM can be set
independent of the relative abundance of helium by assuming ~~at all of
the maximum permitted optical depth in the X-ray spectrum is due to helium.
This is a reasonable assumption in any case, since at 1 keV the HeI cross
section is a factor of ~ 30 greater than that of HI. We may neglect ab-
sorbing matter within our own Galaxy, as 21-cm absorption studies against
3C 273 (Williams, 1965; Hughes, Thomson, and Colvin, 1971; Radhakrishnan
et al., 1971) show that at 1 keV, L% 0·.02 for assumed abundances of the
. .
interstellar medium (Brown and Gould, 1970).
We may avoid assumptions on the ionization mechanism and equilibrium
of intergalactic helium by obtaining separate limits for HeI and Hell.
Following silk (1971), we calculate the density of intergalactic helium,
p , from the relation
He
Ll H Itt.-o Hp.
OcG ( z)
gm cm- 3
5where
z
(S1z < < 1).,
1 + Z
rz1 ....:d...z<=.- -;-__ ~ ---
o (1 + z) 2 (1 + S1z) ~G(z)
In these expressions, S1 = pip 't' 0 is the cross section at 1 keV for
cr~
HeI (Henke, 1967) or Hell (Brown and Gould, 1970), and L l is the
observed up~er limit on the optical depth at 1 keV.
---"The probability contour in Figure 1 permits us to conclude that the
maximum optical depth at 1 keV compatible with the observational data is
Ll = 8.7.
We note that if we use this "worst-case" optical depth, the value of
Ll is independent of the true value of
the photon power law index n. In
this analysis we have assumed that the spectrum of 3C 273 is in fact a
power law within the energy range of our observations. This assumption
appears justified for a number of reasons. First, our spectral data
may be fitted by this model with satisfactory confidence. Second, most
X-ray emission mechanisms proposed for quasi-stellar objects, involving
synchrotron radiation or the inverse Compton effect, result in power-
law spectra in the keV regime. Third, all other reported extragalactic
X-ray spectra of sources in which violent events are occuring, M87
(Lampton et al., 1971 and references therein), NGC 5128 (Lampton et al., 1972),
and NGC 1275 (Fritz et al., 1971), are compatible with power laws. Since
these spectra are also compatible with thermal bremsstrahlung emission
mechanisms. however. we have also compared our data with an exponential
spectrum. We find that all source temperatures T ~ 1.5 x 10 7 OK are
compatible with the observations at the 10 level, and the maximum
permitted optical depth at 1 keV is L = 3.2. Thus if we utilize the1
larger value of Ll derived for power law spectra, we are assured of
calculating valid upper limits on abundances even if the observed X-rays
are due to thermal bremsstrahlung.
In Figure 2 we present the results so derived for helium. Since
the distance to the source as well as the critical density of the IGM
depends upon the uncertain value of HO' we have performed the calculations
for a range of likely values of this parameter. The figure also
indicates the density of matter necessary to close a Friedmann Universe
at each value of HO' For favorable values of HO' the present upper
limits are a factor of about 8 greater than the critical density. These
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7limits are quite crude due to the short time that the source was observed
and the resulting uncertainty in Tl • However, increased exposures obtain-
able even with current rocket-borne instrumentation can set stringent
limits on helium in the IGM.
Another X-ray sourqe to which this technique may be applicable is
NGC 1275 (Fritz et al., 1971; Gursky et al., 1971). At a distance of
approximately 70 Mpc, this object offers only ~O.l the path length to
3C 273; however, the flux at Earth appears to be 11 times greater than
that from 3C 273 in the energy band 1 to 10 keV, and 37 times greater in
the 0.25 to 10 keV band if the spectrum of Fritz et ale is correct. This
source has the disadvantage of definitely having a photoelectric absorp-
tion component due to galactic matter, since it is located at low latitude.
However, column densities of hydrogen within the Galaxy may be derived
from 21-cm data, and thus the necessary correction can in principle be
made with reasonable precision.
There is an existing spectrum of NGC 1275 (Fritz et al., 1971) which
has been fitted with a power law spectrum with no turnover when corrected
for galactic absorption. However, the published spectrum lacks sufficient
detail to subject it to the analysis required to set statistically valid
upper limits on the X-ray optical depth. It does appear that this spectrum
is not incompatible with the results reported here.
The data on 3C 273 reported here have another interesting implication.
It has been shown by silk (1970) that if all quasi-stellar sources have
X-ray luminosities similar to that of 3C 273 and exhibit suitable evolu-
tionary effects, quasars provide sufficient X-ray emission to explain
the intensity of the diffuse X-ray background. We note that the similarity
of the spectral index derived here for 3C 273 to that of the diffuse back-
ground in the 1 to 10 keV band is consistent with this suggestion.
We acknowledge several useful discussions with Dr. J. Silk. This
work has been supported in part by NASA Grant NGR 05-003-278.
8REFERENCES
Arons, J. and McCray, R. 1969, Ap. Lett., ~, 123.
Bahcall, J.N. and Salpeter, E.E. 1965, Ap. J., ~, 1677.
Bowyer, C.S., Lampton, M., Mack, J.E., and deMendonca, F. 1970,
Ap. J. (Letters), l§l, Ll.
Brown, R.L. and Gould, R.J. 1970, Phys. Rev., ~, 2252.
Burbidge, G., and Burbidge, M. 1967, Quasi-Stellar Objects, (San Francisco:
Freeman), p. 146.
Cowsik, R. 1971, paper presented to the High Energy Astrophysics Division,
136th Meeting of the American Astronomical Society, December 1971.
Fritz, G., Davidsen, A., Meekins, J.F., and Friedman, H. 1971, Ap. J.
(Letters), ~, L81.
Gunn, J.E. and Peterson, B.A. 1965, Ap. J., ~, 1633 .
. Gursky, H., Kellogg, E.M., Leong, C., Tananbaum, H., and Giacconi, R. 1971,
Ap. J. (Letters), ~, L43.
Henke, B. 1967, Nore1co Reporter, W' 112.
Hughes, M.P., Thomson, A.R., and Colvin, R.S. 1971, Ap. J. Suppl., 3l,
No. 200.
Kellogg, E., Gursky, H., Leong, C., Schreier, E., Tananbaum, H., and
Giacconi, R. 1971, Ap. J. (Letters), 12§, L49.
Lampton, M., Bowyer, S., Mack, J.E., and Margon, B. 1971, Ap. J. (Letters),
~~, Ll.
Lampton, M., Margon, B., Bowyer, S., Mahoney, W., and Anderson, K. 1971,
Ap. J. (Letters),~, L45.
Radhakrishnan, V., MUJ:ray, ,J.D., Lockhart, P., and Whittle, H.P.;T. J971,
Ap. J. Suppl., ~, No. 203.
Hees, M.,T., Sciama, D.W., and Setti, G. 1968, Nature, ill, 326.
Rees, M.J., and Setti, G. 1970, Ast. and Ap., ~, 410.
Sandage, A.R. 1968, Ap. J. (Letters), ~, L149.
References Continued
Scheuer, P.A.G. 1965, Nature, ~, 963.
Silk, J. 1970, Space Sci. Rev., 11, 671-,....
Silk, J. 1971, Primordial Turbulence and Intergalactic Matter, (Gordon
and Breach, in press) •
WagQner, R.V., Fowler, W.A., and Hoyle, F. 1967, Ap. J., ~, 3.
Williams, D. 1965, in Quasi-Stellar Sources and Gravitational Collapse,
ed. I. Robinson, A. Schild, and E. Schucking (Chicago: University of
Chicago Press), p. 213.
9
Figure Captions
Figu~e 1. X-ray spectrum of 3C273. Error bars are ±l 0 statistical
errors. Solid line: best power-law fit to the data. Inset: contour
of constant probability for the least-squares fitting procedure. The
cross marks the highest confidence model.
Figure 2. Solid lines; upper limits on helium abundance in the inter-
galactic medium as a function of the Hubble constant. Broken lines:
9ritical densities for a Friedmann Universe.
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Limits on intergalactic helium from the 3C 273 X-ray spectrum

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