UvA-DARE (Digital Academic Repository) EXOSAT observations of Z sources Kuulkers, E. Link to publication

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1 UvA-DARE (Digital Academic Repository) EXOSAT observations of Z sources Kuulkers, E. Link to publication Citation for published version (APA): Kuulkers, E. (1995). EXOSAT observations of Z sources. General rights It is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an open content license (like Creative Commons). Disclaimer/Complaints regulations If you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask the Library: or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. You will be contacted as soon as possible. UvA-DARE is a service provided by the library of the University of Amsterdam ( Download date: 05 Apr 2019

2 4 4 Secularr variations in the Z source CygnusX-2 E.. Kuulkers, M. van der Klis k B. A. Vaughan Too be submitted (1995) Abstract t Wee analyzed the variations in the position of the W Z" pattern of Cyg X-2 in its X-rayy colour-colour and hardness-intensity diagrams on time scales of longer than onee day. The presence of these "secular" variations is in accordance with the idea thatt we see this source at a high inclination, i.e. roughly edge-on, where matter near thee equatorial plane modifies the emission. We observed Cyg X-2 to vary between threee intensity "levels" superimposed on the well known horizontal-branch, normalbranchh and flaring-branch variations. We call these levels the "high", "medium" and "low"" levels. They are related to the secular variations in the position of the "Z" pattern.. During both medium and high level episodes we see the horizontal, normal andd flaring branch behaviour characteristic of Cyg X-2, but shifted to higher overall intensityy during the high level episodes than during the medium level episodes. The characterr of the flaring branch is different between high and medium levels. During thee medium level episodes both the colour-colour diagram and hardness intensity diagramm show a clear flaring branch. When Cyg X-2 moves into the upper part of thee flaring branch the intensity dips, while at the same time the colours increase. We calll these dips during the medium level episodes 'colour-dependent dips'. During the highhigh level episodes no clear flaring branch is seen in the colour-colour diagram, but onee of the light curves (and the hardness-intensity diagram) displayed a dip which wee interpret as flaring branch behaviour. This dip in the high level is dubbed a 'colour-independentt dip'. During a low level episode, when the source showed the lowestt overall intensity as compared with all other observations, no Z shape was seen butt only one large, curved branch (reminiscent to a flaring branch). The light curves duringg this low level episode showed flares on time scales of s. Comparisonn with previously reported data from other satellites shows that these threee intensity levels were observed earlier. The levels can be distinguished by source intensity,, and by the characteristic shape of the FB in the colour-colour diagram. Thee occurrence of the medium and high levels is correlated with the occurrence of thee colour-dependent and colour-independent dips in the flaring branch, respectively. Inn total, six high, eleven medium, and three low level episodes have been seen, for whichh sufficient colour-data were available. In total, more than six low levels episodes 67 7

3 Secular variations in the Z source CygnusX-2 occurred.. All happened between orbital phases 0.8 and 0.2 (where phase 0.0 is the X-rayy source superior conjunction), suggesting that the view of the inner disk was partlyy obscured, by e.g. the secondary or the mass transfer stream. We ascribe the occurrencee of the three intensity levels to a component in the system that recurrently obscuress (part of) the emitting regions. We investigate whether the occurrence of thee levels is periodic as would be expected from a precessing disk Introduction Cygnuss X-2 (discovered by Bowyer et al. 1965) is the prototype of the class of Z sources. The ZZ sources are the brightest known low-mass X-ray binaries (LMXBs, see e.g. Van der Klis 1992,, 1995b). Most of them trace out a "Z" in the X-ray colour-colour diagram (CD, see e.g. thee surveys of Z sources by Schulz et al and Hasinger & Van der Klis 1989), which gave Hasingerr (1987a, 1988b) the idea to name them Z sources. The fast timing behaviour of Z sourcess is closely connected to their position in the "Z" pattern (Hasinger & van der Klis 1989). Thee sources move stochastically but smoothly through the "Z", and do not jump from branch too branch (see, however, Dieters & van der Klis 1995). Prom the upper left to the lower right the branchess of the "Z" are called horizontal branch (HB), normal branch (NB) and flaring branch (FB).. All Z sources exhibit all three of these branches, except for GX 349+2, which has not been detectedd in the HB. Recentlyy Kuulkers et al. (1994a, Chapter 3) and Kuulkers & van der Klis (1995a, Chapter 5) elaboratedd on the idea that the Z sources can be divided into two subclasses (Hasinger & van der Kliss 1989, Penninx et al. 1991), by showing that CygX-2, GX5-1 and GX differ from the sourcess ScoX-1, GX and GX17+2 in several distinct properties. Most prominently, the formerr trio shows secular (i.e. long term) variations in the position of the "Z" pattern in the CD andd hardness-intensity diagrams (HIDs), and dipping behaviour in the FB state, whereas the latterr do not show significant variations in the position of the "Z" and really flare up in X-ray intensityy in the flaring branch. These facts, together with studies at other wavelengths, led Kuulkerss et al. (1994a, Chapter 3) to the suggestion that CygX-2, GX5-1 and GX340+0 are viewedd at higher orbital inclinations (i.e. more edge on), while Sco X-l, GX and GX 17+2 aree viewed at lower inclinations (more face on). Duee to the highly eccentric orbit of EXOSAT (see White & Peacock 1988), the EXOSAT dataa provide the longest available uninterrupted high time and medium energy resolution data setss on CygX-2, and are therefore suitable for a detailed investigation of how the source moves throughh its different branches. Several of the EXOSAT ME observations of CygX-2 have been discussedd in previous papers (Hasinger et al. 1985, 1986, van der Klis et al. 1987c, Stella et al. 1986,, Hasinger 1987a,b, 1988a,b, 1991, Chiapetti et al. 1987,1990, Schulz et al. 1989, Hasinger & vann der Klis 1989). We here present the first comprehensive, detailed, and homogeneous account off all the broad-band EXOSAT ME argon data on CygX-2. A comparison with data from other X-rayy satellites provides new insights. Our study provides further evidence for the conclusion off Kuulkers et al. (1994a, Chapter 3) that a relatively high inclination plays an important role inn the observed properties of this source Observations Inn this paper we present observations done with the Medium Energy (ME) Experiment argon detectorss onboard EXOSAT (Turner et al. 1981, White & Peacock 1988), which are sensitive in thee energy range 1-20keV. CygX-2 was observed from 1983 to 1985 in a variety of observing modes,, giving spectral information (HER2, HER3, HER4 and HER5, the so-called HER-modes) andd high resolution timing information (HTR3, HTR4 and HTR5, the so-called HTR-modes),

4 Observations 'S.. QOO (O t* <D N ^ ^ t- ill 55 5???? 5?? 2???? o' o II T T I Tf<< «o <o «i f l N H H öö ooooooooo BC C S.35 5 CO CO mmmm n ^m m S55 S5 m «'m as m m J J I I»» s s V) ) X X nn a ««a 3J3 3 ** * C < S "^ * «B «* n"^^? S B B B B * i o K ) i a i 8 W i o " - ^ i i ) i o i o ««< i ^ * i ; ; ess es ei c4 «o ci r? --^w «pi M '«S H '»r~«'"«r"«*o ooo -* oo ess < ee»» % > t~ I-- t- t- t- t- *"». oo io n n N «JJ >H r WW * ' M e' ^*^ n * i c * n»«^ ^* n n *^ USUS f^* UU W«* * W# *, - ( E! i - l i - l «- l l - l l - I O» " H ^ H ' - I O l - H i - l l oo a onto u> ui O ^ «NN «i-ii ciiorii-tt-t-ijeöi-imeqcj «oo «os^t-^-». -^ -^ ess wc»swi-tt-ne»jw-h«oo mm ej ^" «w nn r4 H H»H i-ii tn in in -,,, CO» «' aa n ^ ^ ^ ^ W K i h i n h i M h h O Z Z Okk O) o Ok Ökk o* o> o>,1 1 ih H V V 71 1 en n SB B > > E E ^1 1 i i JB Ti i * * SB B 1* * & & 3 >> > 11 <o? o» rtrt II lo OS ww S rtrt SB liss I-H «111 ^^ 3 "- 1 S 3 3 «-- "O _, «S ffl ffl ** o S33 o; a 1*1 1 ^^ s» 33 0 J3 aa * ^ "oo -3 & 33 «v 33 U SS J, -- g 8 S 71 1 :-- e e a a 8-a a s s *r"» s "S II I cc s - J i, " a»» SB tt I I 2 Ï a oo aa S is ëë 9 ö

5 Secular variations in the Z source CygnusX-2 orr a combination of the two (HER7). For a more detailed description of these modes we refer to e.g.. Kuulkers et al. (1994a [Chapter 3], 1995b [Chapter 8]). In Table 4.1 we give the observation logg of CygX-2. Inn the analysis in this paper we use CDs and HIDs (see e.g. Schulz et al. 1989, Hasinger & vann der Klis 1989). Almost all our observations were done in HER-modes with a large number off energy channels. Only on 1985 day 205 and 206 (see Table 4.1), most of the data were obtainedd in the HER7 mode. We used these HER7 energy boundaries to define three energy bands:: , and keV. All the data obtained from other observation periods weree rebinned into these same three bands. The low-energy (soft) colour is defined as the ratio off the background- and dead-time corrected count rates in the keV and the kev bands,, and the high-energy (hard) colour as the ratio in the keV and keV bands. Thee intensity is the count rate in the keV band, corrected for background, dead time andd collimator response. We used the new collimator responses determined by Kuulkers et al. (1994a,, see Chapter 1) Results Light curves Inn Figs. 4.1a-d several light curves of the observations are shown with 10 s (1983 day 186) or 55 s (1984 day 205,1985 day 205 and day 318/319) time resolution. The observations obtained on day 186 were performed during the performance verification (PV) and calibration phases of thee EXOSAT mission. These data consisted of a raster scan and were therefore taken at different collimatorr responses. In Fig. 4.1a we only plot data with collimator responses larger than "-30%; notee that data with collimator responses lower than ~60% contain systematic errors of several perr cent. Indicated is which spectral branch the source was in at each instant. We refer to Kuulkerss et al. (1995a, Chapter 6) for the light curves of the other observation periods (1983 dayy 258, 1985 day 176/177, day 177/178, day 206 and day 301/302); these light curves showed severall burst-like events, which interrupt the smoothly varying light curves (see also Fig. 4.1c). Forr light curves of the observation periods 1983 day 256 to day 265 we refer to Chiapetti et al. (1987,, 1990). Differencess in the intensity variations which are characteristic of the different branches can be clearlyy seen in these light curves. In the NB the intensity changes are slow and smooth on time scaless larger than ~1000s. In the HB the changes in intensity are larger and more irregular than inn the NB. In the FB the intensity changes are larger than in the NB and occur on shorter time scaless than in the HB. Moreover, in the FB dips (up to 50% decreases in intensity) occur with durationss of 15~25min. Of the total of 103hr of EXOSAT observation time, CygX-2 spends mostt of its time in the HB (up to 13 hr per observation period 1 ) and the NB (up to 8hr per observationn period), while it spends only up to 1.5 hr in the FB (in three observation periods; day 258, 1985 day 205 and day 318/319), The data obtained on 1985 day 205 (Fig. 4.1c) showw a full sequence of the source moving from the FB, through the NB to the HB, and back intoo the NB in approximately half a day. During twbservation periods (1985 day 205 and dayy 206) CygX-2 had overall intensities which were ~30-40% higher than in other observation periodss (see also Section 4.3.1). On 1983 day 186 (Fig. 4.1a) the source was in a peculiar state: itss overall intensity was lower than normal (~50% lower; even lower than in the dips during the FB),, and it showed flaring behaviour (see also Hasinger et al. 1985, Hasinger 1988a, 1988b). Thiss observation period is therefore denoted by W FB? W. These data will be discussed in more detaill in Section *Thee maximum length of an observation period on CygX-2 is ~14.2hr.

6 Results 71 1 C ó ó CD D in n /187 EE _ coo cc, CU U CC CM ff ff p**t\i p**t\i ièéi NB B HB B * * / NBB FB NB B 2x1CT T 4x11 CT Timee (s) Figuree 4.1. Typical EXOSAT ME light curves of 4 observation peiiods of CygX-2 at a timee resolution of 10 s (1983 day 186) or 5 s (others). Intensity is the dead-time background andd collimator response corrected count rate in the kev range. The branches in whichh the source was during the observations are indicated. Start times for the different observationn periods can be found in Table Gain adjustments AA CD of all EXOSAT argon ME data is displayed in Fig. 4.2a. The data obtained in 1983 andd after 1983 are denoted by different symbols. The points are 200 s averages. The peculiar longg diagonal branch ("FB?"; see Section 4.3.1) can be clearly seen. The data obtained between day 261 and day 265 are at higher soft and hard colours in the CD with respect to the dataa obtained after The 1983 day 256 and day 258 data might be shifted along the NB,

7 Secular variations in tiie Z source CygnusX _ d d X X = 1983 day 186/187 = 1983 day 256 = 1983 day = 1983 day 261 DD = 1983 day 263 ** = 1983 day 265 = 1984/1985 data FB?? CM M NBB J0?» J J FB B a a Softt Colour Figuree 4.2. (a) EXOSAT X-ray colour-colour diagram and (b, c) hardness-intensity diagramss of CygX-2 of the 1983 data. For comparison the 1984/1985 data (see Fig. 3a) aree also plotted in (a). Soft colour is defined as the ratif the counts in the keV andd the kev bands, hard colour as the ratif the counts in the keV and thee keV bands. The intensity (corrected for background, dead-time and collimator response)) is defined as the count rate in the keV band. Different symbols refer to differentt observation periods, which are given in the upper left part of (a). Each point representss a 200s average. Typical error bars are given in each frame. HB (horizontal branch), NBB (normal branch) and FB (flaring branch) are indicated. Peculiar flaring branch-like behaviourr was found during 1983 day 186, and is therefore indicated by "FB?". butt this is not clear from our data. A shift, or "offset", was noted earlier by Chiapetti et al. (1990),, who found that the 1983 observations of CygX-2 were slightly shifted with respect to thee 1984/1985 data of Schulz et al. (1989). They attributed this to a slightly different choice of energyy bands. However, we generated the CDs in a homogeneous way by using the sanergy boundariess for all observation periods (see Section 4.2), so this is not the explanation. CDss of several other X-ray sources reveal a similar "offset" of 1983 data with respect to dataa obtained after 1983: GX5-1 (Kuulkers et al. 1994a, Chapter 3), GX 17+2 (Kuulkers et al b, Chapter 8), GX (Kuulkers et al. 1995d), 4U (see Prins & van der Klis

8 Results 73 3 C 11 1 ' 1 * * bb " ii i c c 3 3 t "oo -* ** FB? if f HB B i i ** ft 3 3 o "".*" FB? '' J HB 1 1 Al»» : (N N NBB 'jjjg,. -- NBB ^^/ _ ii. i i" O.BB Intensityy (cts/s/cm 2 ) Intensityy (cts/s/cm 2 ) Figuree 4.2. (continued) Tablee 4.2: History of the ME argon detector gain adjustments before 1984 day 211 Yearr Day" No. Remarks Dl First ME observation; all detectors same gain setting D2 Approximate alignment of overall gains (see text) D D4 Crab observations to enable checks of overall gains D D6 See D D7 New standard gain setting (see text) "Jann 1 = day ;; see also Damen et al. 1990, van der Klis et al. 1990) and SerX-1 (Jongert et al. 1995a). Thee CDs of these sources were all made in a homogeneous way, as described above. The shift cann be most clearly seen in GX17+2, whose "Z" track was found to exhibit a stable position inn the CD (see Kuulkers et al. 1995b, Chapter 8). Since all sources show the saffects this cannott be related to the sources themselves, but must be instrumental 2. Thee 1983 data of the above sources were all obtained between day 198 and day 265. During the ME argon detector gain settings were changed several times. The gain was dependent onn two factors: the amplifier gain setting, and any "leaks or drifts" in the detectors (see Parmar && Smith 1984, see also Parmar & Smith 1985b). In Table 4.2 we give the history of the argon gainn adjustments up to 1984 day 211, where D1-D7 denote the different points in time at which thee gain was measured and/or changed (see Parmar & Smith 1984, 1985b). The gain settings weree changed to achieve approximate alignment of the overall gains at D2, and checked with Crabb observations at D4 (Table 4.2, see Parmar & Smith 1984). At D7, on 1984 day 211, and afterr this date, a new method of gain adjustment was used: the gain settings were aligned such thatt for each detector the same channel number corresponded to an energy of 6.7keV (Parmar & 2 Thiss does not, however, affect the conclusion of Kuulkers et al. (1994a, Chapter 3) that the "Z" patternn of GX 5-1 shifts.

9 Secular variations in the Z source CygnusX-2 Smithh 1984, see also Parmar & Smith 1985a). The 1983 data of the above sources were obtained betweenn D3 and D4. It is plausible that before checks had been made at D4 using Crab as a calibrationn source, the gain settings were slightly different than at later dates. The different gain settingss affect the values of the energy channel boundaries of each of the eight argon detectors slightlyy differently. Although these differences are small, they become visible in CDs produced off spectral data that were coadded, onboard the satellite, on a channel by channel basis, because suchh diagrams are very sensitive to subtle changes in the spectra. Becausee of these small instrumental differences in data before 1983 day 276 and after this date,, we decided to treat these data sets separately. In the following two Sections we therefore presentt the CD and HIDs of CygX-2 of the 1983 data and of the 1984/1985 data separately in Sectionn and Section 4.3.4, respectively data Thee 1983 day 256 to day 265 data have already been presented by Stella et al. (1986) and Chiapettii et al. (1987, 1990). In Figs. 4.2a-c we show these data together in a CD and HIDs, includingg the data obtained on 1983 day All data points are 200 s averages and the differentt symbols denote the different observation periods. Although the lengths of the observation periodss on day 256 to day 265 were small it was still possible to determine in what spectral branchh CygX-2 was during these observations (Table 4.1, see Chiapetti et al. 1990). Thee data from the observation period 1983 day 186 trace out a large diagonal branch in the CDD (Fig. 4.2a), which seems to differ from the known HB/NB/FB behaviour. This "peculiar" dataa set has already been described in some detail by Hasinger et al. (1985) and Hasinger (1988a, 1988b),, who showed that this branch has characteristics that are in some ways reminiscent of thee FB-behaviour of Sco X-l, GX and GX17+2 (see Section 4.4.1). As can be seen in the HIDss (Figs. 4.2b and c) the overall intensity during this observation period was unusually low (lowerr than during dips in the FB of 1985 day 205 and 318/319, see Section 4.3.4). The 1983 dayy 186 observation occurred between X-ray binary phases and 0.20 (see Table 4.1). No dataa with a sufficiently high time resolution were available during the 1983 day 186 observations too check the fast time variability and the possible presence of QPO /1085 data Thee CD and HTDs of all the 1984/1985 data are shown in Figs. 4.3a-c. The different symbols (2000 s integration time) denote the different observation periods. The lowest energy channel availablee in HERö-mode data during observation periods 1985 day 205 and 206, however, had aa lower energy boundary of 1.4 kev instead of 0.9 kev. This influences the soft colour values. Individuall CD and HIDs of these data are therefore plotted in Figs. 4.4a-c; the 1985 day 205 andd day 206 data plotted in Figs. 4.3a-c are HEB.7 data only. Comparing the NB of the HER7 andd HER5 mode data (Figs. 4.3b and 4.4b), we find that the soft colours of the HER5 data are shiftedd by a factor ~1.06 in soft colour. Wee see, for the first time in CygX-2, an upward curve in the left HB, as previously seen inn GX 5-1 (see Lewin et al. 1992, Kuulkers et al. 1994a, Chapter 3). The upward curve occurs 3 Onlyy data with a collimator response latger than ~60% were used in the diagrams for the observation periodd 1983 day Thee X-ray binary phase is determined from the ephemeris given by Crampton & Cowley (1980): times off expected X-ray superior conjunction (= X-ray phase sero) are T 0 = JD (5) (1)E, where EE is the cycle number and the errors are given between brackets for the last digit. We note that the orbitall phase determination uncertainty at the time of finishing of this paper (february 1995) is ~0.66 inn orbital phase! Follow-up observations determining the orbital phase of CygX-2 are therefore highly desirable,, otherwise the cycle count may be lost.

10 Results 75 5 b% % HB B + + a a CN N d d Z3 3 c> c> " DD day 205 = 1985 day 176/177 ^^ day 177/ = 1985 day 205 =1985 day 206 ** = 19B5 day 301/302 AA = 1985 day 302 QQ day 318/ Softt Colour Figuree 4.3. (a) Colour-colour diagram and (b, c) hardness-intensity diagrams of CygX-2 off the 1984/1985 data. Different symbols refer to different observation periods, which are givenn in the lower light part of 3a. Each point represents a 200 s average. Typical error barss are given in each frame, (d) Intrinsic colour-colour diagram for the data with sufficient spectrall resolution. The colour values are corrected for detector efficiency (see text), and aree estimates of the true photon flux ratios. They are expected to be accurate to ~10%. Differentt symbols denote the different observation periods as given in a. duringg the observation period 1985 day 301/302 and gives the pattern in the CD a kind of "S" shape.. Inn the CD and HIDs of Fig. 4.3a-c similar branches do not fall on top of each other. Between observationn periods similar branches differ by ~8% in hard colour and ~7% in soft colour. The differencess in the position of the "Z" are even more pronounced in the HIDs. The position differs byy up to ~40% in intensity (ignoring the 1983 day 186 data). We investigated to what extent systematicc detector effects could have influenced our CD and HIDs. For this we used data of Crab,, which is assumed to be a steady X-ray source; shifts in the CD and HIDs of Crab between observationn periods can be attributed to changes in the instrument. Wee used the procedures outlined by Kuulkers et a). (1994a, Chapter 3). Crab observation periodss we used are listed by Kuulkers et al. (1994a, Chapter 3). In Figs. 4.5a-c we present the

11 7 44 Secular variations in the Z source Cygnus X-2 m m ««..'^J* FB.,.<-..«1 1 ii I bb " i i C PJ J Ö Ö Ö Ö V HB c +TfcS S r-l l 33 *~ 0 0 OO in ^ ^ m m, mm cmjsy w^ JAJKTT 2? 4«FF J» c5** c "jf 0 n n JH?;.. NB B Intensityy (cts/s/cm ) ss u u?? <N aa oj xx 6 a a d d **** Tm i'^jx FBB "I" '^ 0-88 I Intensityy (cts/s/cm 2 ) Figuree 4.3. (continued) Intrinsicc Soft Colour resultss of this study of the systematic detector effects: from top to bottom we see the average pointss per observation period for the soft colour, hard colour, and intensity, respectively, of Crab.. We conclude that for Crab the soft colour, hard colour and intensity in our chosen bands varyy by ~4%, ~2% and ~5%, respectively. Since these instrumental changes are much smaller thann the shifts in the CD and HIDs of Cyg X-2, we may conclude that intrinsic changes occur inn the position of the branches in the CD and HID (Figs. 4.3a-c) of Cyg X-2. Ass pointed out by Kuulkers et al. (1994a, Chapter 3), estimating systematic changes in CDss and HIDs from changes in the CD and HIDs of Crab is only an approximation. One also hass to take into account the different shapes of the spectra of Crab and Cyg X-2. Changes inn the response of the detectors with time will lead to different changes in the CD position forr Cyg X-2 and Crab, because these sources have different spectral shapes. To estimate this second-orderr effect, we adopted the adhoc procedure (Kuulkers et al. 1994a, Chapter 3) of looking att differences in the observed colours for different detectors at the same time, and taking the relativee differences between Crab and Cyg X-2 in different detectors as indicative for the relative

12 Results ' ' ' a a * NB B * * S S HB B ii 1 i i * * ^ ^ 88 ^ CN N b b 1 1 FB B * + V HB B '.. + +*-- NB Softt Colour Intensityy (cts/s/cm ) d d CM M Ó Ó aa d 00 0 d d CD D " " c c i i FB B HB B *** * *$ ## + NBB. Figuree 4.4. (a) Colour-colour diagram and (b,, c) hardness-intensity diagrams of Cyg X- 22 of the 1985 day 205 and day 206 HER5 data. Thesee data had a lower energy boundary of 1.4keV,, which results in slightly different soft colourss (see text). Different symbols refer to differentt observation periods, which are given inn the lower right part of (a). Each point representss a 200 s average. Typical error bars aree given in each frame. Intensityy (cts/s/cm ) differencess between Crab and Cyg X-2 resulting from the changes in the spectral response over time.. This procedure is based on the assumption that the differences between the detectors are dominatedd by differences in their rates of deterioration. There are twbservations of Cyg X- 22 during which we have separate spectral data for each active detector: 1983 day 186/187 (seee Table 4.1, eight detectors of the whole array) and 1983 day 264 UT 128-UT 01:36 (only fourr detectors of half 1). It was found by Kuulkers et al. (1994a, Chapter 3) that one of the detectorss behaved abnormally during (probably part of) the 1983 period (detector "3" or "C"). Wee therefore ignored the data of this detector in the comparison of the 1983 Crab and Cyg X-2 observations.. Whenn comparing Cyg X-2 and Crab we find that a ~25% difference in the soft colour between differentt detectors for Crab, while we find a ~19% difference in the soft colour between different detectorss for Cyg X-2. The differences in the hard colours are much smaller: there is a ~3% differencee in the hard colour between different detectors for Crab and a ~4% difference in thee hard colour between different detectors for Cyg X-2. The intensity differences in both

13 Secular variations in the Z source CygnusX-2 =!!,-J OO CN oo *- C/) ) C ^o 11 s ó ó (N ^ ^ N ^ EE 2 \\ <N c \\ oo _ * ~~"~~" CN JJ,, "coo LD c:: i"l i>> ~: +JJ CN c c 11 1 ii i i ii I i 0 0 l l i i ii i i J' ' $ $ (i i i i ^ ^.,,. i, i i \ \ 11 1 a a i i,, i 11 1 b b ïï - 11 ii i 1 c c.. t Figuree 4.5. Average Crab soft colour (a),, hard colour (b) and intensity (c) pointss for each EXOSAT ME Crab observationn period as a function of time.. Values were corrected for background,, dead time and collimator response.. Any changes present in these diagramss are attributed to changes in thee instrument and not intrinsic to Crabb itself. Tin iee n (days afterr 1982 d ayy d 2C 7) ) observationss are roughly similar; we find a ~14% difference between different detectors in Crab's intensity,, while we find a ~10% difference in the intensity between different detectors for Cyg X- 2.. Wee conclude that the CD and HIDs of CygX-2 may have been affected to a level of ~3%, ~2.5%,, and ~4%, for the soft colour, hard colour and intensity, respectively. So, our conclusion thatt there are intrinsic changes in the position of the branches of Cyg X-2 is strengthened by thiss analysis of the effects of the differences in spectrum between Cyg X-2 and Crab. Wee also determined an intrinsic colour diagram (ICD; Fig. 4.3d), using the method outlined byy Kuulkers et al. (1994a, Chapter 3). This method estimates colours which are corrected for thee average detector response, using a model independent approach. In the ICD we see again thatt similar branches do not coincide, i.e. the position of the "Z" varies. Since here we have correctedd for the detector response changes of different observation periods, this illustrates our conclusionn that there are secular variations in the position of the "Z" track Intensity levels Iff we compare the HIDs of the different observation periods (Figs. 4.2b,c and 4.3b,c) it becomes apparentt that we have observed Cyg X-2 in three intensity levels. One is the "peculiar" observationn on 1983 day 186 (Figs. 4.2b,c) which corresponds to the lowest intensities (<0.7 cts s _1 cm -2 ) observedd for Cyg X-2 with EXOSAT. As shown in Section 4.3.3, the behaviour in the CD during thiss observation is different than in the other EXOSAT Cyg X-2 observations (Fig. 4.2a). We denotee this the "low level" of Cyg X-2. Inn Figs. 4.3b and c we see clearly twbservation periods at very high intensities (1985 day 2055 and 206; >1.3 cts s _1 cm -2 ), while the other observations (except 1983 day 186 which is much

14 4.44 Discussion 79 9 lower)) are between overall intensities of 0.7 and 1.3ctss _1 cm" 2. We denote the observations duringg 1985 day 205 and 206 the "high level" of CygX-2, while the other observations are denotedd the "medium level". During the observations 1984 day 205,1985 day 301/302 and day 3022 CygX-2 was only found in the HB. Since no NB/FB is present in these data it is not clear iff these observations occurred during the medium or high level. Whenn CygX-2 is in the medium or high level it moves irregularly through the different branches.. Comparing Figs. 4.3a and 4.4a, we see that the lower end of the NB in the high level iss shifted to lower soft and hard colours in the CD with respect to the lower end of the NB in thee medium level. In the HIDs of Figs. 4.3b and c we can discern two "Z" patterns, which are relatedd to the occurrence of the two different levels. In the HIDs, observation 1985 day 301/302 andd 302 (Figs. 4.3b and c) fall along the HB tracks of the high level observations, and may thereforee belong to the high level. The data of 1984 day 205 (whose level is also unknown), however,, belong to different patches of the HB pattern in the HIDS. Becausee in the different branches the source intensity also varies, it is important to use CDss and HIDs (not just light curves) when discriminating between the different intensity levels. EXOSATT seems not to have observed a clear transition between two intensity levels, although it iss possible that the 1984 day 205 observation represents a transition stage between the medium andd high intensity level. Ass noted in Section 4.3.1, EXOSAT observed CygX-2 three times in the FB (1983 day 258, day 205 and day 318/319). If we compare the CDs and HIDs of the observations 1985 day 2055 and day 318/319, we see two different kinds of behaviour. During the medium level 1985 dayy 318/319 observation, a dear FB is seen in the CD (Fig. 4.3a). In the HDD, a slight intensity increasee is followed, when the source moves further into the FB, by a drop in the source intensity byy -30% (Figs. 4.1d and 4.3b,c; see also Kuulkers & van der Klis 1995a, Chapter 5). During this dipp in intensity the soft and hard colour both increase (Figs 4.3a-c, see also Schulz et al. 1989). Thee FB in the medium level observation 1983 day 258 shows similar characteristics, as can be seenn in Chiapetti et al. (1990). During the high level observation of 1985 day 205, however, no dearr FB in the CD is seen (Fig. 4.4a), while a dear drop in intensity by ~50% can be seen in thee HID during this observation (Figs. 4.1c and 4.4b, c). Since this intensity dip occurs from thee end of the NB, we regard this as FB behaviour. We will refer to the two kinds of dips as "colour-dependent"" dips, and "colour-independent" dips, respectively Discussion Secular motion of the Z pattern Wee have performed a study of the long term variations in the position of the W Z" pattern of CygX-22 on the basis of all the CDs and HDDs of the argon EXOSAT ME observations. Longtermm variations in the CD (Hasinger et al. 1990), HID (Hirano et al. 1984, Vrtilek et al. 1986, 1988,, Hasinger 1987a, Hasinger et al. 1990), and in the overall intensity (Holt et al. 1979, Bonnet- Bidaudd & van der Klis 1982, Branduardi-Raymont et al. 1984, Hirano et al. 1984, Vrtilek et al. 1986,1988,, Hasinger et al. 1990, Smale & Lochner 1992) of CygX-2 have been reported earlier. Wee will perform here a comparison between all these reports, including the EXOSAT data, and showw that the way in which CygX-2 varies secularly seems to always follow similar patterns. Apartt from the three well-known branches HB, NB and FB (see Hasinger & van der Klis 1989),, we observed CygX-2 at three overall intensity "levels". We found that the occurrence off these three intensity levds was dosely related to the secular variations of the position of the "Z"" pattern in the CD and HDDs (see Section 4.4.4). Duringg one observation period (1983 day 186) CygX-2 was in a peculiar state. This was alreadyy reported by Hasinger et al. (1985) and Hasinger (1988a). The intensity was lower than

15 Secular variations in the Z source CygnusX-2 inn all other EXOSAT observations; we therefore called it the low level of Cyg X-2. The light curvee of this observation period showed flaring behaviour, especially at high energies (Hasinger ett al. 1985). The corresponding CD of this observation therefore shows a long diagonal branch, displacedd to higher soft and hard colour than all other EXOSAT observations (see also Hasinger 1988a,, 1988b). The Z sources ScoX-1, GX and GX17+2 also show flares in their light curvess which appear to be hard (see e.g. Hasinger 1988b). Because of these characteristics, Hasingerr et al. (1985) ascribed this low level of Cyg X-2 to FB behaviour. As shown by Hasinger (1988b),, the long diagonal branch that is traced out by Cyg X-2 in the CD during the low level episodee is oriented in the same way as the FB of GX However, in contrast to ScoX-1, GXX and GX17+2, the branch in Cyg X-2 is not connected to any of the NBs in the CD andd HIDs, as it is shifted to too high soft and hard colours. Similarr low level episodes have also been found in the Einstein MPC data on Cyg X-2 by Vrtilekk et al. (1986,1988): an HID of these data (Vrtilek et al. 1986; their states A and B) shows aa track that is very similar in shape and position to the EXOSAT low level observation. Vrtilek ett al. (1968, 1988) found that the ~5 low levels had a typical duration of ~3 days. Bovaisky ett al. (1979, Copernicus data) and Marshall & Watson (1979, Ariel 5 data) reported Cyg X-2 observationss at similarly low intensities, flovaisky et al. (1979), Marshall & Watson (1979) and Vrtilekk et al. (1986,1988) found that the low level episodes occurred around binary phases zero, i.e.. near X-ray source superior conjunction when the companion is closest to the observer. Using thee updated ephemeris of Crampton & Cowley (1980) 5 we find that the low level observed with EXOSATT and the low levels reported by Vrtilek et al. (1986, 1988) all occurred between orbital phasess (consistent with the 3 day duration found by Vrtilek et al. 1988). The low levels off Vrtilek et al. (1986,1988) were observed when the UV-luminosity was expected to be also low (ass extrapolated from the UV measurements of Cowley et al. [1979]; note that low UV-intensities occurr near phase zero in each orbital period, while the X-ray low intensity levels do not occur att each orbital period). The low X-ray/UV-intensity levels are in accordance with the idea that att these times the secondary hides its heated face and/or part of our view of the inner disk region.. Also, structure at the outer disc, e.g. a hot spot, could obstruct the inner disc region. Evidencee for absorption during this episode, suggesting obscuration, comes from the observation off absorption features in the X-ray spectra obtained with the Einstein OGS, reported by Vrtilek ett al. (1988). In this situation, most of the radiation may come from a scattering hot corona surroundingg the inner accretion disk, which stays more or less unobscured, while the inner disk itselff is partially hidden (see also Vrtilek et al. 1988). This would explain the hard spectrum andd low intensity in the low level episodes. Onn twccasions (1985 day 205 and day 206) we found that the intensity of Cyg X-2 was higherr (by ~30-40% ) than during other EXOSAT observations (see also Hasinger 1987a). We calll this the high level of Cyg X-2. During these twbservations the source exhibited all three spectrall branches. During 1985 day 205 the source moved through the FB, NB and HB within halff a day. A similar high intensity level episode was also seen in the HID of Vrtilek et al. (1986; theirr observation H, which they called an 'anomalous high state'). Moreover, Hasinger et al. (1990)) found that during two 1988 Ginga observation periods the "Z" shape was observed at differentt intensities. The difference in intensity on the NB was ~30-40%, which is similar to the differencee between our high level observations and other EXOSAT observations. Alll other EXOSAT observations fall in the same part of the HIDs, at intensities intermediate betweenn the low and high level. We therefore call this the medium level of Cyg X-2 (this level is 5 Thee orbital period wasfirstderived by Cowley et al. (1979). An update of the ephemeris was reported byy Crampton & Cowley (1980); see also footnote 4. It is somewhat surprising that up to 1992 all authors usedd the orbital ephemeris of Cowley et al. (1979) and not the updated ephemeris of Crampton & Cowley (1980)..

16 Discussion 81 1 calledd the 'high state' by Vrtilek et al. 1986). In the medium level one also sees the source moving throughh the different spectral branches. It is worth noting that the light curves of the high level day 205 and medium level 1985 day 318/319 observations are very similar in shape and luminosityy to the Ginga high level October 1988 and medium level June 1988 observations of Hasingerr et al. (1990), respectively. Onn three occasions we found CygX-2 only in the HB (1984 day 205 and 1985 day 301/302 andd day 302). In these cases it is not possible to determine whether the observations were obtainedd during a medium or a low level, since the HB could have extended all the way up to thee NB-intensity level of the high level. Wee note that also in the optical, evidence for different brightness levels was found (Goranskii && Lyutyi 1988). Their so-called "quiet" state of CygX-2 was sometimes interrupted by an increasee in brightness for 5-10 days. Moreover, several rare drops in intensity were observed, whichh lasted for several days Character of the FB Wee found different kinds of behaviour in the FB of the medium and the high level during the EXOSATT observations. In the medium level we see a clear FB in the CD. The light curves in thee FB dip when the source moves into the upper FB. When the source intensity dips, both thee soft colour and the hard colour increase which has the effect of extending the FB in the CDD to higher soft and hard colours, whereas in the HID it extends to lower intensities and higherr colours. During such a 'colour-dependent' dip in a medium level observation on 1985 dayy 318/319, a new kind of FB QPO was found (Kuulkers fc van der Klis 1995a, Chapter 5). Ann FB-like dip was alsbserved during the high level (the intensity during this dip was even lowerr than the medium level FB-dip), but no colour changes occurred in this dip so that no FB wass found in the CD. In the HID we see an excursion to lower intensities while the colours stay approximatelyy constant, which we interpret as a FB. Exactlyy the same FB/dip behaviour was seen in the two Ginga observations of Hasinger et al.. (1990). One had a high intensity level and showed an energy-dependent FB-dip, the other a mediumm intensity level and a colour-independent FB-dip. The Ginga observation of Cyg X-2 in (Mitsuda & Dotani 1989) had comparable overall intensities to the high level observation off Hasinger et al. (1990), and may therefore be regarded as a high level observation. This observationn also shows a colour-independent excursion in the HID. Thee colour-dependent dips and the colour-independent dips are related to the so-called N- typee and P-type dips found in OSO 8, HEAO 1 and Einstein observations by Vrtilek et al. (1988). Theirr figures 6 and 7 show that their P-type dips occur at higher intensity levels (~30-40%) thann their N-type dips. These dips are colour-independent and colour-dependent, respectively. Wee note that their so-called I-type dips, and probably some of their P-type dips described by Vrtilekk et al. (1988), are due to intensity variations in the HB. Dipss were also reported by Bonnet-Bidaud & van der Klis (1982) in COS-B data and by Ilovaiskyy et al. (1979) in Copernicus data, and are only observed when the overall intensity is high.. Based on the durations of these dips we deduce that they are related to both FB-dips and HBB intensity variations. Alll in all,, four clear colour-dependent dips have been observed, all of them when Cyg X-2 was inn the medium level, and three clear colour-independent dips, all when Cyg X-2 was in the high level.. We conclude that the 'colour-dependent' dips are correlated with the medium intensity level,, while the 'colour-independent' dips are correlated with the high intensity level. (Whether thiss correlation is strict we can not say; it is certainly strong.) In Fig. 4.6 we summarize the characteristicss of the CD and HID of Cyg X-2 in the medium and high level. Transitionss between intensity levels have only been observed by Vrtilek et al. (1988) between

17 Secular variations in the Z source CygnusX-2 CD D HID D Mediumm High 27 levell level 27 3D D FB: : colour-dependentt dip colour-independent dip Figuree 4.6. Schematic picture characterising the behaviour of the "Z" pattern of CygX-2 inn the CD and HID for the medium (left column) and high (right column) level. Colourdependentt FB-dips occur in the medium level, while colour-independent FB-dips occur in thee high level. thee medium and low intensity levels. From the OS0 8 light curves of Vrtilek et al. (1988) we inferr that these transitions take place within ~6-7hr. A similar transition time was reported byy Ilovaisky et al. (1979) between low intensity levels and higher (in our terminology medium) intensityy levels. The Einstein, EXOSAT and Ginga satellites appear not to have observed such transitions,, implying that they must be relatively rare. We found that possibly one observation (19844 day 205) was at an intermediate stage between the medium and high level. So,, combining the EXOSAT data with data available from other X-ray satellites we can concludee the following: A.. CygX-2 shows three different intensity levels (low, medium and high), at least twf whichh (medium and high) occur independently of the position of the source in the Z track. B.. CygX-2 exhibits long-term variations in the position of its Z track in the CD and HTDs. Thee position of the Z depends on the intensity level the source is in (see D).

18 Discussion 83 3 C.. CygX-2 shows changes in the shape of its Z track. The shape depends on which intensity levell the source is in (see E and F). D.. In the low level the overall soft and hard colours are 5-40% higher than in the medium andd the high level, and the overall intensities are 30-50% lower than in the medium level. Duringg a high level the intensity is 30-40% higher than a medium level. E.. No clear Z track is seen during a low level; only one large curved branch reminiscent of aa FB in e.g. Sco X-l has been observed. In the medium level and the high level the well knownn Z track occurs. F.. During the medium level we see in both the CD and the HIDs a clear FB on which the intensityy drops while the colours increase. In the high level no FB is found in the CD, whereass in the HIDs it is present as a colour-independent excursion to lower intensity. G.. Low level episodes occur in the X-ray binary phase interval They do not occur everyy orbital period. High and medium levels occur at all binary phases (see also Section 4.4.5).. H.. Transitions from and to low levels last between 6 and 7 hours; there is no clear indication forr the duration of transitions between medium and high levels Inclination effects Secularr variations in the position of the Z in the CD and HIDs such as reported here were found inn GX5-1 as well (Kuulkers et al. 1994a, Chapter 3). We note that in the HIDs of GX5-1 the observationss also seem to cluster in two "Z w shapes, remarkably similar tur observations of CygX-2.. Also for GX340+0 sovidence for such secular variations was found (Kuulkers & vann der Klis 1995b, Chapter 7). Using the facts that CygX-2 and GX 5-1 show secular variations inn the CD and/or HIDs, and usually decrease in intensity when the source moves up the FB, Kuulkerss et al. (1994a, Chapter 3) concluded that these sources (together with GX 340+0) are probablyy observed at a high inclination. This view is supported by the inclination of reportedd by Cowley et al. (1979) for CygX-2. The other three Z sources, Sco X-l, GX andd GX 17+2 do not show significant long-term variations in the CD and HIDs (they are less thann several per cent), and in their FB the intensity really flares up (see e.g. Ponman et al. 1988, Penninxx et al. 1990, Hertz et al. 1992). These three sources are therefore thought to be viewed att a lower inclination, which is supported by the inclination of reported by Crampton ett al. (1976) for Sco X-l. The two groups exhibit further differences which may relate to the differencee in inclination (see the discussion in Kuulkers et al. 1994a, Chapter 3). Inn one of the EXOSAT observation periods we found CygX-2 to exhibit a HB that curved upp at the left end. This had not been reported previously. Such a curved HB has been seen in GX5-11 by Lewin et al. (1992, Ginga) and Kuulkers et al. (1994a [Chapter 3], EXOSAT). We notee that these curved HBs occur in two sources which fall in the same, high inclination, group. Iff this characteristic is related to high inclination, such a behaviour would then also be expected inn GX Thee CD of CygX-2 in the high level is similar to that of GX5-1 (small or no FBs, see Kuulkerss et al. 1994a, Chapter 3), while the medium level observations are more similar to GXX (the FB turns around half way through, see Penninx et al. 1991). Recently,, Kuulkers & van der Klis (1995a, Chapter 5) studied the different FB-behaviour off the Z sources. They attributed the differences in the dependence of X-ray brightness on positionn in the FB to differences in inclination. In the sources with the highest inclinations the puffedd up inner accretion disk at near Eddington accretion rates (i.e. in the FB) obscures the

19 Secular variations in the Z source CygnusX-2 emittingg regions and therefore these sources show intensity dips when they move up the FB. In thee sources with the lowest inclinations, nbscuration occurs and therefore these sources really flareflare up in intensity when they move up the FB. Sources with intermediate inclinations show a mixx of these two types of behaviour: they first increase in intensity when they move up the FB, andd then, when obscuration sets in, decrease in intensity while they move further along the FB. KuuLkerss & van der Klis (1995a, Chapter 5) concluded that among the three high-inclination systems,, the orbital inclination of GX5-1 may be the highest, that of GX the lowest (but stilll higher than that of ScoX-1, GX and GX 17+2), and that of CygX-2 intermediate. Thee fact that Cyg X-2 alternates between a GX5-l-like state and a GX340+0-like state would thenn suggest that changes occur in the viewing geometry. This could be due to a precessing disk.. Wee note that one would normally expect to see less radiation at higher inclinations (due to e.g.. obscuration effects), unless radiation is somehow beamed preferentially into the equatorial plane.. We do not further explore this here, and just note that anisotropic emission and beaming intoo the equatorial plane has also been suggested in the case of black-hole-transient systems (van derr Klis 1995c,d). Different anisotropics for different X-ray spectral components can, as in the black-holee transients, be invoked to explain the differences in the colour behaviour of the FBs Changing viewing geometry- Basedd on the observed temporal variations, Vrtilek et al. (1988) has proposed that the secular variationss of Cyg X-2 may be caused by a tilted, slaved accretion disk. They attributed the differencess in the intensity levels to mass accretion related changes in the accretion disk itself. Inn view of the interpretation of the Z track as a consequence of M changes, and since we observe thee HB, NB and FB during both the medium and high levels we do not think that changes in thee mass accretion rate are also responsible for the secular variations leading to the medium and highh levels. A similar conclusion was derived from the study of the secular and timing behaviour inn GX 5-1 (Kuulkers et al. 1994a, Chapter 3). Thee low level episodes of Cyg X-2 do not show HB, NB and FB transitions, but instead one bigg curved branch at low intensity and with a hard spectrum. States with low intensity and hard X-rayy spectra have also been observed in atoll sources (see Van der Klis 1995a, and references therein).. They occur at the lowest mass accretion rates. These states are not associated with a curvedd branch ("banana") in the CD of the atoll sources, but with an isolated patch ("island"), andd do not resemble the low level in Cyg X-2. If the low level episodes in Cyg X-2 would be whatt episodes of low mass accretion look like in a Z source 6, they would be connected to the left HB.. However, this also seems unlikely. The durations of the low level episodes are on the order off three days (Vrtilek et al. 1988), which is much longer than the typical time Cyg X-2 spends inn the HB (<ld). Thee branch seen in the low level episodes shows similarities to the FB in ScoX-1 (see Section 4.4.1).. This branch is, however, not connected to the NB or FB in the CDs and HIDs of Cyg X- 2.. We therefore think that low level episodes cannot be attributed to the usual FB behaviour. Moreover,, the duration of the low level episodes is much longer than the time that Cyg X-2 usuallyy spends in the FB (<2hr), or even the time that the sources ScoX-1, GX17+2 and GXX spend there (<ld). Wee conclude, therefore, that like the high and medium level episodes, also the low level episodess are not connected to mass accretion rate related changes. This conclusion is strengthenedd by the fact that the low level episodes only occur during a particular part ( ) of Inn this case one might expect to see periodic oscillations from the spinning neutron star (van der Klis 1991).. The time resolution of the EXOSAT low-level data is, however, insufficient to search for such an oscillation..

20 Discussion 85 5 thee orbital cycle (see Section 4.4.1). This suggests that rather they are related to the viewing geometry.. Wee now explore the possibility that, as has been suggested for other X-ray binaries (see e.g.. Priedhorsky & Holt 1987), CygX-2 contains a precessing accretion disk, and that this mightt explain the high, medium and low levels we have detected. The inclination of Cyg X-2 is relativelyy high (Cowley et al. 1979, see Section 4.4.3), which makes this a plausible explanation. Iff the disk precesses in Cyg X-2 (see also Hasinger et al. 1990), one would predict that the occurrencee of the different levels is periodic Long-term periodicity? Wee investigated whether the occurrence of high, medium and low levels is periodic by combining thee EXOSAT data with data from other satellites. Although extensive period searches have been performedd previously in Cyg X-2 in data sets of individual X-ray satellites, all reported periods havee been different (Holt et al [Ariel 5]: 11.2 d, plus an inferred precession period of ~80 d; Smalee & Lochner 1992 [VelaSB]: ~77 d, ~51 d and 45 d). All these studies were based on period searchess in the X-ray intensity of Cyg X-2, hence stochastic intensity variations associated with thee HBs, NBs and FBs contaminated the occurrence of the overall low, medium and high levels thatt we have identified here. X-rayy modulations at the orbital period were only reported for observations at low intensities (i.e.. including low level and left HB observations) by Bovaisky et al. (1979), Marshall & Watson (1979)) and Vrtilek et al. (1986, 1988). No modulation at the orbital period was detected using dataa of higher intensity observations (Vrtilek et al. 1988) or all observations (Holt et al. 1979, Smalee & Lochner 1992) of an individual satellite. Fromm the literature, only the Einstein MPC data of Vrtilek et al. (1986), and the Ginga dataa of Mitsuda & Dotani (1989) and Hasinger et al. (1990), are useful to clearly distinguish betweenn the different intensity levels (see Section 4.4.2). These data, together with the EXOSAT data,, and other Ginga data (PC and MPC3) from 1989 to 1991 (Wijnands et al. 1995) are listedd in Table 4.3. In this table we the give the date of the observations, the mid-time of the observationss in Julian Day, the duration of the observation, the intensity level, and the orbital phasee range according the the orbital ephemeris of Crampton & Cowley (1980). We note that thee uncertainty in the orbital phase is up to ~0.5 during the 1991 Ginga observations (see also footnotee 4). Einstein operated from November 1978 to April 1981, EXOSAT from June 1983 to Aprill 1986, and Ginga from April 1987 to October From a Kolmogorov-Smirnov test it waswas found that within the uncertainties the observed high and medium levels can be regarded ass uniformly distributed in orbital phase. Low levels are observed in about one fourth of the observationss which overlap with orbital phase 0.8 < ^or b < 0.2. Using the observations listed in Tablee 4.3 we looked for a possible recurrence period of the different intensity levels. Inn our period analysis we combined any observations that were less than two days apart. Wee redetermined the mid-times and the durations accordingly. This left us with a total of 23 observationn periods: 7 Einstein, 9 EXOSAT and 7 Ginga observations, comprising 3 low levels, 111 medium levels, 6 high levels and three observation periods for which we could not distinguish betweenn the medium and the high level. Wee used the Lomb-Scargle periodogram (Lomb 1976, Scargle 1982) as implemented in the softwaree package developed by Dhillon (1994) for our search. We assigned a value of 1 to the low level,, 2 to the medium level, and 3 to the high level. We searched for periods between 10 and 5000 days. The highest peak we find in the periodogram (indicated with an arrow in Fig. 4.7) is att ~18.02d, with a Lomb-Scargle power of Too investigate the significance of this period, we performed Monte Carlo simulations. We chosee times equal to the mid-times of the observations during the three satellite operation