The detection of very high energy gamma ray sources has been reported in the last few years by means of extensive air shower observations. The Plateau Rosa array for the registration of the arrival directions of extensive air showers has been operating since 1980 and first results on Cygnus X-3 have been reported. Here, the status of observations of Cygnus X-3 and of the Crab Pulsar are reported

Morello, C.

Navarra, G.

Periale, L.

Vallania, P.

English

NASA Technical Reports Server

I 2 7 OG 2.2-12
EXPERIMENTAL RESULTS ON GAMMA-RAY SOURCES AT E = 1013 - 1014 eV
0
(*)
C.Morello, G.Navarra , L.Periale and P.Vallania
Istituto di Cosmo-geofisica del CNR and
(_)Istituto di Fisica Generale dell'Universita, Torino, Italia
l. Introduction. The detection of very high energy gamma ray sources
has been reported in the last few years by means of extensive air show-
er observations (Cerenkov light and EAS particle arrays); as reviews
see (i) and (2).
The Plateau Rosa array for the registration of the arrival directions
of extensive air showers is operating since 1980 and first results on
Cygnus X3 have been reported (3,4). In this note we want to present
the status of observations of Cygnus X3 (_4y) and of the Crab Pulsar
(_2y, for which the ephemeris from radio observations are available).
2.The experiment. The experimental equipment (5) consists of four li-
quid scintillator detectors 1 m each, positioned at the corners of a
rhombus (7x14 m diagonals, 8 m side). The array operates at an alti-
tude of 13500o m a.s.l.; the most probable detected primary energy is
E =3. i0 eV. The accurancy in the determination of the arrival
0
direction of individual EAS is _=5.5 degrees at the zenith (timing
error 6t=4.2 ns, mainly due to the thickness of the shower disk).
Since Dec.'81 for all events the relative delays (t) between different
scintillators ( _t=l nsec step) together with the absolute time (T) of
the event ( AT=I msec) are recorded.
In order to avoid corrections due to the atmospheric absorption, pres-
sure and temperature variations, the data are analyzed in ON/OFF mode,
i.e.: the number of source events is normalized to the counting ratesh
from two "OFF" sources located + 2.5 around the candidate, at the
m
same declination. An event is assigned to a source if the deviation of
relative delays between different scintillators with respect to the
theoretical one is 16t I< 5.9 ns for which the best signal/noise ratio
is expected. The arrival times of the events from the OFF sources are
shifted ; 2.5 h in order that to equal arrival times correspond equal
atmospheric absorptions. Data are analyzed around the source culmina-
tion, up to 30 ° from the zenith, and only complete days of measurement
are considered.
3.Results. C[$nus X3. The arrival times (corrected to the center of
mass of the solar system) are folded following the ephemeris obtained
from X-ray observations (6). The results are shown in Fig.l for the
whole epoch '81-'85, and split into -_i year intervals (also the epoch
Nov'80-Dec'81 is shown, but, due to the insufficient recording system,
the data are analyzed in a different way (4) , and therefore not
1   .  
PERIMENTAL ESULTS  A MA-RAY URCES T O 
C. orello, .Navarra(*), L.Periale and P. allania 
stituto i osmo-geofisica el R d 
(*)Istituto di isica enerale ell' niversita, orino, It lia 
1. tr duction. he etection f ery i h ergy a  urces 
as een r orted i  t  l st fe  ears by eans f tensive ir s -
r bservations ( erenkov li ht d S article rrays); s r iews 
e 1) d ). 
he lateau osa array f r t e r gistration f t e rrival irections 
of extensive air showers is operating since 1980 and first results on 
Cygnus X3 have been reported (3,4). In this note we ant to present 
the status of observations of Cygnus X3 (~4y) and of the Crab Pulsar 
('V 2y, for which the ephemeris from radio observations are available). 
2.The experiment. The experime~tal equipment (5) consists of four li-
id intillator etectors 1  each, ositioned t t e c rners f a 
rho bus (7x14  diagonals, 8  side). The array operates at an alti-
tude of ~500 m a. s.1.; the most probable detected primary energy is 
 3 . 10  e . he accurancy i  t  etermination f t  rrival 
d~rection of individual EAS is Ll1p=5.5 degrees at the zenith (ti ing 
err r at 4. 2 ns, ainly due to the thickness f the sho er isk). 
i ce ec.'Sl f r ll e ents t e r lative elays (t) bet een ifferent 
scintillators ( Llt=l nsec step) together ith the absolute ti e (T) of 
t e e ent ( Ll T=l sec) are recorded. 
I  r er t  oid o rections e t  t  t ospheric sorption, res-
s re and te perature ariations, t e ata are a alyzed i  / FF ode, 
i. .: t  ber f s rce ents i  ormalized t  t  unting r t s 
 t  FF" urces ated  . 5  nd  ndidate, t  
sa e eclination. n ent i  ssi ned t  a s urce i  t  eviation f 
.. relative delays bet een different scintillators ith respect to the 
theoretical one is /ot/< 5.9 ns for which the best Signal/noise ratio 
is expected. The arrival ti es of the events fro  the OFF sources are 
ifted + .   i  rder t at t  ual rri al ti es rrespond ual 
at ospheric absorptions. ata are analyzed around the source cul ina-
ti , up t  30° fro  t e zenith, and only co plete days f easurement 
 nsidered. 
3.Results. Cygnus X3. The arrival times (corrected to the center of 
mass of the solar system) are folded following the ephemeris obtained 
from X-ray observations (6). The results are shown in Fig.1 for the 
whole epoch 'Sl-'S5, and split into rv1 year intervals (also the epoch 
ov'80-Dec'Sl is sho n, but, due to the insufficient recording syste , 
the data are analyzed in a different ay (4) , and therefore not 
https://ntrs.nasa.gov/search.jsp?R=19850025624 2020-03-20T17:53:37+00:00Z
128
OG 2.2-12
TAB.I
Epoch hours Nev ON (ON/OFF-l) _2 A Phase
1
meas. [xlO 6] [xlO-_ B9 d.fJ [xlO -2]
11/80-12/81 1078 1.0 35.9 1.42+0.46 0.32+0.05
m
12/81- 1/83 1177 5.1 3.59+1.71 38.4 0.62+0.24 0.69+0.06
1/83- 3/84 1265 5.8 1.99+1.6 8.0 0.16+0.23 0.59+0.22
3/84- 3/85 990 4.6 -0,65+1.8 25.8 0.85+0.26 0.39+0.05
12/81- 3/85 3432 15.5 1.74+0.98 34.3 0.33+0.14 0.53+0.07
included in the sum). The absolute excesses, the values of _2
obtained by checking for uniformity the phase histograms and the
amplitudes and phases of the first harmonics are shown in Tab.l (in
order to check phase histograms similar to the X-ray light curve).
We notice:
-2
a) the significance of the possible signal in '81-'82 (c.l. i0 from
the phase analysis and 2 i0 from the absolute excess) is decreased
when the data from '83-'84 are added;
b) this might be partly due to different phases of the signal in dif-
ferent epochs (in (4) we tried to analyze such phases in terms of a 34
days modulation, that We cannot confirm);
c) the possibility for the signal to be variable on year's and month's
time scale has been suggested by other experiments (see e.g. 7,8);
d) while the excesses seem to be located between phases 0.5 and 0.7
there is an indication, before 1982, of a signal at phase 0.2-0.3 as
reported in (7,9,10);
e) the overall effect would correspond to a flux (with primary proton
S(E)=2.E-2.75 -2 -icm s sr-iGev-l):spectrum
(E> 3 lol3ev) _ 5-10 -12
_2
• cm s
Crab Pulsar. For obtaining a msec accuracy in the determination
of the arrival times we use a quartz thermostabilized oscillator
(stability r_J 10-9 ) and the detection of radio network signals for
absolute timing and calibration of the oscillator (r.m.s. error includ-
ing the 1 msec step 6T < 0.4 ms, the maximum systematic shift being _,
1
6T = 1 ms). The arrival times corrected to the center of mass of the
2
solar system ( 6T < 0.7 ms) are folded following the Pulsar ephemeris3
obtained from radio observations (ii) (max. error of our interpolation
6T =0.2 ms).
Th_ analysis concerns the data collected between Feb'82 and March'84
corresponding to 1268 h of effective observation, for a total of
4.36_0 events within the source angular bin. No excess arises both
from the absolute counting rate (ON/OFF=I+(O.04_2)IO -3) and the phase
128 
OG 2.2-12 
TAB.1 
Epoch hours Nev ON (ON/OFF-1) X2 A1 Phase 
meas. [X106] [X10-3] [19 d. f.] [X10-2] 
11/80-12/81 1078 1.0 35.9 1.42+0.46 0.32+0.05 12/81- 1/83 1177 5.1 3.59+1. 71 38.4 0.62+0.24 0.69+0.06 1/83- 3/84 1265 5.8 1. 99+1. 6 8.0 0.16+0.23 0.59+0.22 3/84- 3/85 990 4.6 -0.65+1.8 25.8 0.85+0.26 0.39+0.05 12/81- 3/85 432 15.5 1.74+0.98 34.3 0.33+0.14 0.53+0.07 
inclu ed in the sum). The absolute excesses, the values of X 2 
obtained by checking for uniformity the phase histograms and the 
amplitudes and phases of the first harmonics are shown in Tab.l (in 
order to check phase histograms similar to the X-ray light curve). 
We notice: 
a) the significance of the possible signal in '81-'82 (c l. 
-2 the phase analysis and 2 10 from the absolute excess) is 
when the d ta from '83-'84 are added; 
-2 10 from 
decreased 
b) this might be partly due to different phases of the signal in dif-ferent epochs (in (4) we tried to analyze such phases in terms of a 34 days modulation, that we cannot confirm); 
c) the possibility for the signal to be variable on year's and month's 
time scale has been suggested by other experiments (see e.g. 7,8); 
d) while the exces es seem to be located between phases 0.5 and 0.7 
there is an indication, before 1982, of a signal at phase 0.2-0.3 as 
reported in (7,9,10); 
e) the overall effect would correspond to a flux (with primary proton 
-2.75 -2 -1 -1 -1 spectrum S(E)=2·E cm s sr GeV ): 
13 
-12 -2 -1 l/>(E>3'10 eV)AJ 5'10 cm s 
Crab Pulsar. For obtai ing a msec accuracy in the determination 
of the a rival ti es we use a quartz thermostabilized oscillator 
-9 (stab li ty rv 10 ) and the detection of radio network signals for 
absolute ti ing and calibration of the oscillator (r.m.s. error includ-ing the 1 msec step OT < 0.4 ms, the maximum systematic shift being o T ::; 1 s). The arri v~l times corrected to the center of mass of the 2 
solar system ( 0 T 3  0.7 s) are folded following the Pulsar ephemeris 
obtained f  r dio observations (11) (max. error of our interpolation OT 0.2 s). Th~ naly is concerns t e data collected between Feb' 82 and March' 84 
corresgonding t  268  f ffective bservation, for a total of . 6'10 vents i thin t e source angular bin. No excess arises both 
-3 fro  t e bsolute ounting r te ( N/OFF=1+(0.04:t2)10 ) and the phase 
.. 
129 OG 2.2-12
• FqH]
11/80-12/81 _ 0
12/81-1/83 0 +8
-1 -- 0
-8
r=+l /
1/83-3/8_ _ o
"-+1 r +8
3/8_-3185 _ Q _-_ r-iz 0 " 0o I L_I
_, "'L_ _ s
-1 -8
! ; r=-.! _._
"" ] +8
z+ 1 ---0
._. r_.r'- +4
-8
: : I I .... I I i I . I I i
•0 .2 .4 .6 .S 1.0
PHASE
Fig, I Phase histograms of the Extensive Air Showers from the
direction of Cygnus X3,
.. 
1  E)G 2.  
+1 
• 
11/80-12/81 Q 0 1----+----t::::t-1---t-;---.=;;;----t--1 
. 
(/) 
-1 
+1 
-1/83 0 
-  
I-r.. 
I-r..  1 ~ 
/ -3/84 z 0 0 
....., 
b -1 
...... 
'" I-r.. +1 I-r.. 
3/84-3/85 P  "-. :z: 
0 
v -1 
I 
:::.. 
r:... 
0 
...... 
:z +  0 
12/81-3/85 
-- 0 
-1 
.0 .2 .4 
P  
.6 .8 
+8 
0 
 
+8 
0 
-8 
+8 
0 
-8 
+8 
+4 
0 
-4 
-8 
1.0 
Fig. 1 Phase histograms of the Extensive ir Showers from the 
direction of Cygnus X3. 
M 
0 
-x 
-
~ 
t&.. 
~ 
z 
0 
130
OG 2.2-12
MAIN INTERPULSE +10
PULSE
z Phase histograms of
,., o the Extensive Air
< "- • '' 0 Showers from the di-
"JL:_ rection of the Crab,-5 ' Pulsar ("ON events",
-I _ deviations from the
a_ " mean)
z
< -- -10
un -2
I_ I I | I I
•0 .2 .4 .6 .8 1.0
PHASE
analysis ( _2:9.8 / 14 d.f., see fig.2). The upper limits to the
continuous and to the pulsed flux in phase with the main pulse
observed at other wavelenghts, and of duration Z_T<2 ms, are (at 90%
•lol3ev) :
c.l., Eth=3 -ii -2 -i
_cont _ i0 cm sac ;
-12 -2 -i
%uls _ 3 i0 cm sec
6.Acknowledgement. The authors wish to thank N.A.Porter and C.Casta-
gnoli for useful discussions, A.G.Lyne, J.Lloyd-Evans and V.Pettiti
for substantial contributions to the timing of the Crab Pulsar.
We are indebted to the "Cervino" Company for many years of kind
collaboration.
References
l.Stepanian A.A., (1984), Adv. Space Res., 3, 123
2.Porter N.A., (1983), 18th ICRC, Bangalore, Rapp.Paper
3.Morello C. and Navarra G., (1982), 8th European C.R. Symp., Roma
4.Morello C. et al., (1983), 18th ICRC, 1_, 127 ,1
5.More!lo C. and Navarra G., (1981), N.I.M., 18___7,533
6.Van der Klis M. and Bonnet-Bidaud J.M., (1981), Astron.Astrophys.,
9,5, L5
7.Lloyd-Evans J. et al., (1983), Nature, 305, 784
8.Cawley M.F. et al., (1985), Ap.J., in the Press
9.Nesphor Yu.I. et al., (1979), Ap. and Space Science, 61, 349
lO.Samorski M. and Stamm W., (1983), Ap.J.Lett., 268, LI7
ll.Lyne A.G., private communication_
130 
OG 2.2-12 
MAIN INTERPULSE +10 
PULSE 
+1 , t +5 Fig. 2 
II) 
z Phase histograms of 0 (") 
-
0 the Extensive Air f-o 
-< 0 Showers from the di-
-
)( 
:> rection of the Crab 
"-l 
t:l 
- Pulsar ("ON events", 
-5 deviations from the t:l -1 rz 
Qc: "- mean) • < Z 
t:l 
Z 
< -10 
f--
-2 U) 
I I 
.0 .2 .4 .6 .8 1.0 
PHASE 
analysis 2 ( X =9.8 / 14 d.L, see fig.2). The upper limits to the 
continuous and to the pulsed flux in phase 
observed at other wavelenghts, and of duration 13 
c.l., E
th=3'10 eV): 
cP cont'" 
-11 10 -2 -1 cm sec 
cp ::: 3 10-12 cm -2sec-l puIs 
wi th the main pulse 
L1 T< 2 ms, are ( at 90% 
6.Acknowledgement. The authors wish to thank N.A.Porter and C.Casta-
gnoli for useful di cus ions, A.G.Lyne, J.Lloyd-Evans and V.Pettiti 
for subst ntial contributions to the timing of the Crab Pulsar. 
We are indebted to the "Cervino" Company for many years of kind 
collaboration. 
References 
1.Stepanian A.A., (1984), Adv. Space Res., ~, 123 
2.Porter N.A., (1983), 18th l RC, Bangalore, Rap .Paper 
3.Morello C. and Navarra G., (1982), 8th European C.R. Symp., Roma 
4.Morello C. et al., (1983), 18th l RC, !, 127 
5.Morello C. and Navarra G., ( 981), N.l.M., 18 , 533 
6.Van der K1is M. and Bonnet-Bidaud J.M., (1981), Astron.Astrophys., 
95, L5 
7.Lloyd-Evans J. et al., (1983), Nature, ~, 784 
8.Cawley M.F. et al., (1985), Ap.J., in the press 
9.Nesphor Vu.l. et al., (1979), Ap. and Space Science, 61, 349 
10.Samorski M. and Stamm W., (1983), Ap.J.Lett., 268, L17 
11.Lyne A.G., private communication. 
Ii 


Experimental results on gamma-ray sources at E sub 0 = 10(13) - 10(14) eV

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