Measurements of 1000 GeV gamma-rays from Cygnus X-3 made with the University of Durham facility at Dugway, Utah in 1981/82 are reviewed. The light curve of the 4.8 hour modulated emission is updated and shows evidence significant at the 4.4 sigma level for strong emission (9% of the cosmic ray rate) at phase 0.625 and less significant (1.4 sigma level) indications of weaker emission (3% of the cosmic ray rate) at phase 0.125. The effect constituting the excess on the few nights showing the strongest emission appears to arise from the smallest Cerenkov light signals suggesting a steep gamma-ray spectrum. The 1982 data have been searched unsuccessfully for evidence of emission at phase 0.2, in coincidence with the results from the ultra-high energy (extensive Air Showers (EAS) measurements in 1979-1982. A systematic investigation of a long term variation in the strength of the peak of the 4.8 hr modulated 1000 GeV gamma-ray emission has been made. We find that in addition to the approximately 34 d variation reported by us previously, a stronger effect exists at around 19d

Chadwick, P. M.

Dowthwaite, J. C.

Kirkman, I. W.

Mccomb, T. J. L.

Orford, K. J.

Turver, K. E.

English

NASA Technical Reports Server

7g
062.1-8
1000 GeV GAMMA RAYS FROM CYGNUS X-3 - AN UPDATE.
Chadwick, P.M., Dowthwaite, J.C., Kirkman I I.W.,
McComb, T.J.L., Orford, K.J. and Turver, K.E.
Department of Physics, University of Durham,
Durham DHI 3LE, U.K.
ABSTRACT
Measurements of 1000 GeV Y-rays from Cygnus X-3
made with the University of Durham facility at Dugway,
Utah in 1981/82 are reviewed. The light curve of the
4.8 hour modulated emission is updated and shows
evidence significant at the 4.4 _ level for strong
emission (9% of the cosmic ray rate) at phase 0.625 and
less significant (1.4 _ level) indications of weaker
emission (3% of the cosmic ray rate) at phase 0.125.
The effect constituting the excess on the few nights
showing the strongest emission appears to arise from
the smallest Cerenkov light signals suggesting a steep
Y-ray spectrum. The 1982 data have been searched
unsuccessfully for evidence of emission at phase 0.2,
in coincidence with the results from the ultra-high
energy EAS measurements in 1979-1982.
A systematic investigation of a long term variation
in the strength of the peak of the 4.8 hr modulated
I000 GeV Y-ray emission has been made. We find that in
addition to the approximately 34 d variation reported
by us previously, a stronger effect exists at around
19d.
The results of an unsuccessful search for a pulsar
period or pseudo-regular variations in Y-ray emission
are reported.
I.INTRODUCTION.
Cygnus X-3 was observed for 350 hrs in 1981/82 using the Ougway
Y-ray _acility. The initial results on the emission of the I(:00Gev
Y-rays have already been reported _L_. We here update the analysis in
the light of our further understanding of our equipment and the
considerable interest in Cyg X-3 since that time. Most of the data in
1981 were taken with the telescopes in the drift scanning mode, a
reliable but inefficient mode of operation. In contrast to other VHE
Y-ray observations, we made our drift scans of Cyg X-3 at
predetermined phases in the 4.8 hr orbital period using the X-rey
ephemeris _=_. Observations on the other experiments are made with no
preconsideration of the orbital phase and the results _rom many drift
scans, in a typical 10 % phase range, are combined and presented as a
phase histogram. In our case the ON/OFF ratio of counts, being the
basic data of the driftscan, represent the flux of VHE Y-rays in a 10
min time slot (0.035 in phase) at fixed orbital phases. We chose to
make most of our observations at spot phases, separated by 0.125 steps
from 0 to 1.0. A small number were made in 0.125 steps from 0.03 to
1.03. [he observations in 1982 were made using the telescopes to track
the object with the intention of investigating the duration of any
activity detected using the driftscan data and to search for a pulsar
periodicity.
• 
9 
OS 2.1-8 
 6 V   O     . 
had ick, . ., owthwaite, J.e., irk an, I. ., 
c o b, .J. ., rf r , .J. and r er, . . 
epart ent f ysics, niversit  f r , 
a  l , . . 
 
easure ents of iooo e  -rays from ygnus -3 
a  it  t  niversit  f r am ilit  t a , 
ta  in /  are re ie ed. e lig t c r e f the 
.  r odulate  e issi  is ate  a  s s 
evidence si ifica t at the 4.4 u le el f r stro  
issi  (qr, f th  s i  r  r t ) t s  .   
l  i i i t ( .4 u le l) i i ti s f eaker 
e ission (37. of the cos ic ray rate) at phase 0.125. 
The effect constituting the excess on the few nights 
s i  t  tro est issi  ears t  ri  from 
the s allest erenkov lig t si als suggesting a steep 
-  u .   t     
su ce sfully f r i e f issi  t s  . , 
in coincidence ith the results from the ultra-high 
  easure ents  - 82. 
 t atic i esti ti  f  l  term ri ti  
i  t  tre t  f t   f t  .  r odulate  
1   -  issi    ade. e n  t n 
diti  t  t  r xi ately   ri ti  r rt  
by us re i usly, a stro er ff t ists at around 
. 
he res lts of an unsuccessful search for a pulsar 
eri  r do-regular ari ti s i  -r  issi  
 rt . 
1 INTRO TI  . 
ygnus -  as s r  f r  r  i  /  si  t  Dug a  
-r  f ili .  it l lt    issi    100   
-r s  lr   r rt  (1'. e r  dat  t  al sis i  
t e li t of our f rt er nderstanding of our equip ent and t e 
nsi erable i t r st i   -  i  t ti e. ost  t  t  i  
1981 ere taken ith the telescopes in the drift scanning ode, a 
r li l  t i effi i nt ode f perati n. I  ntrast t  t r E 
-ray bservations, e ade ur rift sca s of yg -3 t 
predeter ined phases in the 4.8 hr r ital period using the -ray 
ephe eris (~'. bservations on the other experi ents are ade ith no 
reconsideration of the r ital phase and the res lts fro  any rift 
s, i   t i l  7. hase r e, r  bined  r s t  s  
phase histogra . In our case the N/ FF rati  of counts, being the 
basic data af the driftscan, represent the flux of VHE -rays in a 10 
in ti e sl t (0.035 in phase) at fi e  rbit l hases. e chose t  
ake ast af our observations at spot phases, separated by 0.125 steps 
fro  0 to 1.0.  s all nu ber ere ade in 0.125 steps fro  0.03 to 
. . T e bservations i   er  ade si  t  t l es t  tr  
t  bject it  t  i t ti  f i esti ati g t  urati n af  
acti it  detected using the riftsca  data and t  search for a ulsar 
periodicity. 
https://ntrs.nasa.gov/search.jsp?R=19850025612 2020-03-20T17:54:39+00:00Z
8O
OB2.I-B
2.RESULTS.
We show in Figure I the light curve for 1000 BeV Y-rays based on
all the data recorded on clear nights. The data are similar to those
reported earlier ,I+ but allowance has been made for what we now know
to be the non-uniform background in the Cygnus region ca+ which causes
the effective background for point source detection to be different in
the region of Cygnus X-3. Figure I shows at phase 0.64 + 0.03 an
excess of 9.4 % of the cosmic ray rate for our telescope-s ( a flux of
3xi0 -_°cm -= s-l). The excess is significant at the 4.4 a level if
we assume, in the light of earlier Y-ray observations and a knowledge
of the phase of the X-ray maximum, that this was the expected phase.
The measurements around phases 0.5 and 0.75 show no excess indicating
a duty cycle of < 20%. Consideration of the tracking data _I, suggests
that the activity lasts for only 5 - 10 min. At phase 0.14 + 0.03 we
note a 3.4 % excess significant at the 1.4 o level.
= ....... The origin of a strong
excess (420%) in counts at
4 phase 0.625 in 4 observations
i has been investigated. The
• median value of the Cerenkov
= light amplitudes has been¢;
z evaluated for each detector in
u i each observation This has=
iif! _. _ I 0 been done on the basis of the
events in 20 min OFF SOURCE
i measurement in each scan. the
..... _-_.... _ .... number of flashes in the ON
SOURCE interval for each
- detector in each scan below
.... ' ' this predicted median value0 .-_I_, oI_ .TS *0
r..,= ,4., --_ has been evaluated and summed
over all 4 observations. We
show in Figure 2 the number of
Figure 1. The 4.B hr light events below the predicted
curve, median, expressed as a
fraction of the total, for
' I ' observations in the phase +
o.6 -r range 0.5 - 0.75. If the light
z _ amplitudes from _-rays and
N
-r protons were similar we would
= | I" expect a ratio of 0.5. There
v o.= .- .....t t is clear evidence that thezo events constituting the count
" rate excess at phase 0 625 are
u amongst the smaller Cerenkov
= signals. A complicatedL
0.4 behaviour of the proportion of
= I , 0.625 phase ON SOURCE signals
o.= o._== o.7= within the first octile and
PHAm= C4.= HR_, quartile (again calculated on
Figure 2. The fraction of the basis of the OFF SOURCE
recorded light flashes smaller data) which correlates with
than the median value.
0 
06 2.1-8 
2. . 
We show in Figure 1 the light curve for 1000 GeV -rays based on 
ll t  t  r rded n l ar i hts.  t  r  si ilar t  t s  
e   (11  llow     for t  now now 
to be the non-unifor  background in the ygnus r~gion (31 hich causes 
th  ff ti  r  for i t r  t ii  to e iffer t in 
the region of ygnus -3. igure 1 sho s at phase 0.b4 + 0.03 an 
excess of 9.4 'l. of the cosmic ray rate for our telescopes ( a flux of 
3xl0 -aocm -2 S-l). he e cess is si ifi t at the 4.4 u le el if 
we assume, in the light of earlier Y-ray observations and a knowledge 
of the phase of the X-ray maximum, that this was the expected phase. 
The measurements around phases 0.5 and 0.75 show no excess indicating 
a duty cycle of < 201. Consideration of the tracking data (11 sugge~ts 
that the activity lasts for only 5 - 10 in. t phase 0.14 + 0.03 we 
t   .  Yo s  i i i t t th  .4 a le l. -
• 
4 
.. 
II 
 
· 
:I 
M 
" Z C :a 
u 
.. 
.. 
.. 
z 
• .. 
• 0 
-I 
0 2  .• .70 1.0 
HI., 
Figurl 1. Thl 4.8 hr light 
curvi. 
• I 
PHASE. C04 •• HR) 
Figure 2. The fraction of 
recorded li t fl i ei i aller 
t an t e . i n v.l e. 
The origin of a strong 
 ("'20%) in t  t 
ase . 25 i   servations 
has been investigated. The 
edi  l   t  er  
lig t a plit es has been 
l t  f r  t t r i  
each observation. his has 
been done on the basis of the 
ents in  i   URCE 
easure ent i  eac  sca . The 
ber f l  i  t   
URCE nterval or  
etector in   low 
this predicted edian value 
has been e al ate  and su ed 
er l   servati ons. e 
sha  in Figure 2 the nu ber of 
events below the predicted 
edian, expressed as a 
fracti  of t e t t l, for 
observations in the phase 
range 0.5 - 0.75. If the light 
amplitudes from Y-rays and 
protons were similar we would 
expect a ratio of 0.5. There 
is clear evidence that the 
events constituting the count 
rate excess at phase 0.625 are 
a ongst the s aller Cerenkov 
signals. A complicated 
behaviour of the proportion of 
0.625 phase ON SOURCE signals 
ithin t e first ctile a  
quartile (again calculated on 
the basis of the O  S  
datal hich co relates with 
• 
el 06 2.1-8
the individual detector energy thresholds is noted. The detectors with
the highest threshold energy show the strongest concentration of
Y-ray candidates in near-threshold events. This supports the idea of
a Y-ray spectrum which is steeper than the proton spectrum.
A search has been made of the data taken in 19B2 with the
telescopes in the tracking mode _or emission at phase near to 0.2 ,
the phase at which 10¢= eV emission has been noted ¢4,=) This has
been unsuccessful with only a flux limit {at the 3 a level} of
2x10 -zl cm-1 s -¢ being derived.
X-ray observations of Cyg X-3 have suggested variations in the
amplitude of the 4.8 hr modulation c,) on a time scale of 34 d. In
addition, the magnitude and sign of the phase shift in the time of
occurrence of the peak X-ray emission on a time scale of 19 d _7'has
been noted.
We previously reported the result of folding the values of the
amplitude of the 4.8 hr VHE Y-ray peak at phase 0.625 in individual
scans modulo 34.1 d _*). Recently we have investigated the long term
variation in this peak strength in a more general way.
, _ _ . We have fitted a sinewave to the excess shown
in all the individual
II scans at 0.625 phase with
R the fit constrained to
H have the observed
S 12 strength averaged over
all scans and to have a
peak-to-peak amplitude
13 ranging from zero to
• twice the average. The
rms deviation from such a
sine wave fit for
18 31 188 independent periods in
PERIOD (DRVS) the range 8 to 500 d is
shown in Figure 3. There
are two periods with
small deviations - the
Figure 3. The long term best fit - and these are
variation of the 4.8 hr peak. 19.2 + 0.4 and 36.8 + 1.5
d.
The sine wave with period 19 d has its maximum (i.e. maximum
1000 Gev Y-ray emission at phase 0.625 in the 4.8 hr orbital
period) at JD 2445163 + 0.5. The X-ray phase effect reported
'7' has a maximum phase lead , interpreted as due to apsidal
motion, at JD 2444389 + I. These two possible effects cannot
be reliably linked in phase due to the combined uncertainties
in period and epoch.
81  .1-8 
the individual detector energy thresholds is noted. The detectors ith 
t  i hest t r l  er  s  t  tr est ncentrati n i 
V-ray candidates in ear-t reshold e ents. his s~pports the idea oi 
a V-ray spectrum hich is steeper than the proton spectru •• 
 rch s een  i the ata taken n 8  ith e 
telescopes in the tracking ode ior e ission at phase near to 0.2 , 
t e ase t ich 1D e  ission as ee  oted C4,D'. his as 
 su cessiul ith ly  ilux limit (at t e   level) i 
xl0- 11 c - 1 5- 1 being erived. 
-ray observations of yg -3 have suggested variations in the 
a plitude oi the 4.8 hr odulation c., on a time scale of 34 d. In 
addition, the agnitede and sign of the phase shift in the time of 
occurrence of the peak -ray e ission on a ti e scale of 19 d c71has 
 ot . 
e previously reported the result oi folding the values of the 
a plitude of the 4.8 hr VHE V-ray peak at phase 0.&25 in individual 
s s odul  .   c.,. ecentl  e  i sti t  t  l  hrll 
ri ti  i  t is ea  str t  i  a ore e eral a . 
 
I'll 
11 
 2 
13L1111    
IOD ( Y'S) 
Figure 3. The long term 
variation of the 4.8 hr peak. 
We have iitted a sine 
~    ss  
 l   i al 
s s t .  as  it  
the fit constrained to 
e t e served 
strength averaged over 
l ns     
peak-to-peak a plitude 
ranging from zero to 
t ice the average. The 
 iation    
 a    
independent periods in 
the range 8 to 500 d is 
sho n in igure 3. here 
 t  ri s it  
all viati s  
st it  se e 
.2  .4  .8  5 
 
The sine ave ith period 19 d has its aximu  (i.e. aximu  
1000 Sev V-ray e ission at phase 0.625 in the 4.8 hr orbital 
eri ) at J  2445163  O.S. he -ray ase ff t re rte  
C71 has a maximum phas; lead, interpreted as due to apsidal 
otion, at JD 2444389 + 1. These t o possible effects cannot 
be reliably linked in phase due to the co bined uncertainties 
in period and epoch. 
82
062.1-8
4O
" IFigure 4. The peak =trength at = -
!
0,0 O.D " l.O
P IN4_II[
We have searched a small subset of Cerenkov light data
selected to be rich in Y-ray candidates, those recorded at
the times of maximum VHE Y-ray emission (phase 0.625), for
periodicity on the time scales of I ms - 100 s. Having tested
the 6xtO = independent periods using the Rayleigh test, we find
no evidence for periodicity involving a light curve with a
large duty cycle. Indeed on the basis of the count rate of the
present generation of telescopes, we would no____ttexpect to
obtain sufficient counts in the short time (mins) during the
observed excess at phase 0.625 and which is the maximum
allowable if we are to avoid the effects of the (unknown)
Doppler shift due to a typical orbit. For example, a _t-ray
signal of 11}%of the cosmic ray background would require a
data rate in excess of 3 Hz to produce, in a time of 10 min, a
probability of periodicity of 10-7 on the Rayleigh test which ,
would stand clear of chance expectation.
The Rayleigh test applied to the 1982 tracking data has
provided no evidence for any pseudo-periodic emission in the
range 500 ms to I00s (1200 independent periods tested) of the
type reported for the X-ray emission *=_.
REFERENCES.
41) Dowthwaite J.C., et ai:1983, Astron. Astrophys,12_.__,1.
(2) Van der l'lis,M., et al : 1901, Astron. Astrophys,95,L5.
(3) Dowthwaite J.C., et al:1985,Astron. Astrophys,14__2.2,55.
44) Samoski,M. and Stamm,W.:1983,Ap J.Cett,26B,L17.
(5) Lloyd-Evans, J. et a! :1984, Nature,30__55,784.
46) Molteni,D., et al: 1980, Astron. Astrophys.,87,88.
(7) Bonnet-Bidaud, M., et ai:1981, Astro. Astrophys.,10__.1_1,299.
iS) Van der K]is: 1904 Proc 17th ESLAB Symposium.
 
.0 
A 
:sO II 
igure 4. he ea  s ngth  • 1-... " phise 0.64 folded modulo 19.2 • 20 , 101 
d. u .. 
101 10 
0 .-.. 
0.0 0.11 
e have searched a s all s set of erenkov light ata 
l t  t   i  i  -r  didates, t  r r  t 
1.0 
t e times of aximu  E -ray e issi  ( ase . ), f r 
eri icit  on the time scales of 1 s - 100 s. aving teste  
the 6xl0 S independent periods using the ayleigh test, e find 
 i  f r ri dicit  i l i   li t r  it   
 t  cl .    si    nt    
present generation of telescopes, we would not expect to 
obtain sufficient counts in the short time ( ins) during the 
s r  cess t as  .   hi   t  ~ 
l o able     i   t     
Doppler shift due to a typical orbit. For exa ple, a Y-ray 
signal of 10% of the cos ic ray background ould require a 
ata r t  i  e cess of 3 z t  r uce, i  a time of 10 in, a 
probability of periodicity of 10- 7 on the Rayleigh test hich 
oul  t  l r f ce xpectation. 
he aylei  t t pli  t  t   tr i  at  s 
r vi ed  i ce f r  seudo-periodic issi n i  t  
ra e  s t  1 s (  i ependent eri ds t st ) f t  
type reported for the -ray e ission (81 
EFERENCES. 
(1) owthwaite J. ., et al:1983, stron. strophys,126,1. 
(2) Van der 1(1is,I'I., et al : 1981, Astron.Astrophys,95,L5. 
(3) owthwaite J. ., et al:1985, stron. strophys,142,55. 
(4) Sa oski,l'I. and Sta m,W.:1983,Ap.J.Lett,268,117. 
(S) Lloyd-Evans, J. et al :1984, ature,305,784. 
( 6 ) 1'10 It en i , D., eta 1: 1 980, As t ron. A s t r 0 p h Y s. ,~, 88 • 
(7) onnet-Bidaud, 1'1., et al: 81, stro. strophys.,101,299. 
(8) a  der Kl i 5: 1 8  r c 17t  ES B y posi urn. 
 2.1-8 


1000 GeV gamma rays from Cygnus X-3: An update

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