The newly discovered satellite galaxy located in the Milky Way galactic anti-center region is discussed along with the possibility that a nearby gamma ray source is associated with it. The factors which led to the conclusion that the gamma ray excess is not associated with the galaxy are considered

Fichtel, C. E.

Lamb, R. C.

Thompson, D. J.

English

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NASA T In 7147';1
NON-ASSOCIATION OF A CELESTIAL
Y-RAY SOURCE WITH THE NEW
MILKY WAY SATELLITE GALAXY
(NASA-TM-X-71047) NON-ASSOCIATION OF A
	 N76-17018
CELESTIAL GAMMA RAY SCURCE WITH THE NEW
MILKY WAY SATELLITE GALAXY (NASA)
	 p HC$3.50	 CSCL .)3B	 Unclas
G3/93 13319
R. C. LAMB
D. J. THOMPSON
C. E. c+CHTEL
DECEMBER 1975
--- 
GODDARD SPACE FLIGHT CENTER
GREENBELT, MARYLRND
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Non-Association of A Celestial y-Ray
Source with the New Milky Way Satellite Galaxy
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R.C. Lamb e , D.J. Thompson, C.E, D'ichtel
NASA/Goddard Space Flight Center
Greenbelt, Maryland 20771
* On faculty leave from Iowa State University during 1975-76,
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Non-Association of a Celestial Y-Ray
Source with the new Milky Way Satellite Galaxy
The announcement  of a new satellite galaxy located in the galactic
anti-center region (core position k = 197.3 .1 0.6 0 , b = + 2.1 ± 0.5 0 ) has
stimulated us to re-examine carefully the nearby Y-ray source previously
reported by this group 2,3. Our purpose was to see if there were good
evidence either for or against an association of the Y-ray source with
the new galaxy.
The Y-ray source has an intensity approximately equal to the inten-
sity of the source associated with the Crab Nebula and its pulsar of — 3
+ 1 x 10-6
 (photons > 100 McV)/cm 2s. The error on this intensity includes
possible systematic uncertainties as well as statistical errors. The
confidence level that there is indeed a localized source is greater than
7a. Of the localized sources seen by the Small Astronomy Satellite (SAS_
2), only that associated with the Vela pulsar is stronger. Yet the
source has not been identified with any previously catalogued celestial
object3.
In this note the considerations which have led us to conclude that
the Y-ray excess is not associated with the galaxy will be discussed.
The main points are associated with the exact location and the intensity.
Figure la shows the region of the sky near the galactic anti-center
observed by the SAS-2 detector. Each point on the map represents the
arrival direction for a detected Y-ray whose measured energy was in excess
of 50 MeV. The arrival direction reflects a recently incorporated small
improvement in direction determination. The detection efficiency varies
slowly over the area shown. Within the region bounded by the large
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dashed curve the relative detection efficiency is greater than 50%. The
small circle and ellipse will be discussed later. Two strong sources
are clearly evident in the figure. One of them, near A = 185 0 , b = -500
is identified with the Crab Nebula and its central pulsar PSR 0531 +21.
An analysis of approximately one-half of the events has been published
previously4 . Most of the excess Y-radiation is pulsed at the PSR 0531 +
21 frequency, and therefore the identification of this source with the
Crab is firmly established.
The other source is located near A = 193 °, b = + 3° and may be
identified as Y (193 + 3).
Pigure lb shows the variation in Y-ray intensity along a line be-
tween the two sources. The dashed curves indicate the expected response
to two equal intensity point sources superimposed on a constant back-
ground.
We have analyzed both regions in detail in order to extablish the
precise location of each enhancement. We find that the Crab source is
within 1.4 + 0,7 0 of its known position. The other source is 4.3 + 0.70
away from the quoted position of the galaxy. Thus there is an obvious
incompatibility in position between the new galaxy and the Y-ray source.
One may take an alternative point-of-view that the slight offset of the
Crab is indicative of a systematic error of 1.4 + 0.7 0 that should be
applied as a correction to the events in the surrounding region. If
this offset is treated as a correction the second source would be 4.4 +
0.9° from the position of the new galaxy. We believe that the error
quoted properly accounts for all sources of possible error, both statis-
tical and systematic. In either view this difference in position would
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appear to be decisivly against any association of the two objects. This
conclusion is illustrated in figure la. A circle of 5 0 radius which
should contain 63% of all events from a point source is drawn centered
at the known position of the Crab (R = 184.6, b = -5.8). The circle
contains much of the region of enhanced intensity and is reasonably well
centered.
The core of the satellite galaxy is reported to be elongated with
an angular extent of 7 0 x 2 0 with an orientation c° 40 + 20 0 with re-
spect to the galactic equatorl . If we assume a Uaussian distribution
of matter in the core with dimensions (FWHM) as given by Simonson, an
angular resolution ellipse of semi-major axis 6° and semi-minor axes
5° would be appropriate for the source. Such an ellipse is drawn on the
figure la, There is a clear misalignment by approximately 40.
Another argument against the association is based on a comparison
of the observed intensity with an estimate of the expected y-ray emiss-
ion from the galaxy. There is a discrepancy of approximately two orders
of magnitude, as will oe discussed below. The observed flux from the
y-ray source is about 3 x 10-6
 (photons > 100 McV)/cm2a. Simonsonl has
estimated the mass of hydrogen in the core to be in the range of (0.4 to
1.6) x 107 MO
 and the distance from the sun to be 17 + 4 Kpc. Assuming
the cosmic ray density to be as great as it is locally, a y-ray flux of
about 5+6 x 10-8 (photons > 100 McV)/cm2s would be expected from the
a
interaction of cosmic rays with the matter assuming the source function
as given by Stecker5 . This calculated flux is about (1160) of the flux
actually observed in the nearby region.
Further, it is more reasonable to assume the cosmic ray density is
lower than it is locally. Unless the cosmic ray flux is universal,
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which currently seems to be the less likely possibility, the cosmic ray
density would be expected to be less in thie smaller, less dense galaxy
than in the local spiral arm of our galaxy because, as explained by
Parker6 and discussed further by Bignami et all, the cosmic rays are
held by the matter to which they are tied by the magnetic fields. For
equilibrium to exist the total pressure due to the cosmic rays, magne-
tic fields, and kinetic motion cannot exceed that which can be contained
by gravity. Estimating the volume of the new galaxy to be in the range
(0.5 to 1) x 1065cm3 , the hydrogen gas density would be (1/4) to (1/8)
that locally, and, hence, the cosmic ray density would be expected to be
lower. The best estimate of the Y-ray flux may, therefore, very well be
on the order of 10-8 (photons > 100 McV)/(cm2sec) rather than 5 x 10-8
(photons > 100 McV/cm2see).
It could perhaps be speculated that there are a very large number
of cosmic ray sources and that, even though the lifetime for escape is
short, the density could be high, or it might be suggested that there
are large numbers of high intensity y-ray sources in this small galaxy.
However, both these possibilities seem unlikely because the conditions
for forming objects that are likely to be emitters of cosmic rays or
high energy Y-rays are very likely less favorable in this small galaxy
than in our own.
1101
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REFERENCES
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a
1. Simonson, S.C. III, Astrophys. J. Lett., 201 0 L103 (1975)
2. Fichtel, C.E., Hartman, R.C., Kniffen, D.A., Thompson, D.J., Bignami,
G.F., Ogelman, H., -6zel, M.D., and TUmer, T., Astrophys. J. 198, 163
(1975)
3. Kniffen, D.A., Bign='L, G.F., Fichtel, C.E., Hartman, R.C.,10gelman,
H.B., Thompson, D.J., Ozel, M.E., and Tiimer, T., 14th International
Cosmic Ray Conference Munich, 1, 100 (1975)
4. Kniffen,D.A., Hartman, R.C., Thompson, D.J., Bignami, G.F., Fichtel,
C.E., Cgelman, H., Timer, T., Nature, 251, 397 (1974)
5. Stecker, F.w., Astrophys. J., 185 9 499, (1973)
6. Parker, E.N., Astrophys. J. , 145, 811, (1966)
7. Bignami, G.F., Fichtel, C.E., Kniffen, D.A., Thompson,D.J., Astrophys.
J. 129, 54 9 (1975)
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Figure Caption
Fig. la. Map of arrival directions associated with detected Y-rays
of energy > 50 MeV. The detection efficiency varies slowly
and within the region bounded by the large dashed curve the
relative detection efficiency is greater than 50'%. The cir-
cle and ellipse are centered on the known positions of the
Crab Nebula and the new galaxy respectively.
Fig, lb. Variation in Y-ray intensity along the line between the two
sources. Each point represents the intensity associated
with a V x 8° rectangle. The dashed curves indicate the
expected response to two point sources superimposed on a
constant background. The intensity is in arbitrary units.
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Non-association of a celestial gamma ray source with the new Milky Way satellite galaxy

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