We have obtained differential time series photometry of the cataclysmic
variable GS Pavonis over a timespan of 2 years. These show that this system is
deeply eclipsing (~2-3.5 mag) with an orbital period of 3.72 hr. The eclipse
depth and out-of-eclipse light levels are correlated. From this correlation we
deduce that the disk radius is changing and that the eclipses in the low state
are total. The derived distance to GS Pav is 790+/-90 pc, with a height above
the galactic plane of 420+/-60 pc. We classify GS Pav as a novalike system.Comment: 4 pages, including 2 figures, accepted for publication in A&A Letter

Augusteijn, Thomas

Barziv, Orly

Groot, Paul J.

van Paradijs, Jan

English

arXiv

Groot, P.J.

Augusteijn, T.

Barziv, O.

van Paradijs, J.A.

UvA-DARE

The eclipsing cataclysmic variable GS Pavonis: evidence for disk radius changes

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UvA-DARE is a service provided by the library of the University of Amsterdam (https://dare.uva.nl)UvA-DARE (Digital Academic Repository)The eclipsing cataclysmic variable GS Pavonis: evidence for disk radiuschangesGroot, P.J.; Augusteijn, T.; Barziv, O.; van Paradijs, J.A.Publication date1998Published inAstronomy & AstrophysicsLink to publicationCitation for published version (APA):Groot, P. J., Augusteijn, T., Barziv, O., & van Paradijs, J. A. (1998). The eclipsing cataclysmicvariable GS Pavonis: evidence for disk radius changes. Astronomy & Astrophysics, 340, L31-L34.General rightsIt 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 opencontent license (like Creative Commons).Disclaimer/Complaints regulationsIf you believe that digital publication of certain material infringes any of your rights or (privacy) interests, pleaselet the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the materialinaccessible and/or remove it from the website. Please Ask the Library: https://uba.uva.nl/en/contact, or a letterto: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. Youwill be contacted as soon as possible.Download date:26 Jul 2022Astron. Astrophys. 340, L31–L34 (1998) ASTRONOMYANDASTROPHYSICSLetter to the EditorThe eclipsing cataclysmic variable GS Pavonis:Evidence for disk radius changes?P.J. Groot1, T. Augusteijn2, O. Barziv1,3, and J. van Paradijs1,41 Astronomical Institute ‘Anton Pannekoek’/CHEAF, Kruislaan 403, 1098 SJ, Amsterdam, The Netherlands2 European Southern Observatory, Casilla 19001, Santiago 19, Chile3 European Southern Observatory, Karl-Schwarzschild-Strasse 2, D-85748, Garching bei München, Germany4 Physics Department, University of Alabama in Huntsville, Huntsville, USAReceived 23 September 1998 / Accepted 21 October 1998Abstract. We have obtained differential time series photometryof the cataclysmic variable GS Pavonis over a timespan of 2years. These show that this system is deeply eclipsing (∼2–3.5 mag) with an orbital period of 3.72 hr. The eclipse depth andout-of-eclipse light levels are correlated. From this correlationwe deduce that the disk radius is changing and that the eclipsesin the low state are total. The derived distance to GS Pav is790±90 pc, with a height above the galactic plane of 420±6 pc.We classify GS Pav as a novalike system.Key words: Binaries:eclipsing – stars: GS Pav – cataclysmicvariables1. IntroductionEclipsing non-magnetic Cataclysmic Variables (CVs) (for a re-view see Warner 1995, hereafter W95) are of particular interestnot only because the masses of both stars can be determined, butespecially because studying their eclipses, e.g., by the eclipsemapping method (Horne 1985), gives the opportunity to learnmore about the physics of accretion disks. In thisLetterwe re-port that GS Pav is an eclipsing CV, that shows substantial diskradius changes.GS Pav was first discovered by Hoffmeister (1963) who de-noted it as star S7040 and gave the comment ‘raschwechselnd’(rapidly varying). It was classified as a dwarf nova type CVin the GCVS and in the catalogue of Downes, Webbink andShara (1997), who also give a finding chart for the object. Itwas selected for our observations as a possible member of thehalo population (Augusteijn, 1994). Zwitter and Munari (1995)show that it has a normal CV spectrum. We determined its loca-tion at RA= 20h08m07.s58, Dec=–69◦48′58.′′1 (J2000), almostidentical to that of Downes, Webbink and Shara (1997).Send offprint requests to: Paul Groot (paulgr@astro.uva.nl)? Based on observations with the 0.9m Dutch telescope at ESO, ChileTable 1.Log of V-band observations of GS Pav.Date Start UT Integr. Time (s) No. Obs.Sept 5, 1993 01h50m 120 82Sept 6, 1993 23h21m 120 66Sept 7, 1993 23h11m 120 48Sept 8, 1993 03h57m 120 25March 22, 1995 08h06m 240 25March 23, 1995 07h15m 240 30June 13, 1995 03h20m 240 61July 11, 1995 05h40m 240 65July 25, 1995 04h23m 120 24Aug 16, 1995 04h52m 120 54Sept 15, 1995 03h23m 240 21Sept 19, 1995 02h50m 240 202. ObservationsPhotometric observations were obtained with the Dutch 0.9mtelescope at ESO La Silla, Chile. A log of the observations isgiven in Table 1. All observations were made with a 512x512TEK CCD detector, using a BesselV filter. Standard flatfieldingand debiasing were applied to all observations. A photometriccalibration was obtained on September 6, 1993 using the stan-dard star EG 21 (Landolt 1992). Table 2 gives the coordinatesand magnitudes of the reference stars we have used.3. Photometric ephemerisArrival times of mid-eclipse were determined by fitting a Gaus-sian profile to the eclipses, and by determining the mid-pointbetween the points of steepest ascent and descent. The final ar-rival times listed in Table 3 were taken as the average of theresults from these two methods. We estimate the accuracy ofthese arrival times to be 5·10−4 days, which corresponds to thetypical difference between the results from the two methods.LETTERL32 P.J. Groot et al.: The eclipsing cataclysmic variable GS PavonisTable 2. Reference stars used for the differential photometry of GSPav.No. Name RA (J2000) Dec (J2000) Va1 GAB J200816–6949 20h 8m16.s47 –69◦49.′36.′′0 17.31(4)2 GAB J200810–6949 20h 8m10.s49 –69◦49.′34.′′0 17.41(4)3 GAB J200804–6949 20h 8m04.s83 –69◦49.′40.′′4 17.42(4)4 GAB J200755–6949 20h 7m55.s45 –69◦49.′14.′′4 17.09(4)5 GAB J200802–6948 20h 8m02.s49 –69◦48.′57.′′6 18.54(7)6 GAB J200800–6948 20h 8m00.s64 –69◦48.′49.′′5 16.33(3)7 GAB J200757–6948 20h 7m57.s63 –69◦48.′19.′′6 17.87(5)8 GAB J200810–6947 20h 8m10.s41 –69◦47.′55.′′4 15.45(2)a The quoted errors reflect the internal errors in the brightness mea-surements of the stars which do not include a 0.1 mag uncertainty inthe transformation to standard magnitudes.Table 3. Times of arrival, deduced cycle numbers and the observedminus computed (O–C) residuals for the observations of GS PavCycle No. HJDmin–244 0000 O–C (days)–3062 9235.7370 –0.70·10−3–3050 9237.6014 0.46·10−3–3044 9238.5330 0.44·10−3–3043 9238.6876 –0.23·10−3565 9798.9010 –0.32·10−3571 9799.8340 1.05·10−31098 9881.6603 0.16·10−31099 9881.8147 –0.71· 0−31279 9909.7641 0.13·10−31280 9909.9191 –0.14·10−31369 9923.7376 –0.66·10−31511 9945.7857 –0.87·10−31704 9975.7542 0.56·10−31729 9979.6362 0.82·10−3A linear fit to the arrival times listed in Table 3 yields thefollowing ephemeris:HJDmin = 244 9711.17388(17) + 0.155269817(87) · N, (1)with N the cycle number. The error estimates for the parametersare scaled to give aχ2red = 1.0. The rms value of the arrival timesaround the fit is 6.4·10−4 days, which is in reasonable agreementwith our error estimate.4. Mass ratio, width of the eclipse and inclinationIf the secondary follows the lower main-sequence standardmass-period relation (W95):M2 = 0.065 P5/4orb (h) 1.3 ≤ Porb(h) ≤ 9, (2)with M2 the mass of the secondary,M2 is ∼ 0.34 M. Sincethe mass ratio,q=M2/M1, has to be smaller than 2/3 for stablemass transfer (W95) and the mass of the primary can at most bethe Chandrasekhar mass (1.4 M), the mass ratio is limited tothe range0.67 ≥ q ≥ 0.24.Fig. 1 shows the phase folded eclipse light curves for the 12epochs listed in Table 1. The light curves of June 13 and July11, 1995 contain two eclipses each.The width of the eclipse (∆ϕ) can be estimated by the phasesof steepest descent and ascent. For GS Pav we measure a mean∆ϕ = 0.064± 0.005, where the error is the scatter on the averageof all measurements. From this value and the range in massratio’s we deduce (Horne 1985) a range in orbital inclination of74◦< i < 83◦.5. Correlation between eclipse depthand out-of-eclipse lightFig. 1 shows that GS Pav does not have a constant out-of-eclipse magnitude. In our observations the source varied be-tween V∼14.9 and V∼ 17.1, being mostly at the bright end.Also the eclipse depth is not constant. We have investigated ifthese two variations are correlated. Since the eclipses are wellrepresented by Gaussian functions, we have taken the zero-leveland depth of the Gaussians, used to determine the times of mid-eclipse, as estimates of the depth in magnitudes of mid-eclipse(∆m) and the out-of-eclipse light level (mV ) (Fig. 2). The num-bers in Fig. 2 refer to the eclipses as shown in Fig. 1.In the following we will make a distinction between theobserved radius of the accretion disk, which is the size we inferfrom our observations, and the physical radius of the accretiondisk. The difference between these two is determined by thefractions of the optically thick and thin parts of the accretion diskand the brightness distribution across the disk which determineswhat parts are visible in the chosen passband (here in the V-band). A changing brightness distribution can mimic a changein the physical disk radius, when observed in only one band.In Fig. 2 the straight line labeled ‘Line of Totality’ showswhat the correlation between∆m andmV looks like for a sys-tem in which the eclipse is total. A total eclipse means that theobserved size of the accretion disk is smaller than the size ofthe secondary and that at mid-eclipse the amount of observedlight from the disk is negligible. In a total eclipse the mid-eclipselight level will be constant and equal to the brightness of the sec-ondary. Every magnitude of brightening of the disk will causea magnitude of deepening of the eclipse: the system will followa straight line, with an angle of 45◦, in the∆m-mV diagram.The position of this line in the diagram will be different for eachindividual system, but can be fixed by determining the bright-ness of the secondary. If at any time during our observation GSPas was totally eclipsing, then its mid-eclipse light level will bethe brightness of the secondary. The minimum level occured onJune 13 and July 11, 1995:V =19.9±0.1. We use this point to fixthe position of the ‘Line of Totality’ in Fig. 2. We see that points‘7’ and ‘8’ (which are from June 13 and July 11, 1995) lie onthis line. From the fact that the eclipse depths in point ‘7’ and‘8’ are different, but the brightness at mid-eclipse, we concludethat at these epochs the eclipse is indeed total. It follows thath observed minimum ofV =19.9±0.1 is the brightness of thesecondary. The lack of a flat-bottom in the light curves of point‘7’ and ‘8’ shows that the eclipse is only just total, although theLETTERP.J. Groot et al.: The eclipsing cataclysmic variable GS Pavonis L33Fig. 1.The phasefolded V-band eclipse light curves for the 12 epochs. The heliocentric corrected time of mid-eclipse of the observations is givenin UT.integration time of 4 minutes may be too long to resolve a flatbottom.If we now look at the other points in Fig. 2 we see that theydo not fall on the ‘Line of Totality’. With increasing out-of-eclipse light levels, the depth of the eclipse does not increaseanymore (as it would have on the Line of Totality), but decreases.Apparently, between point ‘7’ and ‘1’ on the track the observedsize of the disk increases to the extent that the eclipse is nolonger total, but that part of the accretion disk remains visible inmid-eclipse. With a further increase of the observed accretiondisk radius, more and more of the disk is visible at mid-eclipseand the eclipse becomes less and less deep. This behaviour wasfirst found by Walker (1963) in his study of RW Tri, whichshows no total eclipses, but does move back and forth on thispart of the track. We have therefore labeled this the ‘WalkerBranch’. Another transition, which to our knowledge has neverbeen noted before, is the one that happens near point ‘10’ inFig. 2. The out-of-eclipse light level reaches a maximum, afterwhich it declines again (the curve bends to the right), but theeclipse depth continues to decrease. We have labeled this partthe ‘Shallow Branch’ because of its decreasing eclipse depth.If the change in the observed radius is caused by a changein the physical size of the disk (rather than a change in thebrightness distribution), then the manifestation of the ShallowBranch may be explained by the effect of self-eclipses. Becauseth height of the disk is correlated with its physical size (Frank,King and Raine 1985), self-eclipses of the hot, and thereforeluminous inner parts of the concave disk by its outer parts, willoccur when the physical size of the disk exceeds a critical valueand therefore a critical height. The out-of-eclipse light leveldecreases because the more luminous parts of the accretion diskare self-eclipsed, and at the same time the eclipse depth cancontinue to decrease if the disk radius continues to increase. Toeclipse the inner parts the disk flaring angle must be (90-i)◦ orgher. In our case this would mean a flaring angle of 7◦–16◦,similar to what has been found in other CVs (e.g. Robinson etal, 1995).The variations appear to trace out a unique track over asubstantial period of time. Changes from one part of the trackto another can occur quite rapidly, e.g. the transition from point‘1’ to ‘4’ took place within 4 days. We are currently modellingthese changes in detail to constrain the geometry of the systemand the changing disk (Groot et al., 1999).LETTERL34 P.J. Groot et al.: The eclipsing cataclysmic variable GS PavonisShallow Branch15 1716 2019181234524567812119103m V∆ m1Line of TotalityWalker BranchFig. 2. The depth of the eclipse in GS Pav as function of the out-of-eclipse light. The numbers correspond to the eclipses in Fig. 1.6. Distance to the systemWith the orbital period of 3.72 hr we can use theMV − Porbrelation of W95 to derive aMV = 10.4 with an estimated error of0.2 magnitudes for the secondary (from Fig. 2.46 in W95). Com-bined with the apparent magnitude ofV =19.9±0.1, this gives adistance to GS Pav of 790±90pc. Given its position on the sky,the implied height above the galactic plane is 420±6 pc. Fromthe distance we can deduce the absolute magnitude of the sys-tem out of eclipse, which is dominated by the accretion disk. Toobtain an absolute magnitude of the accretion disk, we correctfor the inclination of the disk, according to Eq. 2.63 of W95.This correction varies from 1.0 mag (fori=74◦) to 2.1 mag (fori=83◦). The absolute magnitude (MV) lies therefore between6.6≥ MV ≥ 4.4 (for i=74◦) and 5.5≥ MV ≥ 3.3 (for i=83◦).Comparison with the mean absolute magnitudes for NLs andDNe (Fig. 4.16 and 3.9 from W95) shows that the derived rangeis in the normal regime for NLs, but too bright for DN in qui-escence.7. Classification as a novalike systemFrom the shape of the light curve and its absolute magni-tude, we conclude that GS Pav is a novalike system, andconsidering its emission line spectrum (Zwitter and Munari,1995), that it is of the RW Tri subclass, which is definedas having emission line spectra (W95). According to thedefinition in W95 of the VY Scl subclass, as NL systemshaving low states in their long term light curves, and showingno DN outbursts during these low states, we should alsoclassify it as a VY Scl star. This is supported by its orbitalperiod, since almost all known VY Scl stars have peri-ods between 3 and 4 hrs. However, the physical interpretationas outlined in W95 may not apply to GS Pav. In this descriptiona VY Scl system in its low state has a mass-transfer rateṀ , thatis lower than the critical rate,̇Mcrit, below which DN outburstsare expected to occur, but it does not show these outbursts. Thisdistinguishes VY Scl systems from Z Cam systems, that doshow these outbursts in their low state. In our observations GSPav at all times seems to have an absolute magnitude which wasbrighter or equal than that of Z Cam systems during standstill,whereṀ is thought to be larger thaṅMcrit. We therefore cannotconclude if GS Pav is a VY Scl system or not. However, it couldbe that all NL systems with periods between 3 and 4 hours turnout to have low and high states if sufficiently long observed(W95).An interesting example of a system that may be analogousto GS Pav is VZ Scl for which O’Donoghue, Fairall and Warner(1987) concluded that the size of the accretion disk has changedfrom one observation to the other. Unfortunately they only ob-served two eclipses. Since this system, in the low state, is alsototally eclipsing, a comparison with GS Pav would be of interest.8. ConclusionsWe have shown that GS Pav is a deeply eclipsing cataclysmicvariable with a 3.72 hr period. Based on its photometricbehaviour, orbital period and absolute magnitude we classifythe system as a novalike variable. The depth of the eclipseand the brightness of the system out-of-eclipse are correlated.From this relation we infer that the disk radius changes and thatin two of our observations the eclipse is total. Therefore, theapparent visual magnitude of the secondary isV =19.9±0.1,giving a distance to the system of 790±90 pc.ReferencesAugusteijn, T., 1994, PhD thesis, University of AmsterdamDownes, R, Webbink, R.F and Shara, M.M., 1997, PASP 109, 345Frank, J., King, A.R. and Raine, D.J., 1985,Accretion Power in Astro-physics, Cambridge Astrophysics Series Vol. 21, CUPGroot, P.J., Heemskerk, M.H.M., Augusteijn, T. and Van Paradijs, J.,1999,in preparationHoffmeister, C., 1963, Veröff. Sternwarte Sonneberg, 6, 1Horne, K., 1985, MNRAS 213, 129de Jong, J.A., Van Paradijs, J. and Augusteijn, T., A&A 314, 484Landolt, A., 1992, AJ 104, 372O’Donoghue, D., A.P. Fairall and Warner, B., 1987, MNRAS 225, 43Ritter, H. and Kolb, U., 1998, A&AS 129, 83Robinson, E.L., Wood, J.H, Bless, R.C. et al., 1995, ApJ 443, 295Warner, B., 1995,Cataclysmic Variables, Cambridge Astrophysics Se-ries Vol. 28, CUP.Walker, M.F., 1963, ApJ 138, 146Zwitter, T. and Munari, U., 1995, A&AS 114, 575LETTER

UvA-DARE is a service provided by the library of the University of Amsterdam (https://dare.uva.nl)UvA-DARE (Digital Academic Repository)The eclipsing cataclysmic variable GS Pavonis: evidence for disk radiuschangesGroot, P.J.; Augusteijn, T.; Barziv, O.; van Paradijs, J.A.Publication date1998Published inAstronomy & AstrophysicsLink to publicationCitation for published version (APA):Groot, P. J., Augusteijn, T., Barziv, O., & van Paradijs, J. A. (1998). The eclipsing cataclysmicvariable GS Pavonis: evidence for disk radius changes. Astronomy & Astrophysics, 340, L31-L34.General rightsIt 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 opencontent license (like Creative Commons).Disclaimer/Complaints regulationsIf you believe that digital publication of certain material infringes any of your rights or (privacy) interests, pleaselet the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the materialinaccessible and/or remove it from the website. Please Ask the Library: https://uba.uva.nl/en/contact, or a letterto: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. Youwill be contacted as soon as possible.Download date:11 Mar 2023Astron. Astrophys. 340, L31–L34 (1998) ASTRONOMYANDASTROPHYSICSLetter to the EditorThe eclipsing cataclysmic variable GS Pavonis:Evidence for disk radius changes?P.J. Groot1, T. Augusteijn2, O. Barziv1,3, and J. van Paradijs1,41 Astronomical Institute ‘Anton Pannekoek’/CHEAF, Kruislaan 403, 1098 SJ, Amsterdam, The Netherlands2 European Southern Observatory, Casilla 19001, Santiago 19, Chile3 European Southern Observatory, Karl-Schwarzschild-Strasse 2, D-85748, Garching bei München, Germany4 Physics Department, University of Alabama in Huntsville, Huntsville, USAReceived 23 September 1998 / Accepted 21 October 1998Abstract. We have obtained differential time series photometryof the cataclysmic variable GS Pavonis over a timespan of 2years. These show that this system is deeply eclipsing (∼2–3.5 mag) with an orbital period of 3.72 hr. The eclipse depth andout-of-eclipse light levels are correlated. From this correlationwe deduce that the disk radius is changing and that the eclipsesin the low state are total. The derived distance to GS Pav is790±90 pc, with a height above the galactic plane of 420±6 pc.We classify GS Pav as a novalike system.Key words: Binaries:eclipsing – stars: GS Pav – cataclysmicvariables1. IntroductionEclipsing non-magnetic Cataclysmic Variables (CVs) (for a re-view see Warner 1995, hereafter W95) are of particular interestnot only because the masses of both stars can be determined, butespecially because studying their eclipses, e.g., by the eclipsemapping method (Horne 1985), gives the opportunity to learnmore about the physics of accretion disks. In thisLetterwe re-port that GS Pav is an eclipsing CV, that shows substantial diskradius changes.GS Pav was first discovered by Hoffmeister (1963) who de-noted it as star S7040 and gave the comment ‘raschwechselnd’(rapidly varying). It was classified as a dwarf nova type CVin the GCVS and in the catalogue of Downes, Webbink andShara (1997), who also give a finding chart for the object. Itwas selected for our observations as a possible member of thehalo population (Augusteijn, 1994). Zwitter and Munari (1995)show that it has a normal CV spectrum. We determined its loca-tion at RA= 20h08m07.s58, Dec=–69◦48′58.′′1 (J2000), almostidentical to that of Downes, Webbink and Shara (1997).Send offprint requests to: Paul Groot (paulgr@astro.uva.nl)? Based on observations with the 0.9m Dutch telescope at ESO, ChileTable 1.Log of V-band observations of GS Pav.Date Start UT Integr. Time (s) No. Obs.Sept 5, 1993 01h50m 120 82Sept 6, 1993 23h21m 120 66Sept 7, 1993 23h11m 120 48Sept 8, 1993 03h57m 120 25March 22, 1995 08h06m 240 25March 23, 1995 07h15m 240 30June 13, 1995 03h20m 240 61July 11, 1995 05h40m 240 65July 25, 1995 04h23m 120 24Aug 16, 1995 04h52m 120 54Sept 15, 1995 03h23m 240 21Sept 19, 1995 02h50m 240 202. ObservationsPhotometric observations were obtained with the Dutch 0.9mtelescope at ESO La Silla, Chile. A log of the observations isgiven in Table 1. All observations were made with a 512x512TEK CCD detector, using a BesselV filter. Standard flatfieldingand debiasing were applied to all observations. A photometriccalibration was obtained on September 6, 1993 using the stan-dard star EG 21 (Landolt 1992). Table 2 gives the coordinatesand magnitudes of the reference stars we have used.3. Photometric ephemerisArrival times of mid-eclipse were determined by fitting a Gaus-sian profile to the eclipses, and by determining the mid-pointbetween the points of steepest ascent and descent. The final ar-rival times listed in Table 3 were taken as the average of theresults from these two methods. We estimate the accuracy ofthese arrival times to be 5·10−4 days, which corresponds to thetypical difference between the results from the two methods.LETTERL32 P.J. Groot et al.: The eclipsing cataclysmic variable GS PavonisTable 2. Reference stars used for the differential photometry of GSPav.No. Name RA (J2000) Dec (J2000) Va1 GAB J200816–6949 20h 8m16.s47 –69◦49.′36.′′0 17.31(4)2 GAB J200810–6949 20h 8m10.s49 –69◦49.′34.′′0 17.41(4)3 GAB J200804–6949 20h 8m04.s83 –69◦49.′40.′′4 17.42(4)4 GAB J200755–6949 20h 7m55.s45 –69◦49.′14.′′4 17.09(4)5 GAB J200802–6948 20h 8m02.s49 –69◦48.′57.′′6 18.54(7)6 GAB J200800–6948 20h 8m00.s64 –69◦48.′49.′′5 16.33(3)7 GAB J200757–6948 20h 7m57.s63 –69◦48.′19.′′6 17.87(5)8 GAB J200810–6947 20h 8m10.s41 –69◦47.′55.′′4 15.45(2)a The quoted errors reflect the internal errors in the brightness mea-surements of the stars which do not include a 0.1 mag uncertainty inthe transformation to standard magnitudes.Table 3. Times of arrival, deduced cycle numbers and the observedminus computed (O–C) residuals for the observations of GS PavCycle No. HJDmin–244 0000 O–C (days)–3062 9235.7370 –0.70·10−3–3050 9237.6014 0.46·10−3–3044 9238.5330 0.44·10−3–3043 9238.6876 –0.23·10−3565 9798.9010 –0.32·10−3571 9799.8340 1.05·10−31098 9881.6603 0.16·10−31099 9881.8147 –0.71· 0−31279 9909.7641 0.13·10−31280 9909.9191 –0.14·10−31369 9923.7376 –0.66·10−31511 9945.7857 –0.87·10−31704 9975.7542 0.56·10−31729 9979.6362 0.82·10−3A linear fit to the arrival times listed in Table 3 yields thefollowing ephemeris:HJDmin = 244 9711.17388(17) + 0.155269817(87) · N, (1)with N the cycle number. The error estimates for the parametersare scaled to give aχ2red = 1.0. The rms value of the arrival timesaround the fit is 6.4·10−4 days, which is in reasonable agreementwith our error estimate.4. Mass ratio, width of the eclipse and inclinationIf the secondary follows the lower main-sequence standardmass-period relation (W95):M2 = 0.065 P5/4orb (h) 1.3 ≤ Porb(h) ≤ 9, (2)with M2 the mass of the secondary,M2 is ∼ 0.34 M. Sincethe mass ratio,q=M2/M1, has to be smaller than 2/3 for stablemass transfer (W95) and the mass of the primary can at most bethe Chandrasekhar mass (1.4 M), the mass ratio is limited tothe range0.67 ≥ q ≥ 0.24.Fig. 1 shows the phase folded eclipse light curves for the 12epochs listed in Table 1. The light curves of June 13 and July11, 1995 contain two eclipses each.The width of the eclipse (∆ϕ) can be estimated by the phasesof steepest descent and ascent. For GS Pav we measure a mean∆ϕ = 0.064± 0.005, where the error is the scatter on the averageof all measurements. From this value and the range in massratio’s we deduce (Horne 1985) a range in orbital inclination of74◦< i < 83◦.5. Correlation between eclipse depthand out-of-eclipse lightFig. 1 shows that GS Pav does not have a constant out-of-eclipse magnitude. In our observations the source varied be-tween V∼14.9 and V∼ 17.1, being mostly at the bright end.Also the eclipse depth is not constant. We have investigated ifthese two variations are correlated. Since the eclipses are wellrepresented by Gaussian functions, we have taken the zero-leveland depth of the Gaussians, used to determine the times of mid-eclipse, as estimates of the depth in magnitudes of mid-eclipse(∆m) and the out-of-eclipse light level (mV ) (Fig. 2). The num-bers in Fig. 2 refer to the eclipses as shown in Fig. 1.In the following we will make a distinction between theobserved radius of the accretion disk, which is the size we inferfrom our observations, and the physical radius of the accretiondisk. The difference between these two is determined by thefractions of the optically thick and thin parts of the accretion diskand the brightness distribution across the disk which determineswhat parts are visible in the chosen passband (here in the V-band). A changing brightness distribution can mimic a changein the physical disk radius, when observed in only one band.In Fig. 2 the straight line labeled ‘Line of Totality’ showswhat the correlation between∆m andmV looks like for a sys-tem in which the eclipse is total. A total eclipse means that theobserved size of the accretion disk is smaller than the size ofthe secondary and that at mid-eclipse the amount of observedlight from the disk is negligible. In a total eclipse the mid-eclipselight level will be constant and equal to the brightness of the sec-ondary. Every magnitude of brightening of the disk will causea magnitude of deepening of the eclipse: the system will followa straight line, with an angle of 45◦, in the∆m-mV diagram.The position of this line in the diagram will be different for eachindividual system, but can be fixed by determining the bright-ness of the secondary. If at any time during our observation GSPas was totally eclipsing, then its mid-eclipse light level will bethe brightness of the secondary. The minimum level occured onJune 13 and July 11, 1995:V =19.9±0.1. We use this point to fixthe position of the ‘Line of Totality’ in Fig. 2. We see that points‘7’ and ‘8’ (which are from June 13 and July 11, 1995) lie onthis line. From the fact that the eclipse depths in point ‘7’ and‘8’ are different, but the brightness at mid-eclipse, we concludethat at these epochs the eclipse is indeed total. It follows thath observed minimum ofV =19.9±0.1 is the brightness of thesecondary. The lack of a flat-bottom in the light curves of point‘7’ and ‘8’ shows that the eclipse is only just total, although theLETTERP.J. Groot et al.: The eclipsing cataclysmic variable GS Pavonis L33Fig. 1.The phasefolded V-band eclipse light curves for the 12 epochs. The heliocentric corrected time of mid-eclipse of the observations is givenin UT.integration time of 4 minutes may be too long to resolve a flatbottom.If we now look at the other points in Fig. 2 we see that theydo not fall on the ‘Line of Totality’. With increasing out-of-eclipse light levels, the depth of the eclipse does not increaseanymore (as it would have on the Line of Totality), but decreases.Apparently, between point ‘7’ and ‘1’ on the track the observedsize of the disk increases to the extent that the eclipse is nolonger total, but that part of the accretion disk remains visible inmid-eclipse. With a further increase of the observed accretiondisk radius, more and more of the disk is visible at mid-eclipseand the eclipse becomes less and less deep. This behaviour wasfirst found by Walker (1963) in his study of RW Tri, whichshows no total eclipses, but does move back and forth on thispart of the track. We have therefore labeled this the ‘WalkerBranch’. Another transition, which to our knowledge has neverbeen noted before, is the one that happens near point ‘10’ inFig. 2. The out-of-eclipse light level reaches a maximum, afterwhich it declines again (the curve bends to the right), but theeclipse depth continues to decrease. We have labeled this partthe ‘Shallow Branch’ because of its decreasing eclipse depth.If the change in the observed radius is caused by a changein the physical size of the disk (rather than a change in thebrightness distribution), then the manifestation of the ShallowBranch may be explained by the effect of self-eclipses. Becauseth height of the disk is correlated with its physical size (Frank,King and Raine 1985), self-eclipses of the hot, and thereforeluminous inner parts of the concave disk by its outer parts, willoccur when the physical size of the disk exceeds a critical valueand therefore a critical height. The out-of-eclipse light leveldecreases because the more luminous parts of the accretion diskare self-eclipsed, and at the same time the eclipse depth cancontinue to decrease if the disk radius continues to increase. Toeclipse the inner parts the disk flaring angle must be (90-i)◦ orgher. In our case this would mean a flaring angle of 7◦–16◦,similar to what has been found in other CVs (e.g. Robinson etal, 1995).The variations appear to trace out a unique track over asubstantial period of time. Changes from one part of the trackto another can occur quite rapidly, e.g. the transition from point‘1’ to ‘4’ took place within 4 days. We are currently modellingthese changes in detail to constrain the geometry of the systemand the changing disk (Groot et al., 1999).LETTERL34 P.J. Groot et al.: The eclipsing cataclysmic variable GS PavonisShallow Branch15 1716 2019181234524567812119103m V∆ m1Line of TotalityWalker BranchFig. 2. The depth of the eclipse in GS Pav as function of the out-of-eclipse light. The numbers correspond to the eclipses in Fig. 1.6. Distance to the systemWith the orbital period of 3.72 hr we can use theMV − Porbrelation of W95 to derive aMV = 10.4 with an estimated error of0.2 magnitudes for the secondary (from Fig. 2.46 in W95). Com-bined with the apparent magnitude ofV =19.9±0.1, this gives adistance to GS Pav of 790±90pc. Given its position on the sky,the implied height above the galactic plane is 420±6 pc. Fromthe distance we can deduce the absolute magnitude of the sys-tem out of eclipse, which is dominated by the accretion disk. Toobtain an absolute magnitude of the accretion disk, we correctfor the inclination of the disk, according to Eq. 2.63 of W95.This correction varies from 1.0 mag (fori=74◦) to 2.1 mag (fori=83◦). The absolute magnitude (MV) lies therefore between6.6≥ MV ≥ 4.4 (for i=74◦) and 5.5≥ MV ≥ 3.3 (for i=83◦).Comparison with the mean absolute magnitudes for NLs andDNe (Fig. 4.16 and 3.9 from W95) shows that the derived rangeis in the normal regime for NLs, but too bright for DN in qui-escence.7. Classification as a novalike systemFrom the shape of the light curve and its absolute magni-tude, we conclude that GS Pav is a novalike system, andconsidering its emission line spectrum (Zwitter and Munari,1995), that it is of the RW Tri subclass, which is definedas having emission line spectra (W95). According to thedefinition in W95 of the VY Scl subclass, as NL systemshaving low states in their long term light curves, and showingno DN outbursts during these low states, we should alsoclassify it as a VY Scl star. This is supported by its orbitalperiod, since almost all known VY Scl stars have peri-ods between 3 and 4 hrs. However, the physical interpretationas outlined in W95 may not apply to GS Pav. In this descriptiona VY Scl system in its low state has a mass-transfer rateṀ , thatis lower than the critical rate,̇Mcrit, below which DN outburstsare expected to occur, but it does not show these outbursts. Thisdistinguishes VY Scl systems from Z Cam systems, that doshow these outbursts in their low state. In our observations GSPav at all times seems to have an absolute magnitude which wasbrighter or equal than that of Z Cam systems during standstill,whereṀ is thought to be larger thaṅMcrit. We therefore cannotconclude if GS Pav is a VY Scl system or not. However, it couldbe that all NL systems with periods between 3 and 4 hours turnout to have low and high states if sufficiently long observed(W95).An interesting example of a system that may be analogousto GS Pav is VZ Scl for which O’Donoghue, Fairall and Warner(1987) concluded that the size of the accretion disk has changedfrom one observation to the other. Unfortunately they only ob-served two eclipses. Since this system, in the low state, is alsototally eclipsing, a comparison with GS Pav would be of interest.8. ConclusionsWe have shown that GS Pav is a deeply eclipsing cataclysmicvariable with a 3.72 hr period. Based on its photometricbehaviour, orbital period and absolute magnitude we classifythe system as a novalike variable. The depth of the eclipseand the brightness of the system out-of-eclipse are correlated.From this relation we infer that the disk radius changes and thatin two of our observations the eclipse is total. Therefore, theapparent visual magnitude of the secondary isV =19.9±0.1,giving a distance to the system of 790±90 pc.ReferencesAugusteijn, T., 1994, PhD thesis, University of AmsterdamDownes, R, Webbink, R.F and Shara, M.M., 1997, PASP 109, 345Frank, J., King, A.R. and Raine, D.J., 1985,Accretion Power in Astro-physics, Cambridge Astrophysics Series Vol. 21, CUPGroot, P.J., Heemskerk, M.H.M., Augusteijn, T. and Van Paradijs, J.,1999,in preparationHoffmeister, C., 1963, Veröff. Sternwarte Sonneberg, 6, 1Horne, K., 1985, MNRAS 213, 129de Jong, J.A., Van Paradijs, J. and Augusteijn, T., A&A 314, 484Landolt, A., 1992, AJ 104, 372O’Donoghue, D., A.P. Fairall and Warner, B., 1987, MNRAS 225, 43Ritter, H. and Kolb, U., 1998, A&AS 129, 83Robinson, E.L., Wood, J.H, Bless, R.C. et al., 1995, ApJ 443, 295Warner, B., 1995,Cataclysmic Variables, Cambridge Astrophysics Se-ries Vol. 28, CUP.Walker, M.F., 1963, ApJ 138, 146Zwitter, T. and Munari, U., 1995, A&AS 114, 575LETTER

The eclipsing Cataclysmic Variable GS Pavonis: Evidence for disk radius
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The eclipsing Cataclysmic Variable GS Pavonis: Evidence for disk radius changes

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