Counting rate and pulse height distribution spectral data of Crab Nebula telemetered from balloon detector

Jacobson, Allan S.

Pelling, R. M.

Peterson, Laurence E.

English

NASA Technical Reports Server

December 21, 1965 i_ 
I 
The Spectrum of Crab Nebula X-Rays to 120 Kev 
BY 
Laurence E. Peterson, Allan S. Jacobson, and R. M. Pelling 
Physics Department 
0) University of California at San Diego 
La Jolla, California 
W 
it .- 
In this letter, we wish to report a balloon determination of 
the Crab flebula X-ray spectrum over the range 16 to 120 kev. X-rays from 
the Crab were first discovered from a rocket‘l), and were observed from a 
balloon to 60 kev by Clark‘2). In this work the spectrum has been extended 
and determined to a much higher precision. 
The dete~tor‘~) consists of a N a I  crystal 5 mm thick by 24 mm 
diameter. 
9.3 em O.D. by 22.9 em long with a 2 em wall which surrounds the central 
detector and its phototube. 
with the central detector also rejects background events produced by 
cosmic-rays. 
1.5 cm2-steradian, and its resolution, FWHM, 54$, at 30 kev. 
and pre-flight energy calibrations were identical. 
Collimation is provided by a cylindrical CsI scintillation cup 
This collimator, which is in anti-coincidence 
2 The detector area is 9.4 em , its geometry factor is 
Post flight 
Inflight calibrations 
during a previous flight with the same apparatus showed no tendency for 
gain changes. 
a 128 channel pulse-height analyzer. 
Data was telemetered event by event in a digital format from 
The detector was mounted at a fixed 
https://ntrs.nasa.gov/search.jsp?R=19660015394 2020-03-16T20:06:02+00:00Z
0 elevation of 80 , and servo-controlled in azimuth to point true south. 
The reference was obtained from a magnetometer set to compensate for the 
average magnetic declination over Texas. The azimuth was independently 
verified to within about a degree by two additional crossed magnetometers. 
The balloon was launched from Palestine, Texas at 2234 CST, 
September 23, 1965, reached ceiling at 0050 CST, and floated level at 
3.3 ,/em2 until 0610 CST, when the instrument was inadvertently released 
by radio command. The Crab transit on this date, corrected for the drift 
in longitudes of the balloon, was 0545 CST. 
Figure 1 shows the counting rates as a,function of time for 
various energy ranges, averaged over 12 minute intervals. The peak on 
ascent is due to the cosmic ray maximum. The rate increase at lower 
energies as the balloon approaches ceiling is apparently due to a diffuse 
component of cosmic photons. This flux, when corrected to zero depth seems 
in general agreement with the results reported previously. (4’5) This com- 
ponent contributes about half of the measured background in the 20 - 50 kev 
range. 
atmosphere and only about 5 or 1% is true detector background. 
About one-half is due to cosmic ray production in the overlying 
The increase near the end of the flight is attributed to the 
meridian passage of an X-ray source. The dashed line in Figure l(a) shows 
the expected time dependent response for a point object of the observed 
peak intensity at the position of the Crab. 
within about -2 in R.A. and -4 in declination of the Crab Nebula. 
The observed source must be 
+ o  + o  
- Figure 2 shows the pulse height distribution of the background, 
and of the Crab during its transit. The incident flux is obtained by 
2 
subtracting these spectra and correcting for atmospheric absorption, 
efficiency, deadtime, channel width, and geometry factor. The spectrum 
obtained by carrying out this procedure is shown in Figure 3, averaged 
over suitable channel groupings. No statistically significant fluxes 
above background were found above 120 kev. Atmospheric absorption removes 
X-rays below 16 kev. 
exponent of 1.91 -.lo. Therefore a ,  the index of the differential energy 
Plux is 0.91 - .10 over this range. 
A least square fit to the spectrum gives a power law 
4- 
4- 
Clark(2) quotes a spectral index a = 2 with an upper limit at 
60 kev, as shown in Figure 3. Our flux seems significantly above his 
upper limit. His observations were made through a considerably higher 
background and with less energy resolution. Inspection of his results re- 
veals that an a of one is also consistent with his data, if the upper limit 
at 60 kev is excluded. There may possibly be secular variations in the 
Crab Nebula. Also shown in Figure 3 are the rocket observations reported 
by the NRL group(6) and the upper limits obtained during a search for 
solar y-rays . ( 7 )  
Various mechanisms for the production of X-rays in astronomical 
sources have been described by Gould and Burbidge(8). Thermal 
Bremsstrahlung from a hot gas produces an exponential spectrum. A model 
temperature and density distribution to reproduce the power law observed 
here requires a 
nan-*mal electrons gives a 1/E number spectrum. 
temperature on the order of lo9 OK. Bremsstrahlung from 
A distribution of slow 
electrons which will give the observed photon spectrum seems somewhat 
artificial, and would be quickly cooled by collision loss.  The synchrotron 
3 
4 mechanism, with an appropriate time dependent source of 10 Bev electrons 
i n  a 10 -4 gauss f i e l d ,  can produce the observed photon spectrum. (9) 
Clayton and Craddock (lo) have estimated y-ray l i n e  emission from 
the Crab Nebula on the  basis of heavy element production by the  r-process 
i n  supernova explosions. They predict  a l i n e  at  60 kev due t o  Am241 decays 
with an in t ens i ty  about 5.7 x photons cm -sec. Our upper l i m i t  on 
t h i s  l i n e  i s  about 1.1 x photons cm -sec. 
2 
2 
Acknowledgements: 
The authors a re  indebted t o  R. J. Gould, W. H .  Tucker and 
G. R.  Burbidge f o r  helpful  discussion. The apparatus w a s  constructed by 
Rod Jerde,  who a l so  ass i s ted  with the f l i g h t  d e t a i l s .  Balloon f l i g h t  
services were provided by the  National Center for Atmospheric Research. 
This research w a s  supported under Grant NsG 318 from the  National 
Aeronautics and Space Administration. 
\
4 
References: 
1. 
2. 
3. 
4. 
5. 
6. 
7. 
8. 
9. 
10. 
R. Giacconi, H. Gursky, F. R. Paolini, B. R. Rossi, Phys. Rev. 
Letters - 9, 439 (1962). 
G. W. Clark, Phys. Rev. Letters - 14, 91 (1965) 
D. B. Hicks, L. Ried, Jr., L. E. Peterson, Transactions 11th Nuclear 
Science Symposium, IEEE (1965) 
R. Rothenflug, R. Rocchia, L. Koch, presented at 9th Int. Conf. on 
Cosmic Rays (1965) 
A. E. Metzger, E. C. Anderson, M. A. Van Dilla, J. R. Arnold, Nature 
I 204, 766 (1964) 
S. Bowyer, E. T. Byram, T. A. Chub, H. Friedmann, Nature - 201, 1307 
(1964 1 
K. J. Frost, E. D. Rothe , L. E. Peterson, J. Geophys. Res. (to be 
publi shed ) 
R. J. Gould, G. R. Burbidge, Annales d'Astrophysique - 28 171 (1965); 
Handbuch der Physik 46,2 (to be published) 
W. H. Tucker (Private Communication) 
D. D. Clayton, W. L. Craddock, Ap. J. - 142, 189 (1965) 
5 
Figure Captions: 
1. The counting rates of various energy ranges as a function of t i m e .  
Background data w a s  obtained a t  ce i l ing  before the  meridian t r a n s i t  
of the  X-ray source. 
f i l e  f o r  a point  source at t h e  posi t ion of t he  Crab Nebula. 
The pulse height d i s t r ibu t ions  obtained during the background in t e rva l ,  
and during the t r a n s i t  of t he  Crab. No s igni f icant  difference i s  
The dashed l i n e  shows the  expected t r a n s i t  pro- 
2 .  
apparent above 120 kev. 
Observations of the spectrum of the Crab Nebula i n  the  X-ray range. 
The previous measurements of Clark were made with a detector  of 
higher background and may not be inconsistent with t h i s  determina- 
t ion. 
3. 
6 
t .8 
1.6 
1.4 
1.2 
1.0 
0.8 
0.6 
0.4 
0.2 
8 0-O 
; 1.0 
3 8 0.8 
0.6 
0.4 
I I I I I I 1 I I ’  
COUNTING RATES VS TIME 
- - 
- pao”.,, - 
PREDICATED \ \ 
- 0 NORMAL~~-EDT\/~’ RESPONSE \ 0 
- 
\ 
0  - ~o~ooooooo~oo~--~~--.-. 0 - 0 0  00 - 
O 000 
- I I 2- 
LAUNCH CEILING TRANSIT CUT 
- 
0 
0 
- - 
(A) RANGE 20-60 KEV - 0 - 
0 DEPTH (grn/cm*) 0 
I 
17 3.3 
!EO 
li00 
- 
0 
0 0  
1 - 
- o---,,cooooooC>* 0 OO 0 
e - ooo 0 - 0 - 
- 0 - 
i (B) RANGE 60- 120 KEV 0 0.2 
0.0 
0 0 - i  
0.6 
- 0 
0 
0 
o .2 
Oe4 ~ 
0 0 
0 i 
(C) RANGE 120-180 KEV I I 0 1  I 
2200 2300 2400 0100 0200 0300 0400 0500 0600 0700 
TIME (CST) 
SEPT 23-24, 1965 
m l  
Pc- - . I  I I I I I J I l  I I I I 1 I I11 I I I I l l  
0 
, i9 
I I I I I I I l l  I I I ~ l l l l ~  I I I I I I l l -  - - 
- 
FLUX vs ENERGY 
- 
- 
NUMBER SPECTRUM- 
CRAB NEBULA 
- 
I I I I I I111 I I I I I I I l l  I I I I l l l l  


The Spectrum of Crab Nebula X-Rays to 120 Kev

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