Analysis of data from an atmospheric Cerenkov telescope indicated the periodic emission of gamma rays of energy  10 to the 12th power eV, at 60.25 second period, from 2CG 195+4. The gamma ray flux at 99% confidence level is estimated to be 9.5 x 10 to 12 photons/sq cm/s

Bhat, C. L.

Kaul, R. K.

Qazi, R. A.

Rannot, R. C.

Rawat, H. S.

Razdan, H.

Sanecha, V. K.

Sapru, M.

Tickoo, A. K.

Tonwar, S. C.

English

NASA Technical Reports Server

I@| OG 2.4-2
PERIODIC GAMMA-RAY EM_SION FROM 'GEMINGA'
AT _ I0" eV
R.K.Kaul, H.S.Rawat, V.K.Sanecha_ R.C.Rannot, M.L.Sapru, A.K.Tickoo,
R.A,Qaz_, C.L.Bhat, H.Razdan
BARC-NRL, Zakura, Srlnagar-190006, India
S.C.Tonwar
Tata Institute of Fundament_l Research
Bombay-_0000_, India
ABSTRACT
Analysis of data from an atmospheric Cerenkov telescope
at Gulmarg (India), has Indicate_,the periodic emission of
gamma-rays of energy _ L0Ix eV, at 60.23 second perLed, from
2CG 195+4. The gamm al-_ay flux at 99% confidence level isestimated to be 9.5xi0 -photons cm-_ s"I
h Introduction. The high energy gam_ma-ray source 2CG 195+_ (Gemlnga),_
detected by 5AS II and COS B satellites at _ _0 MeV [l,2]j has been tentatively
identified with an unusual se4t x-ray and optlcal_ source9 for which the gamma-
ray to x-ray to optical luminosity ratLo is tO° t 10 t t [3_ This ldehtifl-
catioh has been strengethened by the observation of nearly 60 second periodic
variation In the x-ray source, similar to that reported in gamma-rays.
A recent analysis of Einstein Observatory and Exosat x-ray data [t_]_ along-
With the ground-based observations of Zyskin and Mukanov [_]_ has further
revealed a very .high positive period derivative .for thls.source, which seems
to increase still further between 1.979 and 1993. Here we report the results
of the analysts of our atmospheric Cerenkov telescope data-from Gulmarg
India, which reveals, perlodic gaml3a-ray emission from this 'source ._t a
flux level of 9.5xI_ "'L photons cn__ _" , at photon energles;_ 6.1x10' eV.
2. Experimental Method. The experiment was carried out at Gulmarg,
India (altltude rv 27_3' meters),, during Dec. 19a_ to Februar_ 1985. The
experimental set up, shown in Figure I_ consists mainly of three 90 cm.
diameter (f/2)parabolic searchlight mirrors, each of which has an RCA
8055 photomultipller •tube mounted at its focus. The tubes are provided
with. 3.8 cm. diameter aperture masks to limit the effective field of view
to 2.5". ?dl three mirrors are mounted on individual equatorial mounts,
with a _racking capability of better than 0.5" in.4.hours. An LED lamp
compensator ls. also fixed ,on all three mirrors to ,ensure a constant anode
- current, by compensating for background lig)_t fluctuations due to changes
in night sky intensity and atmospheric transparency. Triple coincidence
rates, integrated over 2 seconds (coincidence resolving time _ 50 ns) alongwlth
the corresponding time are recorded on a printer. The time information
is provided by a temperature controlled crystal clock which Is malntalruDd
accurate to _ t ms by periodic synchronization with standard tlme-code
signals.Alongwlth the prompt coincidence rate_the chance coincidence
rate, obtainedby introducing suitable time delays in the outpiJts, from
'the three channels, is also recorded continuously to monitor the behaviour
of system electronics and the background variations.
Theleffectlve area of the telescope is _g12 cm_ and the energy
.. 
• 
ttl 
I I  MA-RAY ~ N  I I GA' ' 
 ~ 10e  
 2.4-2 
. .Kaul, . .Rawat, CUlecha, . a not, . apr , . lck o, 
.Qazt, .~ , azdan ' 
C-NRL, r , i ar-19 06,  
S. . onwar 
ta titute  ental earch 
ombay-4 00',  ' 
CT 
l sis  ta   pl spheric ov scope 
 ar  ,  indicat~ the dic i sion  
ma-rays  ergy.> 101 , t .2' nd ,perl ,  
2CG 195+4. The gamm!t1a~ flux at R~, confidence level Is ' 
timated   .' l0 photons   • " 
1. ction.   rgy m  r e  . 4 inga)" ' 
tected  S      t lites t ). '  e  1,2],   tatively 
identified with an unusual soft x-ray and opt1c~ sour'cl' for whleh the gamma-
  ray  ical. l inosity i   10 I  I 1 ' ]. I  i n ifi-
n      r      c 
tion i   ra   lar   ~ e   -rays. 
 t         [If.], 
w    s tions     ,]"  r 
vealed  r ,  itive d ative  i  . source, h s 
 rease i l further en 9   8 . r   rt  lts 
 ' i sis   spheric - ov scope from ar  
India, which reveai~,rriodic ga"!.fa:fay e ission fro  this ' 'source Itt a 
 el , 1Q 'ph s m 5 a  t n r ies ~ lO e 
 ~ m ai d.  eriment s' carried out  arg, 
I  itude  4  ters),.  c. &4  ebruary' ,'~ The 
perimental t   in re 1, sists   e ' cm • 
eter ) parabolic archlight irr rs, h  ich h~   
 tomultiplier t  unted,  .its us.  s  d 
,  . eter rture Sks  U lt  fective id . f  
 .5· • Al  e rr rs  ted  idual atorial ts, 
ith  tr ingcapa llty  better  .'· ' If ,hours.   p' 
pensator I  i  d  il e rrors  ure  nstant de 
rrent,  ensating r round h  tuations   s 
 t  i sity  atmospheric t parency.  incidence' 
s, t grated r  onds incidence e ing t e ,'0  gwlth 
 ponding e  rded   ter.  e r ation 
 d   perature tro led stal ck  1.. lntaine  
curate  N 1   r dic chronization· ith ndard e-code 
ignals. l ith  pt  rate, the   
t , t ined' by i t i  it le tI  l  i  t  t uts f  
t   i ,    ti l      
 s st  l t ics  t  r  i ti s. r 
2' e fectlve      .58    t  r  
https://ntrs.nasa.gov/search.jsp?R=19850025636 2020-03-20T17:54:25+00:00Z
166
OG 2.4-2
12
threshold for shower detection has been estimated to be 6.lxl0 eV.
Observations have been taken in the tracking mode, by pointing the mirrors
alternately at the 'source' and an 'off-source' region (6°to the east of
source)) for 24 minute durations. The results reported below refer to the
zenith distance range _zl _ 30°. Since on-source and off-source data refer
to the same zenith angle, no zenith angle dependence Is considered in
the comparison of on-source and off-source rates.
3. Results. Observations were taken from Dec. 20, 1984 to Feb. 20) 1955)
for a total period of 19.75 hours) comprising ll.3 hours of on-source data
and 8.45 hours of off-source data. The three-fold prompt coincidence rate
was adjusted to be _s 0.4 per second, for single channel rates of _ 20kHz.
The delayed coincidence rate was less than 10% of the prompt coincidence
rate throughout the course of these observations. A total of 229730 events
were recorded and the ratio of on-source to off-source counting rate was
found to be 1.17.
The search for periodicity was realized by generating 30-bin
phasograms for 41 periods, in the range of 59-61 seconds) at 0.05 second
interval, both for the on-source and the off-source data sets. In the absence
of an ephemeris for this source) to was chosen to be the start of our obser-
vations (J.D=2446055.27083333)) so that the calculated phases are not
absolute and cannot be compared directly with those derived by earlier
workers [4,5]. As shown In Figure 2) the highest signal-to-noise ratio was
found at a period of 60.25 seconds) where we see a statistically significant
(3.30-) peak at a phase value of 0.46. No such structure Is seen in the
case of off-source data. The probability t_at the peak at 0.46 phase Is
due to chance is estimated to _e 1.01xl0 for a single trial) while the
probability increases to 4.15x10-when the total number of independent
trials is considered.
Figure 3 shows the distribution of %_ values for various periods
in the range of 59-61 seconds. As is obvious) %=attains a maximum value
of 52.3 (for 29 degrees of freedom) at the period of 60.25 seconds, showing
that the phasogram for this period exhibits non-randomness at a confidence
level of 99%. Assuming that the peak at 0.06 phase is due to gamma-rays
of >_ 6..Ixl0 eV, we .can compute the gamma-ray flux from the sourceI'A, _ il0 h ons m at h % nidenc .as 9.5x p ot c- _ t e 99 co f e level
$. Discussion. The Einstein Observatory and Exosat x-ray observations
of 'Gemlnga' reported by Bignami et al. [4] reveal a period Po= 60.06 seconds .
and i_ = 0.65xI0 "I s _I for the Sep. 1953 epoch. If these results are correct
and also valid for our observation period, we should expect Po = 60.26
seconds during the period of our observations. This is very close to the
period of 60.25 seconds derived by us from our preliminary analysis of
the Gulmarg data and confirms both the periodic nature of gamma-ray
emission from this source as well as Its high period derivative. It may
be mentioned here that because of the limitations of our analysis) the
period derived above could be in error by ± 0.05 seconds. A direct comparison
of the light curve derived by us with that derived by earlier workers [4)5]
is not possible due to the different t o used in getting the various results.
However, a 10-bin phasogram constructed from our 30-bin phasogram
88   
 
reshold r  ection   i ated   1x10 e 
bservations have been taken in the tracking ode, by pointing the irrors 
alternately at the 'source' and an 'off-source' region (6°to the east of 
s rce), for  inute rations. e res lts re rted el  refer to t e 
zenith distance range I zl·~ 30°. Since on-source and off-source data refer 
to the sa e zenith angle, no zenith angle dependence is considered in 
the co parison of on-source and off-source rates. 
3. esults. bservations ere taken fro  ec. 20, 1984 to Feb. 20, 1985, 
for a totd period of 19.75 hours, co prising 11.3 hours of on-source data 
and 8.45 hours of off-source data. he three-fold pro pt coincidence rate 
as adjusted to be NO.4 per second, for single channel rates of 6- 20k z. 
The delayed coincidence rate was less than 10  of the pro pt coincidence 
rate throughout the course of these observations.  total of 22,730 events 
were recorded and the ratio of on-source to off-source counting rate was 
    
The search for periodicity as realized by generating 30-bin 
phasogra s for 41 periods, in the range of 59-61 seconds, at 0.05 second 
terval, th r e -source  e f-source ta s.    
of an ephe eris for this source, to as chosen to be the start of our obser-
ations (J. 446055.2708 3), s  t at t e calculated ases are t 
absolute and cannot be co pared directly ith those derived by earller 
workers [4,5]. As shown in Figure 2, the highest signal-to-nolse ratio was 
found at a period of 60.25 seconds, where we see a statistically significant 
(3.3 (j) peak at a phase value of 0.46. o such structure is seen in the 
case. of off-source data. The probablllty !g.at the peak at 0.46 phase is 
due to chance is estimated to _~e 1.01x10 for a single trial, while the 
probablllty increases to 4.15x10 when the total ,number of independent 
ials  sidered. 
Figure 3 shows the distribution ·of -x:- values for various periods 
in the range of 59-61 seconds. s is obvious, "J,.'l. attains  axi um value 
of 52.3 (for 29 degrees of freedom) at the period of 60.25 seconds, sho ing 
that the phasogram for this period exhibits non-randomness at a confidence 
level of 99 'I:tssuming that the peak at 0.46 phase is due to ga ma-rays 
of ~ 6..:-!fl0 e ,!',.e _fan co pute the ga ma-ray flux fro  the source 
as 9.5xlO I p ot  c  IS  t e 99  co fidence level  
4. iscussion.  instein s rvatory  s t -ray s r ations 
of 'Geminga' reP?cfted by Bignami et al. [4] reveal a period p. = 60.06 seconds 
and p = 4.68x lOs 5" for the Sep. 1983 epoch. If these re;ults are correct 
and also valid for our observation period, we should expect Po = 60.26 
seconds during the, period of our observations. This is very ciose to the 
period of 60.25 seconds derived by us fro  our preliminary analysis of 
the Gulmarg data and confirms both the periodic nature of ga ma-ray 
e ission fro  this source as ell as its high period derivative. It ay 
be entioned here that because of the li itations of our analysis, the 
period derived above could be in error by ± 0.05 seconds. A direct co parison 
of the light curve derived by us ith that derived by earlier orkers [4,5] 
is not possible due to the different to used in getting the various results. 
However, a 10-bin phasogram constructed from our 30-bin phasogram 
1 6 7 OG 2._=2
(Figure 2) exhibits a striking similarity with the x-ray light curve [4],
with a main peak at o.4 phase and an intermediate peak at 0.8 phase.
The gamma-ray flux of 9.5x10 photons cm s at 6.1x10 eV,
estimated from the present Study, compares favourably with the value
of (5+3)xl0-11photons cr__ _1 at E >_1012eV, obtained from the ground-based
observations at Tian-Shan [5] and is compatible with a power law energy
spectrum with exponent = -2.3.
References
1. Fitchel, C. E. et al. (1975), Astrophysical 3ournal, 19_._,8165
2. Swanenburg, B.N. et al..(1981), Astrophysical 3ournal Lett. ,
L69-L73
3. Blgnami, G.F. et al. (1983), Astrophysical 3ournal Lett., 272, L9-LI3
4. Bignaml, G.F. etal. (1984), Nature, 310. 464
5. Zyskin, Yu. L. and D.B. Mukanov 1-(T9"83),Proc. 18th ICRC, XG-I, 122
8   4-2 
( igure 2) e i its a stri ing si ilarity ith t e -ra  light c r e [ ], 
with a main peak at 0.4 phase and an intermediate peak at 0.8 phase. 
\ 
-12. -2. -I 12-
e a a-ray fl  f . lO t s c  s t . l  e , 
esti ated ~rom the present study, co pares favourably ith the value 
of (5:t3)xl0- 1 photons cm" 5' at E ~ lOIZeV, obtained from the ground-based 
observations at !ian-Shan [5] and is co patible with a power law energy 
spectru  ith exponent  -2.3. 
erences 
1. Fitchel, . E. et al. (1975), strophysical Journal, 198, 163 
2. S anenburg, . . et al. .(1981), strophysical Journal Lett. , 243, 
-L73 
3. lgna i, .F. et al. (1983), strophysical Journal ett., 272,  9-  13 
4. igna i, .F. et al. (1984), ature, 310 464 
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Periodic gamma-ray emissions from Geminga at  or = 10(12) eV

Periodic gamma-ray emissions from Geminga at or = 10(12) eV

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