arXiv:0806.4392v1 [astro-ph] 26 Jun 2008 o.Nt .Ato.Soc. Astron. R. Not. Mon. RPcoi:a l oasoigsprup n QPOs Schmidtobreick and L. showing old an Pictoris: RR § ‡ † ⋆ of presence the suggesting (4686 by II 1977) He ra- Wehinger the & with of (Wyckoff pho- together variations re- time–resolved observations velocity Metz & dial spot own polarimetric Haefner and their hot structure. tometric explained extended inhomogeneous He however, an an minima. (1982) by with superimposed behaviour gion by this interrupted explained often hump, high very with CV nova–like a rate. 2003) as transfer al. fea- regarded mass et be observational (Schmidtobreick thus several stars Indeed, can show Sex stars. and SW type to for Sex typical found SW period tures the been the of of has above regime period Pic just RR the orbital it into The places and now. gap 1975) by (Vogt state supposed days is quiescence it 0.145025 its nova, slow in a was be it to although maximum and, at 1925 (1931) in such, ther- Jones light Spencer As a by discovered explosion. of was nova Pic observation RR the the outburst, by runaway distinguished late–type monuclear variables are Roche–lobe–filling They a cataclysmic pri from star. dwarf of matter white a receiving subclass with mary stars binary a interacting close are (CVs), novae Classical INTRODUCTION 1 cetdxx.Rcie xx noiia omxxxx form original in xxxx; Received xxxx. Accepted c 3 2 1 rj nvrti rse,Penan2 00Busl,Bel Uni Brussels, Astrof´ısica, Astronom´ıa Pontificia 1050 y de 2, Departamento PLeinlaan 19 Brussel, Santiago Universteit 19001, Vrije Casilla Observatory, Southern European [email protected] [email protected] [email protected] [email protected] 05RAS 2005 otfudtelgtuv oiae yavr broad very a by dominated lightcurve the found Vogt 000 (1) – 20)Pitd2 a 08(NL (MN 2018 May 28 Printed (2005) 1–8 , ⋆ .Papadaki C. , aalsi aibe niiul RPic. RR individual: – variables cataclysmic elwt h l n,ayucranyi o ml oacutfrthe th as for interpreted account usually at to is period small which additional too period, an h is 3.48 find uncertainty the any from from one, Apart derive we shift. old which the period e orbital the with th The that well 0.2. for suggests phase curve evidence around light br located convincing the a be of any with r shape coincides find overall and phase the not eclipse Apa comparing flickering do extrapolated Pic. strong We The RR eclipse. Pic. the nova RR reported show for old curves typical the light ation of the variability, V–photometry hump–like time–resolved present We ABSTRACT eas n niain o h rsneo 3mnqaiproi osc quasi–periodic min 13 a of words: presence also Key the is d for 1.79 indications at find period precession also We corresponding the system; the of )eiso line emission A) ˚ hscldt n rcse:acein crto ic stars – discs accretion accretion, processes: and data Physical (2 , esddCtoia ail 0,Snig 2 Chile 22, Santiago 306, Cat´olica, Casilla versidad gium 1) Chile , † - .Tappert C. , P nteobtlpro.T etteeatraie,w per- we alternatives, these test To change period. a indicate orbital emis- might unstable or the disc to in accretion due years the be two in 0.1 might sources taken this about sion data that of of argue curves shift They velocity a apart. radial noticed the and of phases epochs different of curves the than other period one. any orbital high- reveal Warner’s not the Also does dwarf. in photometry white event speed rotating transient a than a rather likely disc more that concluded was they measure- variation either, X–ray Kubiak’s 1981) in period. Marshall found short & is this (Becker variation longer ments of period sign a min 15 any no on find Since not photometry could time–resolved and time–scale high however, as the (1985) is Schoembs this Pic repeated RR & interpreted that Haefner He concluded polar. and intermediate period. dwarf an white min 15 the of a modulation rotation the the with periodic to optical a addition the found In had in (1984) lightcurves. eclipse Kubiak previous an period, in- the of orbital high in signature a found no be that been to a Note for Pic system. RR evidence showing clination found eclipse, has th irregular he of shallow, distribution addition, isotropic In more radiation. in a place emitted in taken resulting have system, changes the structural (about outburst) 1970s after the yr during 50 photom- that speed concluded high From (1986) was Warner (2006). etry, of Diaz source & al. emission opposite Ribeiro et an and Schmidtobreick disc (2003) such by tomography the of Doppler presence via in confirmed The radiation spot. hot of the source additional an 3 = cmdorike l 20)cmae ailvelocity radial compared (2005) al. et Schmidtobreick . 8h hc eitrrta h uehm period the as interpret we which h, 78 A T E tl l v2.2) file style X (3) ‡ n .Ederoclite A. and , lpesol actually should clipse rsn ntedata. the in present a iiu,but minimum, oad hs aaagrees data these ria n,we one, orbital e osbephase possible illation. no vari- andom previously e tfo the from rt (1) § novae, : e 2 L. Schmidtobreick, C. Papadaki, C. Tappert, A. Ederoclite

Table 1. Summary of the observational details: Date & UT at the start of the first exposure, the number of exposures, the individual exposure time, the covered orbital cycles and the acquisition ID are given.

Date UT #exp texp [s] cycles ID

2005-02-08 00:45:44 483 20 2.03 1 2005-02-09 01:17:57 533 20 1.76 1 Figure 2. The Scargle–diagram for the data. The period of 3.48 h 2005-02-10 01:17:23 495 20 1.63 1 is clearly indicated by the maximum peak. 2005-02-11 01:13:37 530 20 1.74 1 2005-02-12 01:09:17 540 20 1.77 1 2005-02-13 01:07:03 499 20 1.79 1 well as with PERIOD 04 (Lenz & Breger 2005). All three 2005-02-14 01:06:42 534 20 1.76 1 methods agree on the same period Pph = 0.14503(7) which 2005-03-18 01:36:23 240 20 0.80 1 corresponds to a strong and unambiguous peak in the pe- 2005-03-20 01:03:17 320 20 1.08 1 riodograms at f0 = 6.895 cycles/day (see Fig. 2). We sub- 2005-03-25 02:31:58 80 20 0.26 2 divided the observations in different sets but always ob- 2005-03-26 00:24:42 320 20 1.06 2 tained the same result, which thus indicates the robust- 2005-03-26 23:58:29 329 20 1.15 2 2005-03-31 00:38:39 241 20 0.92 2 ness of this peak. We included photometric data taken 2005-03-31 03:52:05 56 20 0.18 1 in 2004 (Schmidtobreick et al. 2005) to increase the accu- 2005-04-09 23:29:27 318 20 1.06 2 racy of the orbital period and derived 0.1450255(1)d. This value agrees very well with the formerly reported one of P = 0.14502545(7) d (Vogt 1975; Kubiak 1984). formed new time–resolved photometry of RR Pic with the We averaged our data with respect to the orbital pe- aim to determine the orbital period and look for a possi- riod P = 0.14502545 d. The orbital phase was computed ble change that could account for the observed phase shift. for each data point using the ephemeris for the eclipse of These data and the results are presented in this paper. Schmidtobreick et al. (2005). The data points have been av- eraged into bins of 0.01 phases. On average, 55 individual data points went into each point of the average . 2 DATA The result is plotted in Fig. 3. For clarity, two orbital cy- cles are plotted (phase 0–2). For the first cycle, the sigma The time resolved photometry was done using a V–filter in of each average point is indicated with an error bar. Apart front of a 512x512 CCD mounted on the 1.0 m SMARTS from the broad hump, the light curve shows two clear fea- telescope at CTIO, Chile. The data were taken in 2005 be- tures, a narrow peak on top of the hump at a phase of 0.42 tween Feb 07 and April 10 and cover about 18 orbital cycles and a sharp minimum at a phase of 0.18. As expected for with a time resolution of 40 s. The details of the observations an average light curve, the flickering, which is present in the are given in Table 1. The reduction was done with IRAF and individual light curves is completely gone. included the usual steps of bias subtraction and division by With PERIOD 04, we checked for the harmonics of the skyflats. orbital period and found peaks close to the theoretical values Aperture photometry for all stars on the CCD field and down to the 6th harmonic at 0.0207 d which has a power was computed using the stand–alone version of DAOPHOT around 2 and is thus at the edge of detection. We used the and DAOMASTER (Stetson 1992). Differential light curves theoretical harmonics to fit the shape of the light curve. We were established with respect to an average light curve of then built an average fit in the same way as the average light those comparison stars, which were present on all frames curve. The result is over–plotted in Fig. 3, the residuals with and checked to be non–variable. While the original idea was the average light curve plotted below. Note that this best to use the same comparison stars for all epochs, we had to harmonic fit also contains the above mentioned features: the settle for two sets of comparison stars, as some of the later narrow peak and the sharp minimum. The residuals of fit data were taken with a different acquisition. The first set and average light curve show high frequency periodic varia- included five, the second set six comparison stars; the two tions with an amplitude around 0.01 mag and harmonic to sets are distinguished by the acquisition IDs 1 and 2 in Table the orbital one (as they would otherwise not appear in a 1, corresponding finding charts are given in the appendix. phase diagram). This indicates, that harmonics of an order The difference in the target’s magnitudes between the two higher than 6 are present in the data even though they do sets were established from three common stars as 0.31. The not appear in the power spectrum. magnitudes of the second set were shifted accordingly. We point out that neither the individual light curves of RR Pic nor the average one show the eclipse that has been observed before. At phase 0, which is the phase of the eclipse 3 RESULTS by Warner (1986), we find a broad minimum. 3.1 The orbital period 3.2 The search for superhumps The obtained light curves are plotted in Figure 1. They show a clear variation. The data were analysed with the Scar- For the further analysis, we subtracted from each data gle and analysis-of-variance (AOV) algorithms implemented point the corresponding value of the average light curve. in MIDAS (Scargle 1982; Schwarzenberg-Czerny 1989) as The resulting residual light curves are plotted in Fig. 4.

c 2005 RAS, MNRAS 000, 1–8 RR Pictoris: an old nova showing superhumps and QPOs 3

Figure 1. The light curves of RR Pic. The phase refers to the orbital period of P = 0.14502545(7) d, φ = 0 corresponds to the eclipse–ephemeris as defined by Schmidtobreick et al. (2005).

c 2005 RAS, MNRAS 000, 1–8 4 L. Schmidtobreick, C. Papadaki, C. Tappert, A. Ederoclite

is present in all combinations of data taken in February, it does not appear in the data taken in March or April. This might in part be explained by the way the observations were performed. The February observing run was dedicated to RR Pic, which was thus observed for at least two orbits every night. In March and April, the observations were done in service mode, and in general, only slightly more than one orbit was observed per night. We averaged the residual light curve for the different data sets using the period of the superhump. In Fig. 6, these average residual light curves are plotted. They clearly show that the superhump is present during all our observing runs. The absence of a clear corresponding peak in the Scargle– diagram is thus only due to the observing strategy and does not indicate the absence of the superhump in the latter data. Figure 3. The average differential V–magnitude of RR Pic is plotted against the phase using the period P = 0.1450 d. Fig. 6 also gives the impression that the superhump is Two cycles are plotted for clarity. The zero-phase was cho- highly structured. Short–periodic variations are present es- sen to correspond to the eclipse–ephemeris as defined by pecially during the early phases (0–0.7), while the later part Schmidtobreick et al. (2005). For the first cycle, error bars repre- of the lightcurve looks rather stable. The amplitude of the senting the sigma of each point are over–plotted, and the fit using high–frequency variations is around 0.01 mag, thus it is sim- the orbital period and its harmonics (see text for details) is given ilar in frequency and amplitude to the variations seen in the as a line. Below, the residuals of fit and average light curve are residuals of Fig. 3. plotted.

3.3 Quasi–periodic oscillations (QPOs) and flickering High–frequency variations present in Fig. 1 and Fig. 4 sug- gest the possible presence of QPOs. The time–scale of these variations is in the order of 15 min, and they seem to re- peat themselves albeit with inconsistency in amplitude and frequency. We therefore searched the periodograms for sig- nals that might indicate the existence of possible QPOs in RR Pic. Although, we did not detect any coherent signal, all nights do show several peaks in the power spectrum in the range between 90 and 130 1/d. As Fourier analysis can omit signals, such as QPOs, that are unstable in amplitude and frequency (see Papadaki et al. (2006) for such examples), we applied the following procedure in order to enhance a pos- sible QPO signal. The power spectra of all nights were av- eraged and plotted in log–log scale (see Fig. 7). In this plot, the harmonics of the orbital period can be followed down to Figure 5. The Scargle–diagram for the data minus the average the 11th harmonic at 82.8 1/d, whose power exceeds the con- light curve. Top diagram: all data, middle diagram: data until Feb 14, lower diagram data from March and April. A new peak tinuum by a factor of 1.9. Around log F = 2.04 1/d a broad becomes visible indicating a period of 3.78 h which is present in peak becomes visible, pointing to the counterpart of a QPO the first part of the data but not in the later observations. as suspected from the inspection of our light curves. The individual average power spectra of February and March– April 2005 (middle and lower plot of Fig. 7, respectively) As expected they are dominated by the strong flickering. show that the 13 min QPO is always there but more promi- However, there seems to be an additional brightness vari- nent in the March–April data. The power of this feature ation present on longer time–scales. We checked the peri- exceeds the continuum by a factor of 4. It is the counterpart odograms for any periodic signal and indeed found a peak of a 13 min oscillation which agrees with the typical value at fsh = 6.34 cycles/day, which corresponds to a period of for the oscillations present in the light curves and even more Psh = 3.78 h (see Fig. 5). The interpretation of this period as visible in the residual light curves (see Fig. 1 and 4). Note the superhump period is supported by the presence of var- that the peak is present in the February data as well as the ious typical frequencies in the numerology of superhumps March–April data but is more dominant in the latter. as described e.g. by Patterson et al. (2005). We find peaks The flickering in CVs, described through a shot noise– at 0.56, 13.21, and 20.14 cycles/day, which correspond to like process, results in the so–called ”red noise” seen in the Ω = 1/Porb − 1/Psh, 2f0 − Ω and 3f0 − Ω. The beat period power spectra of CVs as the exponential decrease of power 1/Ω = 1.79 d is thus interpreted as the probable precession with frequency. It thus follows linearity in a log–log scale di- period of the accretion disc. agram. If all shots have the same duration, the periodogram A detailed investigation on the robustness of this peak of the resulting light curve equals the periodogram of a sin- is not as successful as for the orbital period. While the peak gle shot and thus just describes its shape. The power law

c 2005 RAS, MNRAS 000, 1–8 RR Pictoris: an old nova showing superhumps and QPOs 5

Figure 4. The residual light curves of RR Pic, with the average light curve subtracted.

c 2005 RAS, MNRAS 000, 1–8 6 L. Schmidtobreick, C. Papadaki, C. Tappert, A. Ederoclite

Figure 7. Average power spectra in logarithmic scale of RR Pic’s light curve. Top diagram: all data, middle diagram: data until 14th February, lower diagram: data from March and April. Indi- cated are the positions for the harmonics of the orbital period up to 11th order and the additional bump at 2.04 which we attribute to a QPO feature. The fits to the linear parts are overplotted, the corresponding equation given at the lower left part of each plot.

uncertainty of γ. Within this error, the flickering pattern remains stable within the span of our observations. We checked the photometric data taken in 2004 (Schmidtobreick et al. 2005) for the presence of the QPOs and the flickering pattern. Only two orbits go into this anal- ysis and the average power spectrum is rather noisy. We find no indication for the presence of a QPO. The slope of the linear decrease in the log–log power spectrum has been determined as γ = 1.6.

Figure 6. The average differential V–magnitudes of RR Pic, of which the average orbital variation was subtracted, are plotted against the phase using the period PSH = 0.1577 d. Top diagram: all data, middle diagram: data until 14th February, lower diagram: 4 DISCUSSION data from March and April. The superhump is clearly visible in all plots. 4.1 The orbital period and the radial velocity phase shift With the data spanning two years, we derive an orbital pe- index γ, given by the slope of the linear decrease in the riod of P = 0.1450255(1) d which agrees well with the for- log–log scale, has a value of 2 in the classical ”shot noise”, merly reported ones within our uncertainty. We can thus where the shots have an infinite rise–time and then decay. only give an upper limit to any possible change of the or- − If, however, as expected from mechanisms generating the bital period as 10 7 d or about 0.1s. flickering, the shots’ durations are different and follow some The formerly reported phase shift of 0.1 phases that kind of distribution, then γ gets smaller (Bruch 1992). In was observed in the radial velocities of data taken two this way, γ can be used for the characterisation of flicker- years apart (Schmidtobreick et al. 2005), can thus not be ing activity but so far has been unsuccessful to advance on explained by a change of the orbital period. To obtain a shift understanding the physical origin of flickering activity. For of 0.1 phases after such about 5000 cycles, the period must − more detailed information on the mechanisms and the re- have changed by at least 0.1×0.14502545d/5000 = 3·10 6d, sulting power law, see Papadaki et al. (2006) and references which is in contradiction to the new measurements. This therein. shift is thus best explained by a varying emission structure Clearly, Fig. 7 shows that RR Pic is also characterised in the accretion disc of RR Pic which can influence the shape by ”red noise”. We fitted the linear part for frequencies and phasing of the radial velocity curve. Such variations above 100 c/d by a least–square linear fit and determined have been observed by Schmidtobreick et al. (2006) using γ = 1.40(2) for the average log–log power spectrum. Given Doppler tomography techniques on several emission lines in the noisiness of the power spectrum, the precise choice of the spectra of RR Pic. Also, the structural change suggested the fitting interval becomes more difficult. Applying small by Warner (1986) supports the idea that the accretion disc changes of this interval causes changes in the value of γ up of RR Pic is not stable but undergoes changes of various to 0.13 and therefore 0.1 should be considered as the real extent.

c 2005 RAS, MNRAS 000, 1–8 RR Pictoris: an old nova showing superhumps and QPOs 7

4.2 Superhumps and QPOs 4.3 Is RR Pic eclipsing? RR Pic has been reported by various authors to show eclipse–like features (Warner (1986), The interpretation of the newly found variation with a pe- Haefner & Betzenbichler (1991)), while others have not riod of Psh = 3.78 h as a superhump seems obvious due to noted this (Vogt (1975), Kubiak (1984)). Vogt confirmed the presence of the typical frequencies as described in section that he never saw any evidence for the presence of an 3.2. However, it has to be noted that the resulting value for eclipse in his data (private communication). In fact, Warner Psh − Porb = 18 min is rather large for a nova–like star and (1986) compared the lightcurves taken during the 60s and implies a rather large mass ratio. Still, such large mass ra- beginning of the 70s with those taken later in the 70s and tios are not unlikely for high–mass transfer CVs. Patterson. 80s and suggested a change in the structure of the accretion (2001) relates the mass ratio q to the orbital and super- disc to explain the different appearance of the lightcurves. hump period via an empirical formula ǫ = 0.216 · q with P −P While the early lightcurves were dominated by two humps ǫ = SH orb . In the case of RR Pic, we find ǫ = 0.0860 Porb of about 0.3 mag brightness and several small minima, the and thus derive a mass ratio of q = 0.39. This value would later lightcurves were rather flat (no variation larger than actually be above the critical mass ratio for which super- 0.1 mag) and showed the already mentioned eclipse–like humps are observed and is unlikely to be correct. In fact, feature. Patterson et al. (2005) revised the formula especially for If we put our observations in this context, it seems as large mass ratios where the original one would predict too if RR Pic is back to or at least closer to its state in the large values of q. Using this new formula ǫ = 0.18q + 0.29q2, early 70s. We observe a hump-like feature of about 0.3 mag we derive a mass ratio q = 0.31 for RR Pic, which is more brightness, we see the two minima reported by Vogt, a broad reasonable. This value is slightly higher than the value found one followed by a smaller one, but we find no evidence for by (Ribeiro & Diaz 2006) who used a mass diagram cal- an eclipse. culated from their radial velocity measurements to derive Comparing the overall shape of the average light curve 0.1

c 2005 RAS, MNRAS 000, 1–8 8 L. Schmidtobreick, C. Papadaki, C. Tappert, A. Ederoclite

Doppler map with the shape of the lightcurve, and i.e. the Papadaki C., Boffin H.M.J., Sterken C., Stanishev V., presence of an eclipse. Cuypers J., Boumis P., Akras S., Alikakos J., 2006, A&A 456, 599 Patterson J., 2001, PASP, 113, 736 Patterson J., Kemp J., Harvey D.A., Fried R.E., Rea R., 5 CONCLUSIONS et al., 2005, PASP 117, 1204 We have presented optical lightcurves of RR Pic and shown Ribeiro F.M.A., Diaz M.P., 2006, PASP, 118, 84 that they are dominated by a strong orbital variation. The Scargle J.D., 1982, ApJ, 263, 835 orbital period derived from this data is consistent with the Schmidtobreick L., Tappert C., Ederoclite A., Saviane I., previous reported ones. I.e. a change of this period can not Mason E., 2006, Ap&SS 304, 303 be responsible for the previously observed shift in the phases Schmidtobreick L., Galli L., Saviane I., Tappert C., Whit- of radial velocity curves. Instead, this shift is rather due to ing A., 2005, in Sterken C., ed., The Light-Time Effect in structural changes that are known to occur in the accretion Astrophysics, ASP Conf. Ser. 335, 333 disc of RR Pic. Schmidtobreick L., Tappert C., Saviane I., 2003, MNRAS In addition to the orbital variation, a superhump is 342, 145 found that was used to derive the mass ratio q = 0.31. This Schwarzenberg-Czerny A., 1989, MNRAS 241, 153 value does not agree with a previously reported lower one, Spencer Jones H., 1931, Cape Obs. Ann., 10, Part 9 further observations are needed for clarification. Stetson, P.B., 1992, in Worrall et al., eds, Astronomical QPOs of 13 min are present in all our data taken be- Data Analysis Software and Systems I, ASP Conf. Ser. tween February and April 2005. Older data from 2004, do 25, 291 not show this oscillation. While our analysis thus confirms Vogt N., 1975, A&A, 41, 15 the variations reported earlier, it also shows that these QPOs Warner B., 1986, MNRAS, 219, 751 are a transient phenomenon. Their presence might be con- Wyckoff S., Wehinger P.A., 1977, in Kippenhahn R. et al., nected with the accretion disc’s structure if they occur due eds, The Interaction of Variable Stars with their Environ- to an illumination of blobs in the inner accretion disc from ment, IAU Colloq. 42, p. 201 a spinning white dwarf. From our data we can not confirm the presence of an eclipse. Instead, we note that at least in one historical case the eclipse occurs when the minima of orbital light curve and superhump lightcurve fall together. This might indicate that the observed eclipse is a resonance phenomenon between the two lightcurves and its existence or not-existence is modu- lated with the precession period. We would like to clarify that we do not rule out the presence of a shallow eclipse but insist that either its visibility is enhanced by the resonance or a favourable structure of the accretion disc is needed for an eclipse to be observed. In general we conclude that RR Pic is a highly vari- able system. The previously reported changes that happen in the accretion disc are probably responsible for the various features in the lightcurve that are not present at all times. To really understand what is going on in this system, par- allel time–resolved spectroscopy and photometry would be needed over several cycles.

ACKNOWLEDGEMENTS We thank Rebeccah Winnick for doing a wonderful job in organising the service observations.

REFERENCES Becker R.H., Marshall F.E., 1981, ApJ, 244, L93 Haefner R., Metz K., 1982, A&A, 109, 171 Haefner R., Schoembs R., 1985, A&A, 150, 325 Haefner R., Betzenbichler W., 1991, IBVS 3665 Bruch A., 1992, A&A 266, 237 Kubiak M., 1984, Acta Astron., 34, 331 Lenz P., Breger M., 2005, CoAst, 146, 53

c 2005 RAS, MNRAS 000, 1–8 RR Pictoris: an old nova showing superhumps and QPOs 9

Figure 1. The finding chart of RR Pic (indicated by the square in both images) with the two selected sets of comparison stars (indicated by circles). The left image corresponds to ID1, the right one to ID2 (see table 1 and text).

c 2005 RAS, MNRAS 000, 1–8