SU UMa Research Articles

The disk instability model for dwarf nova outbursts

The development and the present state of the disk instability model for outbursts of dwarf nova are reviewed. Two intrinsic instabilities are known in dwarf nova accretion disks, i.e., the thermal instability and the tidal instability. The thermal-tidal instability model (abbreviated the TTI model) that combines these two intrinsic instabilities was first proposed in 1989 by Osaki to explain the superoutburst phenomenon of SU UMa stars. In this paper a complete account of the TTI model is presented. We first explain the basic concept of the model and how it works for the superoutburst phenomenon. We then discuss recent refinements of the model, in particular on the start and the end of superoutbursts, by which we are now able to explain wide varieties in superoutburst light curves of different stars and of different superoutbursts within one and the same star. We also discuss some exceptional cases that apparently seem to contradict the theory, such as the 1985 superoutburst of U Gem itself. It is argued that the TTI model can after all explain most of the observed phenomena related to superoutbursts and superhumps in dwarf novae. (Communicated by Yoshihide KOZAI, M. J. A.)

Delays in dwarf novae II: VW Hyi, the tidal instability and enhanced mass transfer models

We discuss the multi-wavelength predictions of the two models proposed for SU UMa stars, i.e. the enhanced mass transfer (EMT) and the tidal thermal instability (TTI) models. We focus on the systematic differences of the suggested scenarios before discussing the model predictions together with the observations of the best-studied SU UMa system, VW Hyi. We find that assuming the standard form of the viscosity parameter α, both models predict only outbursts being triggered at the inner edge of the accretion disc. In the TTI model the superoutbursts are triggered when the outer radius of the disc reaches a certain value, i.e. the 3:1 resonance radius. In contrast, the EMT scenario predicts superoutbursts when the disc mass exceeds a critical value. This causes the EMT model to be much more sensitive to mass transfer variations than the TTI model. In both models we find the predicted UV and EUV delays in agreement with the observations of VW Hyi for αh/αc < 4. In addition both models can generate precursor outbursts which are more pronounced at short wavelengths, in agreement with observations. Variations found in the observed light curve of single systems (e.g. VW Hyi) as well as the difference between ordinary SU UMa stars and ER UMa systems are a natural outcome of the EMT model while the TTI model fails to explain them.

IR Geminorum: Complex Disk Behavior During Quiescence

AbstractWe have carried out nearly continuous V-band photometry from Yunnan Observatory (China) and Behlen Observatory (Nebraska, USA) of IR Gem for over six days starting three days after a normal outburst in January 2002. Our observations show that the behavior of this SU UMa star is unexpectedly complicated, and that for IR Gem, quiescence is potentially more interesting than outbursts. We find a photometric modulation with a period of 98.5 min, exactly equal to the spectroscopically determined orbital period. We tentatively attribute this to heating of the secondary. During the first three days a modulation appeared with a period 5% longer than the orbital period. We suggest that this might be a signature of apsidal precession of an eccentric disk. During the middle of our period of observations a modulation with a period 3% shorter than the orbital period appeared. We invoke nodal precession to explain this. A slower modulation we found with a period of about 1.7 d is roughly consistent with the expected period of nodal precession. There is a puzzling 4.3 d period modulation that we suspect may be the result of beating between apsidal and nodal precession frequencies. We also find inexplicable quasi-periodic cycles on timescales drifting from ~ 0.2 to ~ 0.4 days.

A model for superoutbursts in SU UMa-type binaries

We suggest a new mechanism for the superoutbursts in SU UMa binaries, in which the increase in the accretion rate resulting in a superoutburst is associated with the formation of a spiral “precessional” wave in the inner parts of the disk, where gas-dynamical perturbations are negligible. The existence of such waves was suggested by us previously. The results of three-dimensional gas-dynamical simulations have shown that a considerable increase in the accretion rate (by up to an order of magnitude) is associated with the formation of the precessional wave. The features of the precessional spiral wave can explain both the energy release in the superoutburst and all its observational manifestations. One distinguishing feature of superoutbursts in SU UMa-type stars is the formation of a “superhump” in the light curve. Our model reproduces well both the formation of a superhump and its observational features, including its period, which is up to 3–7% longer than the orbital period, and the detectability of the superhump independent of the orbital inclination of the binary.

High-speed photometry of faint cataclysmic variables - IV. V356 Aql, Aqr1, FIRST J1023+0038, Hα 0242-2802, GI Mon, AO Oct, V972 Oph, SDSS 0155+00, SDSS 0233+00, SDSS 1240−01, SDSS 1556−00, SDSS 2050−05, FH Ser

We present results from high-speed photometry of a further 13 faint cataclysmic variables. V356 Aql (Nova Aql 1936) shows flare-like outbursts with recurrence time-scales ∼3000 s, but no coherent periodicities. Aqrl is an intermediate polar with a spin period of 6.7284 min and a probable orbital period P orb = 3.226 h derived from orbital sidebands. Its orbital sideband frequencies show very variable amplitudes. The published spectroscopic period of 2.0 h suggests that Aqrl is similar to GW Lib and FS Aur in having an additional periodicity of unknown origin. FIRST J1023+0038 has P orb = 4.7548 h with an orbital modulation of range 0.45 mag, probably caused by reflection effect from a hot white dwarf primary; as such it may have been a nova sometime in the past few decades. Ha 0242 is a deeply eclipsing very low mass-transfer rate system, probably a dwarf nova of very long outburst interval, with P orb = 1.792 h. GI Mon, an old nova, has optical modulations at 4.325 h and possibly also at 48.6 min and is thus a candidate intermediate polar. AO Oct, an SU UMa type dwarf nova, shows orbital modulation at quiescence with P orb = 94.097 min. V972 Oph (Nova Ophiuchi 1957) showed no flickering activity during one set of observations, but did so at a later time, confirming the correctness of the identification of this object, but it shows no orbital modulation. SDSS 0155 is a deeply eclipsing polar with an orbital period of 87.13 min. SDSS 0233 shows flaring activity but no discernible periodicity. No periodicity was found in the new AM CVn candidate SDSS 1240. SDSS 1556 shows a periodic modulation at 1.778 h, which is possibly due to orbital motion. SDSS 2050 is an eclipsing polar with P orb = 1.5702 h. FH Ser (Nova Serpentis 1970) has strong flickering activity but no detectable orbital modulation.

Mass-transfer bursts and the superhumps in cataclysmic variables

It is suggested that the superhumps in SU UMa stars are generated by a bright region on the red component that rotates asynchronously with the binary period. The bright region would be produced by mass-transfer instability by the red component. A simple experimental illustration of dynamical instability is presented, together with estimates of cooling times after instability.

Detection of Short Fading Episodes in Two Dwarf Novae from VSNET Observations

Abstract From the observations reported to VSNET, we detected short fading episodes in two dwarf novae, RX And (orbital period 5.0 hr) and SU UMa (orbital period 1.8 hr). The episodes in RX And can be naturally explained as the response to a short-term reduction of the mass-transfer rate. A qualitative comparison with VY Scl-type fading is also discussed. In SU UMa, the same explanation is expected to be more difficult to apply. The viable, but still poorly understood, possibilities include a temporary reduction of the quiescent disk viscosity and a temporary reduction of the mass-transfer rate. If the latter possibility is confirmed, we probably need a different mechanism from that of VY Scl-type stars.

Photometric Observations of an SU UMa-type Dwarf Nova, VW Coronae Borealis, during Outbursts

Abstract We report on photometric observations of an SU UMa-type dwarf nova, VW CrB, during two superoutbursts in 2001 and 2003 and a normal outburst in 2003. Superhumps with a period of 0.07287(1) d were observed during the 2003 superoutburst. The change rate of the superhump period was positive. During a normal outburst, there is some hint of modulation up to a 0.2-mag amplitude. However, no periodicity was found. The recurrence cycles of the normal outburst and the superoutburst, and the distance were estimated to be $\ge 50 \,\mathrm{d}$, $270 \sim 500 \,\mathrm{d}$, and $690^{+230}_{-170} \,\mathrm{pc}$, respectively. These recurrence cycles are the usual values for an SU UMa-type dwarf nova having this superhump period. The superhump period of VW CrB was the longest among those of SU UMa stars with positive derivatives of the superhump period. The coverage of our observations was, however, not sufficient, and the variation of the $P_\mathrm{SH}$ change rate of VW CrB is still unknown. A superhump regrowth and a brightening were seen near the end of the plateau phase. Measuring the deviation of the start timings of the brightening and the superhump regrowth ($ \gt 2$ days in VW CrB) will be a key to reveal the mechanism of these phenomena.

A Deeply Eclipsing SU UMa-Type Dwarf Nova with the Shortest Orbital Period, XZ Eridani

Abstract We discovered superhumps in an eclipsing dwarf nova, XZ Eridani, during an outburst in 2003 January–February. We determined the orbital and average superhump periods to be $0.061159602 \pm 0.000000044 \,\mathrm{d}$ and $0.062808\pm 0.000017 \,\mathrm{d}$, respectively. Our observations thus established that XZ Eri belongs to a rare class of deeply eclipsing SU UMa-type dwarf novae. The object has the shortest orbital period among seven known objects in this class. The superhump period decreased with time during a mid-plateau phase of the superoutburst, as observed in a number of ordinary, long-period SU UMa stars. During an early phase of the superoutburst, however, the superhump period increased, as observed in several short-period systems. XZ Eri is a unique object with which we can study the evolution of accretion-disk structures through eclipses during the phase of the superhump period increase.

A New SU UMa-Type Dwarf Nova, QW Serpentis (= TmzV46)

Abstract We report on the results of the QW Ser campaign, which has been continued from 2000 to 2003 by the VSNET collaboration team. Four long outbursts and many short ones were caught during this period. Our intensive photometric observations revealed superhumps with a period of $0.07700(\pm 0.00004) \,\mathrm{d}$ during all four superoutbursts, proving the SU UMa nature of this star. The recurrence cycles of the normal outbursts and the superoutbursts were measured to be $\sim 50$ days and $240(\pm 30)$ days, respectively. The change rate of the superhump period was $-5.8 \times 10^{-5}$. The distance and the X-ray luminosity in the range of 0.5–2.4 keV are estimated to be $380(\pm60) \,\mathrm{pc}$ and $\log L_\mathrm{X} = 31.0 \pm 0.1 \,\mathrm{erg} \,\mathrm{s}^{-1}$. These properties have typical values for an SU UMa-type dwarf nova with this superhump period.

A Photometry Campaign for IR Geminorum in Quiescence

We report a V band photometry of the SU UMa star IR Gem at quiescence in January 2002. The observations were made with two telescopes spaced ~ 160° apart in longitude. Several photometric modulations have been found. One gives a period of 98.50(13) min, exactly equal to the orbital period determined spectroscopically. Two others occasionally strengthen and seem to be positive and negative superhumps with periods of 103.6(4) and 95.4(4) min, 5.2 % longer and 3.1 % shorter than the orbital period, respectively. A signal at ~ 0.6 c/d in the power spectrum is roughly consistent with the expected period of nodal precession of the disk. There is a puzzling peak at 0.21(3) c/d corresponding to the ~ 4.3 d sine wave seen in the raw light curve. We suspect it to be a beat frequency between the frequencies of apsidal and nodal precessions of the disk. Quasi-periodic cycles with amplitudes 0.15–0.6 mag can be seen in the light curve. The mechanism underlying this modulation is not clear.

Erupting Dwarf Novae in the Large Magellanic Cloud

We report the first likely detections of erupting dwarf novae (DNe) in an external galaxy: the Large Magellanic Cloud. Six candidates were isolated from approximately a million stars observed every second night over 11 nights with the Cerro Tololo Inter-American Observatory 8K × 8K Mosaic2 CCD imager. Artificial dwarf nova and completeness tests suggest that we are seeing only the brightest of the LMC DNe, probably SS Cygni–like cataclysmic variables (CVs), but possibly SU UMa type cataclysmics undergoing superoutbursts. We derive crude but useful limits on the LMC DN surface density and on the number of DNe in the LMC. Many thousands of CVs in the Magellanic Clouds can be discovered and characterized with 8 m class telescopes.

An atlas of line profile studies for SU UMa type cataclysmic variables

We present Hα line-profile analyses for the seven SU UMa type dwarf novae AK Cnc, WX Cet, AQ Eri, VW Hyi, RZ Leo, TU Men, and HS Vir. All data sets are treated in the same manner, applying a sequence of techniques for each system. The basic ingredients of this sequence are the diagnostic diagram to determine the zero point of the orbital phase, and Doppler tomography to visualise the emission distribution. We furthermore introduce a new qualitative way of to evaluate the Doppler fit, by comparing the line profile of the reconstructed with the original spectrum in the form of the plot. We present the results of the analysis in the compact form of an atlas, allowing a direct comparison of the emission distribution in our targets. Although most of the data sets were not taken with the intention of a line-profile analysis, we obtain significant results and are able to indicate the type of the additional emission in these systems. Our objects should have in principle very similar physical properties, i.e. they cover only a small range in orbital periods, mass ratios, and mass-transfer rates. Nevertheless, we find a large variety of phenomena both with respect to the individual systems and also within individual data sets of the same object. This includes “canonical” additional emission components from the secondary star and the bright spot, but also emission from the leading side of the accretion disc.

Helical Tomography of an Accretion Disk by Superhump Light Curves of the 2001 Outburst of WZ Sagittae

Abstract A new method for analyzing complex superhump light curves for the 2001 outburst of WZ Sagittae is proposed. The complexity arises because intrinsically time-varying and non-axisymmetric distributions of superhump light sources are coupled with the aspect effects around the binary orbital phase because of its high orbital inclination. The new method can disentangle these complexities by separating the non-axisymmetric spatial distribution in the disk from the time variation with the superhump period. It may be called a helical tomography of an accretion disk because it can reconstruct a series of disk images (i.e., disk’s azimuthal structures) at different superhump phases. The power spectral data of superhump light curves of the 2001 outburst of WZ Sge by Patterson et al. (2002, PASP, 114, 721) are now interpreted under a new light based on the concept of helical tomography, and the azimuthal wave numbers of various frequency modes are identified. In particular, a frequency component, $n\omega_0-\Omega$, where $\omega_0$ and $\Omega$ are the orbital frequency and a low frequency of the apsidal precession of the eccentric disk, is understood as an $(n-1)$-armed traveling wave in the disk. A vigorous excitation of a wave component of $\cos(2\Theta-3\omega_0 t)$ in the first week of the superhump era of WZ Sge, where $\Theta$ is the azimuthal angle, supports Lubow’s (1991, AAA 54.064.175) theory of non-linear wave coupling of the eccentric Lindblad resonance for the superhump phenomenon. This method can in principle be applied to other SU UMa stars with high orbital inclination if light curves are fully covered over the beat cycle.

FS Aurigae: A New Class of Cataclysmic Variables or the Missing Link between Intermediate Polars and SW Sextantis Objects?

ABSTRACTFS Aur is a known dwarf nova with an orbital period of about 85.7 minutes. It has been assumed to be a member of the SU UMa subclass of cataclysmic variables, but previous searches for superhumps and superoutbursts have been unsuccessful. We conducted a series of photometric and spectroscopic observations of FS Aur during quiescence. We confirmed its short orbital period from radial velocity measurements. However, the long‐term photometry revealed an unexpected result: the system also shows a distinct 0.24 mag modulation in the BVR photometric bands with a period of 205.5 minutes, which is 2.4 times longer than the orbital period. We discuss various possible causes for such a peculiar behavior.

In-the-gap SU UMa-type dwarf nova, Var73 Dra with a supercycle of about 60 days

An intensive photometric-observation campaign of the recently discovered SU UMa-type dwarf nova, Var73 Dra was conducted from 2002 August to 2003 February. We caught three superoutbursts in 2002 October, December and 2003 February. The recurrence cycle of the superoutburst (supercycle) is indicated to be60 d, the shortest among the values known so far in SU UMa stars and close to those of ER UMa stars. The superhump periods measured during the first two superoutbursts were 0.104885(93) d, and 0.10623(16) d, respectively. A 0.10424(3)-d periodicity was detected in quiescence. The change rate of the superhump period during the second superoutburst was 1:7 10 3 , which is an order of magnitude larger than the largest value ever known. Outburst activity has changed from a phase of frequent normal outbursts and infrequent superoutbursts in 2001 to a phase of infrequent normal outbursts and frequent superoutbursts in 2002. Our observations are negative to an idea that this star is an related object to ER UMa stars in terms of the duty cycle of the superoutburst and the recurrence cycle of the normal outburst. However, to trace the superhump evolution throughout a superoutburst, and from quiescence more eectively, may give a fruitful result on this matter.

The SU UMa Nature of the Dwarf Nova, DM Lyrae

Abstract We carried out time-resolved $V$-band photometry of DM Lyr during long outbursts in 1996 July and in 1996 February–March at Ouda Station, Kyoto University, and at Osaka Kyoiku University. Since superhumps were clearly detected in the light curves, DM Lyr was first identified as being an SU UMa-type dwarf nova. The superhump period is 0.0673(2) d, and the superhump excess is 2.8(3)%. The duration of the superoutburst, the outburst amplitude, the decline rate in the plateau phase, and the superhump excess were typical values for a usual SU UMa star. According to visual and CCD observations reported to VSNET, this star has experienced a dramatic change of the outburst pattern from a superoutburst phase to a normal outburst phase. There may exist mechanisms to decrease the number of normal outbursts between two successive superoutbursts and to elongate the recurrence cycle of the superoutburst.

X-RAY SPECTRAL ANALYSIS OF SU UMa TYPE DWARF NOVAE OBSERVED WITH ROSAT

X-ray spectral parameters were determined for eight SU UMa type Dwarf Novae observed with the ROSAT PSPC. The raw data were fitted with various spectral models and the best fit spectral models are found to be that of Raymond–Smith and Thermal Bremsstrahlung. The best fit temperatures were estimated to be between kT ~ 1.1-1.8 keV while the Column Densities were found to be between NH ~ 2.4×1020-4.1×1020 cm -2. The estimated 0.1-2.4 keV fluxes were in the range of log FX=-13 to -11 ergs cm-2 s-1. FX/F UV and FX/F opt rates were calculated to be between ~0.09 and ~0.37. This shows that most of the energy is radiated in the Optical and Ultraviolet band from the accretion disk in the quiescent state. Many of the SU UMa type Dwarf Novae show an Ultraviolet lag in their outburst spectrum, the Coronal Siphon Flow Model of Meyer and Meyer-Hofmeister may explain this phenomenon. This model proposes a corona at the boundary layer of a system when it is a quiescent state and suggests that some parts of the X-rays come from the corona. For these reasons, the equations of this model were applied to the results of the spectral analysis. Using this model, the mass accretion rates, the mass evaporation rates, and the radii of the coronas were calculated to be ~10-12.3-10-11.3 M⊙ yr -1, ~10-6.5-10-5.5 g cm -2 s -1 and ~109.1-109.9 cm , respectively. The pressures in the coronas were less than ~1200 g cm -2 s -1 for (z) up to ~10×109 cm . The obtained values suggest that the Corona model can indeed operate in SU UMa type Dwarf Novae.

Is evidence for enhanced mass transfer during dwarf-nova outbursts well substantiated?

Outburst mechanisms of SU UMa-type dwarf novae are discussed. Two competing models were proposed; a pure disk instability model called the thermal-tidal instability model (TTI model) and the enhanced mass transfer model (EMT model). Observational evidence for enhanced mass transfer from the secondary star during outbursts is critically examined. It is demonstrated that most evidence for enhanced mass transfer is not well substantiated. Patterson et al. (2002) have recently claimed to have found evidence for enhanced mass transfer during the 2001 outburst of WZ Sge. We show that their evidence is probably due to a misinterpretation of their observed light curves. Our theoretical analysis also shows that irradiation during outburst should not affect the mass transfer rate. A refinement of the TTI model is proposed that can explain why superhumps appear a few days after the superoutburst maximum in some SU UMa stars. We present our own interpretation of the overall development of the 2001 outburst of WZ Sge based on the TTI model that does not require the assumption of an unproved enhanced mass transfer.

High-speed photometry of faint cataclysmic variables -- III.V842 Cen, BY Cir, DD Cir, TV Crv, V655 CrA, CP Cru, V794 Oph, V992 Sco, EU Sct and V373 Sct

We present further results from a high-speed photometric survey of faint cataclysmic variables. We find that V842 Cen (Nova Cen 1986 No. 2) is highly active, but with no evident orbital modulation. BY Cir (Nova Cir 1995)is an eclipsing system with an orbital period (P o r b ) of 6.76 h. TV Crv, an SU UMa type dwarf nova, is found to have P o r b = 1.509 h from photometry at quiescence. DD Cir (Nova Cir 1999) is an eclipsing system with P o r b = 2.339 h, a possible secondary eclipse, and a ∼670 s photometric modulation. V655 CrA is highly active but shows no orbital modulation. The identification of V794 Oph is probably incorrect as we find no photometric variability. CP Cru (Nova Cru 1996) is an eclipsing system with P o r b = 22.7 h. V992 Sco has P o r b = 3.686 h from its periodic modulation. The supposed identification of EU Sct is probably incorrect. Finally, during one run V373 Sct (Nova Sct 1975) had a 258.3-s coherent periodicity, making it a candidate DQ Herculis star.