. . . - ' e s . Vol 5 No. 3 . p p . 1 0 9 - 1 1 2 1985 ()2-~-1177/8-~ S t 3 0 0 ÷ . 5 0 P r i n t e d i n G r e a t B r i t a i n . A l l r i g h t s r e s e r v e d . C o v > r t g h t c COSP.\R
IS C Y G N U S X - 3 A L O W - M A S S X - R A Y
B I N A R Y ?
M van der Klis" and F Jansen
Astronornlcal lnstttute 'A~lton Pantlel, oe/, . Unt~ el ~lll o/ A m s t e r d a m . Roetersstraat 15. 1018 14'B Anl~terdaot, The
Vetherlands
" Laboratory f o r Space Research. Letden The Netherhuld~
ABSTRACT
We present examples of the quasi-perlodic variations in the X-ray flux of Cyg X-3 which we have recently found during observations of this source with EXOSAT. Amplitudes and periods of the variations range from 5% to 20% of the total flux and from 50 to 1500 s, respectively. Our tentative interpretation of these quasl-periodicities, the occurrence of quasl-perlodic phenomena in an accretion disk which partially occults the X-ray source, points towards an analogy of Cyg X-3 with certain 'dipping' low-mass X-ray binaries such as 4U 1822-37, as suggested
]I/
previously. We point out, however, that there are also fundamental differences between Cyg X-3 and this type of low-mass X-ray binary.INTRODUCTION
Cygnus X-3 is a famous bright X-ray source, which occasionally shows very large radio outbursts /2[, and which may be emitting y-rays in the 108
/3:',
10 12 /4/ and 10 16 /5/ eV bands. The source has an intrinsic X-ray luminosity which sometimes exceeds I038 erg/s, and always shows a smooth, large-amplitude X-ray modulation with a period of 4.fl hr/6/.
This period is probably the orbital period of a binary system, in which the X-rays are generated by either accretion onto a compact object or by a young, Crab-like pulsar /7/. In most models proposed, the smoothness of the X-ray modulation is explained in terms of scattering material surrounding the X-ray source, with various geometries being invoked for the distribution of this material/8/,19/,/I0/,/II/,/I/.
The X-ray light curve varies strongly from one 4.8 hr cycle to the next /12/ but in the long run these variations average out, giving an average light curve w~ich is very constant in shape, but not in amplitude /13/. On the basis of a statistical analysis of a large amount of low-sensitivity X-ray data, the cycle-to-cycle variability of Cyg X-3 was estimated to occur mainly on time scales below 3000 s, with an anplitude of 5-10% of the amplitude of the 4.8 hr modulation, i.e., 7.5-15% of the total flux /12/. In this paper we show examples of the transient, q u a s l - p e r l o d l c oscillations which we have found to be a component of the cycle-to-cycle variability of Cyg X-3, and consider some possible consequences of this behaviour.OBSERVATIONAL RESULTS
Cyg X-3 was observed on 5 occasions between October 1983 and January 1984 with the EXOSAT Medium Energy instrument. During four of these observations, which each lasted between 0.6 and 1.4 lO ~ s, a total of six different q u a s i - o s c i l l a t i o n s were seen. The oscillations covered a range in period of 50-1500 s and had amplitudes between 5% and 20% of the total flux. They always occurred in the phase interval 0.O to 0.75 (with phase 0.0 at light curve minimum). Examples are given in Flg. I. A partlcularly interesting case is illustrated in Fig. 2, where a 25% intensity drop on a i00 s time scale is preceded by two simultaneous quasi-oseillatlons, with periods of - 500 s and - 70 s, respectlvely. Insets show the enhancements caused by the quasl-osclllations in the power spectra of the data. Notice that the oscillations are in soma cases quite coherent (only a few bins in the power spectrum) and in other cases cover a considerable range in frequency.
DISCUSSION
I l O 51 ~ a n d e r K I z ' , a n d F . J a n s e n , J . _ x Z i ~Zc ~ i 1 ~ 8 0 I ,
t,, i!
laO [
4
~2D i
1 i , - ! I I I i i i , _ ~ -~L i 0 0 ~ 1500 ZOO0 ?SO0 loom 3500 gOOC ~500 SECt c52 ~r~r,T l n g ( S E ~ 1
,oc [ 0"a
N
g,
/ ' ¢ i , 45~ . I i -i II i3so
' ~ ' ~ "bi),
l
cream-',r, P ' , , L i: 1 0 0 " ; 0 ZOO0 3000 ~1000 50C~ SO00 "~000 ~:JOr
T I t l E ,'SEE
• ~ e ~ a ~ o I a q ~ u I p a m q ~ i i q ~ i q s u o ~ m m I I ! O s o - ! s e n b s 0 0 ~ - ~ a m ° I s a q 3 u o p a s o d ] a d n s a ~ e
qoTq~ s 0 8 - 0 ~ ~o 8mT3 8 o u a ~ n ~ 8 ~ i e o T d ~ 3 e q~T~ s ~ e [ ~ ~o s~T~as e ~uT~oqs ' 8 m e ~ ~a~o I aq~ UT e3ep aq~ ~o ] ~ d ] o d n - ~ o l q p a I T e ~ a P a~o~ e s u ~ e ] u o o a m e ~ ~addn aq~. " I "8T3 sV "~ "ST3
0000[ 0 0 0 6 0 0 0 8 f I
F
! i I 3OOKrh
i("Z~3S) 3Nil
O00L 0009 O00S 00011 000£ I I I I I'i'il
s O O q - 0 0 ~ ) r i I i i i Joo,. oo.°. oo., ... . oo° oo,.. oo.,L oo.*,
to be more ~n a c c o r d a n c e wlth the n o n - s l n u s o l d a l character whlch f r e q u e n t l y c h a r a c E e r l z e s the q u a s i - o s c l l l l a t l o n s .
Thls i n t e r p r e t a t l o n i s in accordance wlth the a c c r e t l o n - d l s k corona model which was applied by White and Holt /1/ to Cyg X-3 as well as to the low-mass X-ray b i n a r i e s 4U 1822-37 and 4U 2129+47, whlch show periodic X-ray dlps. S i m i l a r e x p l a n a t i o n s have been proposed for a 10 3 s q u a s l - p e r i o d l c i t y observed in the p u l s a t i n g source 4U 1626-67 /14/, and recently for f l u c t u a t i o n s seen /15/ in 4U 1755-33 /16/ and 4U 1705-44 /17/. These s o u r c e s are all low-mass X-ray binaries believed to be powered by the R o c h e - l o b e o v e r f l o w of a r e d - d w a r f companion to the X-ray source. The m e c h a n i s m which is causing the red dwarf to o v e r f l o w its Roche lobe in these systems is not yet clear. G r a v i t a t i o n a l r a d i a t i o n /18/ and m a g n e t i c b r a k i n g /19/ have been proposed to take care of the n e c e s s a r y loss of a n g u l a r m o m e n t u m from the system. I n d e p e n d e n t of the precise nature of the a n g u l a r m o m e n t u m sink, we can c a l c u l a t e what the o r b i t a l e v o l u t i o n of the system must be if it is p o w e r e d by the Roche lobe o v e r f l o w of a m a i n - s e q u e n c e red dwarf.
A d o p t i n g the a p p r o x i m a t e m a s s - r a d l u s relation of m a i n - s e q u e n c e stars for the red dwarf" M r d / M o = Rrd/Ro, and using the e x p r e s s i o n for the R o c h e - l o b e radlu~ of the l e a s t - m a s s l v e b i n a r y component (in this case the red dwarf) / 2 0 / , R R ~ a = 0 . 4 6 ( M r d / M ) I / 3 , where M is the total mass of the system, and a the binary separation, one finds, with K e p l e r s law, that the c ~ n d i t i o n that the red dwarf f111s its Roche lobe (i.e., RRL = Rrd) implies:
( P / P ) o r b = (M/M)rd' where Porb is the orbital period. This result i m p l i e s that all low-mass X-ray binaries in which a m a l n - s e q u e n c e red dwarf is t r a n s f e r r l n g m a t t e r to a neu[ron star more m a s s i v e than itself by R o c h e - l o b e overflow~ the period must be d e c r e a s i n g (and the orbit must be shrlnking~. The rate of period decrease depends on the m a s ~ - t r a n s f e r rate and could be as high as - ( P / P ) o r b = iO-8/yr' This result has not, as yet, been v e r i f i e d for any of Ehe
'canonical' low-mass X-ray blnarles.
In Cyg X-3, however, the orbital period is i n c r e a s i n ~ /21/ at a rate of - lO-9/vr, whlch shows that the source can not be powered by R o c h e - l o b e o v e r f l o w of a m a l n - s e q u e n c e red dwarf. Therefore, we must conclude that (even ignoring the u n i q u e radio-, y-ray and IR properties of the source) Cyg X-3 does not flt in with the 'standard' model for l o w - m a s s X-ray blnarles. The discovery of q u a s i - p e r l o d l c fluctuations i n the X-ray flux of the source does strengthen the suspicion, on the other hand, that there is a s i m i l a r i t y b e t w e e n the way in w~leh the X-ra~ m o d u l a t i o n is produced in Cyg X-3 and in c e r t a i n 'dipping' low'mass X-ray blnarles with a s i m i l a r orbital period.
R E F E R E N C E S
I. White, N.E., Holt,S.S., Astrophys. J. 257, 318 (1982). 2. Gregory, P.C., et al., N a t u r e 239, 440 (1972).
3. Lamb, R.C., et al., Astrophys. J. 212, L63 (1977),
4. V l m d l m l r s k y et al., Proc. 14 th Int. Conf. C o s m i c Rays I_~ I18 (1975). 5. Samorskl, M., Stanm, W., Astrophys. J. 268, LIT (1983).
6. Parslgnault, D.R., et at., Nat. Phys, Sc. 239p 123 (1972). 7. Basko, M.M., et al., Astron. Astrophys. 3 1 , 249 (1977). 8. Pringle, J.E., N a t u r e 247, 21 (1974).
9. Davldsen, A.O., Ostrlker, J.P., ~strophys. 3. 189, 331 (1974). IO. Bignaml, C.F., et al., Astron. Astrophys. 55, 155 (1977). ii. M i l g r o m , M., Pines, D., Astrophys. J. 220, 272 (1978)o
12. Van der Klis, M., B o n n e t - B 1 d a u d , J.M., Astron, A s t r o p h y s . Suppl. Set. 5 0 , 129 (19@2). 13. B o n n e t - B i d a u d , J.M., van der Klzs, M., Astron. A s t r o p h y a . i01, 299 (1981).
14. Joss, P.C., Avnl, Y., Rappaport, S., A s t r o p h y s . J. 221~ 645 (1978). 15. W'hlte, N.E., et al., Astrophys. J. Left., in press (1984).
16. Frank, J., Sztajno, M., Astron. Astrophys. 138p LI5 (1984).
17. Langmeler, A., S2tajno, M., Trunper, J., Proc. Conf. "X-ray Astron. '84", Bologra, in press (1984).
18. Faulkner, J., Astrophys. J. 170, L99 (1981),
19. Verbunt, F., Zwaan, C., Astron. Astrophys. lOOp L7 (1981). 20. Psczynskl, B., Acts Astron. 17, 287 (1967).
