Balloon measurement of high energy X-ray spectrum from Crab Nebul

Craddock, W. L., Jr.

Haymes, R. C.

English

NASA Technical Reports Server

J 
H i g h - E n e r g y  X-rays from t h e  
C r a b  Nebula 
R. C. Haymes  and W. L. C r a d d o c k ,  Jr. 
D e p a r t m e n t  of Space Science 
R i c e  U n i v e r s i t y  
Houston, T e x a s  
8 Februa ry  1966 
https://ntrs.nasa.gov/search.jsp?R=19660014463 2020-03-16T19:43:19+00:00Z
HIGH-ENERGY X-RAYS FROM THE CRAB NEBULA 
by 
1 
R. C. Haymes and W. L. Craddock, Jr. 
Space Science  Department 
R i c e  U n i v e r s i t y  
Houston, Texas 
W e  have conducted a b a l l o o n  measurement of t h e  x - r a d i a t i o n  
from t h e  Crab Nebula. Photons of h igh  energy  (-50 keV) from 
t h e  Crab w e r e  f i r s t  d e t e c t e d  by C la rk  (1965):  t h e  p r e s e n t  
experiment  h a s  examined t h e  energy r ange  23  keV t o  455 keV w i t h  
i n c r e a s e d  s e n s i t i v i t y  and deduced a spectrum over  a b r o a d e r  
r a n g e  t h a n  t h e n  possible. T h i s  letter d i s c u s s e s  t h e  x-ray 
spectrum a t  photon e n e r g i e s  u p  t o  90 keV. 
The in s t rumen t  employs a NaI ( T I )  s c i n t i l l a t o r  10 c m  i n  
d i a m e t e r  and 5 c m  t h i c k .  
i s  abou t  27%, f u l l - w i d t h  half-maximum, a t  50 keV and about  16% 
a t  150 keV. This s c i n t i l l a t o r  and a s s o c i a t e d  p h o t o m u l t i p l i e r  
a r e  surrounded by ano the r  NaI (T l )  c r y s t a l  t h a t  i s  30 c m  long  
and 25.4 c m  i n  o u t e r  diameter :  it ac t s  a s  a c o l l i m a t o r .  The 
w a l l  t h i c k n e s s  of t h e  col l imator  i s  7.5 c m  on t h e  sides and 10  c m  
i n  t h e  back;  s ix  p h o t o m u l t i p l i e r s  a r e  a s s o c i a t e d  w i t h  it. The 
t w o  c r y s t a l s  a r e  o p t i c a l l y  decoupled from each o t h e r ,  and t h e  
p h o t o m u l t i p l i e r s  viewing t h e  t w o  a r e  connected i n  an t i - co inc idence .  
A t h i n  (0.64 gm/cm ) p l a s t i c  s c i n t i l l a t o r  cove r s  t h e  75 c m  
a p e r t u r e .  The p h o t o m u l t i p l i e r s  viewing t h e  p l a s t i c  are a l s o  
The energy  r e s o l u t i o n  of t h e  system 
2 2 
1. NASA P r e d o c t o r a l  Fe l low 
-2- 
. 
c o n n e c t e d  i n  a n t i - c o i n c i d e n c e  w i t h  t h e  t u b e  v iewing  t h e  c e n t r a l  
c r y s t a l .  
o ther  r a d i a t i o n  t y p e s  such  a s  cosmic r a y s .  A d d i t i o n a l  a n g u l a r  
c o l l i m a t i o n  i s  p r o v i d e d  by  t h r e e  7.5 c m  t h i c k  to ro ida l  p l a s t i c  
s c i n t i l l a t o r s  spaced  i n  f r o n t  of the  main assembly  (see F i g .  1). 
Each of these three s c i n t i l l a t o r s  has air i n n e r  d i a m e t e r  of i0 c m  
a n d  a n  o u t e r  d i a m e t e r  of 25.4 c m .  
v i ewing  them a re  a l s o  connec ted  i n  a n t i - c o i n c i d e n c e  w i t h  t h e  
c e n t r a l  s c i n t i l l a t o r .  The  a c c e p t a n c e  cone of t h e  i n s t r u m e n t  
v a r i e s  s l i g h t l y  w i t h  energy:  a t y p i c a l  v a l u e  i s  0.006 ster. 
The a n t i - c o i n c i d e n c e  a r r angemen t  rejects p u l s e s  due  t o  
The s ix  p h o t o m u l t i p l i e r s  
P u l s e s  t h a t  a r e  due  t o  pho tons  a r e  a n a l y z e d  by a b a l l o o n  
b o r n e  128-channel  p u l s e  b g h t  a n a l y z e r ,  t h e  g a i n  of which i s  
set t o  c o v e r  t h e  desired energy  r a n g e  (23 t o  455 k e V ) .  The 
o u t p u t  is  d i g i t a l l y  telemetered. 
The i n s t r u m e n t  i s  p o i n t e d  i n  f l i g h t  a t  the  Crab by  a 
s e r v o  sys tem t h a t  l o c k s  on t h e  geomagnet ic  f i e l d  i n  az imuth .  
An e q u a t o r i a l  mount i s  p rov ided .  The p o l a r  a x i s  i s  p a r a l l e l  
t o  t h e  e a r t h ' s  axis  of  r o t a t i o n  t h r o u g h  compensat ion for  t h e  
a v e r a g e  geomagnet ic  d e c l i n a t i o n  a l o n g  the t r a j e c t o r y .  
i n s t r u m e n t  i s  c lock-d r iven  i n  r i g h t  a s c e n s i o n  a b o u t  t h i s  axis  
a t  a r a t e  of one r e v o l u t i o n  per day. Radio commands a r e  
p r o v i d e d  t o  change t h e  r a t e  a s  t h e  b a l l o o n  changes  l o n g i t u d e  
and  a l s o  t o  p r o v i d e  a n  o f fse t  so t h a t  t h e  background may be 
measured. 
The 
The b a l l o o n  w a s  launched  f r o m  P a l e s t i n e ,  Texas ,  a t  2205 
C.S.T., on October 1 9 ,  1965,  and a scended  t o  p r e s s u r e  a l t i t u d e s  
v a r y i n g  between 4.35 mbar a n d  6.0 mbar,  a s  measured b y  a photo-  
b a r o g r a p h  c a r r i e d  a l o f t  w i t h  t h e  i n s t r u m e n t .  The f l i g h t  was 
t e r m i n a t e d  by r a d i o  command a t  0902 t h e  fo l lowing  day. 
cu lmina ted  a t  0353 on 20 October (when corrected f o r  b a l l o o n  
l o n g i t u d e ) ,  a t  10  d e g r e e s  sou th  of z e n i t h .  
The Crab 
The i n s t r u m e n t  was recovered i n  good c o n d i t i o n  and t h e  
p o s t - f l i g h t  c a l i b r a t i o n s  ag ree  w i t h  t h o s e  conducted prior t o  
t h e  f l i g h t .  T h e r i r o s t a t i c a i i y  cwriti-wiled lieat was prov ided  
d u r i n g  t h e  f l i g h t  i n  o r d e r  t h a t  t h e  system g a i n  should  be 
c o n s t a n t ,  and t h e  f l i g h t  d a t a  e x h i b i t  no t i m e  dependence con- 
s i s t e n t  w i t h  a g a i n  change. 
A b a t t e r y  f a i l u r e  occurred  a t  0610 C.S.T., t h u s  l i m i t i n g  
t h e  t i m e  f o r  measurement of t h e  background a t  t h e  f l o a t i n g  
a l t i t u d e  t o  40 minutes ,  compared w i t h  320 minutes  of expcsure 
t o  t h e  Crab ( p l u s  background) .  The background was o b t a i n e d  i n  
t h e  i n t e r v a l  0010 t o  0050, a t  an ave rage  z e n i t h  a n g l e  of 4S0, 
j u s t  b e f o r e  t h e  p o i n t i n g  system w a s  t u r n e d  on by r a d i o  command. 
The background i s  due t o  a tmospheric  r a d i a t i o n  and t o  photons  
l o c a l l y  produced i n  t h e  suppor t ing  s t r u c t u r e ,  b a t t e r i e s ,  e tc .  
I n  t h e  d i s c u s s i o n  t h a t  fo l lows ,  t h e  background has  been  
normalized by a f a c t o r  of 6.18 t h a t  i s  t h e  r a t i o  of t h e  t w o  
ave rage  f l u x e s  i n  channels  80-100 (255 keV-363 keV). The 
d i f f e r e n c e  between t h i s  and t h e  8.00 r a t i o  of t h e  measurement 
periods presumably reflects t h e  zen i th -ang le  dependence of t h e  
a tmospher ic  background ( F r o s t  e t  a l . ,  1963) ,  s i n c e  (6.18/8.00) 
i s  e q u a l  t o  t h e  r a t i o  of t h e  s e c a n t s  of t h e  t w o  ave rage  z e n i t h  
a n g l e s .  
F i g u r e  2 shows t h e  spectrum o b t a i n e d  w h i l e  p o i n t i n g  a t  
t h e  Crab and t h e  (normal ized)  background spectrum. The atmo- 
s p h e r e  and p l a s t i c  s c i n t i l l a t o r  s t r o n g l y  abso rb  x- rays  a t  l o w  
energies;  t h i s  r e s u l t s  i n  t h e  appa ren t  peak a t  about  33 keV. 
-4- 
. 
The d i f f e r e n c e  between t h e  t w o  s p e c t r a  h a s  been  corrected 
t o  t h e  top o f  t h e  atmosphere,  t a k i n g  i n t o  accoun t  t h e  a b s o r p t i o n  
f a c t o r ,  t h e  p a t h  l e n g t h  through t h e  atmosphere (X sec 8 ,  where X 
i s  t h e  d e p t h  and 6 t h e  z e n i t h  a n g l e ) ,  t h e  dead  t i m e  of t h e  
sys tem (- 2%) and t h e  geomet r i c  f a c t o r .  Data a t  e n e r g i e s  less 
L I ~ ~ I I  33 keV have been rejected b e c a u s e  of t h e  v e r y  i a r g e  
c o r r e c t i o n s  n e c e s s a r y  for  a tmospher ic  a b s o r p t i o n .  The r e s u l t  
i s  shown i n  F i g u r e  3 .  
LL-- 
W e  have shown i n  F i g u r e  3 f o r  p u r p o s e s  of comparison t h e  
r e s u l t s  of C l a r k  (1965) ,  who found a n  E-3 dependence of  t h e  
pho ton  f l u x  on energy  b e t w e e n  20  keV and 40 keV. There i s  
r a t h e r  good agreement i n  t h e  r e g i o n  of o v e r l a p .  The p r e s e n t  
exper iment  y i e l d s  a f l u x  below t h e  upper  l i m i t  set by t h e  
M I T  exper iment  f o r  e n e r g i e s  i n  excess of 62 keV. 
The r e c e n t  d e t e r m i n a t i o n  by P e t e r s o n  e t  a l .  (1966) is  a l s o  
shown i n  t h e  f i g u r e .  That  experiment  took place abou t  26 days  
p r i o r  t o  ou r  measurement. The d i s c r e p a n c i e s  between t h a t  
d e t e r m i n a t i o n  and o u r s  ( a s  w e l l  a s  t h e  C l a r k  exper iment )  a r e  
n o t  r e a d i l y  r e s o l v a b l e .  
It i s  possible t o  approximate ou r  spectrum w i t h  t w o  power 
-4.4 
laws.  Between 30 keV and 50 keV, o n e  f i n d s  an  E d i f f e r e n t i a l  
spectrum: a n  E v a r i a t i o n  may be f i t t e d  t o  the d a t a  between 
50 keV and 74 keV, w h e r e  t h e s e  refer t o  t h e  photon f l u x  a t  t h e  
kop of t h e  atmosphere.  But w e  shou ld  n o t e  t h a t  t h e  r e s u l t s  can 
be best approximated by a s i n g l e  e x p o n e n t i a l  ove r  t h e  e n t i r e  
r ange .  
c m  sec keV , where E i s  expres sed  i n  keV (see F i g u r e  4 ) .  
-9.2 
The e x p r e s s i o n  i s  (1.1 * ( 3 . 3 )  exp f.119 f .OOjE) pho tons  
-2 -1 -1 
The synchro t ron  mechanism p roduces  a power l a w  spectrum. 
However, a s  Burbidqe e t  a l .  (1965) and Gould and Burbidqe (1965) 
-5- 
have recently remarked, the electron lifetime against synchro- 
tron emission is only - 30 years << 911 years, the age of the 
Crab. A synchrotron source for high energy x-rays thus requires 
an active source of very high energy (- 10 ev) electrons 
assuming a 10 gauss magnetic field). 
14 
-4 
Bremsstrahlung from a hot gas, however, produces an 
exponential spectrum (Allen, 1963). The temperature deduced 
from the slope of the exponential curve fitted to the data by 
least squares in Figure 4 is * 0 . 5 ) ~  10 7 0  K. 
One possible mechanism for generating this temperature is 
suggested by the observed high expansion velocity of the nebula 
(- 1100 km/sec). A plasma streaming this fast will encounter 
shock waves wherever there are obstacles to the flow, such as 
the interstellar medium and the filaments in the Crab. A rather 
analogous situation exists at the earth's magnetosphere, where 
the solar wind velocity of - 400 km/sec results in temperatures 
of lo7 OK immediately behind the collisionless shock that is in 
front of the magnetosphere (Bridqe et al. 1965). The analogy 
suggests that a plasma speed of 1100 km/sec will produce tempera- 
tures (11/4) as great, or about 10 "K at the shocks in and 
around the Crab. It should also be noted that Heiles (1964) 
and Burbidqe et al. (1965) have suggested that a thermal 
bremsstrahlung model should be considered. 
2 8 
. 
-6- 
Acknowledgments: 
W e  would l i k e  t o  express o u r  ve ry  deep a p p r e c i a t i o n  
t o  M r .  D. R. Oehme f o r  h i s  i n v a l u a b l e  a s s i s t a n c e  i n  t h e  
d e s i g n  and c o n s t r u c t i o n  of t h e  e l e c t r o n i c s .  W e  a l s o  t h a n k  
Messrs. D. V. E l l i s ,  G. 3 .  Fishiiian a i d  R. L. Showen f o r  
h e l p  i n  conduc t ing  t h e  expe r imen t ,  and D r s .  D. D. C lay ton  
and  A. J. Dessler f o r  v a l u a b l e  d i s c u s s i o n s .  The r e s e a r c h  
w a s  s u p p o r t e d  i n  p a r t  by t h e  A i r  Fo rce  O f f i c e  of S c i e n t i f i c  
Resea rch ,  O f f i c e  of Aerospace  Resea rch ,  U n i t e d  S t a t e s  A i r  
F o r c e ,  under  Gran t  Number AF-AFOSR-858-65. 
-7- 
REFERENCES 
C. W. A l l e n ,  A s t r o p h y s i c a l  Q u a n t i t i e s  (The Ath lone  P r e s s ,  
London 1963), 2nd e d . ,  p. 100. 
H. B r i d g e ,  A. E g i d i ,  A .  Lazarus  and E. Lyon, P r e l i m i n a r y  
i i e s u i t s  of Plasma i j ieasurements on imp A, Space Research  V 
(North-Holland P u b l i s h i n g  Company, A m s t e r d a m ,  1 9 6 5 ) ,  969-978. 
G. R. Burb idge ,  R. J. Gould and W.  H. Tucker ,  G a l a c t i c  X-Ray 
S o u r c e s ,  Phys. Rev. L e t t e r s  14,  289-292 (1965).  
G. W. C l a r k ,  Ba l loon  Observa t ion  of t h e  X-Ray Spectrum of t h e  
C r a b  Nebula,  Phys.  Rev. L e t t e r s  1 4 ,  91-94 (1965) .  
K. J, F r o s t ,  E. D. Rothe and L. E. P e t e r s o n ,  Low-Energy Gamma-Ray 
F l u x ,  Spectrum, and S p a t i a l  D i s t r i b u t i o n  Observed a t  3.5 g 
R e s i d u a l  Atmosphere o v e r  Minneapol i s ,  Trans .  Am. Geophys. U. 
- 44, 74-75 (1963) .  
R. J. Gould and G. R. Burbidge, H i g h  Energy Cosmic Photons  and 
N e u t r i n o s ,  Annales  d 'As t rophys ique  28, 171-201 (1965) .  
C.  H e i l e s ,  Supernova S h e l l s  and  G a l a c t i c  X-Rays, As t rophys .  J. 
- 140,  470-476 (1964) .  
L. E. P e t e r s o n ,  A .  S. Jacobson and R. M. P e l l i n g ,  Spectrum of 
Crab  Nebula X-Rays t o  120 Kev, Phys.  Rev. L e t t e r s  16, 142-144 
(1966) . 
-8- 
FIGURE CAPTIONS 
F i g u r e  1, Schematic view of  t h e  gamma-ray d e t e c t o r .  The 
p h o t o m u l t i p l i e r s  viewing t h e  p l a s t i c  s c i n t i l l a t o r s  
a s  w e l l  as t h o s e  viewing t h e  cup-shaped N a I  
s c i n t i l l a t o r  a r e  connected i n  an t i - co inc idence  w i t h  
t h e  p h o t o m u l t i p l i e r  a s s o c i a t e d  w i t h  t h e  c e n t r a l  NaI 
s c i n t i l l a  t o r  - 
F i g u r e  2 .  P u l s e  h e i g h t  s p e c t r a  o b t a i n e d  w h i l e  p o i n t i n g  a t  
t h e  Crab and whi le  p o i n t i n g  away from t h e  nebula .  
The exposure t i m e  was 320 minutes .  
F i g u r e  3 .  The x-ray spectrum from t h e  Crab Nebula, c o r r e c t e d  
t o  t h e  top of t h e  e a r t h ' s  atmosphere.  The p r e s e n t  
d e t e r m i n a t i o n  a g r e e s  w e l l  w i t h  t h a t  by C l a r k ,  i n  
t h e  r e g i o n  of ove r l ap .  It is  a l s o  seen  t h a t  a 
power-law spectrum i s  v a l i d  only over  a narrow 
range  of e n e r g i e s .  The NRL d a t a  have been  quoted  
by C la rk  and t h e  F r o s t  e t  a l .  r e s u l t s  have been 
noted  by P e t e r s o n  e t  a l .  
F i g u r e  4. The d a t a  of F i g u r e  3 ,  replotted on semi- logar i thmic  
c o o r d i n a t e s .  I t  is  seen  t h a t  a s i n g l e  e x p o n e n t i a l  
i s  a good approximation t o  t h e  spectrum over  t h e  
e n t i r e  energy range. The slope of t h e  e x p o n e n t i a l  
cor responds  t o  a t empera ture  of (9.5 * 0 . 9 ~  10 7 0  K. 
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High-energy X-rays from the Crab Nebula

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