Optical observations of close binary systems with a compact component - 3 Coordinated X-ray and Optical observations of Sco X-1

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Optical observations of close binary systems with a compact component

Augusteijn, T.

Publication date

1994

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Citation for published version (APA):

Augusteijn, T. (1994). Optical observations of close binary systems with a compact

component.

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3 3

Coordinatedd X-ray and Optical observations of

Scoo X-l

T.. Augusteijn, K. Karatasos, M. Papadakis, G. Paterakis, S. Kikuchi, N. Brosch, E. Leibowitz, P.. Hertz, K. Mitsuda, T. Dotani, W.H.G. Lewin, M. van der Klis, J. van Paradijs

AstronomyAstronomy & Astrophysics 265, 177 (1992)

Abstract t

Wee present the results of coordinated, partly simultaneous, optical and X-ray (Ginga)) observations of the low-mass X-ray binary Sco X-l. We find that the di-visionn between the optically bright and faint state, at a blue magnitude B = 12.8, correspondss to the change from the normal to the flaring branch in the X-ray colour-colourr diagram as proposed by Priedhorsky et al. (1986). From archival Walraven dataa we find that in both optical states the orbital light curve is approximately sinusoidal,, and have similar amplitudes.

3.11 Introduction

Inn the decade after the optical identification of the low-mass X-ray binary Sco X-l (Sandage et al.. 1966 ) many optical studies were made of this source. In a number of cases these observa-tionss were coordinated with X-ray observations [detailed reports on campaigns of coordinated X-ray/opticall observations have been given by Canizares et al. (1975), Bradt et al. (1975), Mook ett al. (1975), White et al. (1976), Willis et al. (1980), Ilovaisky et al. (1980); for general reviews off optical observations of Sco X-l we refer to Miyamoto and Matsuoka (1977) and Van Paradijs (1983)].. It was found that Sco X-l is X-ray active, showing flares in X-ray intensity, when its bluee magnitude B<12.8, and inactive when optically fainter than this. In the active state the X-rayy and optical brightness (averaged over a few minutes) often show a correlation, roughly accordingg to Fo p t (:) F ^5. In the inactive state relatively large optical brightness variations

occurr without a corresponding change in X-rays. From the results presented by Canizares et al. (1975),, Bradt et al. (1975) and Mook et al. (1975) it appears that also the radio behaviour is correlatedd with this optical threshold: radio flares appear only when Sco X-l is in the optically faintt state.

Followingg the discovery of QPO in the X-ray intensity variations of LMXB (Van der Klis et al.. 1985, see Lewin et al. 1989, and Van der Klis 1990 for reviews) it has become clear that there aree two groups of LMXB, each of which is characterized by its own type of correlation between thee X-ray spectral behaviour and the fast-variability properties of the source (Hasinger and Van

30 0 _{33 Coordinated X-ray and Optical observations ofSco X-l}

Tablee 3.1 Log of observations Observatory y Dodaira a u u ESO O Dodaira a Wise e ESO O Dodaira a Start(UT) ) 88 March 17:45 99 March 16:29 100 March 05:40 100 March 17:15 100 March 23:15 111 March 05:37 111 March 16:55 Duration(hrs) ) 2:03 3 3:18 8 1:00 0 2:50 0 4:15 5 3:41 1 2:40 0

derr Klis 1989). These groups are the Z sources and the atoll sources, so called after the shapes off the tracks which they trace out in X-ray colour-colour diagrams. Sco X-l is classified as a Z source. .

Priedhorskyy et al. (1986) proposed that the optically faint and bright states of Sco X-l are correlatedd with the normal and flaring branch, respectively, in the X-ray colour-colour diagram. Subsequentt coordinated X-ray and optical (and UV) observations of Cyg X-2 show that also thiss Z-type source is brightest in the optical (and UV) when it is on the flaring branch (Hasinger ett al. 1990; Vrtilek et al. 1990). X-ray/radio observations of most Z sources have shown that thee radio brightness decreases along the Z track in the direction from the horizontal branch, via thee normal branch to the flaring branch (Penninx et al. 1988; Hjellming et al. 1990a, 1990b; however,, for GX 5-1 this appears not to be the case, Tan et al. 1991).

Heree we report on coordinated X-ray and optical observations of Sco X-l made during a multi-- wavelength campaign in March 1989. We also report on a reanalysis of historical Walraven photometricc data (Van Genderen et al. 1969; 1976), motivated by the new insights in the optical behaviourr of Sco X-l. A preliminary description of the campaign has been given by Wood et al.. (1989). A detailed spectral and temporal analysis of these X-ray data has been made by Hertzz et al. (1991). Results of the coordinated radio/X-ray observations obtained during this campaignn have been published by Hjellming et al. (1990b). The results of the UV observations obtainedd with TUE have been published by Vrtilek et al. (1991).

3.22 Observations

Thee Ginga X-ray observations were made between 1989 March 9, UT 20h and March 11, UT 8h. Thee Large Area Counters (Turner et al. 1987) were used in several instrumental configurations (thee so-called MPC and PC modes), providing different combinations of spectral and time resolutions.. For the purpose of the present paper it is important that except for the last six hours off the observation (when it was in the normal-branch state) Sco X-l was in the flaring-branch state. .

Coordinatedd optical photometric observations were made at ESO, Dodaira Observatory and Wisee Observatory (see Table 3.1 for an observing log). At ESO the 90 cm Dutch telescope was usedd with the Walraven photometer, which provides simultaneous photometry in five passbands betweenn 3250 and 6000 A. A diaphragm of 16" diameter was used, the integration time was 16 seconds.. At the Dodaira Observatory data were taken with the 91 cm telescope simultaneously inn eight passbands between 3600 and 8800 A. A diaphragm of 18" diameter was used, the integrationn time was 23.2 seconds. The observations were made continuously, with interruptions everyy ~20-30 minutes for measurements of the sky background and comparison stars. The

3.33.3 Results 31 1

Figuree 3.1. Optical light curve of

Scoo X-l during the March 1989 campaign.. Plotted as a function of timee is the Johnson B magnitude of Scoo X-l

75944 7595 7596 7597 Timee (JD0-244OOOO.)

d a t aa from ESO a n d t h e D o d a i r a Observatory were reduced differentially with respect t o BD -15°4301,, a n d transformed t o t h e s t a n d a r d Johnson system using the t r a n s f o r m a t i o n formulae givenn by Pel (1987), for t h e ESO d a t a , a n d Kikuchi (1991, private c o m m u n i c a t i o n ) , for the D o d a i r aa observatory d a t a . T h e visual m a g n i t u d e s a n d colour indices B - V a n d U - B , in the s t a n d a r dd J o h n s o n UBV p h o t o m e t r i c system, for BD -15°4301 are V = 9.90, B - V = 0.324 and UU - B = 0.255 (Van Genderen 1969, Pel 1987 a n d 1989 [private communication]).

Att t h e Wise Observatory d a t a were obtained in t h e Johnson U B V R I system. Unfortunately t h ee observations were affected by poor weather conditions. Typical B m a g n i t u d e s were compa-rablee t o those obtained shortly before at t h e D o d a i r a Observatory (see Table 3.1). However, the d a t aa showed a m u c h larger scatter and quite different colours a n d were therefore n o t included inn our analysis.

3.33 Results

Inn Fig. 3.1 we show t h e optical light curve (we have selected the B b a n d since most of the previouslyy published optical p h o t o m e t r y of Sco X - l was in this b a n d ) . During one of t h e six nightss t h e source was relatively faint (i.e., B > 1 2 . 8 ) , during t h e remainder of t h e nights it was opticallyy bright .

Wee have folded the d a t a using t h e ephemeris of G o t t l i e b , Wright and Liller (1975), which is basedd on optical brightness variations during t h e interval 1890 - 1974 (in this ephemeris phase zeroo corresponds t o m i n i m u m light). T h e folded light curve of the optically bright s t a t e is shown inn Fig. 3.2, from which we see t h a t an orbital light variation is present. T h e light curve seems t oo be slightly shifted with respect to the ephemeris of G o t t l i e b , Wright and Liller (1975). A sinusoidall fit t o the d a t a , excluding t h e "flares" occurring at orbital phase ~ 0 . 3 5 and ~0.75 (seee also below), result in a m i n i m u m at orbital phase 0.87(1), a n d a full a m p l i t u d e of 0.126(4) m a g ,, somewhat smaller t h a n t h a t r e p o r t e d by G o t t l i e b , Wright a n d Liller (1975). As t h e phase coveragee is not complete, and the light curve shows s u b s t a n t i a l irregularities (see Fig. 3.2), we doo not consider this a p p a r e n t phase-shift significant.

Forr t h e orbital U-B colour curve given in Fig. 3.2 we used only the d a t a from the D o d a i r a Observatoryy as no proper transformation formula exist from t h e Walraven system to the Johnson UU p a s s b a n d . This excludes a small part of t h e d a t a between orbital phase ~ 0 . 6 a n d ~ 0 . 7 with respectt t o the B light curve. T h e colour curve shows two interesting features; (i) during t h e two "flares"" (at phase ~ 0 . 3 5 and ~ 0 . 7 5 ) the source gets bluer (this is also marginally seen in the B -- V indices); (ii) a clear orbital colour variation is present. A sinusoidal fit t o t h e colour indices, excludingg again the "flares", result in a m a x i m u m a t orbital phase 0.42(1) (i.e. t h e source is redderr a t m a x i m u m light) a n d a full a m p l i t u d e of 0.041(3) m a g .

32 2 _{33 Coordinated X-ray and Optical observations of Sco X-l}

__ ' I ' ' ' ' I '

ii i , , , , i . , . , i ,"

00 0.5 1 1.5 2 0 0.5 1 1.5 2

^ O r b i tt Porbit

F i g u r ee 3.2. Optical light and colour curve of Sco X-l. Plotted as a function of orbital phasee is (left) the B Johnson magnitude, and (right) the U - B colour indices. U - B colour indicess are averages of 10 consecutive points. The phases have been calculated according too the ephemeris of Gottlieb et al. (1975)

Radial-velocityy studies of Sco X - l ( C r a m p t o n et al. 1976, LaSala a n d Thorstensen 1985) show t h a tt inferior conjunction of t h e emission line region occurs near p h o t o m e t r i c m a x i m u m . Assum-ingg t h a t the emission lines originate from t h e accretion disc this indicates t h a t t h e p h o t o m e t r i c variationss are due t o t h e varying aspect of the X-ray h e a t e d side of t h e companion.

T h ee inclination of t h e Sco X - l system h a s been e s t i m a t e d t o be fairly low (i~30° C r a m p t o n ett al. 1976), so t h a t t h e accretion disc is in full view during t h e entire o r b i t a l period. If t h e orbital b r i g h t n e s ss m o d u l a t i o n is caused by the variable visibility of the heated side of t h e companion s t a r ,, it follows t h a t t h e emission from this h e a t e d side is redder t h a n t h a t from t h e accretion disk. F r o mm t h e o r b i t a l intensity a n d colour variations we derive a colour for the varying component off (U - B )0= -0.6(1) (we used EB—v= 0.35(5), see Willis et al. 1980). Interpreting this colour

ass t h a t of a n o r m a l star one finds an effective t e m p e r a t u r e of ~ 19000 K (as t h e side of t h e c o m p a n i o nn p o i n t i n g away from t h e X-ray source is not expected t o contribute significantly t o t h ee t o t a l light from t h e system, this value can be considered as a [very rough] e s t i m a t e for t h e t e m p e r a t u r ee of t h e X-ray h e a t e d side of t h e companion).

Too investigate t h e relation between the X-ray a n d optical properties we have (see Wood et al. 1989)) characterized t h e X-ray s t a t e using a "soft" a n d a " h a r d " X-ray colour, which a r e defined a ss t h e ratios of the count r a t e s in t h e photon energy bands (3.5 - 5.8) keV a n d (1.2 - 3.5) keV, a n dd in t h e b a n d s (9.2 - 18.4) keV a n d (5.6 - 9.2) keV, respectively. For d a t a which have been t a k e nn simultaneously we have in Fig. 3.3 p l o t t e d t h e " h a r d " X-ray colour as a function of t h e BB m a g n i t u d e . We see t h a t in t h e optically faint state (i.e., B>12.8) these two quantities are a n t i - c o r r e l a t e d .. In t h e bright s t a t e the X-ray colour varies substantially, b u t t h e B m a g n i t u d e iss a p p r o x i m a t e l y constant at B ~ 1 2 . 5 .

T h i ss confirms t h a t t h e s e p a r a t i o n between t h e two modes of the bi-modal X-ray behaviour iss defined by a n optical threshold at B~12.8 (Canizares et al. 1975, B r a d t et al. 1975; see also H i l t n e rr and Mook 1970).

Too show t h e n a t u r e of t h e X - r a y / o p t i c a l b i m o d a l behaviour more clearly we have p l o t t e d t h e o p t i c a ll d a t a in a n X-ray colour-colour diagram (Fig. 3.4); t h e size of t h e symbols is a measure off t h e B m a g n i t u d e (ranging from B = 13.1 for t h e smallest symbol t o B = 12.5 for t h e largest) a n dd t h e i r location represents their X-ray spectral s t a t e (the distribution of t h e symbols in this plott shows t h e flaring b r a n c h a n d t h e normal branch, as earlier presented by Wood et al. 1989).

T h ee two p o i n t s located at t h e vertex of the flaring a n d n o r m a l b r a n c h in Fig. 3.4 b o t h

3.43.4 Historical WaJraven data 33 3 I I x x o o I D D f) f) O O K l l O O CM M ff) ) O O K5 5 O O 1 1

--. --.

' '

* *

** *

«. .

--— --—

--_ --_

Figuree 3.3. Relation between the

Johnsonn B magnitude of Sco X-l andd its "hard" X-ray colour,, as obtained from simulta-neouss X-ray/optical observations in Marchh 1989. The hard X-ray colour iss denned as the ratio of the count ratess in the photon energy bands (9.22 - 18.4) keV and (5.6 - 9.2) keV

I I x x o o X X CD D O O 6 6 ro o 6 6 to o o o '' i ' ~i' i ' i

_ _

AOO

Softt X—ray colour 1.2 2

F i g u r ee 3.4. Variation of the Johnsonn B magnitude of Sco X-l alongg the normal and flaring branch inn the X-ray colour-colour diagram. Thee size of the symbol is a measure off the B magnitude. The smallest symboll corresponds to B = 13.1, thee largest to B = 12.5. The "soft" andd a "hard" X-ray colours are de-finedfined as the ratios of the count rates inn the photon energy bands (3.5 -5.8)) keV and (1.2 - 3.5) keV, and in thee bands (9.2 - 18.4) keV and (5.6 -- 9.2) keV, respectively

correspondd t o m a g n i t u d e B = 12.80. It is clear from t h e figure t h a t t h e separation between t h ee optical bright a n d faint states correspond t o the separation between t h e flaring and n o r m a l branchess in t h e X-ray colour-colour d i a g r a m , as previously suggested by Priedhorsky et al. (1986). .

Unfortunatelyy no detailed comparison can be m a d e of t h e optical d a t a a n d t h e d a t a from IUE,IUE, as only very little d a t a was taken simultaneously. However, t h e over-all shape of t h e light curvee in the optical (Fig. 3.1), a n d the UV (Vrtilek et al. 1991; their Fig. 1) is very similar.

3.44 Historical Walraven d a t a

Histogramss of t h e optical brightness of Sco X - l have often shown a double peak, i.e., it a p p e a r s t h a tt not only are there optically bright a n d faint states b u t also preferred optical brightness levelss (see, e.g., Hiltner a n d Mook 1970). T h e distribution is not always b i m o d a l , a n d if it is, t h ee two peak m a g n i t u d e s are not always t h e same. E.g., t h e archival-plate p h o t o m e t r i c d a t a fromm which G o t t l i e b , Wright a n d Liller (1975) determined t h e orbital period of Sco X - l show a single-peakedd m a g n i t u d e distribution (Wright, Gottlieb a n d Liller 1976). T h e optical threshold betweenn the bright and faint s t a t e is consistently found t o be in t h e range B = 12.75 - 12.80 (seee M i y a m o t o and Matsuoka 1977).

T h ee presence of "preferred" optical brightness levels makes it feasible t o separate o r b i t a l and secularr brightness variations, a n d study t h e dependence of t h e orbital light curve on t h e optical s t a t e .. This we have a t t e m p t e d t o do using already published Walraven d a t a , comprising t h e

34 4 _{33 Coordinated X-ray and Optical observations of Sco X-l} in in COO ^ i n n C\j j CDD fO i n n ro' ' i n n mm fo i n n

:: : ^

"v.

ft

"'M

."" ."

_J..

_*;

^ B f s s

F i g u r ee 3.5. Optical light curves off Sco X-l. Plotted as a function off orbital phase is the Johnson B magnitudee of Sco X-l. The orbital lightt curves are shown twice for clarity.. The phases have been cal-culatedd according to the ephemeris off Gottlieb et al. (1975). (a) Data obtainedd during 23 nights in 1966, 1967,, and 1968 (Van Genderen 1969).. (b) Data obtained during approximatelyy 150 nights in 1971, 1972,, and 1973 (Van Genderen 1976).. (c) Data obtained during sevenn nights in 1972 (Canizares et al.. 1975)

? 0 r r

followingg d a t a sets.

(i)) P h o t o m e t r y published Van Genderen (1969), comprising 380 d a t a points obtained on 23 n i g h t ss in 1966, 1966 a n d 1968; all d a t a have been taken relative t o BD -15°4300.

(ii)) D a t a o b t a i n e d in 1971, 1972, and 1973, used by Van Genderen (1976). Between one a n dd five d a t a p o i n t s have been t a k e n per night, during two weeks per m o n t h , a n d five m o n t h s p e rr year. T h e t o t a l d a t a set consists of 476 individual d a t a points. T h e comparison star is BD -15°4300. .

(iii)) Walraven p h o t o m e t r y obtained as p a r t of a multi-wavelength observing campaign con-d u c t e con-dcon-d in 1972, covering seven nights (see Canizares et al. 1975) The con-d a t a set contains 1654 p o i n t s . .

Ass these d a t a were only available in p r i n t e d form we limited ourselves t o t h e Walraven V a n dd B m a g n i t u d e s , which were transformed to J o n h s o n V a n d B m a g n i t u d e s in the s a m e way ass before.

Wee have folded t h e d a t a p o i n t s in each of these d a t a sets using t h e orbital ephemeris of G o t l i e b ,, Wright a n d Liller (1975); t h e resulting B light curves are shown in Fig. 3.5.

F r o mm this F i g u r e it is a p p a r e n t t h a t t h e light curve for d a t a set (ii) shows a separation i n t o h i g h - s t a t ee a n d low-state d a t a p o i n t s , each of which follows an average, approximately sinusoidal o r b i t a ll light curve. Such a s e p a r a t i o n is n o t a p p a r e n t for t h e folded light curves for d a t a sets

3.55 Conclusions 35 5 f f CM M T — — CD D CM M T— — 00 0 CM M m m '' 1 ' "" tt

--|| |

'' 1

.l'VV

1

ii I ' l i l

11 ' M

i l l ''

i '

t l '

!

1

Hii ' '1

' i i

i

--' --'

lll

-1'' .

Figuree 3.6. Optical light curve of

Scoo X-l. Plotted as a function of orbitall phase is the average John-sonn B magnitude in 15 phase bins forr the optical bright state (top), andd the optical faint state (bot-tom).. The error bars indicate the errorr in the mean in each phase bin.. The orbital light curves are shownn twice for clarity. A thresh-oldd optical flux corresponding to B=12.800 is assumed. The phases havee been calculated according to thee ephemeris of Gottlieb et al. (1975) )

(i)) a n d (iii). This is likely caused by t h e fact t h a t these are based on d a t a obtained during a relativelyy small n u m b e r of nights. As a result the folded light curves for these two d a t a sets aree strongly influenced by correlations between consecutive d a t a points obtained during a single night. .

Inn Fig. 3.6 we show the average B m a g n i t u d e of d a t a set (ii) as function of o r b i t a l phase forr the optical bright a n d faint s t a t e , assuming an optical threshold flux corresponding t o B = 12.80.. F r o m sinusoidal fits t o the d a t a we derive full amplitudes of 0.13(2) a n d 0.13(3) for the opticallyy bright and faint s t a t e respectively. P h o t o m e t r i c m i n i m u m occurs at phase 0.96(3) in t h ee bright s t a t e , a n d 0.97(5) in t h e faint s t a t e . No significant variations of t h e B - V indices withh orbital phase were found, a n d we derive average colour indices of B - V = 0.179(3) and 0.203(4)) for the optically bright a n d faint s t a t e , respectively.

Wee performed similar fits to t h e d a t a for threshold fluxes corresponding t o B = 12.70, 12.75 a n dd 12.85. No significant change in any of t h e above results was found.

Cygg X2 is t h e only other Ztype source which is well studied in t h e optical (Cowley, C r a m p -t o nn a n d Hu-tchings 1979, Goranskii a n d Lyu-tyi 1988). T h e orbi-tal ligh-t curve of -this source consistt of a double-peaked ellipsoidal curve (full a m p l i t u d e ~ 0 . 2 5 m a g in B; Goranskii a n d Lyu-tyii 1988), due t o the g r a v i t a t i o n a l distortion of the secondary star, with a variable contribution fromm t h e X-ray h e a t i n g of t h e secondary, superposed on which are irregular variations. T h e t o t a l brightnesss variation of Cyg X-2 ( ~ 1 m a g in J o h n s o n B) is similar t o t h a t in Sco X - l . Cowley, C r a m p t o nn and Hutchings 1979 found t h a t during a period of relative high optical a n d X-ray brightnesss Cyg X-2 showed only a single-peaked orbital light curve in t h e J o h n s o n U p a s s b a n d

Cygg X-2 does not show any clear division between optically bright a n d faint s t a t e s . However, t h ee relatively strong ellipsoidal light m o d u l a t i o n could m a s k t h e presence of a b i m o d a l optical brightnesss distribution similar t o t h a t found for Sco X - l .

3.55 Conclusions

T h ee division at B ~ 1 2 . 8 between the optically bright and faint states of Sco X - l corresponds t o t h ee change from the n o r m a l t o t h e flaring branches in t h e X-ray colour-colour d i a g r a m . This confirmss the suggestion by Priedhorsky et al. (1986). In each of t h e two states the B m a g n i t u d e s showw a n approximately sinusoidal variation with orbital phase, with similar a m p l i t u d e s of 0.13 m a g ,, superposed on which are s u b s t a n t i a l irregular variations (see Fig. 3.5).

36 6 _{References s} thee source being bluer at photometric minimum. We interpret this as being due to the varying contributionn of the X-ray heated companion which is slightly redder than the constant contribu-tionn from the accretion disc. No orbital variations were found in the B - V indices. The source iss on average bluer in the optically bright state.

Acknowledgements Acknowledgements

Wee are indebted to Dr. A.M. van Genderen for supplying the historical Walraven data. TA acknowledgess support by the Netherlands Foundation for Research in Astronomy (NFRA) with financiall aid from the Netherlands Organisation for Scientific Research (NWO). NB and EML acknowledgee with thanks the support of the Basic Research Foundation of the Israeli Academy off Sciences. WHGL acknowledges support by the United States National Aeronautics and Space Administrationn under grants NAG8-571, NAG8-674, and NSG-7643. This work was supported inn part by the Netherlands Organisation for Scientific Research (NWO) with grant PGS 78-277.

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