High energy X ray sky survey data on decreased intensity of Cen XR-2, and X ray energy flux from Magellanic clouds - Mildura, Australia, October 15 and 24, 196

Clark, G. W.

Lewin, W. H. G.

Smith, W. B.

English

NASA Technical Reports Server

https://ntrs.nasa.gov/search.jsp?R=19690001933 2020-03-12T06:10:30+00:00Z
C . S . R .  P68-1 
Decrease in the High Energy X-Ray 
Search for X-Rays from the 
Large and Small Magellanic 
Clouds 
Flux from Cen XR-2 
W.H.G. Lewin,G.W. Clark,W,B. Smith 
This work was supported in part by a grant from the National 
Aeronautics and Space Administration, in part under contract 
At(30-112098 from the United States Atomic Energy Commission, 
and in part by a grant from the Sloan Fund of M.I.T. 
Two high energy x-ray sky surveys w e  
ba l loon  f l i g h t s  from Mildura,  A u s t r a l i a ,  on O c  
1967. W e  observed a 30% decrease  i n  t h e  h igh  energy x-ray 
i n t e n s i t y  of  Cen XR-2 between t h e  t w o  ba l loon  observa t ions .  
W e  a lso placed upper l i m i t s  on t h e  high energy x-ray i n t e n s i t i e s  
from t h e  Large and S m a l l  Magellanic Clouds t h a t  are less than  
1 / 1 2  and 1/8 of t h e  i n t e n s i t y  from Tau X - 1 ,  r e s p e c t i v e l y ,  
Resul t s  on Cen XR-2 from t h e  October 1 5 t h  f l i g h t  have 
been publ ished elsewhere (Lewin e t  a l ,  1968a).  I n  t h i s  paper  
w e  d i s c u s s  t h e  r e s u l t s  from t h e  October 24th f l i g h t .  W e  used 
fou r  N a I ( T 1 )  s c i n t i l l a t i o n  d e t e c t o r s  w i th  a t o t a l  s e n s i t i v e  
a rea  of 368 c m 2  and an a n t i c o i n c i d e n t  j a c k e t  of p l a s t i c  s c in -  
- 
t i l l a t o r .  The f i e l d  of view had an angular  width of 17.5O FWHM. 
The instrument  w a s  suspended f r o m  t h e  ba l loon  wi th  i t s  a x i s  in- 
c l i n e d  a t  40' from t h e  v e r t i c a l ,  and it w a s  rotated cont inuous ly  
w i t h  an average per iod  of  about  8 minutes.  The p u l s e s  w e r e  
analyzed i n  7 channels covering t h e  energy range from 20 to  
110  keV. 
source demonstrated t h a t  no change of  any s i g n i f i c a n c e  occurred  
i n  t h e  channel boundaries.  W e  determined t h e  background rates 
by averaging over  400 sec i n t e r v a l s ,  excluding t h o s e  pe r iods  
when obvious sources  w e r e  i n  t h e  f i e l d  of v i e w .  The azimuth of 
t h e  t e l e scope  a x i s  w a s  measured cont inuously by magnetometers 
which w e r e  c a l i b r a t e d  i n  f l i g h t  by r e f e r e n c e  t o  a sunsensor .  
The d i r e c t i o n  of ope a x i s  w a s  d rmined w i t h i n  an  un- 
I n - f l i g h t  c a l i b r a t i o n  every 20 minutes w i t h  an Am241  
c e r t a i n t y  r a d i u s  of approximately 2O a t  every i n s t a n t  throughout  
t h e  f l i g h t .  A m o r e  d e t a i l e d  d e s c r i p t i o n  of t h e  ins t ruments  
and matters of a n a l y s i s  i s  given elsewhere (Clark e t  a l ,  1968; 
Lewin e t  al, 1968a).  
W e  der ived  t h e  spectrum and i n t e n s i t y  of t h e  Centaurus 
source  from only those  d a t a  recorded when t h e  axis of the de- 
t e c t o r  w a s  w i t h i n  1 2 O  of t h e  source.  The t o t a l  e f f e c t i v e  ex- 
posure accumulated under t h i s  cond i t ion  i n  1 9  scans  over t h e  
source w a s  1 .6  x 105cm2sec.  Taking i n t o  account  t h e  angular  
response func t ion  of t h e  d e t e c t o r  a t  each moment as t h e  data 
w a s  recorded, and c o r r e c t i n g  f o r  smal l  v a r i a t i o n s  i n  t h e  at- 
mospheric a t t e n u a t i o n  due t o  a l t i t u d e  v a r i a t i o n s  (2.17 t o  
2 .19  mm Hg), w e  ob ta ined  t h e  average observed ra te  above back- 
ground i n  each of t h e  7 channels a t  t h e  average ba l loon  a l t i -  
tude.  These d a t a  w e r e  converted i n t o  t h e  e s t ima ted  va lues  
f o r  t h e  energy i n t e n s i t y  (keV/keV c m 2 s e c )  shown i n  f i g u r e  1 
using t h e  atmospheric a t t e n u a t i o n  and t h e  response func t ion  of 
t h e  s c i n t i l l a t i o n  d e t e c t o r .  Figure 1 also shows our  r e s u l t s  
obtained 1 0  days e a r l i e r  on October 15, 1967 when w e  achieved 
an exposure of 3 . 3  x 105cm2sec  i n  35 scans  over  t h e  source.  
s p e c t r a l  d a t a  der ived  from the  l a s t  p o s i t i v e  r o c k e t  obse rva t ion  
- 
The 
on May L8 ,  1 9 6 7  (Chodil ,  e t  a l ,  1 9 6 7 )  are also shown i n  t h e  f i g u r e .  
Table 1 g ives  t k e  s p e c t r a l  d a t a  on both the October 15  
and t h e  October 24, 1967 f l i g h t .  Columns 1 and 3 show the energy 
channel s e t t i n g s ,  columns 2 and 4 show t h e  c a l c u l a t e d  energy in -  
t e n s i t y  co r rec t ed  t o  zero  atmospheric th ickness .  The ratios of 
t h e  i n t e n s i t i e s  as  observed dur ing  t h e  t w o  f l i g h t s  are g iven  i n  
- 2 -  
column 5. Lumping t h e  d a t a  f r o m  20 t o  52  keV, 5 2  t o  -107 keV, 
and 20 t o  -107  k e V ,  w e  f i n d  r a t i o s  of 1.53kO.25,  1 .17kO.29,  and 
1.38.tO.19, r e spec t ive ly .  Thus w e  conclude t h a t  t h e  i n t e n s i t y  
decreased by approximately 30% between October 15,  1 9 6 7  and O c t o -  
b e r  2 4 ,  1967.  
When f i r s t  observed i n  A p r i l  1 9 6 7 ,  by Francey, e t  a l ,  
Cen XR-2 was nea r ly  as b r i g h t  as Sco X-1 .  I t s  i n t e n s i t y  de- 
creased by an o rde r  of magnitude and t h e  spectrum form changed 
s i g n i f i c a n t l y  i n  a per iod  of s i x  weeks (Francey e t  a l ,  1967; 
Cooke e t  a l ,  1967 ;  Chodil  e t  a l ,  1 9 6 7 ) .  Seventeen days be fo re  
our October 15th  observa t ion ,  on September 28 ,  1 9 6 7 ,  t h e  r eg ion  
was scanned by LRL (Chodil ,  e t  a l ,  1 9 6 8 )  and no source  a t  t h e  
p o s i t i o n  of Cen XR-2 w a s  observed. 
on October 15, 1967 from 20 t o  52  keV and 52  t o  1 0 5  keV w a s  
- 4  x 1 0  and -2.7 x 10-9ergs/cm2sec,  r e s p e c t i v e l y .  T h i s  is  
about h a l f  t h e  f l u x  observed i n  t h e  same energy i n t e r v a l  f r o m  
Tau X - 1  (Peterson e t  a l ,  1968 ;  Lewin e t  al, 1 9 6 8 ~ ) .  
- 
The high energy x-ray f l u x  
-9 
The measured r a t e  of change i n ' t h e  energy i n t e n s i t y  from 
20 t o  52 keV is  35+17% i n  9 . 8  days. Ex t r apo la t ions  back i n  t i m e  
(based only on our October 15th  and October 2 4 t h  obse rva t ions )  t o  
May 18 ,  1 9 6 7 ,  assuming e i ther  an exponent ia l  o r  a power l a w t j - m e  deca l  
func t ion  lead t o  high va lues  t h a t  do n o t  match the rocket r e s u l t s  
f r o m  May 18 ,  1 9 6 7  (see f i g .  1). W e  therefore conclude t h a t  t h e  
r a t e  of change a s  measured by us  i s  e i t h e r  a c c i d e n t a l l y  high or ,  
which seems m o r e  l i k e l y ,  t h e  high energy x-ray f l u x  o r i g i n a t e d  a t  
a la ter  t i m e  than t h e  low energy x - r a y ' f l u x .  This  would e l i m i n a t e  
t h e  p o s s i b i l i t y  t h a t  t h e  high energy spectrum has  a m o r e  per- 
manent c h a r a c t e r  and is  o l d e r  i n  o r i g i n  than  t h e  r e c e n t  nova- 
l i k e  x-ray ou tbur s t .  Manley (1968) has  suggested t h a t  h 
energy x-ray spectrum arises from an expanding s h e l l  whose 
shock f r o n t  accelerates, gene ra t ing  i n c r e a s i n g  temperatures  as 
it propagates i n t o  t h e  i n t e r s t e l l a r  material around t h e  Centaurus 
source.  This  sugges t ion  is worked o u t  i n  m o r e  d e t a i l  i n  t h e  
following paper.. 
An a n a l y s i s  s i m i l a r  t o  t h e  one desc r ibed  elsewhere (Lewin 
e t  a l ,  1968b) f o r  t h e  1 9  scans over  t h e  sou rce  on O c t o b e r . 2 4 ,  
1 9 6 7  between 21h 05m and 23 
t e r m  i n t e n s i t y  v a r i a t i o n s .  
h 20m U.T. showed no evidence of s h o r t  
From t h e  October 15  and 24 f l i g h t  d a t a  w e  have found t h e  
maximum l i k e l i h o o d  p o s i t i o n s  of t h e  Cen XR-2 source  t o  be 
a= 196?5+3O, 6 = - 6 4 O + 2 O  and a= 1 9 7 O + 6 O ,  6=-64:5+3O, r e s p e c t i v e l y .  
* 
I n  o rde r  t o  compare our obse rva t ions  w i t h  t h e  r o c k e t  ob- 
s e r v a t i o n  of Chodil  e t  a1 (1968) made 1 7  days  be fo re  our  October 
15th  f l i g h t ,  w e  have ex t r apo la t ed  t h e  high energy spectrum t o  
lower ene rg ie s  and f i n d  va lues  f o r  t he  energy i n t e n s i t y  on O c t o -  
ber (15, 1967  between 2 and 5 k e V  of -1.8 x 10-9ergs/cm2sec and 
-6 .4  x 10’9ergs/cm2sec f o r  an assumed exponent ia l  (kT=20 k e V )  and 
a power law (a= 1 . 2 )  spectrum form, r e s p e c t i v e l y  (see f ig .  1). 
S i m i l a r ,  though somewhat lower va lues ,  can be i n f e r r e d  from t h e  
* 
This  p o s i t i o n  ( r ev i sed )  d i f f e r s  somewhat f r o m  t h e  p o s i t i o n  
a= 197:2+3O, 6=-62O+lP5 as r epor t ed  earlier by us  (Lewin e t  a l ,  
1968a) 
- 4 -  
October 24th r e s u l t s .  An upper l i m i t  on t h e  energy f l u x  
(2-5 keV) of - 3  x l o m 9  ergs/cm2sec w a s  r epor t ed  as  a r e s u l t  of 
t h e  rocke t  observa t ion  on September 28, 1967 (Chodil  e t  a1,1968). 
W e  t h e r e f o r e  conclude t h a t  t h e  energy f l u x  (2-5 keV) on Sep- 
tember 28, 1967 w a s  m o s t  l i k e l y  ( 2 + 1 )  x 10-9ergs/cm2sec. 
t u a l  f l u x  could have been-s-ignificantly l o w e r  on t h i s  date if 
t h e  s p e c t r a  as observed by u s  b e n t  over  towards l o w e r  ene rg ie s ,  
or i f  Cen XR-2, or  a source  w i t h i n  a 'few degrees  from Cen XR-2 ,  
increased i ts  x-ray f l u x  s i g n i f i c a n t l y  between September 28 and 
October 15, 1967. 
The ac- 
If an exponent ia l  func t ion  is  forced  t o  f i t  t h e  above re- 
s u l t  and a l l  previous f l u x e s  (2-5 keV) r epor t ed  f o r  Cen XR-2 
i t s  cons t an t  r is  about 4 5  days. A va lue  of f - 2 5  days w a s  ob ta ined  
by Harries e t  a1 (1967)  from t h e  A p r i l  and May 1967 data only.  
Manley (1968) cons iders  t h a t  a power l a w  t i m e  decay of t h e  energy 
i n t e n s i t y  for Cen XR-2 is a more .p lausable  assumption. I f  such 
a l a w  of  t h e  forrn,twa is forced  t o  f i t  t h e  a v a i l a b l e  l o w  energy 
d a t a  (2-5 keV) inc luding  t h e  one der ived  here  f o r  September 28, 
1967, then t h e  va lues  for a vary f r o m  -2 t o  -6 under va r ious  as- 
sumptions regarding t h e  moment of  t h e  o r i g i n  of t h e  i n t e n s e  x-ray 
emission which must l i e  between October 28, 1965 and A p r i l  4 ,  1967. 
Values fo r  a of  2.3,  2 .3 ,  4 . 4  and 6.2 w e r e ,  calculated, assuming 
. t h a t  t h e  x-ray o u t b u r s t  o r i g i n a t e d  on March 1, 1966,  J u l y  1, 1956,  
November 1, 1966 and March 1, 1967, r e s p e c t i v e l y .  
N o  evidence w a s  found of d i s c r e t e  sources  i n  t h e  d i r e c t i o n s  
of  t h e  Large and S m a l l  Magellanic Clouds. T h e  upper l i m i t s  on t h e  
- 5 -  
x-ray energy flux between 20 and 52 keV from the Large and 
Small Magellanic 'C 
that of Tau X-1.  
s/cm2sec. The hbov 
on the Large Magellanic Cloud is a facto 
only other reported upper limit (Seward and Toor, 1967) in the 
energy range above 20 keV. 
We thank E. Curwood, D. Scott, and the staff of the Bal- 
loon Launch facility at Mildura, Australia, for carrying out 
balloon flight, and the Australian Ministry of Supply for their 
coroperation. 
analysis. 
We also thank M. Gerassimenko for assisting in the 
- 
. .  
- 6 -  
October 15 ,  1967 
Energy 
channel  
s e t t i n g s  
(keW 
20-30 
30-41 
41-52 
52-63 
63-74 
74-90 
Energy 
i n t e n s i  
keV/keV 
TABLE 1. 
October 24, 1967 
Energy Energy 
c m  sec s e t t i n g s  keV/keV c m  sec 2 
t Y " 2  channel  i n t e n s i t y "  
(keW 
13,851.7 20-31 7.8k2.2 
6.8k0.7 31-42 4.3kl .O 
5.0k0.6 42-52 3 .7k1.0  
- 
3.2k0.7- 52-64 2.8k0.9 
3 .9k0.8  64-75 3.0k1.3 
3.650.8 75-92 3 .3k1.3  
90-105 3 .6k1.2  92-110 1 .3k1.7  
Flux ratios 
Oct. 15 
O c t .  25 
1 .850 .5  
1 .3k0.4  
1 , 3 5 0 - 6  
1.150,s 
2 .853.8  
* 
Corrected t o  z e r o  atmospheric t h i ckness .  
IO 
IO0 
IO' ' 
IO'* 
CEN X R - 2  
Chodil et al 5 
c 
P 
0 
May 
Oct. 
Oct. 
18, 1967 
15, 1967 
24,1967 
= 1.2 
20 40 60 80 100 i 20 
keV 
F i g .  1, - Spectra from Cen XR-2 
REFERENCES 
Chodil ,  G . ,  Mark, H. ,  Rodrigues, R., Seward, F., 
and Swi f t ,  C.D. ,  1967 ,  Phys. Rev.  Letters, 19 ,  681. 
Chodil ,  G . ,  Mark,. H., Rodrigues, R., Swi f t ,  C.D., 
1968, Ap.  J. (Letters),  152, L45. 
Clark ,  G.W., L e w i n ,  W.H.G.,  Smith, W.B., 1968, 
A p .  J., 151, 21.  
Cooke, B . A . ,  Pounds, K . A . ,  Stewardson, E.A.,  and 
Adams, D . J . ,  1967, A p .  J. (Letters),  150, L189. 
Francey, R . J . ,  Fenton, A.G., Harries, J . R . ,  and 
McCracken, K.G. ,  1967, Nature ,  216, 773, 
Harries, J . R . ,  McCracken, K.G. ,  Francey, R . J . ,  and 
Fenton, A . G . ,  1967, Nature ,  215, 38.. 
Lewin, W.H.G.,  Clark,  G.W., and Smith, W.B., 1968a, 
Ap.  J. (Letters) 152, L49. 
Lewin ,  W.H.G., C lark ,  G.W., 'and Smith,  W.B., 1968b, 
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Lewin ,  W.H.G., C la rk ,  G.W., and Smith,  W.B., 1968c, 
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- 
Manley, O.P., 1968, p r i v a t e  communication. 
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Decrease in the high energy X-ray flux from Cen XR-2. Search for X-rays from the large and small Magellanic clouds

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