Data from a scan of the galactic plane by the SAS-2 high energy gamma ray experiment in the region 250 deg  l2  290 deg show a statistically-significant excess over the general radiation from the galactic plane for gamma radiation of energy  100 MeV in the region 260 deg  l2 270 deg and -7.5 deg  b2  0 deg. If the enhanced gamma radiation results from interactions of cosmic rays with galactic matter, as the energy spectrum suggests, it seems reasonable to associate the enhancement with large scale galactic features, such as spiral arm segments in that direction, or with the region surrounding the Vela supernova remnant, with which PSR 0833-45 is associated. If the excess is attributed to cosmic rays released from this supernova interacting with the interstellar matter in that region, then on the order of 3.10 to the 50th power ergs would be released by that supernova in the form of cosmic rays

Bignami, G. F.

Fichtel, C. E.

Kniffen, D. A.

Thompson, D. J.

English

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SAS-II OBSERVATIONS We THE GALACTIC
GAMMA RADIATION FROM THE VELA REGION
D. J. Thompson, G. F. Bignami, C. E. Fichtel, and D. A. Kniffe
NASA/Goddard Space Flight Center, Greenbelt, Maryland 20771
ABSTRACT
Data from a scan of the galactic plane by the SAS-II high energy
(E > 30 MeV) gamma ray experiment in the region 250 0 < III < 290 0 show
a statistically-significant (7Q) excess over the general radiation from
the galactic plane for gamma radiation of energy > 100 MeV in the region
260°<EII<270° and - 7.5 0<1,I< 0% If the enhanced gamma radiation results
from interactions of cosmic rays with galactic matter, as the energy
spectrum suggests, it seems reasonable to associate the enhancenent
with large-scale galactic features, such as spiral arm segments in that
direction, or with the region surrounding the Vela supernova remnant,
with which PSR 0833-45 is associated. If the excess is attributed to
cosmic rays released from this supernova interacting with the interstellar
matter in that region, then on the order of 3.10 50 ergs would have been
released by that supernova in the form of cosmic rays.
I. INTRODUCTION
One of the objectives of ►.he gamma-ray astronomy satellite, SAS-2,
has been the investigation of the emission from the galactic plane dis-
covered by OSO-III (Kraushaar et al., 1973). The first results from
SAS-II (Kniffen et al., 1973) covered the .ongitude intervals
320°GQ II< 50° and 160°< 11< 230% These observations revealed a broad,
(NASA-TM-X-72582) SAS-2 OBSERVATIONS OF THE
	 N76-15012
GALACTIC GAMMA RADIATION FROM THE VELA
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flat maximum in the longitude distribution around the galactic center
region which is a factor of about six higher than the plane emission
from the anticenter region, The latitude distribution was compatible
with emission from the very thin galactic disc. The energy information
available revealed a spectrum which is much flatter than the one measured
for the extragalactic gamma rays, and is consistent with a n° decay
hypothesis for the galactic disc photons.
These data, especially when combined with the 21-cm and continuum
radio evidence, have been used recently in constructing models for the
production of the high energy gamma radiation in our galaxy. Influenced
primarily by the OSO-III data, Strong et al. (1973) suggested a general
.	 increase in cosmic ray density toward the galactic center, correlating
•	 the increase with an estimate of the average magnetic field strength.
More recently, with the SAS-II data also available, Stecker et al (1974)
and Puget and Stecker (1974) attempted to explain the observed emission
especially in the central region by assuming that the galactic cosmic-
ray flux varies with the radial distance from the galactic center and
is about an order of magnitude higher than the local value in a toroidal
region between 4 and 5 kpc from the galactic center. Bignami and Fichtel
(1974) start with the hypothesis that the cosmic rays in our galaxy are
preferentially tied to the magnetic fields, which are in turn largely
bound to regions of high matter density (Parker, 1966). The high energy
gamma radiation is then predicted to be strongly correlated with the
distribution of the interstellar matter and hence with galactic arm
segments and clouds. The probable importance of clouds for ga=ra ray
emission has also been emphasized by Black and Fazio (1973).
3
i
In this letter, data from a scan of the galactic plane for the
•	 longitude region 250°< III< 290° are presented and the observed dis-
tribution is discussed in terms of possible origins.
II, EXPERIMENTAL RESULTS
A brief summary of the SAS-II high-energy gamma-ray experiment is
given by Fichtel et al. (1973), a more complete description of the
detector system by Derdeyn et al. (1972), and a discussion of calibra-
tion and data analysis procedures by Fichtel et al. (1972) and Thompson
(1973). During the period February 15 to 20, 1973, the region of the sky
including the galactic plane from III - 250° to I II a 290° was examined.
The flux of gamma rays above 100 MeV has been calculated in ten-degree
intervals along the galactic plane and with a 7.5° interval on each side
of the plane for the portion of the data examined thus far. * The results
are summarized in Table 1(a).
The most striking feature is the large excess in the interval
(260 0< III< 270°, -7.5 0 < bII< 0°) where the intensity is seen to be
three times the level in surrounding intervals. In that interval, the
intensity of 3.0
	
10"4 gamma rays/(cm2sr sec) is slightly over seven
standard deviations above the average level of approximately (.95 ± .26)
10-4 . The intensities in the other regions given in :able I are similar
to those in the galactic plane anticen.ter direction. (Kniffen et al., 1973).
For a more direct comparison to previous results, Table 1(b) gives the
'kApproximately 40% of the data during this period was lost due to inter-
ference by the SAS-I satellite transmitter.
intensities summarized over-the interval -10°< b II< 10% Here the
excess is still evident, but it is not so marked because the intensity
represents a sum over positive and negative galactic latitudes.
TAKE I
(a) Intensity of gamma rays/(cmz ster sec) above 100 MeV in
indicated solid angle interval multiplied by 104.
gII	 250°	 260°-	 2700	 2800	 290°
-7.5°<bII<0° .8+.3 3.0 ±5 1.08±126 1.15 +.36
0°<b1S<7.5° .5+.3 .95 +.26 .83+.23 1.04 +.3f
(b) Intensity of gamma rays/(cm rad sec) above 100 MeV for
-10°< b1l< 10°) in the indicated interval multiplied by 105.
11 11 1600-2300 2500-2600 260°-2700 2700-2800 2800-290°J
I x 105 2.5 +.3 1.8 +.6 5.4 + 0.7 2.9 +0.5 3.6 +0.7
The energy spectrum of the gamma rays within 7.5° of b11 = 0 is
consistent with a ,t° decay type spectrum expected from cosmic ray col-
lisions with interstellar matter. The energy spectrum of the gamma rays
with bII' > 7.5 0 measured from 35 MeV upwards is consistent with the
very steep energy spectrum observed for the diffuse radiation off the
galactic plane (Fichtel et al., 1973) both in spectral shape (assuming
the form dJ/dE = AE a , a is about 3.0 +.5) and intensity (J[Ey > 100 MeV]
[3.1 +.63- 10
-5 
gamma rays! [cm sr sec]). Within the limitations of
statistics, the spectral shape of the enhanced region (260°< LII< 270°,
-7.5°< I 11< 0°) does not differ from the rest of the galactic plane
4
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with l bil l < 7.5°. Specifically, the spectrum is rather flat and
definitely inconsistent with a steep spectrum, e.g., of the type of
E
the diffuse radiation.
The limited extent in solid angle of the observed radiation, 4°
t
to 8° in diameter, together with the gamma-ray detector resolution
i
suggests that the source of the radiation is either more than about
-arm segment feature, or that it is a2 kpcs away, if it is a galac ic
close compact source. The center of the excess radiation can be placed
at about 6 = -(46 +1)° and cx - 129.5° +1°, (1II - 264.5° and bII = -30).
For a V radius circle centered at the above coordinates, the excess is
an 8.56 effect. If the entire excess is due to a compact source, the
flux above 100 MeV would be about 0.5 - 10-5/cm2sec. .
.	 III. DISCUSSION
The results presented in Section II show an enhancement of the
gamma radiation from the galactic plane in one particular interval.
Two possible explanations for this radiation seem to present themselves
naturally. One is cosmic ray interactions in the large-scale galactic
structure features in that region, especially in view of the "hat brim"
effect of the galactic plane at those longitudes wherein the matter
distribution of the plane tends to dip below b II = 0. The other is
cosmic ray interactions in the region of the Vela X supernova remnant.
Although the Milky Way in the region III 260° to 270° has not
been studied as thoroughly as other regions,21-cm radio data does point
to a maximum of emission around 265° and bII - -2° or less (Hindman
S
6	 (;
and Kerr, 1970; Goniadski and Jech, 1970; Kerr et al., 1974). That a
maximum of 21 cm emission should appear around 270° had also been pre-
dicted by several models of the galactic structure, from the pioneering
work of Mills (1959) to the most recent applications of the density
wave theory (Simonson, 1974). Also, radio continuum data at different
frequencies [e.g., Wilson and Bolton (1960) at 960 MHz , Landecker and
Wielebiski (1970) at 150 MHz ; and Mathewson et al. (1962) at 1440 MHZ]
show evidence for a peak at approximately 270° in agreement with the
first results of Mills at 85 MHz . All these data point to a concentra-
tion of galactic hydrogen along the line of sight in the 260° to 2700
direction, either because of a tangential. view of a single arm or as a
result of the sum of several more distant arm segments. The excess gamma
ray emission could then be explained in terms of cosmic ray interactions
with the interstellar gas. The narrow extent of the excess gamma radia-
tion would seem to be more consistent with an end-on view of a single
arm than a sum of several arm segments, and the radio data do suggest
such a perspective (Kerr, 1973).
The center of the gamma ray excess lies close to both the Vela X
supernova remnant, at a - 130.5°,and 6 - -45.0% and the second fastest
pulsar known, PSR 0833-45 (period about 89.2 msec), at a = 128.8° and
& - -45.0% A study of this portion of the sky in ultraviolet light
by Hiller (1973) has revealed that the supernova remnant is approxi-
mately 5.1° in diameter. Assuming a distance of 460 parsecs (Brandt et
al., 1971) 0 the diameter is 40 parsecs. The region has a complex fila-
7has a complex non-thermal radio source geometry (Milne, 1968).
Both soft and hard continuum X-rays have been observed from the region
(Seward et al., 1971; Bunner, 1973; Kellogg et al., 1973), as well as
a pulsating hard X-ray component (Iiarnden et al., 1972; Harnden and
Gorenstein, 1973). The pulsed component, however, accounts for only
about 6% of the total radiation in the 20 to 80 keV energy interval.
The optical and low energy X-ray emissions appear centered on the same
point in the sky.
The X-ray emission in the range 2-6 keV from the SNR is weak,
according to the UHURU catalog, less than one percent that of the Crab.
The spectrum for the nebula in the X-ray region below 2 keV is quite
steep (Seward et al., 1971; Bunner, 1971), while the UIIURU measurements
from 2-10 keV indicate a flatter spectrum for the compact source. An
extrapolation even of the latter spectrum to the gamma-ray region lies
well below the results presented here, implying that some new production
mechanism would be required.
The supernova remnant and the flat gamma ray energy spectrum both
suggest cosmic rays interacting with the local interstellar matter.
Supernovae have long been thought to be the most likely source of cosmic
rays, and theoretical studies of the hydromagnetic shock theory of
supernovae (Colgate and Johnson, 1960; Colgate and White, 1966) have
shown that it should be possible to generate enough energy in the form
of cosmic rays (1049
 to 1051 ergs) per supernova to keep the galaxy
resupplied against losses. The exact manne-- in which the cosmic rays
A
8expand after the supernova is not clear because of the complex inter-
actions with the local environment. Nonetheless, it is likely that the
observed approximately 5° diameter region about the Vela supernova
remnant represents the maximum volume in which the cosmic rays are
contained. The very high density cosmic rays will, of course, be inter-
actin- with the local matter (Pinkau, 1970) yielding strong hard gamma
ray emission from the region. -
Assuming the excess gamma radiation observed from the Vela region
tr be due to cosmic rays associated with the supernova, assuming the
remnant to be 460 parsecs away, and assuming the matter density to be
about 1..5 protons/cm3 , 3 1050 ergs of energy would be in the form of
cosmic rays from the supernova. (Because the parameters involved, par-
ticularly the matter density in the Vela region, are not well-known, this
number should be taken as an estimate of the energy involved rather than
as a definitive calculation.) This number does lie in the energy
range mentioned above if supernovae are to be the main source of galactic
cosmic rays.
For the moment, the question of which explanation (cosmic rays in
one or more galactic arm segments or cosmic rays expanding about the Vela
supernova remnant) accounts for most of the Vela excess must remain open.
The question can perhaps be resolved by improved angular resolution gamma
ray studies in conjunction with radio and optical measurements to determine
which galactic features best correlate With the observedge.;,xa ray excess.
"t  •. ', . ♦
REFERENCES
Bignami, G. F., and Fichtel, C. E., 1974, to be published in Ap. J.
Lett.
Black, J. M., and Fazio, G. G., 1973, Ap. J. (Letters), 185, W. i
Brandt, J. C., Stecher, T. P., Crawford, D. L., and Maran, S. P., 1971,
Ap. J. (Letters), 163, L99.	 1
Bunner, A. N., 1973, in The Gum.Ncbula and Related Problems, ad. by
S. P. Maran, J. C. Brandt, and T. P. Stocker (National Technical
Information. Scrvice),NASA SP-332, p. 158.
Colgate, S. A. and Johnson, M. H., 1960, Phys. Rev. Letters 5, 235.
Colgate, S. A., and White, R. H., 1966, Ap. J. 143, 626.
Derdeyn, S. M., Ehrmann, C. H., Fichtel, C. E., Kniffen, D. A., and
•	 Ross, R. W., 1972, Nuclear Instruments and Methods 98, 557.
,Fichtel, C. E.;-Hartman, R. C., Kniffen, D. A., and Sommer, M., 1972,
Astrophys. J. 171, 31.
Fichtel, C. E., Hartman, R. C., and Kniffen, D. A., 1973, Astrophys. J.
(Letters) 186, L101.
Goniadzki, A., Jech, A., 1970, I.A.U. Symposium No. 38, 157.
Harnden, F. R., Jr., Johnson, W. J., III, and Haymes,'R. C., 1972,
Ap. J. (Letters) 172, 191.
Harnden, F. R., Jr., and Gorenstein, P., 1973, Nature 241, 107.
Hindman, J. V., and Kerr, F. J., 1970, Aust. J. Physics Astrophysics
Suppl. 18, 43.
Kellogg, E., Tananbaum, H., Harnden, F. R., Jr., Gursky, H., and
Giacconi, R.,1973, Ap. J. 183, 935.
PRECEDING PAGE BLANK NOT FR"
fREFERENCES - continued
Kerr, F. J., Harten, R. 11., and Ball, D. L., 1974, "21 cm Line Obsor-
vations in 1970 on Parkes (CSIRO) 210 ft. Telescope", U. of Maryland
preprint.	
I
Kerr, F. J., 1973, The Gum Nebula and Related Problems, National Tech-
nical Information Service, %.%SA SP-332, p. 38.
I
Kniffen, D. A., Hartman, R. C., Thompson, D. J., and Fichtel, C. E.
1973, Ap. J. (Letters) 186, L105.
,raushaar, W. L., Clark, G. W., Garmire, G. P., Borken, R., Higbio, P.,
Leong, V., and Thorsos, T., 1973, Ap. J. 177, 341.
Landecker, T. L., Wielebiski, R., 1970, Aust. J. Phys. Astrophys.
Suppl. 21, 169.
Mathewson, D. S., Healey, J. R., Rome, J. M., 1962, Aust. J. Phys. 15,
353.
Miller, E. W., 1973, "The Vela Supernova Remnant in Ultraviolet Light";
Steward Observatory, University of Arizona preprint.
Mills, B. y., 1959, Publ. Astr. Soc. Pac., 71, 267.
Milne, D. K., 1968, Australian J. Physics, 21, 201.
Parker, E. N., 1966, Ap. J. 145, 811.
Pinkau, K., 1970, Phys. Rev. Letters 25, 603.
Puget, J. L., and Stocker, F. W., 1974, (GSFC Report X-640-74-17).
Seward, F. D., Burginyon, G. A., Gradcr, R. J., :Hill, R. W., Palmieri,
T. M., and Stoering, J. P., 1971, Ap. J. 169, 515.
Simonson, S. C., 111, 1974, "A Density-Wave :Sap of the Galactic Spiral
Structure", University of Maryland preprint.
REFERENCES - continued
Stocker, F. W., Puget, J. L., Strong, A. W., and Bredekamp, J. L.,
1974, Ap. J. Letters 188, in press
Strong, A. W., Wdowczyk, J., and Wolfendale, A. W., 1973, Carina Ray
Astrophysics, U. S. Covernment P.O., Washington, D. C. NASA SP-339.
Thompson, D. J., 1973, Univ. of Maryland PhD Thesis, to be published
in shortened form in J. Ceophys. Res.
Wilson, R. W., and Bolton, J. C., 1960, Publ. Astr. Soc. Pac. 72, 331.


SAS-2 observations of the galactic gamma radiation from the Vela region

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