Electromagnetic radiation from the Crab nebula were observed, showing that the Crab is unique among strong X-ray sources in that major component in the low energy range (1 to 10 KeV) shows little or no temporal variation. Observations of the Crab above 35 MeV were made with the high energy gamma ray telescope flown on SAS-2. The detector and technique are described in detail

Bignami, G. F.

Fichtel, C. E.

Hartman, R. C.

Kniffen, D. A.

Oegelman, H.

Thompson, D. J.

Tuemer, T.

English

NASA Technical Reports Server

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https://ntrs.nasa.gov/search.jsp?R=19740024128 2020-03-23T04:03:10+00:00Z
Gamma Radiation From the
Crab Nebula above 35 MeV
D. A. Kniffen, R. C. Hartman, D. J. Thompson,
G. F. Bignami*, C. E. Fichtel.
NASA/Goddard Space Flight Center
Greenbelt, Maryland 20771
H. Ogelman, T. Tiimer
Middle East Technical University
Ankara, Turkey
ESRO Postdoctoral Fellow on leave from LFCTR, Instituto di Fisca dell'
Universita di Milano, Italy
0Observations of electromagnetic radiation from the Crab have been
made to at least a GeV(l) and possibly as high as 1012 eV(2,3). The
Crab is unique among strong X-ray sources in that the major component
in the low energy range (1 to 10 KeV) shows little or no temporal
variation. A small ( 10% at 10 KeV) pulsed component from NP0532 is
observed in the X-ray region; but its strength relative to the constant
component increases with increasing X-ray energy.. Gamma ray observations
of the Crab in the 15 to 100 MeV range have yielded rather uncertain
results with reported upper limits (4,5) and fluxes (6 to o10) in appar-
ent or near conflict. Since the-reported intensities were just marginally
detectable above atmospheric background with the balloon experiments
which produced the observations reported thus far, this seems under-
standable.
Here we report observations of the. Crab above 35 MeV made with the
high energy gamma ray telescope flown on SAS-2. The detector and tech-
nique are described in detail by Derdeyn et al. (11), and in summary
by Fichtel et al. (12)
The data were obtained during the week of December 14-21, 1972.
The Crab direction remained about 50 from the experiment axis for the
entire exposure. The absolute timing accuracy of the SAS-2 satellite
and the ground station was sufficient to allow separation of the pulsed
component. Because of the finite angular resolution of the SAS-2
detector, an angular cone of acceptance for gamma rays must be included
in the final analysis. We used a 60 cone for gamma rays with measured
energies from 35 to.100 MeV and 4 o for energies above 100 MeV. Appro-
priate account was taken of the portion of the flux which is calculated
2on the basis of the known detector angular resolution to fall outside
these circles.
The arrival time of each gamma ray was compared to the radio
pulse time using predicted pulse arrival times provided by J. M. Rankin,
Arecibo Radio Observatory, and derived as described in ref. 13, (Fig. 1).
An independent timing analysis was made using a program developed at
GSFC based on a pulse prediction program provided by D. C. Backer.
Notice that peaks do occur at both the positions of the main and
secondary radio pulses, and that there is a constant component above
the general background from the Crab region. Although the statistics
are limited, the probability of finding by chance sixteen gamma rays
in a bin where a peak was predicted, when the average based on the
eight bins where no peak was expected is five, is less than 10- 4 . The
probability of finding peaks of the observed size in both bins is then
extraordinarily small. Since the absolute timing uncertainty for SAS-2
was about 1 millisecond, selecting markedly smaller intervals of time
than those shown in Fig. 1 does not produce peaks of increased signifi-
cance.
The total flux of gammd rays above 35 MeV is estimated to be
(15.8 + 4.2)- 102'6 photons cm"2 sec" I and that of the pulsed component,
using bins 1 and 5 only in Fig. 1, to be (6.2 + 2.8)o10- 6 photons cm-2
sec - . The total flux above 100 MeV is (3.2 + O.9)'10- 6 photons cm-2
sec , and thepulsed component above 100 MeV is (2.2 + 0.7) 10-  cmM
sec" . These results are shown in Fig. 2, along with data from other
experiments. The unpulsed component appears to become a progressively
smaller fraction of the total flux as the energy increases. Above
3140 MeV, only a 95% confidence upper limit of 0.7-10 - 6 photons cm -
see 1 can be set for unpulsed component whereas a finite flux of
(1.30 + 0.46) - 10T photons cm 2 see- was seen for the pulsed component.
The situation contrasts markedly with the low energy X-ray region, where
only 10% of the radiation is pulsed (ref. 14).
The most striking feature of the spectral distribution of the elec-
tromagnetic radiation from the Crab between 104 and 109 eV is the shift
from a predominantly unpulsed total flux at the lower energy,, to one
which is predominantly pulsed. The spectral shape of the pulsed emission,
although only poorly known, can be approximated by a power law with
energy spectral index of a - 1 to about aGeV. Qualitatively, this picture
is consistent with the Shklovsky (15) model of synchrotron emission from
the speed of light cylinder with a steady flux of much lower energy
photons from the region of smaller field. With a magnetic field in the
Nebula between 1074 and 10- G one needs electrons accelerated up to
10C 2 1013 eV to produce gammas in the 100 MeV energy region.. Such
electron energies have indeed been predicted in models of particle accel-
eration for the Crab pulsar.
It is interesting to compare the gamma ray results discussed here
with those for the Vela region reported by Thompson et al. (16). The
gamma ray fluxes from the two regions above 100 MeV are nearly equal,
the Vela flux being (5.0 + 1.2) • 10 - 6 photons cm- sece. But the
Crab, spectrum in the gamma ray region seems to be a simple extension
of the X-ray spectrum, whereas in the case of the Vela source the gamma
radiation is quite hard and substantially larger than an extension of
4the X-ray spectrum to that energy region. The Vela intensity and spectra
are consistent with the gamma radiation being the result of about 3 * 10sc
.ergs of cosmic ray nuclei from the Vela supernova interacting with the
local matter.
It is not inconsistent with the data from the Crab supernova also
to have released a few times 105o ergs of cosmic rays, because being
about four times further away, the gamma radiation from cosmic ray
interactions would be approximately 16 times less intense, assuming
the local matter density is not grossly different from 1 to 2 nuclei
IM- The gamma ray intensity near 100 MeV from supernova nucleonic
cosmic rays, then, would be more than an order of magnitude less than
the pulsed radiation at the present time.
So, a consistent picture of the present data ascribes a very
different origin to the gamma ray intensity from the Crab to that from
Vela, with the Crab gamma radiation being due to the same mechanism
responsible for the X-ray emission and the Vela gamma radiation being
due to the nucleonic component of the cosmic rays interacting with
the local matter.
The authors would like to express their appreciation to J. M. Rankin
of the University of Iowa and Robert Payne of the Arecibo National
Astronomy and lonosphere Center for providing NP0532 predicted pulse
arrival times. We also thank D. C. Backer of GSFC for assistance in
the development of timing programs.
5RFEFERENCES,
1. McBreen, B., Ball, S. E., Jr., Campbell, M., Greisen, K., and
Koch, D., Astrophys. J. 184,.571-580 (1973).
2. Grindlay, J. E., Astrophys. J. Lett. 174, L9-L17 (1972).
3. Fazio, G. G.-, Hemken, H. F., O'Mongain,; E., and Weekes, T. E.,
Astrophys. J. Lett. 175, L117-LI22 (1972).
4. Frye, G. M., Jr., and Wang, C. P., Astrophys. J. 158, 925-937 (1969).
5. Kettenring, G., Mayer-Hasselwander, H. A., Pfeffermann, E., Pinka , K.,
Rothermel, H., and Sommer, M., Proc. 12th Internat. Cosmic Ray Conf.
1, 57-62 (Hobart, 1971).
6. Browning, R., Ramsden, D., and Wright, P. J., Nature 232, 99-101
(1971).
7. Albats, P., Frye, G. M., Jr., Zych, A. D., Mace, 0. B., Hopper, V. D.,
and Thomas, J. A., Nature 240, 221-224 (1972).
8. Kinzer, R. L., Share,.G. H., and Seeman, N., Astrophys. J. 180,
547-549 (1973).
9. Parlier, B., Agrinier, B., Forichon, M., Leray, J. P., Boella, G.,
Marschi, L., Bucheri, R., Robba, N. R., and Scarsi, L., Nature
Phys. Sci., 242, 117-120 (1973).
10. Helmken, H., and Hoffman, J., Proc. 13th Internat. Cosmic Ray Conf.
1, 31-35 (Denver 1973).
11ii. Derdeyn, S. M., Erhmann, C. H., Fichtel, C. E., Kniffen, D. A., and
Ross, R. W., Nucl. Instr. Meth. 98, 557-566 (1972).
12 Fichtel, C. E., Hartman , R. C., and Kniffen, D. A., Astrophys. J.
Lett. 186, L99-L103 (1973).
613. Rankin, J. M., Counselmann, C. C., III, and Richards, D. W.,
Astron. J. 76, 686-690 (1971),
14. Fishman, G. J., Harnden, R. F., Jr., and Haymes, R. C., Astrophys.
J. Lett. 156, L17-LIIO (1969).
15. Shklovsky, I. S., Astrophys. J. 159, (1970).
16. Thompson, D. J., Bignami, G. F., Fichtel, C. E., and Kniffen, D. A.,
Astrophys. J. Lett. 190, L51-L53 (1974).
17. Kurfess, J. D., Astrophys. J. Lett. 168, L39-L42 (1971).
18. Orwig, L. E., Chupp, E. L., and Forrest, D. J., Nature Sci. 231
171-172 (1971).
19. Laros, J. G., Matteson, J. L., and Pelling, R. M., Nature Sci. 246,
109-111 (1973).
7Fig. 1. Distribution of gamma ray arrival time in fractions of a pulse
period for gamma rays above 35 MeV from the direction of NP0532.
M and S refer to the predicted main and secondary pulse times
as determined from radio observations (Rankin, private communi-
cation).
Fig. 2. Spectrum distribution of fluxes observed from the region of the
Crab nebula.
= 0.7E- 0. pulsed flux, ref. 19
... . 21.8E-I 9 total X-ray flux, ref. 19
= SAS-2 total flux
* = SAS-2 pulsed flux
S = ref. 17
[ = ref. 1
0 = ref. 10
A = ref. 7
3 = ref. 6
< = ref. 8
=ref. 9
= ref. 18.
= ref. 14
T = ref. 5
NP0532
GAMMA RAYS >35 MeV
M S
16 - -->16
14 -
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w 6
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TIME IN FRACTIONS
OF A PULSE PERIOD
Fig. 1
IO b un I ul t i uu HuI  I I im a wu l I I I irt I I ullilli I  1-I0-
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-6
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PHOTON ENERGY (eV)
Fig. 2 NASA-GSFC
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Gamma radiation from the Crab nebula above 35 MeV

Gamma radiation from the Crab nebula above 35 MeV

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