Local supergalaxy as structured aspect of X-ray backgroun

Boldt, E.

English

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THE LOCAL SUPERGALAXY AS THE STRUCTURED 
ASPECT OF A UNIVERSAL BACKGROUND OF X-RAYS 
Elihu Boldt 
September 1967 
. GODDARD SPACE FLIGHT CENTER 
Greenbelt, Maryland 
THE LOCAL SUPERGALAXY AS THE STRUCTURED 
ASPECT OF A UNIVERSAL BACKGROUND OF X-RAYS 
Elihu Boldt 
ABSTRACT 
It is shown that Silk's model for a universal X-ray background 
leads to a measurable structure arisingfrom the local supergalaxy. 
A telescope tuned for detecting this structure would have a field of 
view 21 arc degree with a resolution SlO arc  minutes. The ratio of 
the expected signal to the observed diffuse background is of the 
order of unity. 
iii 
THE LOCAL SUPERGALAXY AS THE STRUCTURED 
ASPECT OF A UNIVERSAL BACKGROUND OF X-RAYS 
An important application for an X-ray with a resolution of 
$10 arc  minutes is the measurement of the fine structure characteristic of a 
universal background. Silk(3) attributes the observed background to the super- 
posed contributions from all normal galaxies. The following discussion describes 
how this model leads to a measurable structure. 
Definitions: 
I 
4 
a 
r = average galactic radius 
q 
For the situation where the observed background is the superposition of the 
observed diffuse background flux (ergs/cm2-sec -sr) 
half-angle subtended by an average galaxy 
half-angle for the average field of view required to  observe one galaxy 
X-ray power output for an average galaxy (e rgdsec)  
contributions from all galaxies, it is expected that 
The X-ray background flux is 
I = 9 x ergs/cm2-sec-sr . 
1 
Silk(3) estimates that the X-ray power output from our own galaxy is 
q 2: ergs/sec . 
Insertion of I and q, as given by (2) and (3), into Equation (1) yields 
(3) 
a = 6 x r . (4) 
For r - 3 x cm., this Equation (4) indicates that $ and a are  of the same 
order. Hence, for the -lolo galaxies of the metagalaxy (697) 
- d 2 4 arc seconds . (5) 
This angular size ($24  a rc  seconds) for an object of radius equal to that of 
our galaxy (r * 3 x cm) would correspond to a distance of -2 x lo2' cm., 
comparable to the radius (R) of the metagalaxy. The fineness of this structure 
( $ 2 4  arc seconds) would be difficult to detect. 
The situation for the local supergalaxy(7) of -lo4 galaxies is promising as 
regards detectable structure. The radius (R')  of the local supergalaxy is 
R' * 2 x lo2' cm. Therefore, the half-angle 4' subtended by a representative 
galaxy would be 
r 
$ 1  - 
R'  
-+ d' 2 5 arc  minutes . 
2 
The half-angle ( a ' )  for the average field of view required to observe one 
such galaxy is given by 
- a' 2 1 degree . (7) 
Over a field of view for which a' - 1 degree, it is expected that the average 
background (2) is achieved, corresponding to -9 x 
ever, over the -5 arc minutes half-angle subtended by a representitive galaxy 
of the local supergalaxy it is expected that the background increases by an 
amount 
ergs/cm2-sec- sr. How- 
= 3 x ~ o - ~ e r g s / c m ~ - s e c - s r  , 9 A I  2 
(2-rr  o2 
for q = lo3' ergs/sec and r"3 x 10' cm. 
Comparing (8) with (2), it is important to note that AI  is comparable to I. 
The flux (J) from a representative galaxy of the local supergalaxy is given 
by 
- J 2: 2 x 10-13ergs/cm2-sec. (9) 
For a spectrum of the form given by Seward et al.(4) for photons 24 keV, 
this energy flux (9) corresponds to -2 x photons/cm2-sec. 
3 
In summary, the galaxies of the local supergalaxy are  expected to give a 
significant structure to the X-ray background, with the following features: 
i) The deviation in the flux is comparable to the general sky background. 
ii) The enhanced flux is associated with angular regions -5 arc  minutes, 
half-angle. 
iii) The average field of view required to observe one such enhanced region 
is -1 degree half-angle. 
iv) The photon flux (L4 keV) anticipated from a representative galaxy of 
the local supergalaxy is -2 x lo-’ photons/cm2-sec. 
REFERENCES 
1. 
2. 
3 .  
4.  
5 .  
6 .  
7 .  
Giacconi, R. and Rossi B., J. Geophys. Res. 65, 773 (1960). 
Giacconi, R.,  Harmon, N. F., Lacey, R. F., and Szilagyi, Z., NASA Con- 
tractor Report NASA CR-41 (1965). 
Silk, J., submitted to the Astrophysical Journal, August 2, 1967. American 
Science and Engineering, Inc., Publication ASE- 1697. 
Seward, F. D., Chadil, G., Mark, H., Swift, C., and Toor, A., Ap. J. 1967 
(in the press). 
Byram, E. T., Chubb, T. A., and Friedman, H., Science 152, 66 (1966). 
Allen, C. W., Astrophysical Quantities, University of London (Athlone Press) 
1955. 
Ginzburg, V. L., and Syroaitskii, S. I., The Origin of Cosmic Rays, Pergamon 
Press  (Oxford) 1964. 
4 


The local supergalaxy as the structured aspect of a universal background of X-rays

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