Energetic (E35 MeV) Gamma rays were observed from Cyg X-3 with the SAS-2 Gamma ray telescope. They are modulated at the 4.8 sup h period observed in the X-ray and infrared regions, and within the statistical error are in phase with this emission. The flux above 100 MeV has an average value of (4.4 + or - 1.1)x 10 to the -6 power/sq cm/sec. If the distance to Cyg X-3 is 10 kpcs, this flux implies a luminosity of more than 10 to the 37th power ergs/s if the radiation is isotropic and about 10 to the 36th power ergs/s if the radiation is restricted to a cone of one steradian, as it might be in a pulsar

Fichtel, C. E.

Hartman, R. C.

Kniffen, D. A.

Lamb, R. C.

Thompson, D. J.

English

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OBSERVATION OF GAMMA RAYS
WITH A 4.8 HR PERIODICITY
FROM CYG X-3
Unclas
G3/93 06447
R. C. LAMB
C. E. FICHTEL
R. C. HARTMAN
D. A. KNIFFEN
D. J. THOMPSON
(NASA-TM-X-71201) OBSERVATION OF GAMMA RAYS
	 N76-33122
WITH A 4.8 HOUR PFRIOLICITY FROM CYG X-3
(NASA) 1 4 p HC $ 3.50	 CSCL 03C
SEPTEMBER 1976
4D ------ GODDARD SPACE FLIGHT CENTERGREENBELT, MARYLAND
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Observation of Gamma Rays With a 4.8 Hour
Periodicity From CYG X-3
R. C. Lamb*, C. E. Fichtel, R. C. Hartman,
D. A. Kniffen, and D. J. Thompson
•	 NASA, Goddard Space Flight Center, Greenbelt, Md. 20771
ABSTRACT
Energetic (E s 35	 y -,-ays have been observed from Cyg X-3
with the SAS-2 7-ray telescope. They are modulated at the 4.8h
 period
observed in the X-ray and infrared regions, and within the statistical
error are in phase with this emission. The flux above 100 MeV has an
average value of (4.4 +1.1)x10 -6
 photons cm 2s-1 . If the distance to
Cyg X-3 is 10 kpcs, this flux implies a luminosity of more than 1037
ergs s-1 if the radiation is isotropic and about 10 36 ergs s-1 if the
if the radiation is restricted to a cone of one steradian, as it might
be in a pulsar.
Subject headings: y-rays, binaries, pulsars, Cygnus X-3.
*On faculty leave from Iowa State University, August 1975-August 1976.
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1. INTRODUCTION
Cygnus X-3 is a particularly interesting object for several
reasons,including the synchronized 4.8h	modulation of infrared and
X-ray emission (Becklin et al., 1973; Mason et al., 1976), the high
stability of the 4.8h	X-ray period (Parsignault et al., 1976a),
and the erratic, frequency-dependent radio outbursts of up to 2x104
.	 times the quiescent flux level with no evidence for a 4.8h	period
(cf. Gregory et al., 1972).
In the y-ray energy range, Galper et al. (1975) reported a 3.6a
excess above 40 MeV of (2.0 +0.8)x10-4 cm-2s-1 from an October 1972
balloon flight, with a 4.8h	modulation in which the minimum y-ray
emission occurred near the time of the X-ray maximum. From balloon
flights in September 1972 and July 1973, however, McKechnie, Mount,
and Ramsden (1976) report an upper limit to the Cygnus X-3 flux above
70 MeV of 6.5 x 10-6 cm 2s-1 , well below the result of Galper et al.
(1975). Vladimirsky et al. (1975) have reported a positive flux from
Cygnus X-3 above 1012 eV.
In the course of a search of the SAS-2 y-ray data for emission
from suspected X-ray binaries, strong evidence '.^ qs been found for
radiation from the Cygnus X-3 region in the energy range above 35 MeV.
The specific identification with Cygnus X-3 is based on the observa-
tion of a 4.8 hr. periodicity in the y-ray data in phase with the
X-ray emission. On the basis of the observed y-ray flux and a distance
estimate of 10 kpc (Laque, Laqueux, and Nguyen-Quant-Rieu, 1972),
Cyg X-3 is the most luminous y-ray point source yet identified by
13
well over an order of magnitude. Upper limits reported in this paper
from the other X-ray sources in the survey further distinguish Cyg X-3
from conventional accreting binaries and raise anew the question of the
underlying physical characteristics of this object.
The telescope used to collect the data reported here is a 32-level
wire-grid, magnetic-core spark chamber assembly covered by an anti-
coincidence scintillator and triggered by any one of four independent
directional scintillator-Cerenkov counter telescopes in anticoincidence
with the outer scintillator (Derdeyn et al., 1972). A general discussion
of the data analysis procedure, the experiment calibration, and the detec-
tor response is given by Fichtel et al. (1975) and Thompson et al. (1976).
II. RESULTS
The majority of the data used in the Cyg X-3 study came from the
observing period 7-13 March 1973 with a small portion from 1-6 March
1973. Figure 1 presents the histogram of y-ray arrival times for y-rays
from within an error circle of approximately 1.2a of the direction of
Cyg X-3, plotted as a function of phase within the (0.1996814 +0.0000005)d
period seen in X-rays (Parsignault et al., 1976x). The zero phase is
chosen to coincide with the X-ray minimum as defined by Parsignaul.t et
al. (1976a). A small correction is included in the histogram in
Fig. 1 to take into account the non-uniform exposure to the source as
a function of phase resulting from the 4.8h source period being close
to three times the SAS-2 earth orbital period. The probability that
the distribution shown in Fig. 1 is random is less than one part in a
thousand, and the argument becomes more persuasive when it is noted
I4
that the two adjacent low intervals are consistent in intensity level
with the celestial and galactic diffuse emission and that the X-ray
minimum falls within these two intervals.
The flux from Cyg X-3 averaged over the 4.8h
 period is found to
be (4.4 +1.1)x10-6 photons cm 2sec-1
 for E > 130 MeV and (10.9 +3)
x 10-6 photons cm 2 sec -1 for E > 35 MeV, where the uncertainty
includes both statistical and systematic errors.
As noted in the introduction, the y-ray emission from Cyg X-3 was
revealed in a systematic search of the SAS-2 data for radiation from
periodic X-ray sources. Table 1 lists all known or suspected binary
sources with an intensity greater than 50 UHURU counts/sec. (Giacconi
et al. 1974). The last three entries are only considered to be possible
binaries. In particular for Cyg X-3, the 4.8h
 variation may very well
not be orbital in nature, as will be discussed later.
Cyg X-3 is seen to be the only source on the list for which a
positive flux of 7-rays was seen. If.the distance to Cyg X-3 is 10 kpc,
the observed flux value implies that the power emitted in 7 -rays with
energies above 35 MeV is in excess of 1037
 ergs s -1 if the radiation is
isotropic and over 1036 ergs s-1 if the radiation is restricted to a
cone of one steradian as it might be in a pulsar. For comparison, the
next most luminous identified y-ray point source is the Crab pulsar
with a radiated y-ray power above 35 MeV of approximately 6x1034 ergs s-1,
where the emission is assumed to be uniform within a solid angle of 1 sr.
The differential 7 -ray flux is shown in Fig. 2 together with
measurements in the radio, infrared, and X-ray energy intervals.
5Table I
Observed UHURU Intensity l y-Ray Flux4
Source Periods (ct/sec) 105xy's(9>100 McV)cm 2s-1
3U 1700-37 3.4d 102 < 1.0 (3.4d)
300900-40 2838
(Vela XR-1) d9.0 100 < 0.8	 (9.0d)
Her X-1 1.28
1.7d
100 < 0.5
35.Od
SMC X-1 0.78 78 < 0.7
3.9d
Cen X-3 4.8s 160. < 2.53
2.1d
Cyg X-1 5.6d 1176 < 1.1
Sco X-1 0.8d 17000 < 1.02
Cyg X-2 13.6d 540 < 1.22
Cyg X-3 4.8h 194 4.4 +1.1
1. Giacconi et al. 1974.
2. Fichtel et al. 1975
3. The relatively high upper limit is due to the source being near
the edge of the field of view and in the gblactic plane.
4. If the upper limit applies to a specific period, it is given in
parenthesis; otherwise, the upper limit applies to the time
averaged flux,
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Although it is tempting to draw a smooth curve through these data,
it must be remembered that the radio emission is highly variable, and
a wide variation in X-ray spectra has been observed, including the
identification of X-ray emission lines superimposed on a hard continuum
at some times and consistency with a black-body distribution at others
(Serlemitsos et al. 1975). Thus, as will be discussed in the next
section, the origin of the radiation in different parts of the spectrum
may be quite different.
III. DISCUSSION
As noted in the in the introduction, there is also a 4.8h modu-
lation seen in the infrared region which is apparently only present
at some times and not others (Mason et al. 1976). In general, when
the modulation is present in the infrared, its amplitude is sig-
nificantly less than that seen in X-rays. On the other hand the modu-
lation of the y-rays as seen in Fig. 1 is nearly complete, indicating
a possible natural progression in the amplitude of the modulation as
the energy of the radiation increases. The absence of any detectable
radio modulation reported by Mason et al. (1976) is consistent with
this characteristic. This feature may indicate a progression in the
size of the emitting region as the energy changes,with the highest
energy y-rays originating from the smallest volume of space.
The shape of the 7 -ray modulation above the lowest intensity
level is consistent with the shape as seen in X-rays (Mason et al.
1976). The minimum in the 7 -ray distribution occurs from 0.9 to 0.3
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in phase. The observations of Parsignault et al (1976b) show that in the
phase interval from 0 . 85 to 0.25 the X -ray intensity has its lowest
values and shows little variation. (Note: Add .25 to the phase used
by Parsignault et al. to obtain that used here.) Except for a dif-
ference in phase of 0.05 which could easily be a statistical fluctua-
tion in the y-ray data, the y-ray emission, therefore, would seem to
correspond with the period in which X-ray variability was seen.
The mechanism for producing the Y-rays is uncertain. Binary models
which have been proposed foi X-ray emission include those in which
there is a smaller object, either a white dwarf (e.g. Davidsen and
Ostriker, 1974) or a neutron star (e.g. Shklov sky, 1967; Pringle,
1974) and one in which the observed X-rays are those reflected off
the large companion star (Basko, Surryaev, and Titarchuk, 1974). The
extension of most of these models to include y-ray emission is
generally difficult. Accretion onto either a white dwarf or a neutron
star as the source of y-rays seems unlikely, both in view of absence
f y-rays from the other entries in Table 1 and the energy required to
produce the y-rays. Other difficulties associated with binary hypo-
theses for Cyg X-3 include the very clear and apparently unchanging
asymmetric nature of the X-ray phase plot and the rather small sepa-
ration of the centers of the two objects dictated by the length of
the observed period unless at least one of the objects is quite
massive.
Accretion onto a black hole has also been proposed as a source of
y-ray (Dahlbacka, Chaplin, and Weaver 1974); however, the total
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energy in y-rays observed here is very much larger than that predicted
for this process.
If the previous SAS-2 results on localized y-ray sources (Thompson
et al. 1975; Ugelman et al. 1976; Thompson et al. 1976) are used to
suggest possibilities, then a pulsar origin would seem to be a likely
possibility. Four pulsars are now known to emit y-rays and the possi-
bility that Cyg X-3 is a fifth certainly exists. It should be
emphasized that it seems quite unlikely that the 4.8h	period is
the one to be associated with the pulsar since all known radio pulsars
have periods of several seconds or less, and also a much larger period
than those observed would imply a much smaller rate of energy emission.
Pursuing the pulsar possibility, Treves (1973) has proposed that the
4.8h
 period is a free precession of the spin axis of a neutron star,
and, on this basis, he predicts that the rotation period of the neutron
star would be approximately 13 cosec. A search of the radio emission
from Cyg-3 for a pulsar in this period range would be desirable despite
the dispersion and low flux problems associated with the large distance.
Another possible model is one in which a young pulsar is part of
a binary system. Again, the 7 -ray emission might be associated with
the pulsar itself. Detection of a radio counterpart would be even more
difficult in this case due to absorption by the plasma in the source
region. In this model in particular, the X-ray and infrared emission
may very well have a different origin, for example, accretion onto the
neutron star. On the basis of the other 7 -ray sources observed and
these last two models t che fast pulsar possibility seems to deserve
serious attention as a candidate for explaining the 7-ray emission
from Cyg X-3.
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ACKNOWLEDGMENTS
We are happy to acknowledge the helpful discussions that we have
had with Elihu Boldt and Joseph Taylor.
FIGURE CAPTIONS
Fig. 1 - Distribution of y---ay arrival times in fractions of the
(0.1996814 +.000000")d period for y-rays above 35 MeV
from the region of Cygnus X-3. The zero of time corresponds
to the X-r minimum defined by Parsignault et al. (1976a)
dashed portions of the plot are repeated parts of the single
period distribution, shown to emphasize the periodicity.
The dotted lines show the number of y-rays normalized to the
average SAS-2 exposure to Cyg X-3. The dot-dashed line shows
the estimated contribution from diffuse celestial and
galactic radiation, together with its uncertainty.
Fig. 2 - The differential photon energy flux spectrum observed for
Cygnus X-3. Upward pointing arrows on the radio fluxes
indicate that flaring sometimes increases these fluxes by
over an order of magnitude. The X-ray intensities are
represented by cross-hatched regions which bracket the reported
values. The radio observations show no indication of the 4.8h
pulsation; however, at higher frequencies there is a pulsed
component which ap;arently increases relative to the constant
component as a function of energy.
4
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REFERENCES
Basko, M. M., Sunyaev, R. A., and Titarchuk, L. G. 1974, Astron. and
Astrophys., 31, 249.
Becklin, E. E., Hawkins, F. J., Mason, K. 0., Mathews, K., Neugebauer,
G., Sandford, P. W., and Wynn-Williams, C. G., 1973; Nature Phys.
Sci., 245, 302.
Dahlbacka, G. H., Chapline, G. F., and Weaver, T. A. 1974, Nature,
250, 36.
Davidsen, A., and Ostriker, J. P. 1974, Ap. J. 189, 331.
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Luchkov, B. I., and YurAn, Yu. T., 1975, 14th International Cosmic
Ray Conference, Munich, Vol. 1, 95.
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Gregory, P. C., Kronberg, P. P., Seaquist, E. R., Hughes, V. A.,
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6
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Ma. ns F. 0., Sanford, P., and Ives, J. 1975, GSFC X-Ray Binery
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bgelman, H. B., Fichtel, C. E., Kniffen, D. A., and Thompson, D. J.
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Parsignault, D. R., Schreier, E., Grindlay, J., and Gursky, H. 1976a,
"On the Stability of the Period of Cygnus X-3," submitted to
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Parsignault, D. R., Grindley, J., and Gursky, H., 1976b, 'Observations
of Phase Dependent VariabiliVy in Cygnus X-3," submitted to
Ap. J. (Letters).
Pringle, J. E. 1974, Nature, 247, 21.
Serlemitsos, P. J., Boldt, E. A., :colt, S. S., Rothschild, R. E., and
Saba, J. L. R. 1975, Ap. J. (Letters), 201.p L9.
Shklovsky, I. S., 1957, Ap. J. (Letters),148, U.
Thompson, D. J., Fichtel, C. E., Kniffen, D. A., and Ugelman, H. B.
1975, Ap. J. (Letters), 200, L79.
Thompson, D. J., Fichtel, C. E., Kniffen, D. A., and Lamb, R. C.
1976, submitted to Ap. J.
Treves, A. 1973, Nature Phys. Sci., 242, 121.
Vladimirsky, B. 11., Neshpor, Yu. I., Stepanian, A. A., and Fomin, V.P.,
1975, 14th International Cosmic Ray Conference, Munich, Vol. 1, 118.
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Observation of gamma rays with a 4.8 hour periodicity from CYG X-3

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