SU UMa Research Articles

Multiple Periods in the Dwarf Nova GZ Cancri

Abstract TESS light curves of the SU UMa type dwarf nova GZ Cnc are analyzed. They reveal positive superhumps which persist until long after a superoutburst, variations at the orbital period restricted to outbursts, and transient negative superhumps. But none of these variations are seen simultaneously.

A Phenomenon Resembling Early Superhumps in a New SU UMa-type Dwarf Nova with a 2 hr Orbital Period

We investigate K2BS5, an optical transient that we identified in Campaign 13 of the Kepler/K2 archives by the K2 Background Survey, and classify it as a new SU UMa-type dwarf nova. Using the light curve generated from Keplers long-cadence observation mode, we analyze the dwarf nova during quiescence and superoutburst. Following 20 days of quiescence at the start of the observation, the system entered a superoutburst lasting 12 days, after which it experienced at least one rebrightening. K2BS5 clearly meets the criteria for an SU UMa star, but at the peak of the superoutburst, it also shows double-wave oscillations consistent with the spectroscopic orbital period, a phenomenon that closely resembles early superhumps in WZ Sge stars. While we do not classify K2BS5 as a WZ Sge system, we discuss how this phenomenon could complicate efforts to use the suspected detection of early superhumps to distinguish SU UMa-type dwarf novae from the recently recognized class of long-orbital-period WZ Sge systems.

White Dwarf Photospheric Abundances in Cataclysmic Variables. IV. Deriving the [N/C] Ratio* *Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the data archive at the Space Telescope Science Institute. STScI is operated by the Association of University for Research in Astronomy, Inc. under NASA contract NAS 5-26555.

We present results from our ongoing far-ultraviolet archival analysis of cataclysmic variable white dwarf (WD) abundances for six more systems: four SU UMa dwarf novae (BW Scl, SW UMa, BC UMa, and VW Hyi) together with the dwarf nova RX And, and the novalike DW UMa. To derive a reliable nitrogen abundance, for the four SU UMa systems (with a WD temperature T wd ∼ 14,000 to ∼22,000 K), we use the dominant N i (1492 Å) absorption line; for DW UMa (with T wd possibly as high as 60,000 K), we use the N v (∼1240 Å) doublet; and for RX And (with T wd = 33,800 K), we use the N iii (1183.0 and 1184.6 Å) absorption lines. We find a [N/C] ratio of the order of 1–100 (in solar units). Oxygen, silicon, phosphorus are mostly underabundant while aluminum is mostly overabundant. We also derive magnesium, sulfur, calcium, and iron for a few systems. VW Hyi has a solar composition secondary implying the suprasolar [N/C] ratio very likely originates in the WD itself, e.g., accretion-driven dredge-up, mixing, and convection bringing material from deeper regions to the WD surface. If the donor star in the other five systems is nonevolved, the WD is the origin for the [N/C] ratio in these systems, either directly as in VW Hyi or due to contamination of the donor and accretion disk by the repetitive explosive CNO burning during the common envelope stage of the nova explosions.

Photometric analysis of the dwarf nova SU UMa based on TESS

We report the photometric analysis of SU UMa based on the observations of the Transiting Exoplanet Survey Satellite (TESS). TESS has released a large amount of data, which contains the light curves of a complete superoutburst and three normal outbursts of SU UMa. Based on the observations, the evolution of superhumps during the superoutburst was analyzed. By using the O - C method, the three stages were determined, and the superhump period for each stage was calculated. The mass ratio q=0.137(1) was estimated based on the stage A superhump method. A periodic oscillation with a period of 2.17(9) days was found in the superhump minimum O - C diagram, which is related to the precession of the accretion disk. We investigated the frequency information of quasiperiodic oscillations (QPOs) in the light curves. An ultralong period QPO of 0.18 days at the time of normal outburst and two ultralong periods QPOs of 0.19 days and 0.285 days in quiescence were found. No QPO was found in the superoutburst.

A Surprising Periodicity Detected during a Super-outburst of V844 Herculis by TESS

We identify a previously undetected periodicity at a frequency of 49.08 ± 0.01 days−1 (period of 29.34 ± 0.01 minutes) during a super-outburst of V844 Her observed by TESS. V844 Her is an SU UMa type cataclysmic variable with an orbital period of 78.69 minutes, near the period minimum. The frequency of this new signal is constant in contrast to the superhump oscillations commonly seen in SU UMa outbursts. We searched without success for oscillations during quiescence using MDM, TESS, and XMM-Newton data. The lack of a periodic signal in the XMM light curve and the relatively low X-ray luminosity of V844 Her suggest that it is not a typical IP. We consider the possibility that the 29-minute signal is the result of super-Nyquist sampling of a dwarf nova oscillation with a period near the 2-minute cadence of the TESS data. Our analysis of archival AAVSO photometry from a 2006 super-outburst supports the existence of a 29-minute oscillation, although a published study of an earlier super-outburst did not detect the signal. We compare the X-ray properties of V844 Her with short orbital period intermediate polars (IP), V1025 Cen and DW Cnc. We conclude that the new signal is a real photometric oscillation coming from the V844 Her system and that it is unlikely to be an aliased high-frequency oscillation. The steady frequency of the new signal suggests that its origin is related to an asynchronously rotating white dwarf in V844 Her, although the precise mechanism producing the flux variations remains unclear.

Investigation of Superhumps in SU UMa-type Dwarf Novae Based on the Observations of TESS

We report a superhumps analysis of seven SU UMa-type dwarf novae based on observations made by the Transiting Exoplanet Survey Satellite (TESS). Superhumps are seen during the superoutbursts of SU UMa-type dwarf novae. The monthlong data sets of TESS are well suited for studying the variations of superhumps. We selected seven non-eclipsing SU UMa-type dwarf novae with superhumps in which TESS light curves are available and have not yet been studied. The A, B, and C stages of superhumps in superoutbursts were determined by the O − C method. The results indicate that not all complete superoutbursts show three obvious stages, such as DT Oct and the second superoutburst of J1730+6247. We calculated the superhump period for each stage and the mean period for all superoutbursts. Taking the stage A superhump method, the mass ratio (M2/M1) was estimated. According to the results, the seven stars are pre-bounce systems with mass ratios ranging from 0.1 to 0.2. By combining the orbital periods and the mean superhump periods, the precession periods were calculated. The results show that the precession periods of the seven SU UMa stars are about 2 days.

Photometric long-term variations of dwarf novae during quiescence

ABSTRACT Using the Catalina Sky Survey and Gaia alert data bases, we have carried out a systematic search for long-term variability of the brightness of dwarf novae in quiescence. A small, but significant portion of them revealed variability on time-scales between 2 and 14 yr which can be fitted, in some of the targets, by linear slopes of 0.14 mag yr−1 on average and up to 0.6 mag yr−1 in extreme cases. Another group of dwarf novae showed quasi-periodic variations in their quiescent brightness, with periods of 3.7 yr on average and up to 13 yr in extreme cases, and mean amplitudes of 0.5 mag, reaching up to 1.3 mag. The observed slopes could be understood as portions of a quasi-sinusoidal variability with larger periods, not covered by our data. SU UMa stars have, on average, smaller slope values and longer periods, compared to other dwarf novae. These variations could be interpreted as caused by magnetic activity of the secondary companion star, similar to the 11-yr solar cycle, but one could consider also other origins of this phenomenon.

Symbiotic Star T CrB as an Extreme SU UMa–type Dwarf Nova

T CrB is a symbiotic recurrent nova that exhibits quiescent and active phases between its classical nova eruptions. The statistical properties of these active phases have been poorly studied thus far. Because of that their nature remained unknown. Here we study statistical properties of the active phases and show that they are consistent with outburst and superoutbursts observed in SU UMa–type dwarf novae. The recurrence time of these outbursts is consistent with theoretical predictions for similar systems. Moreover, the visual and X-ray evolution of the last active phase is consistent with a superoutburst. This suggests that T CrB is a dwarf nova with an extremely long orbital period, closely related to SU UMa dwarf novae. The similarities between the last superoutburst and the reported activity preceding the 1946 nova eruption may suggest that the next classical nova eruption in T CrB could be indeed soon expected.

Updated Orbital Ephemeris and Detection of Superhump Modulation in X-Ray Band for the Ultra-compact Low Mass X-Ray Binary 4U 1820-30

The 4U 1820-30 is an ultra-compact low-mass X-ray binary (LMXB) near the center of the globular cluster NGC 6624. Its negative orbital period derivative, observed from the phase evolution of its sinusoidal-like orbital variation, contradicts the positive value that was obtained from the theoretical prediction. In this paper, we present the analysis of the 4U 1820-30 orbital modulation from light curves obtained from the Neutron star Interior Composition ExploreR (NICER) observations from 2017 to mid-2022. Combined with historical records, the orbital derivative is measured from the orbital phase evolution between 1976 and 2002 is yr−1. No significant second order orbital period derivative is detected with a 2σ upper limit of s s−2. We discuss the possible intrinsic orbital period derivative of 4U 1820-30 and suggest that this binary system may have a significant mass outflow similar to some other LMXBs. In addition, a periodic modulation with a period of 691.6 ± 0.7 s, which is consistent with the superhump period discovered in the far-ultraviolet band of the Hubble Space Telescope, was also detected in the X-ray light curves that were collected by NICER. We conclude that this modulation is probably caused by a period of 0.8 ± 0.1 day apsidal precession of the accretion disk, similar to the SU UMa type dwarf novae and some LMXBs. However we cannot exclude the possibility that it is induced by a hierarchical third star that orbits around the binary system.

TESS light curves of cataclysmic variables – II – Superhumps in old novae and novalike variables

ABSTRACT Superhumps are among the abundant variable phenomena observed in the light curves of cataclysmic variables (CVs). They come in two flavours as positive and negative superhumps, distinguished by periods slightly longer or shorter, respectively, than the orbital periods of these interacting binary systems. Positive superhumps are ubiquitous in superoutbursting short-period dwarf novae of the SU UMa type but are less common in longer period systems with accretion discs in a permanent bright state, such as novalike variables and most old novae. Negative superhumps do not seem to have a preference for a particular type of CV. Here, I take advantage of the long high-cadence light curves provided by TESS for huge number of stars, selecting all old novae and novalike variables with past reported superhumps for which TESSlight curves are available and have not yet been analysed in previous publications in order to study their superhump behaviour. In combination with information taken from the literature, the results enable to compile the most complete census of superhumps in these stars so far. As a corollary, for the eclipsing systems in the present sample of objects, eclipse epochs derived from the TESS light curves and in some cases from archival light curves are listed and used to update orbital ephemeris and to discuss period changes.

Transient behaviour of three SU UMa-type dwarf novae: AR Pic, QW Ser, and V521 Peg

ABSTRACT Changes in the supercycle lengths of some SU UMa-type dwarf novae have been detected by other studies, and indicate that the mass transfer rates noticeably decrease over time. We investigated the supercycle lengths of three SU UMa-type dwarf novae: AR Pic, QW Ser, and V521 Peg, to determine if they have detectable changes in their supercycles. We present the results of optical spectroscopic and photometric observations of these sources. Our observations were conducted in 2016 and 2017 at the Boyden Observatory and the Sutherland station of the South African Astronomical Observatory. The quiescent results indicated that all three sources are typical SU UMa-type dwarf novae. We also present results of AR Pic and QW Ser in outburst and of V521 Peg during a precursor outburst and superoutburst. Light curves were supplemented by the Catalina Real-Time Transient Survey, the ASAS-3 and ASAS-SN archives, and the AAVSO International Database in order to investigate the long-term behaviour of these sources. Our results combined with catalogued properties for all short-period dwarf novae show a possible relationship between the supercycle time in SU UMa systems and their orbital periods, which is interpreted as the decline in the mass transfer rate as systems evolve towards and away from the ‘period minimum’. At the shortest orbital periods, SU UMa systems are almost indistinguishable from WZ Sge systems. However, we propose that the scaleheight between the secondary’s photosphere and L1 may be a factor that distinguish the SU UMa subclasses.

FIVE NEW FAINT CATACLYSMIC VARIABLES FROM THE ZWICKY TRANSIENT FACILITY

A search of faint, large-amplitude variables in the Zwicky Transient Facility data has led to the identication of five new CVs. Four of the stars, ZTFJ010013.34+610809.8, ZTFJ051913.86+293006.0, ZTFJ210441.02+394052.7 and ZTFJ210705.14+394617.7 are probbly SS Cyg-type systems, while ZTFJ173854.93+175136.3 is a likely SU UMa system.

Determination of QW Ser and QZ Lib parameters from optical spectra

The paper investigates a new technique for determining dwarf nova parameters based on the analysis of their optical spectra with a signal-to-noise ratio S/N ≈ 80. We have determined parameters for the QW Ser and QZ Lib systems related to the SU UMa and WZ Sge dwarf novae. Moderate resolution spectra of both systems were obtained at BTA SAO during their low state with an optically thin accretion disk and the dominance of white dwarf radiation. As a result of the model analysis of the spectra, the parameters of the accretor atmosphere of QW Ser (Teff = 23500 ± 400 K and log g = 8.36 ± 0.05 dex) and constraints on the parameters for QZ Lib (Teff = 9500 ± 300 K and log g > 8.55 dex) were found. The presence of moderate noise in the studied spectra is not shown to affect the accuracy of determining parameters. The low temperature of the white dwarf does not allow to obtain a correct estimate of its surface gravity. Based on the determined parameters of the accretor atmospheres, a complete set of QW Ser parameters and estimates of QZ Lib parameters were found. The secondary components of both systems are defined to be red dwarfs of spectral type M.

Changes in the Period of Negative Superhumps of Type SU UMa Dwarf Novae. II. NY Her (2017 and 2020)

Changes in the Period of Negative Superhumps of Type SU UMa Dwarf Novae. II. NY Her (2017 and 2020)

The determination of white dwarf parameters in dwarf novae by optical spectral modeling

This paper presents the results of a model analysis of optical spectra and determination of the parameters of three individual SU UMa and WZ Sge type dwarf novae. The moderate resolution spectra of TY Psc, FL Psc and V455 And were obtained at the 6-m BTA of the SAO RAS in the low state of these systems with the determination of white dwarf radiation. The theoretical spectra were calculated using the grid models of hydrogen dwarf atmospheres of white dwarfs by varying the parameters (T eff and log g) to reach the best agreement with the observed ones. We highlight different effects of the parameters on the shape and intensity of the HI lines. Therefore, it is possible to unambiguously determine T eff and log g from the analysis of observations. The fundamental parameters of white dwarfs (M and R) were found by comparing the parameters of atmospheres with theoretical models of the internal structure. The obtained parameters of the primaries of TY Psc, FL Psc and V455 And are consistent with the average values for SU UMa and WZ Sge systems. As a result, we demonstrate the efficiency of the method for determining the parameters of such systems based on the analysis of a limited set of observed optical spectra.

The period-gap cataclysmic variable CzeV404 Her: A link between SW Sex and SU UMa systems

Context. We present a new study of the eclipsing cataclysmic variable CzeV404 Her (Porb = 0.098 d) that is located in the period gap. Aims. This report determines the origin of the object and the system parameters and probes the accretion flow structure of the system. Methods. We conducted simultaneous time-resolved photometric and spectroscopic observations of CzeV404 Her. We applied our light-curve modelling techniques and the Doppler tomography method to determine the system parameters and analyse the structure of the accretion disk. Results. We found that the system has a massive white dwarf MWD = 1.00(2) M⊙, a mass ratio of q = 0.16, and a relatively hot secondary with an effective temperature T2 = 4100(50) K. The system inclination is i = 78.8°. The accretion disk spreads out to the tidal limitation radius and has an extended hot spot or line region. The hot spot or line is hotter than the remaining outer part of the disk in quiescence or in intermediate state, but does not stand out completely from the disk flux in (super)outbursts. Conclusions. We claim that this object represents a link between two distinct classes of SU UMa-type and SW Sex-type cataclysmic variables. The accretion flow structure in the disk corresponds to the SW Sex systems, but the physical conditions inside the disk fit the behaviour of SU UMa-type objects.

Evidence that short-period AM CVn systems are diverse in outburst behaviour

ABSTRACT We present results of our analysis of up to 15 yr of photometric data from eight AM CVn systems with orbital periods between 22.5 and 26.8 min. Our data have been collected from the GOTO, ZTF, Pan-STARRS, ASAS-SN, and Catalina all-sky surveys and amateur observations collated by the AAVSO. We find evidence that these interacting ultracompact binaries show a similar diversity of long-term optical properties as the hydrogen accreting dwarf novae. We found that AM CVn systems in the previously identified accretion disc instability region are not a homogenous group. Various members of the analysed sample exhibit behaviour reminiscent of Z Cam systems with long superoutbursts (SOs) and standstills, SU UMa systems with regular, shorter SOs, and nova-like systems that appear only in a high state. The addition of TESS full frame images of one of these systems, KL Dra, reveals the first evidence for normal outbursts appearing as a precursor to SOs in an AM CVn system. Our results will inform theoretical modelling of the outbursts of hydrogen deficient systems.

White Dwarf Photospheric Abundances in Cataclysmic Variables. I. SS Aurigae and TU Mensae*

Abstract Chemical abundance studies of cataclysmic variables have revealed high nitrogen to carbon ratios in a number of cataclysmic variable white dwarfs (based on ultraviolet emission and absorption lines), as well as possible carbon deficiency in many secondaries (based on the absence of infrared CO absorption lines). These indicate that the accreted material on the white dwarf surface and the donor itself might be contaminated with CNO processed material. To further understand the origin of this abundance anomaly, there is a need for further chemical abundance study. In the present work, we carry out a far-ultraviolet spectral analysis of the extreme SU UMa dwarf nova TU Men and the U Gem dwarf nova SS Aur using archival spectra. We derive the mass and temperature of the WD using the recently available DR2 Gaia parallaxes. The analysis of HST STIS spectra yields a WD mass with a temperature of 27,750 ± 1000 K for TU Men and a WD mass M wd ∼ 0.80 ± 0.15 M ⊙ with a temperature of ∼30,000 ± 1000 K for SS Aur. However, the analysis of a FUSE spectrum for SS Aur gives a higher temperature of ∼33,375 ± 1875 K, yielding a higher WD mass of ∼1 ± 0.25 M ⊙, which could be due to the effect of a second hot emitting component present in the short wavelengths of FUSE. Most importantly, based on the white dwarf far-ultraviolet absorption lines, we find that both systems have subsolar carbon and silicon abundances. For TU Men, we also find suprasolar nitrogen abundance, evidence of CNO processing.

Determination of the superoutburst cycle lengths of 206 SU UMa-type dwarf novae

ABSTRACT SU UMa stars are characterized by ‘superoutbursts’, which are brighter at maximum light and which last much longer than the more frequent ‘ordinary’ outbursts of these dwarf novae. Although there are now more than 1180 SU UMa-type dwarf novae catalogued, our knowledge on their superoutburst cycle length Cso has hitherto been limited to about 6 ${{\ \rm per\ cent}}$ of the entire sample of known SU UMa stars. Using public data bases we have determined new Cso values for a total of 206 additional SU UMa stars in the range 17 d < Cso < 4590 d (including some ER UMa and WZ Sge-type representants) within total time intervals between 2 and 57 yr, and with an estimated uncertainty of $\pm 11{{\ \rm per\ cent}}$. This way, we are increasing our present knowledge of Cso values by a factor of ∼3.8. Their distribution is characterized by a broad maximum around Cso ≈ 270 d, and slowly decreasing numbers up to Cso ≈ 800 d. The domain Cso > 450 d was unexplored until now; we add here 106 cases ($\sim \! 51{{\ \rm per\ cent}}$ of our total sample) in this range of long cycles, implying a better statistical basis for future studies of their distribution. Our sample contains 16 known WZ Sge stars, and we propose WZ Sge membership for five others hitherto classified as ordinary SU UMa stars. Individual superoutburst timings deviate on average by about $\pm 7{{\ \rm per\ cent}}$ of the cycle length from their overall linear ephemeris, confirming the pronounced quasi-periodic repeatability of superoutbursts. All relevant parameters are listed with their errors, and a table with individual superoutburst epochs of our targets is given, enabling future researchers to combine our results with other (past or future) observations.

Long-Term Activity of a Little Known Dwarf Nova DT Octantis

We analyze the long-term activity of a little known dwarf nova (DN) DT Oct. We use photographic observations from the Bamberg Observatory (Southern Patrol Photographic Sky Survey), CCD data from ASAS-3 survey, and AAVSO data (both CCD and visual). We determined the recurrence times of the superoutbursts, TCso, and of the normal outbursts, TCno, to be approximately 311 d and about 30 d, respectively. Although the length of TCso evolves gradually and undergoes only small variations (increases and decreases) of the nearby or neighbouring events (on the timescale of a few epochs), the evolution of TCso displays considerably larger changes on the longer timescales. They suggest a dominant trend of a decrease of TCso on the timescale of tens of years. The possibly magnetized white dwarf does not prevent DT Oct to show the long-term activity of the SU UMa systems. We place the time evolution of the superoutbursts of DT Oct in the context of DNe of various sub-types.