SU UMa-type Dwarf Novae 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.

V1006 Cyg: SU UMa-type dwarf nova in the period gap that wobbles between subclasses

V1006 Cyg: SU UMa-type dwarf nova in the period gap that wobbles between subclasses

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.

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.

Optical Studies of Seven Bright Southern Cataclysmic Variable Stars

We report spectroscopic observations of seven bright southern cataclysmic variable stars, collected on a single two-week observing run using the 1.9 m Radcliffe telescope at the South African Astronomical Observatory. We used a radial-velocity time series, in some cases in combination with other data, to determine or clarify orbital periods for five of them, namely ATO J061.1478−31.0634, BMAM-V547, MGAB-V202, NSV 4202, and V1147 Cen. For BMAM-V547, we use data from the Transiting Exoplanet Survey Satellite (TESS) to corroborate and sharpen the orbital period; the TESS data also show a photometric period near 3.93 days, likely indicating precession of the accretion disk. Also, we find a periodic modulation in the radial velocities of the SU UMa-type dwarf nova Var Ret2005, but are unable to specify a unique cycle count. Finally, we show a spectrum of ASASSN-V J061528.41−412007.3 that appears typical of a luminous novalike variable.

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.

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.

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.

BO Ceti: Dwarf nova showing both IW And-type and SU UMa-Type features

Abstract IW And stars are a recently recognized subgroup of dwarf novae which are characterized by (often repetitive) slowly rising standstills terminated by brightening, but the exact mechanism for this variation is not yet identified. We have identified BO Cet, which had been considered as a novalike cataclysmic variable, as a new member of IW And stars based on its behavior in 2019–2020. In addition to this, the object showed dwarf nova-type outbursts in 2020–2021, and superhumps that had periods $7.8\%$ longer than the orbital one developed during at least one long outburst. This object has been confirmed as an SU UMa-type dwarf nova with an exceptionally long orbital period (0.1398 d). BO Cet is thus the first cataclysmic variable showing both SU UMa-type and IW And-type features. We obtained a mass ratio (q) of 0.31–0.34 from the superhumps in the growing phase (stage A superhumps). At this q, the radius of the 3 : 1 resonance, responsible for tidal instability and superhumps, and the tidal truncation radius are very similar. We interpret that in some occasions this object showed IW And-type variation when the disk size was not large enough, but that the radius of the 3 : 1 resonance could be reached as a result of thermal instability. We also discuss that there are SU UMa-type dwarf novae above q = 0.30, which is above the previously considered limit (∼0.25) derived from numerical simulations and that this is possible since the radius of the 3 : 1 resonance is inside the tidal truncation radius. We constrained the mass of the white dwarf larger than 1.0 M⊙, which may be responsible for the IW And-type behavior and the observed strength of the He ii emission. The exact reason, however, why this object is unique in that it shows both SU UMa-type and IW And-type features is still unsolved.

Spectroscopic and photometric observations of dwarf nova superoutbursts by the 3.8 m telescope Seimei and the Variable Star Network

Abstract We present spectroscopic and photometric observations of 17 dwarf-nova superoutbursts obtained by KOOLS-IFU mounted on the 3.8 m telescope Seimei at the Okayama Observatory of Kyoto University and through the Variable Star Network collaboration (VSNET). Our spectroscopic observations for six outbursts were performed within 1 d of their optical peak. 11 objects (TCP J00590972+3438357, ASASSN-19ado, TCP J06073081−0101501, ZTF20aavnpug, ASASSN-19ady, MASTER OT J061642.05+435617.9, TCP J20034647+1335125, ASASSN-20kv, ASASSN-20kw, MASTER OT J213908.79+161240.2, and ASASSN-20mf) were previously unknown systems, and our observations enabled quick classification of their transient type. These results illustrate that the Seimei telescope has the capability to conduct quick follow-up observations of unknown transients. Our photometric observations yielded that 11 of the objects are WZ Sge-type dwarf novae and their candidates, and the other six are SU UMa-type dwarf novae and their candidates. The He ii 4686 Å emission line was clearly detected among ASASSN-19ado, TCP J06073081−0101501 and MASTER OT J213908.79+161240.2, the association of which with a spiral arm structure in an accretion disk has been suggested in previous studies. Our result suggests that a higher-inclination system shows a stronger emission line of He ii 4686 Å, as well as larger-amplitude early superhumps.

ASASSN-18aan: An eclipsing SU UMa-type cataclysmic variable with a 3.6-hr orbital period and a late G-type secondary star

Abstract We report photometric and spectroscopic observations of the eclipsing SU UMa-type dwarf nova ASASSN-18aan. We observed the 2018 superoutburst with 2.3 mag brightening and found the orbital period (Porb) to be 0.149454(3) d, or 3.59 hr. This is longward of the period gap, establishing ASASSN-18aan as one of a small number of long-Porb SU UMa-type dwarf novae. The estimated mass ratio, [q = M2/M1 = 0.278(1)], is almost identical to the upper limit of tidal instability by the 3 : 1 resonance. From eclipses, we found that the accretion disk at the onset of the superoutburst may reach the 3 : 1 resonance radius, suggesting that the superoutburst of ASASSN-18aan results from the tidal instability. Considering the case of long-Porb WZ Sge-type dwarf novae, we suggest that the tidal dissipation at the tidal truncation radius is enough to induce SU UMa-like behavior in relatively high-q systems such as SU UMa-type dwarf novae, but that this is no longer effective in low-q systems such as WZ Sge-type dwarf novae. The unusual nature of the system extends to the secondary star, for which we find a spectral type of G9, much earlier than typical for the orbital period, and a secondary mass M2 of around 0.18 M⊙, smaller than expected for the orbital period and the secondary’s spectral type. We also see indications of enhanced sodium abundance in the secondary’s spectrum. Anomalously hot secondaries are seen in a modest number of other CVs and related objects. These systems evidently underwent significant nuclear evolution before the onset of mass transfer. In the case of ASASSN-18aan, this apparently resulted in a mass ratio lower than typically found at the system’s Porb, which may account for the occurrence of a superoutburst at this relatively long period.

Humps and Superhumps in the SU UMa-Type Dwarf Nova System IRXS J161659.5+620014

Photometric studies of the SU UMa-type dwarf nova IRXS J161659.5+620014 have been made on the 2.6-m and 1.25-m telescopes at the Crimean Astrophysical Observatory during 2017 and 2018 in the quiescent state of the system, during normal outbursts, and during superoutbursts. It is found that the interval between adjacent normal outbursts was about 11 days during 2017. Observations in the quiescent state during 2017 yielded an orbital period of 0.06888(2) days. Observations during 2018 manifested good reproducibility of the characteristics of the superoutbursts compared to those of previous years. A refined period of 0.071044(15) days was obtained for the positive superhumps in stage “C” and their color indices were determined.

Variations in the Period of Negative Superhumps in SU UMa-Type Dwarf Novae. I. Mn Dra (2012-2017)

This is a photometric study of the dwarf nova MN Dra made during 2012-2017 on nine telescopes over 152 nights. Overall, the observations covered 4 superoutbursts, 7 normal outbursts and the quiescent state. The interval between neighboring superoutbursts in 2017 was 65 days, and between neighboring normal bursts, 15 days. During the superoutbursts of 2012 and 2017, positive superhumps with a period of 0.10558(6) and 0.10500(2) days, respectively, were observed and in the quiescent state, negative superhumps with an average period of 0.095921(3) days, It is shown that the period of the negative superhumps varied cyclically between normal outbursts: sharply decreasing during an outburst and gradually increasing toward the onset of the next outburst. This feature of the variation in the period of the negative superhumps may correspond to a rapid increase in the radius of the accretion disk during the time of an outburst followed by a slow decrease, in agreement with the theory of thermal-tidal instability.

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.

Survey of period variations of superhumps in SU UMa-type dwarf novae. X. The tenth year (2017)

Abstract Continuing the project described by Kato et al. (2009, PASJ, 61, S395), we collected times of superhump maxima for 102 SU UMa-type dwarf novae observed mainly during the 2017 season, and characterized these objects. WZ Sge-type stars identified in this study are PT And, ASASSN-17ei, ASASSN-17el, ASASSN-17es, ASASSN-17fn, ASASSN-17fz, ASASSN-17hw, ASASSN-17kd, ASASSN-17la, PNV J20205397$+$2508145, and TCP J00332502$-$3518565. We obtained new mass ratios for seven objects using growing superhumps (stage A). ASASSN-17gf is an EI Psc-type object below the period minimum. CRTS J080941.3$+$171528 and DDE 51 are objects in the period gap, and both showed a long-lasting phase of stage A superhumps. We also summarize the recent advances in understanding of SU UMa-type and WZ Sge-type dwarf novae.

ASASSN-14ho: Longest-period dwarf nova with multiple rebrightenings

Abstract The post-outburst rebrightening phenomenon in dwarf novae and X-ray novae is still one of the most challenging subjects for theories of accretion disks. It has been widely recognized that post-outburst rebrightenings are a key feature of WZ Sge-type dwarf novae, which predominantly have short (≲0.06 d) orbital periods. The author found four post-outburst rebrightenings in ASASSN-14ho during its 2014 outburst, whose orbital period was recently measured to be exceptionally long [0.24315(10) d]. Using the formal solution of the radial velocity study in the literature, this paper discusses the possibility that this object could be an SU UMa-type dwarf nova near the stability border of the 3 : 1 resonance, despite its exceptionally long orbital period. Such objects are considered to be produced if mass transfer occurs after the secondary has undergone significant nuclear evolution, and they may be hidden in a significant number of dwarf novae showing multiple post-outburst rebrightenings.

CS Indi: SU UMa-type dwarf nova with long precursor outburst

Abstract We observed the 2018 November outburst of CS Ind and confirmed that it was a genuine superoutburst with a very long [0.12471(1) d on average] superhump period. The superoutburst was preceded by a long precursor, which was recorded for the first time in SU UMa-type dwarf novae. Our interpretation is that the combination of a sufficient amount of mass in the disk before the ignition of the outburst and the slow development of tidal instability near the borderline of the 3 : 1 resonance caused a cooling front to start before the full development of tidal instability. This finding provides more support to the recent interpretation of slow development of the tidal instability causing various phenomena similar to WZ Sge-type dwarf novae in SU UMa-type dwarf novae with very long orbital periods.

FUSE and IUE Spectroscopy of the Prototype Dwarf Nova ER Ursa Majoris During Quiescence

Abstract ER Ursae Majoris is the prototype for a subset of SU UMa-type dwarf novae characterized by short cycle times between outburst, high outburst frequency, and negative superhumps. It suffers superoutbursts every 43 days, lasting 20 days, normal outbursts every 4 days, and has an outburst amplitude of 3 mag. We have carried out a far-ultraviolet (FUV) spectral analysis of ER UMa in quiescence, by fitting Far Ultraviolet Spectroscopic Explorer and International Ultraviolet Explorer spectra with model accretion disks and high-gravity photosphere models. Using the Gaia parallax distance and an orbital inclination of 50°, we find that during the brief quiescence of only four days, the accretion rate is 7.3 × 10−11 M ⊙ yr−1, with the ER UMa white dwarf contributing 55% of the FUV flux and the accretion disk contributing the remaining 45% of the flux. The white dwarf in ER UMa is markedly hotter (32,000 K) than the other white dwarfs in dwarf novae below the cataclymic variable (CV) period gap, which have typical temperatures of ∼15,000 K. For higher inclinations of 60°–75°, the accretion rates that we derive are roughly an order of magnitude higher, (1–3) × 10−10 M ⊙ yr−1.

KSP-OT-201611a: A Distant Population II Dwarf Nova Candidate Discovered by the KMTNet Supernova Program

Abstract We present a multicolor, high-cadence photometric study of a distant dwarf nova KSP-OT-201611a discovered by the Korea Microlensing Telescope Network Supernova Program. From 2016 October to 2017 May, two outbursts, which comprise a super/long outburst followed by a normal/short outburst separated by ∼91 days, were detected in the BVI bands. The shapes and amplitudes of the outbursts reveal the nature of KSP-OT-201611a to be an SU UMa- or U Gem-type dwarf nova. Color variations of periodic humps in the super/long outburst possibly indicate that KSP-OT-201611a is an SU UMa-type dwarf nova. The super and normal outbursts show distinctively different color evolutions during the outbursts due most likely to the difference in time when the cooling wave is formed in the accretion disk. The outburst peak magnitudes and the orbital period of the dwarf nova indicate that it is at a large Galactocentric distance (∼13.8 kpc) and height (∼1.7 kpc) from the Galactic plane. KSP-OT-201611a, therefore, may provide a rare opportunity to study the accretion disk process of Population II dwarf novae.