Observations of Geminga made at the Whipple Observatory using the atmospheric Cherenkov technique during the moonless periods of November 1983 to February 1984 and November 1984 till February 1985 were examined for evidence for the emission of gamma rays with energy in excess of approx 400 GeV. Evidence of either a steady flux or a flux pulsed with a period near 60 seconds were studied. In neither case was any significant effect observed, enabling the establishment 3 of sigma upper limits of 5.5 x 10 to the -11th power photons/sq cm/s and 2.0 x 10 to the -11th power photons/sq cm/s for the steady and pulsed emission respectively. The limit to the pulsed flux is approximately a factor of six below that predicted

Cawley, M. F.

Fegan, D. J.

Gibbs, K.

Gorham, P. W.

Lamb, R. C.

Liebing, D. F.

Mackeown, P. K.

Porter, N. A.

Stenger, V. J.

Weekes, T. C.

English

NASA Technical Reports Server

1 78 OG 2.4-4
SEARCH FOR G/tibIA-RAYS ABOVE 400 GeV FROH GENINGA
M.F. Cawley(1), D.J. Fegan(1), K. Gibbs(2), P.W. Gorham(3),
R.C. Lamb(_) D.F. Liebing(4), P.K. MacKeown(5)
N.A. Porter (_), V.J. Stenger(3), T.C. Weekes(2)
(1) Physics Department, University College, Dublin
(2) Harvard-Smithsonian Center for Astrophysics
(3) Department of Physics and Astronomy, Univ. of Hawaii
(4) Department of Physics, Iowa State University
(5) Dept. of Physics, University of Hong Kong
ABSTRACT
Observations of Geminga made at the Whipple Observatory
using the atmospheric Cherenkov technique during the
moonless periods of November 1983 - February 1984 and
November 1984 - February 1985 have been examined for
evidence for the emission of gamma rays with energy in
excess of _ 400 GeV. The data were searched for evidence of
either a steady flux or a flux pulsed with a period near 60
seconds. In neither case was any significant effect
observed, enabling us to establish 3 sigma upper limits of
5.5x10-ii photons cm-2 s-i and 2.0x10-ii photons cm-2 s-i
for the steady and pulsed emission respectively. The limit
to the pulsed flux is approximately a factor of six below
that predicted by Zyskin and Mukanov (1983) at this energy.
i. Introduction. The high energy gamma-ray source Geminga
(2CG 195+4) was discovered by the SAS-II experiment in 1975
(Kniffen et al, 1975; Thompson et al, 1977) and was indepen-
dently detected and studied in greater detail by the COS-B
experiment (Hermsen 1983). With an integral flux of _5.10-6
photons cm-2 s-I at i00 MeV it remains, ten years after its
discovery, the brightest unidentified object (and second
brightest overall) in the sky at these energies. Its inten-
sity, combined with its location in the direction of the
galactic anticenter, have made it a prime candidate for
identification with objects detected at other wavelengths.
At the present time the available evidence points to, but is
incapable of deciding between either a nearby (_,100pc) soft
spectrum x-ray Source, possibly a collapsed star, (Bignami
et al, 1983) or an extragalactic, flat-spectrum radio source
(Spelstra and Hermsen, 1984; Moffat et al, 1983).
Since the early suggestion of a possible, _59-second,
periodicity in the 100 MeV gamma-ray flux (Thompson et al,
1977) further information on the nature and possible
counterpart of Geminga has been sought from studies of the
time variability of its emission. Again the present
experimental situation appears confused with evidence being
presented both in favor of (Zyskin and Mukanov 1983; Bignami
et al, 1984) and against (Buccheri et al, 1984) the
existence of such a periodicity.
.. 
.. 
.. 
. F. 
. . 
. A. 
173  . -4 
EARaJ R AMM YS OVE 0 eV M EMINGA 
awley(l), .J. Fegan(l), K. ibbs(2), P. . orham(3), 
Lamb(~), .F. iebing(4), P. . acKeown(5) 
orter(l), .J. tenger(3), . . eekes(2) 
(1) Physics epartment, niversity ollege, ublin 
( ) arvard-Smithsonian enter f r strophysics 
(3) epartment of Physics and stronomy, niv. of awaii 
(4) epartment of hysics, Io a tate niversity 
(5) ept. of hysics, niversity of ong ong 
 B S T RAe T 
bservations f eminga ade t t  hipple bservatory 
using the at ospheric herenkov technique during the 
oonless periods of ovember 1983 - February 1984 and 
ovember 1984 - ebruary 1985 have been examined f r 
e idence f r t e ission f ga a r s ith e ergy i  
ce s f ~  e . he ata ere rched r idence f 
i ther  dy  r  x l.sed ith  eriod ear  
s conds. I  either se as a y i nificant ffect 
served, abling us t  stablish  si a pper li its f 
5.5xlO-11 photons c -2 s-l and 2.0xlO-ll photons c -2 s-l 
f r t  t ady d ulsed i sion r spectively. he li it 
to the pulsed flux is approximately a factor of six below 
t at predicted by yskin and ukanov (1983) at t is energy. 
1. I troduction. he high energy ga ma-ray source eminga 
(2  +4) as iscov red  t  S-II e periment i  5 
( niffen t I, 75; hompson t I, 77) d as i epen-
dently detected and studied in greater detail by the COS-B 
experiment ( er sen 1983). ith an integral flux of~5.10-6 
photons c -2 s-l at 100 MeV it re ains, ten years after its 
iscovery, t e rightest nidentified ject (and second 
rightest verall) i  t e sky t t ese e ergies. Its i ten-
sit , co bined ith it  l cation i  t e irection of t e 
galactic anticenter, have ade it a pri e candidate for 
i entification ith jects etected t t er avelengths • 
t t  resent ti e t  ailable i ence oints t , t i  
incapable of deciding bet een eit er a nearby (~lOOpc) s ft 
s ectrum -ray s urce, ssibly a c llapsed st r, ( ignami 
t I, 1983) r a  trag la tic, fl t-s ectrum ra io s rce 
( pelstra and ermsen, 1984; offat et aI, 1983). 
i ce t  rly s ggestion f a ssible, '" 59-second, 
periodicity in the 100 eV ga ma-ray flux ( ho pson et aI, 
77) f rther i f rmation  t  ture a d ssible 
counterpart of eminga has been sought fro  studies of the 
ti e ariability f it  e ission. gain t e resent 
experimental situation appears confused ith evidence being 
resented th i  f or f ( yskin d ukanov 83, i nami 
t I, 1984) and ainst ( uccheri t I, 1984) t e 
istence f s ch a eriodicity. 
https://ntrs.nasa.gov/search.jsp?R=19850025638 2020-03-20T17:53:10+00:00Z
174 OG 2.4-4
In this paper we present results of a search for a flux of
gamma rays, either continuous or pulsed, with energies in
excess of _400 GeV from the direction of Geminga.
2. Data. The data were taken with the F.L. Whipple
Observatory 10-meter reflector (Cawley et al, paper
OG 9.5-4, this conference) on clear moonless nights during
the periods Nov 1983 - Feb 1984 and Nov 1984 - Feb 1985.
Observations were generally made by alternately tracking the
source ("ON" SCAN) and a comparison region ("OFF" SCAN) over
the same range of zenith and azimuth angles for periods of
28 sidereal minutes. On occasion the source was tracked
continuously for periods of _ 3-6 hours to facilitate the
search for pulsed emission. On all occasions the arrival
time of each air shower was recorded, with the aid of a
clock synchronised to WWVB, to an absolute accuracy of +_0.5
msec, with a resolution of i/_s.
3. _I_. Before being incorporated in any analysis all
scans, for both "ON" and "OFF" regions, were examined for
evidence of possible systematic effects e.g. significant
deviation from Poisson counting statistics. This resulted
in the elimination of 10 "ON-OFF" scan pairs from the
original sample of 38 pairs taken under good observing
conditions. Three scans, in which Geminga was tracked
continuously were also judged suitable for analysis.
3.1 Steady Emission. The raw data from the 28 "ON-OFF"
scan pairs comprise a total of 87,983 showers "ON" and
87,419 "OFF" an excess of 564 events (1.4o9 implying a 3
sigma upper limit to the flux above 400 GeV of 3.9xi0-I0
photons cm-2 s-l. The raw data were then subjected to a cut
designed to preferentially pass gamma-ray initiated showers
whilst rejecting nuclear initiated showers with an effi-
ciency of_90% (Cawley et al paper OG 2.3-1 these proceed- °
ings). The total numbers of events in this enhanced sample
were 1,352 "ON" and 1,253 "OFF" -- the excess of 99 events
is not judged to be significant, differing from zeroby 1.9
sigma. Assuming a collection area of ix104 m2 for these °
selected events (Cawley et al. OG 9.5-4 these proceedings)
the observing time of 766 minutes implies a 3 sigma upper
limit of 5.5x10-ii photons cm-2 s-l. This limit is plotted
in figure 1 along with the results of other experiments.
3.2 Periodic Emission. Fifteen hours of data, from four
separate nights, in which Geminga was tracked have been
examined, independently, for evidence of possible pulsed
emission near a period of sixty seconds. The time of
arrival of each event passing the cut designed to preferen-
tially select gamma rays was folded at periods in the range
58.75-61.75 seconds and, to facilitate comparison with other
experiments, the resultant phase distribution displayed as a
twenty bin histogram and tested against uniformity with
chisquare. The period was incremented uniformly over the
174 G 2.  
I  i  er  sent ults   s r h f   flu  ~  
gamma rays, either continuous or pulsed, with energies in 
excess of 'V 400 GeV from the direction of Geminga. 
2. Qata. The data ere taken ith the F.L. hipple 
bservatory lO- eter reflector (Cawley et aI, paper 
OG 9.5-4, this conference) on clear moonless nights during 
t  ri ds . 3 - e  1  a   4 - e  5. 
Observations were generally made by alternately tracking the 
source ("O " S N) and a co parison region (" FF" S N) over 
the same range of zenith and azimuth angles for periods of 
28 si ereal inutes. n ccasion t  s rce as tr ed 
c ntinuously f r eriods f ~ 3-6 hours t  f cilitate t e 
s rch f r lsed ission. n ll casions t  rri al 
ti e f h ir s er as r rded, ith t  i  f  
clock synchronised to WWVB, to an absolute accuracy of to.5 
msec, with a resolution of ~. 
3. Analysis. Before being incorporated in any analysis all 
scans, f r both " " and " FF" re ions, ere examined f r 
evidence of possible systematic effects e.g. significant 
eviation fro  oisson c unting st tistics. his resulted 
i  t e eli ination f 10 " -OFF" scan airs fro  t e 
original sa ple of 38 pairs taken under good observing 
conditions. Three scans, in hich eminga as tracked 
continuously ere also judged suitable for analysis. 
.  teady missiorl. he ra  ata fro  t e 28 li N-O F I. 
an airs mprise  tal f 7,983 owers " d 
87,419 "OFF" an excess of 564 events (1. 401 i plying a 3 
Si a pper it  e x ove 0 e  f . xlO-lO 
photons cm-2 s-l. The raw data were then subjected to a cut 
esigned t  referentially ass ga ma-ray i itiated showers 
hilst r jecting uclear i itiated s owers ith a  ffi-
ciency of IV 90% (Cawley et al paper OG 2.3-1 these proceed-
ings). The total numbers of events in this enhanced sample 
ere , 52 " " a d , 53 " F" -- t e xcess f 9 vents 
is not judged to be significant, differing from zero ·by 1.9 
si a. ssuming a ollection rea f l l 4 2 f r t ese 
selected events (Cawley et ale OG 9.5-4 these proceedings) 
the observing time of 766 minutes implies a 3 sigma upper 
li it of 5.5xlO-l l photons cm-2 s-l. This li it is plotted 
in figure 1 along with the results of other experiments. 
3.2 eriodic mission. ifteen hours of data, fro  four 
separate ights, i  hich eminga as tracked have been 
examined, independently, for evidence of possible pulsed 
e ission ear a eriod f sixty seconds. he ti e of 
arrival of each event passing the cut designed to preferen-
tially select gamma rays was folded at periods in the range 
58.75-61.75 seconds and, t  facilitate comparison ith other 
experiments, the resultant phase distribution displayed as a 
t enty bin histogram and tested against uniformity ith 
chisquare. The period was incremented uniformly over the 
1 78 OG 2.4-4
range scanned in steps of (58.75)_/(2x20xT) seconds, where T
is the duration of the scan in seconds. The maximum reduced
chisquare observed in any scan was 2.3 (19 dof) correspond-
ing to a chance occurence probability of 10-3, as there
were 300 independent trial periods used in the analysis of
this scan the probability reduces to 0.3. No systematic
trend in the period associated with the maximum chisquare of
individual scans was noted.
From the four nights' observations a 3 sigma upper limit to
the pulsed flux of 2.3xi0-II photons cm-2 s-i was estab-
lished.
4. __Q_. From a comparison of 13 hours of "ON"
and "OFF" source data and 15 hours of tracking data we
deduce 3 sigma upper limits of 5x10-11 photons cm-2 s-i
and 2.3xi0-ii plates cm-2 to the steady and pulsed flux of
gamma rays above 400 GeV from Geminga. The latter flux is
approximately a factor of six below that predicted by the
E-I.3 extrapolation of Zyskin and Mukanov(1983) between
their observations at i000 GeV and those of COS-B at 0.i
GeV. Thus Geminga has not been confirmed as a source of VHE
gamma rays.
Acknowledgements. This work is supported by the
U.S. Department of Energy, the Smithsonian Scholarly Studies
Fund and the National Board of Science and Technology of
Ireland. We acknowledge the assistance of Kevin Harris in
making observations.
Buccheri, R., D'Amico, N., Hermsen, W., Sacco, B.,
(pre-print, submitted to Nature)
Bignami, G.F., Caraveo, P.A., Lamb, R.C.,
Ap.J. Letter. _7_2, Lg, 1983
Bignami, G.F., Caraveo, P.A., Paul, J.A.,
Nature, 310, 464, 1984
Hermsen, W., Space Science Reviews, 36, 61, 1983
Kniffen, D.A., Bignami, G.F., Fichtel, C.E., Hartman,
R.C., Ogelman, H., Thompson, D.J., Ozel, M.E., Tumer, T.,
Proc. International Cosmic Ray Conf. Munchen, [, i00, 1975
Moffat, A.F.J., Schlickeiser, R., Shara, M.M., Sieber, W.,
Tuffs, R., Kuhr, M., Ap.J. Letter, 271, L45, 1983
Spelstra, T.A. Th., Hermsen, W., Astron. Astrophys., 135,
135, 1984
Thompson, D.I., Fichtel, C.E., Hartman, R.C., Kniffen,
D.A., Lamb, R.C., Ap.J., 213, 252, 1977
Weekes, T.C. and Helmken, H.F. Proc. 12th ESLAB Symposium,
Frascati, 1977
Zyskin, Yu.L., Mukanov, D.B., Soviet Astron. Lett., _,
117, 1983
• 
.. 
175 G .4-4 
range scanned i  steps of (58.7 5?- / (2x20xT) seconds, here  
 e uration f t e an  conds. he aximum duced 
chisquare observed in any scan as 2.3 (19 dof) correspond-
ing to a chance occurence probability of 10-3 , as there 
ere 300 independent trial periods used in the analysis of 
t is scan the probability reduces t  0.3. o systematic 
trend i  t e eriod associated ith t e axi um c isquare f 
dividual ans as oted. 
Fro  the four nights' observations a 3 sig a upper li it to 
the pulsed flux of 2.3xlO-ll photons c -2 s-1 as estab-
ed. 
. Conclusions. Fro  a comparison of 13 hours of hON h 
d h F h urce ata nd  ours f t king ata e 
educe  si a pper li its f xlO-ll hotons c -2 -l 
and 2.3xlO-ll plates c -2 to the steady and pulsed flux of 
ga ma rays above 400 GeV fro  eminga. The latter flux is 
approximately a factor of six below that predicted by the 
-l.3 xtrapolation f yskin a d ukanov(1983} et een 
eir bservations t 1 00 e  d se f S-B t .1 
eV. hus eminga has not been confirmed as a source of E 
ga ma rays. 
cknowledgements. This ork is supported by the 
u.s. epartment of Energy, the S ithsonian Scholarly Studies 
und d e ational oard f cience d echnology f 
Ireland. e acknowledge t e assistance f evin arris i  
aking observations. 
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u cheri, ., ' Amico, ., ermsen, ., a co, ., 
<pre-print, submitted to ature} 
ignami, . .F., araveo, . ., a b, . ., 
p.J. etter. ~, 9, 1983 
ignami, .F., araveo, . ., aul, J. ., 
ature, llQ, 4, 4 
ermsen, ., ace cience eviews, l[, , 3 
niffen, . ., i nami, .F., ichtel, .E., art an, 
.C., gelman, ., hompson, .J., zel, .E., urner, ., 
roc. t rnational osmic ay onf. unchen, L, 1 0, 75 
offat, .F.J., chlickeiser, ., hara, . ., i ber, w., 
uffs, ., uhr, ., p.J. etter, 1Il, 5, 3 
pelstra, . . h., ermsen, ., stron. strophys., 112, 
135, 1984 
Thompson, .I., ichtel, .E., art an, . ., niffen, 
.A., a b, . ., p.J., ll, ,  
eekes, . . a  el ken, .F. r c. th B posium, 
rascati, 7 
yskin, u.L., ukanov, .B., viet stIon. ett., ~, 
7, 3 
176 OG2.4-4
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Search for gamma-rays above 400 GeV from Geminga

Search for gamma-rays above 400 GeV from Geminga

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