SU UMa-type Research Articles

Unusual Phase Reversal of Superhumps in ER Ursae Majoris

Abstract We studied the evolution of superhumps in the peculiar SU UMa-type dwarf nova, ER UMa. Contrary to the canonical picture of the SU UMa-type superhump phenomena, the superhumps of ER UMa show an unexpected phase reversal during the very early stage ($\sim 5 \,\mathrm{}$d after the superoutburst maximum). We interpret that a sudden switch to so-called late superhumps occurs during the very early stage of a superoutburst. What had been believed to be (ordinary) superhumps during the superoutburst plateau of ER UMa were actually late superhumps. The implication of this discovery is briefly discussed.

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.

Interpretation of light curves of the cataclysmic variable OY Car in a model with shockless interaction between a gaseous stream and the disk

To determine the parameters of the accretion disk and shock-wave region responsible for the formation of the orbital peak in the light curve of the binary system OY Car (an SU UMa-type variable), we have analyzed its U BV R and JK light curves using two gas-dynamical models with different regions of shock interaction: one with a hot line along the stream from the Lagrange point L1 and one with a hot spot on the accretion disk. The hot-line model can better describe the quiescent state of the system: the maximum X2 for the optical light curves does not exceed 207, whereas the minimum residual for the hot-spot model is X2>290. The shape of the eclipse is almost identical in both models; the main differences are in interpreting out-of-eclipse portions of the light curves, whose shape can varyin the transition from one orbital cycle to another. The hot-spot model is not able to describe variations of the system’s brightness at orbital phases ϕ∼0.1–0.6. The rather complex behavior of the observed flux in this phase interval can be explained in the hot-line model as being due to variations of the temperature and size of the system. Based on the analysis of a sequence of 20 B curves of OY Car, we conclude that the flux variations in the primary minimum are due to variations of the luminosity of the accretion disk, whereas the flux variability in the vicinity of the orbital peak is due to the combined effect of the radiation of the disk and hot line. The JK light curves of OY Car in the quiescent state and during a small flare also indicate preference for the hot-line model, since the primaryminimum and the flux near quadratures calculated using the hot-spot model are not consistent with the observations.

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.

Dramatic Changes in the Outburst Properties in V503 Cygni

Abstract We examined the VSNET light curve of the unusual SU UMa-type dwarf nova V503 Cyg, which is known to show a short (89 d) supercycle length and exceptionally small (a few) normal outbursts within a supercycle. In 1999–2000, V503 Cyg displayed frequent normal outbursts with typical recurrence times of 7–9 d. The behavior during this period is characteristic to an usual SU UMa-type dwarf nova with a short supercycle length. On the other hand, V503 Cyg showed very infrequent normal outbursts in 2001–2002. Some of the superoutbursts during this period were observed to be shorter than usual. The remarkable alternations of the outbursting states in V503 Cyg support the presence of mechanisms of suppressing normal outbursts and prematurely quenching superoutbursts, which have been proposed to explain some unusual SU UMa-type outbursts. The observed temporal variability of the suppressing/quenching mechanisms in the same object suggests that these mechanisms are not primarily governed by a fixed system parameter, but more reflect the state changes in the accretion disk.

Photometric study of new southern SU UMa-type dwarf novae and candidates: V877 Ara, KK Tel and PU CMa

We photometrically observed three dwarf novae V877 Ara, KK Tel and PU CMa. We discovered undisputed presence of superhumps in V877 Ara and KK Tel, with mean periods of 0.08411(2) d and 0.08808(3) d, respectively. Both V877 Ara and KK Tel are confirmed to belong to long-period SU UMa-type dwarf novae. In V877 Ara, we found a large decrease of the superhump period (dot(P)/P = -14.5 +/- 2.1 x 10^(-5)). There is evidence that the period of KK Tel decreased at a similar or a more exceptional rate. Coupled with the past studies of superhump period changes, these findings suggest that a previously neglected diversity of phenomena is present in long-period SU UMa-type dwarf novae. The present discovery of a diversity in long-period SU UMa-type systems would become an additional step toward a full understanding the dwarf nova phenomenon. PU CMa is shown to be an excellent candidate for an SU UMa-type dwarf nova. We examined the outburst properties of these dwarf novae, and derived characteristic outburst recurrence times. Combined with the recently published measurement of the orbital period of PU CMa, we propose that PU CMa is the first object filling the gap between the extreme WZ Sge-type and ER UMa-type stars.

Discovery of a Dwarf Nova Breaking the Standard Sequence of Compact Binary Evolution

Abstract We revealed that the dwarf nova 1RXS J232953.9+062814 is an SU UMa-type system with a superhump period of $ 66.774\pm 0.010 \,\mathrm{min}$. The short period strongly indicates that the orbital period of this object is below the period minimum of cataclysmic variables. The superhump period is $ 4.04\pm 0.02$% longer than the photometric period during quiescence ($ 64.184\pm 0.003 \,\mathrm{min}$), which is probably associated with the orbital period. Although the standard evolutionary scenario of cataclysmic variables predicts lower mass-transfer rates in systems with shorter orbital periods, we found firm evidence of a relatively high mass-transfer rate from its large proper motion and bright apparent magnitude. Its proximity indicates that we have overlooked a number of objects in this new class. With the analogous system of V485 Cen, these objects establish the first subpopulation in hydrogen-rich cataclysmic variables below the period minimum.

The detection of the donor star in IY UMa

We present the results of a search for the donor star in the high-inclination SU UMa type cataclysmic variable IY UMa. We detect absorption features in the near infrared consistent with an M type dwarf donor star. Using the skew mapping technique to exploit the velocity information provided by the 8183–8194 Å Na i absorption doublet, we locate the absorption at the expected donor velocity of IY UMa.

Photometry of the 1991 Superoutburst of EF Pegasi: Super-Quasi-Periodic Oscillations with Rapidly Decaying Periods

Abstract We observed the 1991 October outburst of EF Peg. Prominent superhumps with a period of 0.08705(1)d were observed, qualifying EF Peg as being a long-period SU UMa-type dwarf nova. The superhump period showed a monotonic decrease during the superoutburst, which makes a contrast to the virtually zero period change observed during the 1997 superoutburst of the same object. Large-amplitude and highly coherent quasi-periodic oscillations (super–QPOs) were observed on October 18, when superhumps were still growing in amplitude. Most strikingly, the QPOs showed a rapid decrease in the period from 18 min to 6.8 min within the 3.2-hr observing run. Such a rapid change in the period has not been observed in any class of QPOs in cataclysmic variables. We propose a hypothesis that the rapid decrease of the QPO period reflects a rapid removal of the angular momentum from an orbiting blob in the accretion disk, via viscosity in a turbulent disk. A brief comparison is given with the QPOs in X-ray binaries, some of which are known to show a similar rapid decrease in the periods.

IR Com: Deeply Eclipsing Dwarf Nova Below the Period Gap —A Twin of HT Cas?

Abstract We observed an X-ray selected, deeply eclipsing cataclysmic variable IR Com ($=\mathrm{S}~10932$). We detected an outburst occurring on 1996 January 1. The light curve of the outburst closely resembled that of a normal outburst of an SU UMa-type dwarf nova, rather than that of an intermediate polar. Time-resolved photometry during the outburst showed that eclipses became systematically deeper and narrower as the outburst faded. Full-orbit light curves in quiescence showed little evidence of orbital humps or asymmetry of the eclipses. In addition to the presence of high–low transitions in quiescence, the overall behavior of the outbursts and the characteristics of the eclipse profiles suggest that IR Com can be best understood as a twin of HT Cas, a famous eclipsing SU UMa-type dwarf nova with a number of peculiarities.

Photometrical Study of SU UMa-type Binary RZ Lmi During the Superoutburst in 2001

Photometrical Study of SU UMa-type Binary RZ Lmi During the Superoutburst in 2001

1998 Superoutburst of the Large-Amplitude SU UMa-Type Dwarf Nova WX Ceti

Abstract We observed the 1998 November superoutburst of WX Cet, a dwarf nova originally proposed as a WZ Sge-like system. The observation established that WX Cet is an SU UMa-type dwarf nova with a mean superhump period of 0.05949(1)d, which is 2.1% longer than the reported orbital period. The lack of early superhumps at the earliest stage of the superoutburst, the rapid development of usual superhumps, and the possible rapid decay of late superhumps seem to support that WX Cet is a fairly normal large-amplitude SU UMa-type dwarf nova, rather than a WZ Sge-type dwarf nova with a number of peculiarities. However, a period increase of superhumps at a rate $\dot{P}/P = +8.5\pm 1.0 \times 10^{-5}$ was observed, which is one of the largest $\dot{P}/P$ ever observed in SU UMa-type dwarf novae. A linear decline of light, with a rate of $0.10 \,\mathrm{mag} \,\mathrm{d}^{-1}$, was observed in the post-superoutburst stage. This may be an exemplification of the decay of the viscosity in the accretion disk after the termination of a superoutburst, mechanism of which is proposed to explain a variety of post-superoutburst phenomena in some SU UMa-type dwarf novae.

A ROSAT WFC observation of SW UMa: the EUV behaviour of dwarfnovae in superoutburst explained

During re-processing and analysis of the entire ROSAT Wide Field Camera (WFC) pointed observations database, we discovered a serendipitous, off-axis detection of the cataclysmic variable SW UMa at the onset of its 1997 October superoutburst. Although long outbursts in this SU UMa-type system are known to occur every ~450 days, none had ever been previously observed in the extreme ultra-violet (EUV) by ROSAT. The WFC observations began just ~13 hr after the optical rise was detected. With a peak count rate of 4.5 count/s in the S1 filter, SW UMa was temporarily the third brightest object in the sky in this waveband. Over the next ~19 hr the measured EUV flux dropped to <2 count/s while the optical brightness remained essentially static at V~11. Similar behaviour has also been recently reported in the EUV light curve of the related SU UMa-type binary OY Car during superoutburst (Mauche and Raymond 2000). In contrast, U Gem-type dwarf novae show no such early EUV dip during normal outbursts. Therefore, this feature may be common in superoutbursts of SU UMa-like systems. We expand on ideas first put forward by Osaki (1994, 1995) and Mauche and Raymond (2000) and offer an explanation for this behaviour by examining the interplay between the thermal and tidal instabilities which affect the accretion disks in these systems.

Photometric Observation of a New in-the-Gap SU UMa-Type Dwarf Nova V 725 Aquilae during the 1999 Superoutburst

Abstract We report our photometric observation of the 1999 September outburst of a dwarf nova, V725 Aql. Our detection of superhumps established that it belongs to SUUMa stars. By performing a period analysis of the superhumps, we revealed a long superhump period of $0.09909 \pm 0.00015\,\mathrm{d}$, which implies that its orbital period is well over 2 hr. The outburst light curve is typical for a superoutburst, though, after the superoutburst, the object experienced a re-brightening which can be seen in some SUUMa stars with short orbital periods. The 1999 superoutburst occurred about 1650 d after the last possible superoutburst in 1995. This indicates the supercycle of V725 Aql to be long, over 1000 d. This long supercycle implies a low mass-transfer rate of this star.

On Echo Outbursts and ER UMa Supercycles in SU UMa–Type Cataclysmic Variables

ABSTRACTI present a variation on Osaki’s tidal thermal instability model for SU UMa behavior. I suggest that in systems with the lowest mass ratios, the angular momentum dissipation in an eccentric disk is unable to sustain the disk on the hot side of the thermal instability. This decoupling of the tidal and thermal instabilities in systems with q≲0.07 allows a better explanation of the “echo” outbursts of EG Cnc and the short supercycles of RZ LMi and DI UMa. The idea might also apply to the soft X‐ray transients.

Extreme Ultraviolet ExplorerObservations of OY Carinae in Superoutburst

The Extreme Ultraviolet Explorer (EUVE) satellite was used for three days beginning on 1997 March 26.96 UT to obtain photometric and spectroscopic observations of the eclipsing SU UMa-type dwarf nova OY Carinae in superoutburst. Because of the longer time on source (143 ks), the larger number of eclipses observed (17), and the higher count rate in the detector (0.5-2.2 counts s -1), we are able to significantly strengthen previous reports that there is little or no eclipse by the secondary of the EUV emission region of OY Car in superoutburst. The mean EUVE spectrum extends from 70 to 190 A and contains broad (FWHM ≈ 1 A) emission lines of N V, O V-O VI, Ne V-Ne VII, Mg IV-Mg VI, Fe VI-Fe VIII, and possibly Fe XXIII. Good fits of the observed spectrum are obtained with a model (similar to that of Seyfert 2 galaxies) wherein radiation from the boundary layer and accretion disk is scattered into the line of sight by the system's photoionized accretion disk wind. It is possible to trade off continuum luminosity for wind optical depth, but reasonable models have a boundary layer temperature Tbl ≈ 90-130 kK and a boundary layer and accretion disk luminosity Lbl = Ldisk 4 × 1034 ergs s-1 ≈ 10 L☉ corresponding to a mass accretion rate a 10-8 M☉ yr-1; an absorbing column density NH ≈ 1.6-3.5 × 1019 cm-2; and a wind mass-loss rate w 10-10 M☉ yr-1 ≈ 0.01a. Because radiation pressure alone falls an order of magnitude short of driving such a wind, magnetic forces must also play a role in driving the wind of OY Car in superoutburst.

Outburst Characteristics in the Dwarf Nova SU Ursae Majoris

ABSTRACTThe 1973–1999 V‐band light curve from AAVSO visual estimates and RoboScope CCD exposures is examined to study the systematic properties of normal outbursts and superoutbursts in this prototype of the SU UMa–type dwarf novae. A number of the outbursts’ correlations previously reported in VW Hyi and in SU UMa are not present in these data. Several lines of evidence suggest that, during this 26 year interval, variations in the mass‐transfer rate served to obscure some of the expected correlations. The M ˙ variation apparently caused a nearly complete cessation of outbursting in 1980–1983. We do find a correlation of the time since the last superoutburst with the mean brightness in the interval since the last superoutburst, supporting the notion that superoutbursts are a disk‐initiated phenomenon that occurs when the gas remaining in the disk after each normal outburst has accumulated to some critical value.

Discovery of a New Deeply Eclipsing SU UMa-Type Dwarf Nova, IY UMa (= TmzV 85)

We discovered a new deeply eclipsing SU UMa-type dwarf nova, IY UMa, which experienced a superoutburst in 2000 January. Our monitoring revealed two distinct outbursts, which suggest a superoutburst interval of ∼ 800 d, or its half, and an outburst amplitude of 5.4 mag. From time-series photometry during the superoutburst, we determined a superhump and orbital period of 0.07588 d and 0.0739132 d, respectively.

The 1996 Superoutburst of the New SU UMa–Type Dwarf Nova HS Virginis

ABSTRACTWe report long‐term and time‐resolved photometric observations of the dwarf nova HS Vir (=PG 1341−079). Time‐resolved photometry during the bright outburst in 1996 March revealed superhumps with a period of 0.08059 ± 0.00003 days, which first firmly confirmed HS Vir as a genuine member of SU UMa–type dwarf novae. HS Vir is rather unusual in the smallness (2.7–3.5 mag for brightest superoutbursts) of outburst amplitudes. The marked diversity inferred from the comparison with SX LMi may reflect different mechanisms responsible for the smallness of outburst amplitudes in these dwarf novae.

Two New SU UMa-Type Dwarf Novae with Superhump Periods of Over Two Hours: GX Cassiopeiae and V419 Lyrae

We succeeded in detecting superhumps in GX Cas and V419 Lyr during the 1994 long outburst and the 1995 outburst, respectively, and identified those stars as SU UMa stars. A period analysis gives 0.09297 (± 0.00005) d as the best-estimated superhump period of GX Cas, which is the third-longest one following TU Men and NY Ser. This superhump period suggests GX Cas to be located at the upper edge of the period gap of dwarf novae. The superhump period of V419 Lyr is estimated to be close to GX Cas, though the short coverage of superhumps prevented us from determining an accurate value. The recurrence cycle of the superoutburst (supercycle) and the normal outburst of GX Cas is estimated to be 386 d and 50–70 d, respectively, based on the VSNET data archive. This supercycle predicts that the next superoutburst in GX Cas will occur around 1998 November 10. Detailed observations of these stars may give important information concerning the study of dwarf-nova evolution as well as the criterion of the tidal instability.