15th European Workshop on White Dwarfs ASP Conference Series, Vol. 372, 2007 R. Napiwotzki and M. R. Burleigh

HS 2331+3905, the Brightest CV White Dwarf Pulsator

Boris T. G¨ansicke Departments of Physics, University of Warwick, Coventry CV4 7AL, UK

Abstract. We report preliminary results of infrared spectroscopy and of a multi-site photometric campaign on the brightest pulsating white dwarf in a cataclysmic variable (CV), HS 2331+3905. The Gemini J, H, and K spectra fail to reveal a clear signature of the donor star, constraining its spectral type to be L2 or later. The presence of two short-period signals at 67.62 s and 67.24 s is confirmed, and we identify the 67.62 s signal unambiguously as the white dwarf spin. Our photometric data shows that the ZZ Ceti pulsations detected near 5 min are extremely complex, with the dominant frequencies varying on time scales of hours. We suggest that the complex pulsation pattern is related to the extremely rapid rotation of the white dwarf.

1. Introduction

The properties of white dwarfs in cataclysmic variables (CVs) are notoriously difficult to be determined, as the spectrum of the white dwarf is contaminated by emission from the accretion disc/stream and/or the companion star (G¨ansicke 2000). Therefore, standard methods such as Balmer line profile fitting, which give reasonably accurate values for the effective temperatures and surface grav- ities of single white dwarfs, can be applied only to a limited extend in CVs. In particular the determination of the white dwarf mass is practically impos- sible from spectral modelling alone. Single white dwarfs pass through an in- stability strip as they cool, which is located around ≃ 11 000 − 12 500 K for pure hydrogen atmospheres (Mukadam et al. 2004; Gianninas et al. 2006) and around 22 400−27 800 K fore pure helium atmospheres (Beauchamp et al. 1999). The analysis of these non-radial oscillations can, in principle, provide measure- ments of the white dwarf masses, envelope masses, spin, and magnetic fields (e.g. Bradley 2001). Given the difficulty in determining fundamental param- eters such as the white dwarf masses in CVs, the discovery of pulsating white dwarfs in CVs (van Zyl et al. 2000; Woudt & Warner 2004; Patterson et al. 2005; G¨ansicke et al. 2006; Mukdadam et al. 2007) stirred up a flutter of activity to adopt the methods of asteroseismology to accreting white dwarfs. Here we re- port preliminary results of infrared spectroscopy and of a co-ordinated multi-site photometric campaign on the brightest CV white dwarf pulsator, HS 2331+3905.

2. HS2331+3905

HS 2331+3905 has been discovered as a CV in the Hamburg Quasar Survey (HQS), and a first assessment of its properties has been given by Araujo-Be- 597 598 G¨ansicke

Figure 1. Spectral energy distribution of HS 2331+3905 (gray lines). The far-ultraviolet spectrum was obtained with STIS on board HST, the opti- cal spectrum at Calar Alto, and the infrared spectrum with Gemini-North. Shown as dotted curves are the model spectra of a 0.6 M⊙ white dwarf of 10 500 K, an isothermal/isobaric hydrogen slab of 6500 K, and an L2 dwarf template. Shown as black line is the sum of the white dwarf and the slab models, black dots are the infrared magnitudes of the L2 dwarf. tancor et al. (2005). The orbital period of the system is 81.08 min, close to the observed minimum period of CVs, accurately determined from shallow eclipses. The optical spectrum reveals the broad Balmer absorption lines of the white dwarf, superimposed by double-peaked Balmer and weak helium emission lines, typical for the origin in an accretion disc. No spectroscopic sign of the donor star has detected in the optical spectrum of the system. To date, no dwarf nova outburst of HS 2331+3905 has been recorded. All observational evidence suggests that HS 2331+3905 is a CV with a very low-mass donor star and a very low accretion rate, similar in many aspects to the prototypical system WZ Sge. However, HS 2331+3905 did reveal a very unusual property: the radial velocity variation of the Balmer emission lines is strongly modulated at a period of ∼ 3.5 h, which is not the orbital period, moreover this spectroscopic period is not a stable clock, but drifts by a few per cent on time scales of days.

2.1. The Spectral Energy Distribution A fit to the spectral energy distribution (SED) composed of an HST/STIS far- ultraviolet spectrum, an optical spectrum, and 2MASS J, H, and KS magnitudes provided an estimate of the white dwarf temperature, TWD ≃ 10500 K for a dis- tance of d = 90±15 pc. The absence of TiO band-heads in the red part of the op- tical spectrum and the lack of a significant infrared flux excess strongly suggested that the donor star in HS 2331+3905 is a brown dwarf. In order to test this hy- pothesis, we have obtained Gemini J, H, and K spectroscopy of HS 2331+3905. Figure 1 shows the ultraviolet to infrared SED of HS 2331+3905, which can be reproduced relatively well with a three-component model consisting of a 10 500 K white dwarf of 0.6M⊙ (Hubeny & Lanz 1995), an isothermal/isobaric pure hy- drogen slab (G¨ansicke et al. 1997), which accounts at first order for the emission HS 2331+3905, the Brightest CV White Dwarf Pulsator 599 of the accretion disc, and a late-type stellar template (Beuermann et al. 1998; Kirkpatrick et al. 1999). No significant features that could be ascribed to the donor star are detected in the Gemini spectra, and the flux level of the IR data sets a rather conservative ”early” limit on its spectral type of L2. The model fails most severely for λ < 1500 A˚, where bound–free and free–free absorption from elements heavier than hydrogen is likely to contribute to the continuum emission, and near the Balmer, Paschen, and Brackett jumps, which are clearly more pronounced in the observations compared to our simple model. A possible reason for this discrepancy is that our single-temperature/single-density model for the accretion disc emission is too simple. However, lacking a satisfying theory for the structure of quiescent accretion discs, we refrain from adding additional free parameters such as multi-temperature/density zones.

2.2. A Multi-longitude Photometric Campaign Motivated by the detection of ZZ Ceti pulsations with periods near 5 min and two short-period signals near 70 s (Araujo-Betancor et al. 2005), we have organ- ised a multi-longitude campaign on HS 2339+3905 spanning mid-August/mid- September 2004. Participating telescopes were the 0.8 m at Wendelstein Obser- vatory (Germany), the 1.2 m at Kryoneri Observatory (Greece), the 1.2 m Oskar Luhn¨ ing at the Hamburger Sternwarte (Germany), the 1.0 m Optical Ground Station at the Observatorio del Teide (Tenerife), the 1.52 m at Loiano Obser- vatory (Italy), the 1.5 m Russian–Turkish on Mount Bakyrlytepe (Turkey), the 1.0 m at Mount Laguna Observatory (California), the 1.5 m at San Pedro Mar- tir (Mexico), the 1.0 m Ritchey-Chr´etien at the US Naval Observatory Flagstaff Station (Arizona), and the K-380 Cassegrain of the Crimean Astrophysical Ob- servatory. The combined data from all sites provides > 60 000 CCD frames with an average temporal cover of ∼ 40 % over the 25 days/nights of the campaign. Unfortunately, no single run longer than ∼ 15 h has been achieved due to the lack of participating observing sites in the Asian region. Figure 2 shows the Scargle periodogram obtained from the combined data set. The dominant signal is found at 40.54 min, i.e. 1/2×Porb, due to the strong double-hump morphology of the orbital brightness modulation. An independent signal is detected near 84 min. This signal displays a complex structure in fre- quency space, which implies that it is not strictly coherent in period. Given that this period is somewhat longer than the orbital period, we believe that it represents a permanent superhump modulation, which is most likely related to the slow precession of the slightly elliptical accretion disc.

2.3. The White Dwarf Spin and a Warped Disc The power spectrum of HS 2331+3905 contains a pair of signals at 1277.73 cy- cles/day (67.62 s) and 1284.96 cycles/day (67.24 s), as well as power at the re- spective second harmonics. While we interpret the power seen near 5 min as ZZ Ceti pulsations (see below), we argue that these shortest-period signals are related to the white dwarf spin, for a number of reasons. Firstly, extremely short pulsation periods are very rarely observed in single ZZ Ceti white dwarfs, and to our knowledge the shortest pulsation period detected so far is 70.9 s in G 185−32 (Castanheira et al. 2004). In HS 2331+3905, we find signals at 33 s which seems too short do be explained by non-radial oscillations. Secondly, we 600 G¨ansicke

Figure 2. The complex power spectrum of HS 2331+3905. Top panel: full frequency range. Middle panel: the dominant signal at 1/2×Porb (left) and the permanent superhump near 84 min (right). Bottom panel: Complex ZZ Ceti pulsations near 5 min (left); white dwarf spin plus beat with the inner disc rotation (middle) and their second harmonics (right). tested the degree of coherence in the two short-period signals by pre-whitening the data set with their respective frequencies, and computing new Scargle peri- odograms. Figure 3 illustrates that no power is left near the 67.62 s period after the pre-whitening, implying that the signal is extremely coherent over the course of our 25 d-long campaign. In contrast to this, the 67.24 s signal is incoherent, as substantial power is left after pre-whitening. Also iterative per-whitening is not able to remove the residual power in that frequency range. Hence, we are left with two closely-spaced short-period signals, one of which is a stable clock in the system, and the other one is drifting. The separation between the two signals is ∼ 7.2 cycles/day, or 3.3 h – which is close to the period of the strong modu- lation detected in the radial velocity modulation of the optical emission lines. HS 2331+3905, the Brightest CV White Dwarf Pulsator 601

We interpret the radial velocity modulation and the two short photometric pe- riods as follows: the white dwarf in HS 2331+3905 is rapidly spinning at 67.62 s, and mildly magnetic, causing the observed photometric period. At the low mass transfer rate in HS 2331+3905, the weak magnetic field of the white dwarf warps the inner accretion disc. The permanent superhump signal near 84 min shows that the outer part of the disc is precessing with a period of ∼ 1.5 d, the inner warped disc will precess more rapidly – at the ∼ 3.5 h period detected in the radial velocities. The rotating white dwarf acts like a light house, illuminating the warped inner disc, causing a beat signal in the photometry. The fact that the beat signal is found at a period shorter than the white dwarf spin indicates that the warp is precessing retrogradely, as predicted from theoretical models (Shirakawa & Lai 2002).

2.4. ZZ Ceti Pulsations HS 2331+3905 exhibits a clear excess of power at periods near 5 min (Fig. 2). Araujo-Betancor et al. (2005) suggested that variability is caused by non-radial pulsations of the white dwarf, and tentatively identified a number of pulsation frequencies on the base of a limited set of photometric time-series. The main motivation behind our multi-longitude campaign was to resolve the pulsation spectrum and identify individual pulsation modes. However, it turned out that the power spectrum is even more complex than the initial data suggested: while several strong signals are present in each single data set, their total number and positions in frequency space change from day to day. On occasions, individual signals fade or appear over the course of several hours during a single observation. As stated above, the lack of participating observing sites in Asia caused gaps in the daily light curves, preventing the tracking of individual signals over longer periods of time. Analysing either the entire data set together by an iterative pre-whitening procedure, or the longest uninterrupted runs on their own, shows that the strongest frequencies identified by Araujo-Betancor et al. (2005) recur frequently, but the much larger volume of data from this campaign reveals a multitude of additional signals.

3. Discussion

HS 2331+3905 being a CV, one has to critically assess whether the variability observed around periods of 5 min could be related to accretion activity, rather than ZZ Ceti pulsations. A variety of quasi-coherent oscillations are indeed ob- served in CVs, such as QPOs with periods in the range of a few 1000 s in novalike variables, dwarf nova oscillations (DNOs) in dwarf novae during outburst with periods of several tens of seconds (Pretorius et al. 2006; Piro & Bildsten 2004), or quasi-periodic variability in strongly magnetic CVs, again on time scales of tens of seconds (Larsson 1992). All these signals differ dramatically from the variability observed in HS 2331+3905 as they show up as a single-frequency sig- nal at a given time, whereas HS 2331+3905 displays always several well-defined frequencies simultaneously. A plausible explanation for the extremely complex pulsation spectrum of HS 2331+3905 is its very fast rotation, which implies that simple frequency splitting of pulsations modes is no longer valid (see Townsley & Bildsten 2007). 602 G¨ansicke

Figure 3. Close-up of the spin of the WD in HS 2331+3905. The two strongest signals are 1277.73 and 1284.96 cycles/day. Left panel: Scargle periodogram of the combined data from the 2004 campaign. Middle: Scargle periodogram obtained after pre-whitening the data with 1277.73 cycles/day. No significant power is left at that frequency. Right: Scargle periodogram obtained after pre-whitening the data with 1284.96 cycles/day. Substantial power remains in this frequency range even after iterative pre-whitening.

Acknowledgments. I am greatly indebted to all the observers who con- tributed to the photometric data set of HS 2331+3905: A. Aungwerojwit, S. Balman, H. Barwig, D. de Martino, D. Engels, O. Giannakis, A. Jiannis, E.T. Harlaftis, S.Howell, I. Nestoras, G. Tovmassian, S. Zharikov, W. Welsh. S. Araujo-Betancor, S. Howell, and J. Thorstensen are acknowledged for their help with the Gemini observations. BTG has been supported by a PPARC Advanced Fellowship.

References

Araujo-Betancor, S., G¨ansicke, B. T., Hagen, H.-J., et al. 2005, A&A, 430, 629 Beauchamp, A., Wesemael, F., Bergeron, P., et al. 1999, ApJ, 516, 887 Beuermann, K., Baraffe, I., Kolb, U., & Weichhold, M. 1998, A&A, 339, 518 Bradley, P. A. 2001, ApJ, 552, 326 Castanheira, B. G., Kepler, S. O., Moskalik, P., et al. 2004, A&A, 413, 623 G¨ansicke, B. T. 2000, Reviews of Modern Astronomy, 13, 151 G¨ansicke, B. T., Beuermann, K., & Thomas, H. C. 1997, MNRAS, 289, 388 G¨ansicke, B. T., Rodr´ıguez-Gil, P., Marsh, T. R., et al. 2006, MNRAS, 365, 969 Gianninas, A., Bergeron, P., & Fontaine, G. 2006, AJ, 132, 831 Hubeny, I., & Lanz, T. 1995, ApJ, 439, 875 Kirkpatrick, J. D., Reid, I. N., Liebert, J., et al. 1999, ApJ, 519, 802 Larsson, S. 1992, A&A, 265, 133 Mukadam, A. S., Winget, D. E., von Hippel, T., et al. 2004, ApJ, 612, 1052 Mukadam, A., Szkody, P., Fraser, O. J., et al. 2007, these proceedings, 603 Patterson, J., Thorstensen, J. R., Armstrong, E., et al. 2005, PASP, 117, 922 Piro, A. L., & Bildsten, L. 2004, ApJ, 616, L155 Pretorius, M. L., Warner, B., & Woudt, P. A. 2006, MNRAS, 368, 361 Shirakawa, A. & Lai, D. 2002, ApJ, 565, 1134 Townsley, D. M. & Bildsten, L. 2007, these proceedings, 557 van Zyl, L., Warner, B., O’Donoghue, D., et al. 2000, Baltic Astronomy, 9, 231 Woudt, P. A. & Warner, B. 2004, MNRAS, 348, 599