SU UMa-type Dwarf Novae Research Articles

The 2006 November Outburst of EG Aquarii: the SU UMa Nature Revealed

Abstract We report on time-resolved CCD photometry of the cataclysmic variable EG Aquarii during a 2006 November outburst. During the outburst, superhumps were unambiguously detected with a mean period of 0.078828(6) d, allowing for the first time to classify the object as an SU UMa-type dwarf nova. It also turned out that the outburst contained a precursor. At the end of the precursor, immature profiles of humps were observed. By a phase analysis of these humps, we interpreted the features as being superhumps. This is the second example that the superhumps were shown during a precursor. Near the maximum stage of the outburst, we discovered an abrupt shift of the superhump period by ${\sim\;}$0.002 d. After the supermaximum, the superhump period decreased at a rate of $\dot{P}/P =$$-$8.2${\;\times\;}$10$^{-5}$, which is typical for SU UMa-type dwarf novae. Although the outburst light curve was characteristic of SU UMa-type dwarf novae, long-term monitoring of the variable shows no outbursts over the past decade. We also comment on the basic properties of long period and inactive SU UMa-type dwarf novae.

Late Superhumps in WZ Sge-Type Dwarf Novae

Abstract We report on the detection of very stable modulations with periods unexpectedly ($\sim\;$0.5%) longer than the superhump periods during the slowly fading stage of WZ Sge-type superoutbursts in three systems: GW Lib, V455 And, and WZ Sge. The periods are naturally explained by assuming that these modulations are superhumps arising from matter near the tidal truncation radius. This finding provides additional support for the hypothetical idea concerning expansion of the accretion disk well beyond the 3:1 orbital resonance in some low mass-ratio systems. Combined with the effect of a 2:1 resonance, we explain the origin of positive period derivatives in certain short-period SU UMa-type dwarf novae.

Discovery of a WZ Sge-Type Dwarf Nova, SDSS J102146.44+234926.3: Unprecedented Infrared Activity during a Rebrightening Phase

Abstract Several SU UMa-type dwarf novae and WZ Sge-type stars tend to exhibit rebrightenings after superoutbursts. The rebrightening phenomenon is problematic for the disk instability theory of dwarf novae, since it requires a large amount of remnant matter in the disk, even after superoutbursts. Here, we report on our optical and infrared observations during the first-ever outburst of a new dwarf nova, SDSS J102146.44+234926.3. During the outburst, we detected superhumps with a period of 0.056281$\pm$0.000015 d, which is typical for superhump periods in WZ Sge stars. In conjunction with the appearance of a long-lived rebrightening, we concluded that the object is a new member of WZ Sge stars. Our observations, furthermore, revealed infrared behaviors for the first time in the rebrightening phase of WZ Sge stars. We discovered prominent infrared superhumps. We calculated the color temperature of the infrared superhump source to be 4600–6400 K. These temperatures are too low to be explained by a fully ionized disk appearing during dwarf-nova outbursts. We also found a $K_{s}$-band excess over the hot disk component. These unprecedented infrared activities provide evidence for the presence of mass reservoir at the outermost part of the accretion disk. We propose that a moderately high mass-accretion rate at this infrared active region leads to the long-lived rebrightening observed in SDSS J102146.44+234926.3.

Photometric Studies of New Southern SU UMa-Type Dwarf Novae, FL Trianguli Australis and CTCV J0549–4921

We report on the time-resolved optical CCD photometry of newly discovered SU UMa-type dwarf novae, FL TrA and CTCVJ0549$-$4921. During a 2006 August outburst, we detected superhumps with a period of 0.59897(11) days for FL TrA, clarifying the SU UMa nature of the system. On the first night of our observations of FL TrA, the object showed no superhumps. This implies that it takes a few days for the full development of superhumps. The superhump period variation diagram of FL TrA was similar to that observed in some WZ Sge stars and short-period SU UMa-type stars. This indicates that the system is closely related to WZ Sge stars and SU UMa stars having short orbital periods. For CTCV J0549$-$4921, the candidates of the mean superhump period are 0.083249(10) days and 0.084257(8) days, respectively. Due to a lack of observations, we cannot determine the true superhump period, but the latter period is favorable. Using the ASAS-3 archive, it turned out that the system showed only four outbursts over the past 6 years. The outburst amplitude of CTCV J0549$-$4921 was relatively small, with about 4.5 mag. One possibility is that mass evaporation may play a role during quiescence.

Long-Term Monitoring of the Short Period SU UMa-Type Dwarf Nova, V844 Herculis

Abstract We report on the time-resolved CCD photometry of four outbursts of a short-period SU UMa-type dwarf nova, V844 Herculis. We successfully determined the mean superhump periods to be 0.05584(64) d, and 0.055883(3) d for a 2002 May superoutburst, and a 2006 April–May superoutburst, respectively. During the 2002 October observations, we confirmed that the outburst was a normal outburst, which was the first recorded normal outburst in V844 Her. We also examined superhump period changes during the 2002 May and 2006 April–May superoutbursts, both of which showed an increasing superhump period over the course of the plateau stage. In order to examine the long-term behavior of V844 Her, we analyzed archival data over the past ten years since the discovery of this binary. Although photometry is not satisfactory for some superoutbursts, we found that V844 Her showed no precursors or rebrightenings. Based on the long-term light curve, we further confirmed that V844 Her has shown almost no normal outbursts despite the fact that the supercycle of the system is estimated to be about 300 d. In order to explain the long-term light curves of V844 Her, evaporation in the accretion disk may play a role in the avoidance of several normal outbursts, which does not contradict with the relatively a large X-ray luminosity of V844 Her.

The Incidence of Dwarf Novae in Large Area Transient Searches

Understanding and quantifying the contribution of known classes of transient and variable sources is an important lesson to be learned from the manifold of precursor programs of the near-future large synoptic sky survey programs such as SkyMapper, Pan-STARRS, and LSST. With this goal in mind, we undertook photometric and spectroscopic follow-up observations of four recently reported unidentified transients. For two sources, WFI J132813.7-214237 and WFI J161953.3+031909, we show that unfortunate coincidences led to their previous designation as transients. While the former is now interpreted as the spatial coincidence of a solar system object with a faint background star, the latter is merely a cataclysmic variable unfortunately caught in and out of eclipse. The third candidate, ROTSE3 J160213.1-021311.7, is identified as an SU UMa type dwarf nova with quiescent brightness of R ~ 22.7 and an outburst amplitude of ~5 mag. The fourth event, SDSS-SN 15207, similarly shows evidence for a dwarf nova origin. Our main conclusion is that cataclysmic variables in their various avatars will contribute moderately to the population of transient objects.

Photometric Observation and Numerical Simulation of Early Superhumps in BC Ursae Majoris during the 2003 Superoutburst

Abstract We carried out time-resolved photometric observations of an SU UMa-type dwarf nova, BC UMa, during its superoutburst in 2003 February. The period of early superhumps is 0.06258(13)d and the amplitude is 0.04 mag. After the early superhump phase, common superhumps with an amplitude of 0.3 mag and a period of 0.064466(16) d developed. The change rate of the superhump periods ($\dot{P}_{\rm SH}/P_{\rm SH}$) ($P_{\rm SH}$: superhump period) was positive throughout the superoutburst. The superhump period excess ($\varepsilon \equiv P_{\rm SH}P_{\rm orb}^{-1}-1$) is 3% and we derived a mass ratio of 0.13. We have modeled early superhump light curves including irradiation effects of the accretion disk and secondary star. The observed early superhumps can be reproduced when two-armed spirals appear on the accretion disk. The long-term data taken from AAVSO, VSOLJ, and VSNET indicate that BC UMa shows normal outbursts and two types of superoutbursts (short-faint and long-bright). BC UMa is the first example to show such two types of superoutbursts. The supercycle of BC UMa is between 600 d and 1000d. This suggests that BC UMa is an intermediate type of dwarf novae between WZ Sge and SU UMa.

WX Ceti: a closer look at its behaviour in quiescence and outburst

Context. WX Cet is a dwarf nova with rare outbursts of large amplitude. Aims. We compile the available data of WX Cet, compare the results with other SU UMa stars, and discuss our findings in the context of current theories of superhumps and superoutbursts to progress with our understanding of SU UMa stars. Methods. We analyse all recorded outbursts of WX Cet, based on the AAVSO archive and other published sources, and present new CCD photometry during two recent superoutbursts, including the determination of the corresponding periodicities. We perform numerical disc instability model calculations and compare its predictions with the observations. Results. WX Cet is a SU UMa type dwarf nova with a superoutburst cycle of 880 days on average, and short eruptions every 200 days. It seems that the outburst cycle length increased by nearly a factor of 2 during the past 70 years. According to our numerical simulations, this can be explained in the context of the disc instability model by assuming enhanced mass transfer during outburst and a decreasing mean mass transfer rate during the last decades. Using the data available, we refine the orbital period of WX Cet to 0.0582610 ± 0.0000002 days and interpret the orbital hump found in quiescence as emission from the hot spot. During two recent superoutbursts in July 2001 and December 2004 we observed superhumps, with a rather large positive period derivative of u Ps/Ps = 1.6 × 10 −4 , present only during the first 9 days of a superoutburst. Afterwards and during decline from the “plateau” phase, a constant superhump period of about 0.05922 days was observed. Late superhumps are present for at least 12 days after the decline from the “plateau”, with a period of 0.05927 days. We find this phenomenology difficult to interpret in the context of the standard explanation for superhumps, i.e. the thermal tidal instability model. Conclusions. We interpret the long-term light curve of WX Cet as the result of a significantly decreasing mean mass transfer rate. Highlighting the complexity of the observed superhump light curves, we emphasise the importance of WX Cet for a proper understanding of the SU UMa star outburst physics and the evolution of ultra-short period cataclysmic variables.

Discovery of a New Dwarf Nova, TSS J022216.4+412259.9: WZ Sge-Type Dwarf Nova Breaking the Shortest Superhump Period Record

Abstract We report on the time-resolved CCD photometry of a newly discovered variable star, TSS J022216.4$+$412259.9 during an outburst in a 2005 November–December brightening. The obtained light curves unambiguously show 0.2–0.3 mag modulations, which we confirmed to be the superhump observed among SU UMa-type dwarf novae. We also performed a period search for data obtained during the outburst plateau phase, and revealed the existence of the two periodicities: 0.054868(98)d for the first two nights and 0.055544(26)d for the following plateau phase. This bi-periodicity is hardly observed in the usual SU UMa-type dwarf novae, but is characteristic of WZ Sge-type stars. We undoubtedly detected a rebrightening in the post-outburst stage, which is typical of short-period SU UMa-type dwarf novae, including WZ Sge-type stars. These observations suggest that TSS J022216.4$+$412259.9 may be a new WZ Sge star breaking the shortest superhump period so far of 0.05648 d for V592 Her among this class with a known superhump period.

J-, H-, and K-band Spectra of Three SU UMa-Type Dwarf Novae

AbstractWe present the results of J-, H-, and K-band spectroscopy on three SU UMa-type dwarf novae with orbital periods of 1.33 – 1.37 hr. We performed the SED fitting for the obtained spectra by assuming a power-law distribution for the accretion disk and using template spectra of late-type dwarfs for secondary star. ASAS 002511+1217.2 and EQ J183926+260409 are WZ Sge-type or WX Cet-type dwarf novae. We found strong water absorption features in their spectra, which are characteristic in late M- or L-type dwarfs. The SED fitting suggests that their secondary contributions to the overall SED are less than one third. We identified the secondary stars as M9 and L1 type dwarfs, which are rather less massive but still normal stars. The spectrum of SDSS J013701−091235 is dominated by the secondary component. Spectral features of this object are similar to those of an early M-type dwarf in spite of its short orbital period. The spectrum of SDSS 0137 strongly suggests that the evolutionary path of this object is different from that of ordinary CVs, and this object is a candidate of EI Psc-like systems.

The 2005 July Superoutburst of the Dwarf Nova 2QZ J021927.9−304545: the SU UMa Nature Confirmed

Abstract We report on the time-resolved photometry of the 2005 July superoutburst of a dwarf nova, 2QZ J021927.9$-$304545. The resultant light curves showed conspicuous superhumps with a period of 0.081113(19) days, confirming the SU UMa nature of the object. Although we missed the maximum phase of the outburst, the amplitude of the superoutburst well exceeded 5 mag. This value is slightly larger than that of typical SU UMa-type dwarf novae. The superhump period decreased as the time elapsed, as can be seen in most SU UMa-type dwarf novae. Based on the archive of ASAS-3, the recurrence time of a superoutburst of the variable turned out to be about 400 days. This value is typical of well-known SU UMa stars. The distance to this system was roughly estimated as $370\,(+20,-60) \,\mathrm{pc}$ using an empirical relation.

Superhumps Behavior during Normal Outbursts in ER Ursae Majoris: Spectroscopy and Photometry

Abstract We undertook a 4-day spectroscopic observation and conducted a 13-day photometric study of an SU UMa-type dwarf nova, ER Ursae Majoris. The mean $K$-amplitude for the emission lines is $54 \pm 8 \,\mathrm{km} \,\mathrm{s}^{-1}$ and the $\gamma$ velocity is $8 \pm 4 \,\mathrm{km} \,\mathrm{s}^{-1}$ based on our spectroscopic results. A phase shift of 0.22 was also obtained. Our photometric observation confirms superhumps in normal outbursts of ER UMa, and reconciles contradictory observational results obtained by different authors. We find that superhumps possibly develop near each normal outburst maximum and fade out before the next outburst maximum. If the observed humps were not late superhumps, but ordinary superhumps, much more theoretical work should be done to explain the new phenomena.

The 2003/2004 Superoutburst of SDSS J013701.06−091234.9

Abstract We report on time-resolved photometry of the superoutburst of an SU UMa-type dwarf nova, SDSS J013701.06$-$091234.9, in 2003 December–2004 January. The obtained light curves definitely show superhumps with a period of 0.056686(12)d, which is one of the shortest superhump periods among those of SU UMa-type dwarf novae ever observed. Considering quiescent photometric studies, we estimated the fractional superhump excess to be 0.024. Spectroscopic observations by Szkody et al. (2003, AJ, 126, 1499) provided evidence for TiO bands despite the short orbital period, implying that the system has a luminous secondary star. We have drawn a color-color diagram of SU UMa-type dwarf novae in quiescence using 2MASS archives, revealing that the location of this star in the color-color diagram deviates from the general trend. The distance to the system was roughly estimated to be $300 \pm 80 \,\mathrm{pc}$, using the empirical period–absolute magnitude relation and based on the proper motion.

A Spectroscopic Study of the SU UMa-type Dwarf Nova YZ Cnc during its 2002 Superoutburst

We report on time-resolved spectroscopic observations of the SU Ursae Majoris dwarf nova, YZ Cnc for over 11 h on two nights during its 2002 January superoutburst. The spectra on the first day only showed absorption-line profiles, while on the second day the lines showed ``W" profiles with blue and red troughs. The radial velocity curve of the absorption troughs and emission peaks of Hβ has an amplitude of 49±10 km s−1 and a phase offset of −0.07±0.04, which are very similar to those measured in quiescence. However, the γ velocity deviates strongly from the systemic velocity measured in quiescence, by some ±60 km s−1. Large shifts of ∼70 km s−1 in the orbital-averaged velocity and ∼0.09 in the phase are also found in our observations. All these features can be well explained by a precessing, eccentric disk.

The 2003 Superoutburst of an SU UMa-Type Dwarf Nova, GO Comae Berenicis

Abstract We photometrically observed a 2003 June superoutburst of GO Gom. The mean superhump period was 0.063059(13) d. The resultant data revealed that (1) the obtained light curve contained a precursor, (2) a plateau stage of the object lasted for 8 days, which is remarkably shorter than that of ordinary SU UMa-type dwarf novae, and (3) the amplitude of the superoutburst was less than 5 mag, which is unpredictably small when taking into account the fact that the supercycle of GO Com is about 2800 days. In order to explain these anomalies, a mass-elimination process from the accretion disk, such as evaporation, may play an important role.

Detection of the white dwarf and the secondary star in the new SU UMa dwarf nova HS 2219+1824

We report the discovery of a new, non-eclipsing SU UMa-type dwarf nova, HS 2219+1824. Photometry obtained in quiescence () reveals a double-humped light curve from which we derive an orbital period of 86.2 min. Additional photometry obtained during a superoutburst reaching clearly shows superhumps with a period of 89.05 min. The optical spectrum contains double-peaked Balmer and He I emission lines from the accretion disc as well as broad absorption troughs of , , and from the white dwarf primary star. Modelling of the optical spectrum implies a white dwarf temperature of , a distance of , and suggests that the spectral type of the donor star is later than M 5. Phase-resolved spectroscopy obtained during quiescence reveals a narrow emission line component which has a radial velocity amplitude and phase consistent with an origin on the secondary star, possibly on the irradiated hemisphere facing the white dwarf. This constitutes the first detection of line emission from the secondary star in a quiescent SU UMa star.

Dwarf Nova-Like Outburst of Short-Period Intermediate Polar HT Camelopardalis

Our time-series photometric observations of a short outburst of HT Cam in 2001 strongly suggest that disk instabilities occurred during the outburst.HT Cam is a cataclysmic variable identified as the optical counterpart of the hard X-ray source RX J0757.0+6306, discovered during the ROSAT All-Sky Survey. Tovmassian et al. (1998) suggested that this object is an intermediate polar with a shortest orbital period of 80.92min and a spin period of 8.52min. However, the existence of dwarf nova-like outbursts and the short orbital period allowed an alternative interpretation that it may be an SU UMa-type dwarf nova or WZ Sge-type stars (Tovmassian et al. 1998).

1RXS J232953.9+062814: A Possible Progenitor of AM CVn Stars

AbstractWe revealed that the hydrogen-rich cataclysmic variable lRXS J232953.9+062814 is an SU UMa-type dwarf nova with a superbump period of 66.774±0.010 min. A photometric orbital period is determined to be 64.184± 0.003 min, which is below the period minimum. Although the standard evolutionary scenario of cataclysmic variables predicts lower mass-transfer rates in systems with shorter orbital periods, its short recurrence time of outbursts and bright apparent magnitude indicate that this object has a relatively high mass-transfer rate. With the analogous system V485 Cen, these objects establish the first subpopulation in hydrogen-rich cataclysmic variables below the period minimum. Concerning the evolutionary status of them, we propose that they are progenitors of AM CVn stars on evolutionary courses in which systems have an evolved secondary star with a hydrogen-exhausted core.

The 2003 Superoutburst of a New SU UMa-Type Dwarf Nova, GO Comae Berenices

We photometrically observed the 2003 superoutburst of GO Com, which is a candidate for WZ Sge-type dwarf novae (WZ). However, the obtained light curve is atypical of both types: a plateau stage is too short for SU UMa-type dwarf novae (SU) while the amplitude of the superoutburst is too small for WZ. In order to explain this behavior, it might be taken into account that the mass accretion onto the white dwarf is efficient during quiescence.

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.