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Secular Changes in the Quiescence of WZ Sagittae: the Development of a Cavity in the Inner Disk

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Secular Changes in the Quiescence of WZ Sagittae: the Development of a Cavity in the Inner Disk A&A 528, A152 (2011) Astronomy DOI: 10.1051/0004-6361/201014141 & c ESO 2011 Astrophysics Secular changes in the quiescence of WZ Sagittae: the development of a cavity in the inner disk E. Kuulkers1,A.A.Henden2, R. K. Honeycutt3, W. Skidmore4, E. O. Waagen2, and G. A. Wynn5 1 European Space Astronomy Centre, SRE-O, PO Box 78, 28691 Villanueva de la Cañada, Madrid, Spain e-mail: [email protected] 2 American Association of Variable Star Observers, 49 Bay State Rd., Cambridge, MA 02138, USA 3 Astronomy Department, Indiana University, Swain Hall West 319, 727 East 3rd Street, Bloomington, IN 47405-7105, USA 4 TMT Observatory Corporation, 2632 E Washington Blvd, Pasadena, CA 91107, USA 5 Department of Physics and Astronomy, University of Leicester, Leicester, LE1 7RH, UK Received 27 January 2010 / Accepted 22 January 2011 ABSTRACT We find a dimming during optical quiescence of the cataclysmic variable WZ Sge by about half a magnitude between superoutbursts. We connect the dimming with the development of a cavity in the inner part of the accretion disk. We suggest that, when the cavity is big enough, accretion of material is governed by the magnetic field of the white dwarf and pulsations from the weakly magnetic white dwarf appear. The time scale of forming the cavity is about a decade, and it persists throughout the whole quiescent phase. Such a cavity can be accommodated well by the proposed magnetic propeller model for WZ Sge, where during quiescence mass is being expelled by the magnetic white dwarf from the inner regions of the accretion disk to larger radii. Key words. accretion, accretion disks – binaries: close – stars: dwarf novae – Stars: individual: WZ Sge – novae, cataclysmic variables – white dwarfs 1. Introduction WZ Sge has an orbital period of 81.6 min (Krzeminski´ 1962; Warner 1976). One sees it at an inclination of about 75◦,just Cataclysmic variables (CVs; for a review see Warner 1995) high enough for the donor star to obscure the accretion disk but are binary systems wherein a white dwarf gains material from not the white dwarf (Krzeminski´ 1962;Smak1993). So far it has a gravitationally bound companion star. Some of these CVs displayed (super)outbursts in 1913, 1946, 1978 and 2001 (e.g., (called dwarf novae) show recurrent enhancements in bright- Mayall 1946; Patterson et al. 1981; 2002,Katoetal.2009; see, ness by several magnitudes (i.e., outbursts) which come from e.g., Kuulkers et al. 2002, for the outburst light curves). enhanced accretion of material, onto the white dwarf through an The standard disk-instability model (Osaki 1974), with some accretion disk. If the inclination of the binary is high enough, modifications (see, e.g., Lasota 2001; Schreiber & Lasota 2007, periodic increases in flux (“humps”) are discerned in the light for discussions), can explain the normal outbursts of dwarf no- curves when the system is at rest (i.e., in quiescence). These vae. Two main competing models have been put forward for the humps have their origin in the region where the stream of ma- superoutbursts. On the one hand, superoutbursts are thought to terial from the companion star interacts with the accretion disk occur when the outer disk reaches the tidal radius, and enhanced (so-called bright spot), and they appear when that site comes into tidal dissipation occurs due to a resonance between the orbit of our view, once per orbital period. the outer disk and the orbit of the secondary star, increasing the WZ Sge is an intriguing CV. It is an extreme member of mass transfer rate even more (e.g., Osaki 1989, 1996;thermal a subclass of the dwarf novae, called SU UMa stars. SU UMa tidal instability model). On the other hand, the enhanced mass stars show two kinds of outbursts: 1) normal outbursts, which transfer model predicts a pure enhanced mass transfer from the typically last several days; and 2) superoutbursts with maxi- secondary star, owing to, e.g., irradiation of the secondary (see mum brightness being about a magnitude brighter than normal Hameury et al. 2000). Also, a “hybrid” solution between the two outbursts, which last a few weeks (hence the prefix “super”). has been suggested (e.g., Smak 2000). For a review of the differ- Superoutbursts occur less frequently than the normal outbursts. ent flavours of outburst models we refer to Lasota (2001), among During the main part of the superoutburst, periodic humps are others. discerned in the light curves (irrespective of the binary’s inclina- Neither the thermal tidal instability model nor the enhanced tion) with a period that is a few percent longer than the orbital pe- mass transfer model can fully explain what is happening in riod; they are referred to as superhumps. The extreme members WZ Sge (see, e.g., Schreiber et al. 2004; Matthews et al. 2007, of the SU UMa stars usually do not show normal outbursts, but for recent discussions). The observed recurrence times are quite only superoutbursts. These superoutbursts recur after very long long, and all outbursts are seen to start immediately as super- intervals (years to decades) and show large amplitudes, typically outbursts (similar to what is seen in low-mass X-ray binary five magnitudes or more, from quiescence to peak of the out- transients, see, e.g., Lasota 2001). Somehow the inner disk re- burst. These systems are given yet another label: WZ Sge stars gions need to be cleared during quiescence, in order to explain (e.g., Bailey 1979) or TOADs (Howell et al. 1995). the observations (Angelini & Verbunt 1989;Lasotaetal.1995; Article published by EDP Sciences A152, page 1 of 10 A&A 528, A152 (2011) Warner et al. 1996; Hameury et al. 1997; Meyer-Hofmeister Table 1. Log of observations of WZ Sge using the Stiening Photometer. et al. 1998; Matthews et al. 2007). Monitoring the quiescence is thus important for constraining Date Band Telescope Ap. tint tdur modelling of the (super)outburst behaviour. This can be a prob- 1982 Jun. 23 UBR 1.5-m Mt. Lemmon 17 1.25 s 2.8 h lem because most of the systems in quiescence are too faint to 1988 Aug. 18 UBVR 2-m McDonald 6.7 1.00 s 2.0 h be observed by moderate equipment. However, since WZ Sge is 1988 Aug. 19 UBVR 2-m McDonald 6.7 1.00 s 4.0 h 1991 Aug. 16 UBVR 2-m McDonald 6.7 1.00 s 1.3 h nearby ( 43 pc; Thorstensen 2003;Harrisonetal.2004), it can rather easily be observed during quiescence (V ∼ 15 mag, see 1991 Aug. 17 UBVR 2-m McDonald 6.7 1.00 s 2.6 h 1997 Aug. 29 UBVR 2-m McDonald 26.8 1.00 s 4.1 h also Sect. 3). A huge amateur database exists on this source. Part 1997 Aug. 30 UBVR 2-m McDonald 26.8 1.00 s 5.5 h of the available data has already been presented at the conference 1997 Aug. 31 UBVR 2-m McDonald 26.8 1.00 s 3.9 h “The Physics of Cataclysmic Variables and Related Objects”, in 1997 Sep. 1 UBVR 2-m McDonald 26.8 1.00 s 3.8 h 2001, in Göttingen, Germany. One of us (EK) showed the qui- escence data and pointed out a decrease in brightness several Notes. Ap. = aperture size used, tint = integration time per measurement, = years before the 2001 superoutburst. It was questioned, how- tdur duration of the observing run. ever, whether this apparent drop may have been caused by a change of data acquisition methods of the amateurs in the mid- 90’s (see Marsh 2002). In this paper we come back to this issue the error on each WZ Sge’s measured differential magnitude is and present data also beyond the 2001 superoutburst, as well as about 0.03 mag. The resulting light curve has an arbitrary zero prior to the 1978 superoutburst. point, which is established to within 0.01 mag using 14 sec- ondary standard stars in the field from Henden & Honeycutt (1997). A transformation coefficient for the V filter (evaluated 2. Observations from regular observations of standard fields) of +0.05 is applied in the zero-point calculation, assuming a B − V colour value of 2.1. Amateur observations 0.0 (see Appendix A) for each observation of WZ Sge. There The American Association of Variable Star Observers (AAVSO) were 1078 usable RoboScope images of the field of WZ Sge, holds a huge international database of observations of variable acquired during the interval November 1990 to December 2004 stars by thousands of observers from all over the world, dating over 635 nights. back to its foundation over 90 years ago (e.g., Henden 2006). Visual estimates (by eye) of these stars typically have an uncer- 2.3. Multicolour Stiening photometry tainty of about 0.2–0.3 mag, although some of the more expe- rienced amateur observers can reach a precision down to about High-speed multicolour photometry of WZ Sge was obtained 0.02 mag. The uncertainty in CCD observations varies typically with the Stiening photometer (see, e.g., Horne & Stiening 1985) between 0.1 to 0.01 mag, but some professional and experienced in 1982, 1988, 1991 (Skidmore et al. 1997), and 1997 (Skidmore amateur observers can obtain milli-magnitude precision (see, 1998). As far as we are aware, this dataset provides the only e.g., Price et al. 2009). Observers are provided with a common more or less homogeneous multicolour coverage over a single observing chart to ensure a homogeneous set of comparison and quiescent interval.
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