Permanent Superhump Research Articles

Characteristics of the Permanent Superhumps in V533 Herculis

We analyze two sectors of Transiting Exoplanet Sky Survey (TESS) photometry of the nova-like cataclysmic variable star V533 Her. We detect a periodicity consistent with the binary orbital period and estimate a revised value of 3.53709(2) hr. We also detect a strong signal near a period of 3.8 hr that we associate with positive superhumps. The superhump frequency varies over the TESS observations with the fractional difference between the superhump and orbital periods, ϵ, ranging between 0.055 ≤ ϵ ≤ 0.080. The superhump amplitude is correlated with its frequency such that the amplitude increases as ϵ decreases. Positive superhumps result from an instability that generates an eccentric accretion disk and ϵ is a measure of the disk precession rate in the binary rest frame. The observed correlation implies that as the disk precession rate slows, the disk eccentricity increases.

A comparative study of the strength of flickering in cataclysmic variables

ABSTRACT Flickering is a universal phenomenon in accreting astronomical systems that still defies detailed physical understanding. It is particularly evident in cataclysmic variables (CVs). Attempting to define boundary conditions for models, the strength of the flickering is measured in several thousand light curves of more than 100 CVs. The flickering amplitude is parametrized by the full width at half-maximum of a Gaussian fit to the magnitude distribution of data points in a light curve. This quantity requires several corrections before a comparison between different sources can be made. While no correlations of the flickering strength with simple parameters such as component masses, orbital inclination, or period were detected, a dependence on the absolute magnitude of the primary component and on the CV subtype is found. In particular, flickering in VY Scl type novalike variables is systematically stronger than in UX UMa type novalikes. The broad-band spectrum of the flickering light source can be fit by simple models but shows excess in the U band. When the data permitted to investigate the flickering strength as a function of orbital phase in eclipsing CVs, such a dependence was found, but it is different for different systems. Surprisingly, there are also indications for variations of the flickering strength with the superhump phase in novalike variables with permanent superhumps. In dwarf novae, the flickering amplitude is high during quiescence, drops quickly at an intermediate magnitude when the system enters into (or returns from) an outburst and, on average, remains constant above a given brightness threshold.

UU Aqr - No superhumps but variations on the time scale of days

Recently, brightness variations occurring on twice the accretion disk precession period in the old nova and permanent superhump system V603 Aql have been observed by Bruch & Cook (2018). In an attempt to detect a similar effect in other cataclysmic variables reported to contain permanent superhumps the novalike variable UU Aqr was observed during 11 nights in September 2018. While no traces of superhumps were seen in the data, rendering the quest for variations related to the disk precession period obsolete, the system exhibits regular variations with a period of ~4 days.

RZ Leonis Minoris bridging between ER Ursae Majoris-type dwarf nova and nova-like system

AbstractWe observed RZ LMi, which is renowned for its extremely short (∼19 d) supercycle and is a member of a small, unusual class of cataclysmic variables called ER UMa-type dwarf novae, in 2013 and 2016. In 2016, the supercycles of this object substantially lengthened in comparison to the previous measurements to 35, 32, and 60 d for three consecutive superoutbursts. We consider that the object virtually experienced a transition to the nova-like state (permanent superhumper). This observed behavior reproduced the prediction of the thermal-tidal instability model extremely well. We detected a precursor in the 2016 superoutburst and detected growing (stage A) superhumps with a mean period of 0.0602(1) d in 2016 and in 2013. Combined with the period of superhumps immediately after the superoutburst, the mass ratio is not as small as in WZ Sge-type dwarf novae, having orbital periods similar to RZ LMi. By using least absolute shrinkage and selection operator (Lasso) two-dimensional power spectra, we detected possible negative superhumps with a period of 0.05710(1) d. We estimated an orbital period of 0.05792 d, which suggests a mass ratio of 0.105(5). This relatively large mass ratio is even above that of ordinary SU UMa-type dwarf novae, and it is also possible that the exceptionally high mass-transfer rate in RZ LMi may be a result of a stripped secondary with an evolved core in a system evolving toward an AM CVn-type object.

Extensive photometry of the WZ Sge-type dwarf nova V455 And (HS2331+3905): Detection of negative superhumps and coherence features of the short-period oscillations

We report the results of photometry of the WZ Sge-type dwarf nova V455 And. Observations were obtained over 19 nights in 2013 and 2014. The total duration of the observations was 96 h. We clearly detected three coherent oscillations with periods of 80.376 ± 0.003 min, 40.5431 ± 0.0004 min and 67.619685 ± 0.000040 s. The 67.619685-s period can be the spin period of the white dwarf. The 40.5431-minute period is the first harmonic of the orbital period. The 80.376-minute oscillation can be a negative superhump because its period is 0.9% less than the orbital period. This oscillation was evident both in the data of 2013 and in the data of 2014. These results make V455 And a permanent superhump system which shows negative superhumps. This is also the first detection of persistent negative superhumps in a WZ Sge-type dwarf nova. In addition, the analysis of our data revealed incoherent oscillations with periods in the range 5–6 min, which were observed earlier and accounted for by non-radial pulsations of the white dwarf. Moreover, we clearly detected an oscillation with a period of 67.28 ± 0.03 s, which was of a low degree of coherence. This oscillation conforms to the beat between the spin period of the white dwarf and the 3.5-h spectroscopic period, which was discovered earlier and accounted for by the free precession of the white dwarf. Because the 67.28-s period is shorter than the spin period and because the free precession of the white dwarf is retrograde, we account for the 67.28-s oscillation by the free precession of the white dwarf.

A photometric study of the nova‐like variable TT Arietis with the MOST satellite

AbstractVariability on all time scales between seconds and decades is typical for cataclysmic variables (CVs). One of the brightest and best studied CVs is TT Ari, a nova‐like variable which belongs to the VY Scl subclass, characterized by occasional low states in their light curves. It is also known as a permanent superhumper at high state, revealing “positive” (PS > P0) as well as “negative” (PS < P0) superhumps, where PS is the period of the superhump and P0 the orbital period. TT Ari was observed by the Canadian space telescope MOST for about 230 hours nearly continuously in 2007, with a time resolution of 48 seconds. Here we analyze these data, obtaining a dominant “negative” superhump signal with a period PS = 0.1331 days and a mean amplitude of 0.09 mag. Strong flickering with amplitudes up to 0.2 mag and peak‐to‐peak time scales of 15–20 minutes is superimposed on the periodic variations. We found no indications for significant quasi‐periodic oscillations with periods around 15 minutes, reported by other authors. We discuss the known superhump behaviour of TTAri during the last five decades and conclude that our period value is at the upper limit of all hitherto determined “negative” superhump periods of TTAri, before and after the MOST run. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Detection of superhumps in the cataclysmic variable V378 Peg (PG 2337+300)

The cataclysmic variable V378 Peg is known since 15 years. Although V378 Peg is a rather bright star (14 mag), it underwent no detailed study. We performed photometric observations of V378 Peg during 75 h with the goal to detect periodic brightness variations. The obtained light-curves clearly showed changes with a period of about 3 h. The Fourier analysis reveals that this oscillation occurs with a period of 3.238 h and a semiamplitude of 0.07 mag. Although the detected oscillation possesses certain coherence, it appears to have a slightly unstable period or phase. Therefore, the detected period cannot be the orbital period of the V378 Peg system. Because such instability is typical of superhumps, we must consider the detected oscillation as superhumps. Furthermore, V378 Peg shows no outbursts and has to be a nova-like variable rather than a dwarf nova. Hence, the detected superhumps have to be regarded as permanent superhumps. Because superhump periods in cataclysmic variables are close to orbital periods, we can find the place of V378 Peg in the orbital period distribution of cataclysmic variables. V378 Peg is a permanent superhump system above the upper edge of the 2–3 h period gap in the orbital period distribution.

A photometric study of V1193 Ori: Detection of orbital and negative superhump periods

We present the results obtained from unfiltered photometric CCD observations of V1193 Ori made during 24 nights between November 2002 and January 2003. We found periods of 0.1430 d , 0.1362 d and possibly 2.98 d in the data. The 0.1430 d period is consistent with the 1 d −1 alias of the proposed orbital period of P orb = 0.165 d (Ringwald, F.A., Thorstensen, J.R., Hamwey, R.M., 1994. MNRAS 271, 323; Papadaki, C., Boffin, H.M.J., Cuypers, J., Stanishev, V., Kraicheva, Z., Genkov, V., 2004. In: Hilditch, R.W., Hensberge, H., Pavlovski, K. (Eds.), Spectroscopically and Spatially Resolving the Components of Close Binary Stars, ASP Conference Series, vol. 318, Astronomical Society of the Pacific, San Francisco, p. 399.). Thus and using the known relation between the orbital and superhump periods, we interpret these periods as the orbital period of 0.1430 d , the negative superhump period of 0.1362 d and the precession period of 2.98 d . V1193 Ori can then be classified as a permanent superhump system.

HS 2331+3905: The cataclysmic variable that has it all

We report detailed follow-up observations of the cataclysmic variable HS 2331+3905, identified as an emissionline object in the Hamburg Quasar Survey. An orbital period of 81.08 min is unambiguously determined from the detection of eclipses in the light curves of HS 2331+3905. A second photometric period is consistently detected at P 83.38 min,∼2.8% longer than P orb , which we tentatively relate to the presence of permanent superhumps. High time resolution photometry exhibits short-timescale variability on time scales of 5−6 min which we interpret as non-radial white dwarf pulsations, as well as a coherent signal at 1.12 min, which is likely to be the white dwarf spin period. A large-amplitude quasi-sinusoidal radial velocity modulation of the Balmer and Helium lines with a period ∼3.5 h is persistently detected throughout three seasons of time-resolved spectroscopy. However, this spectroscopic period, which is in no way related to the orbital period, is not strictly coherent but drifts in period and/or phase on time scales of a few days. Modeling the far-ultraviolet to infrared spectral energy distribution of HS 2331+3905, we determine a white dwarf temperature of T eff 10 500 K (assuming M wd = 0.6 M ), close to the ZZ Ceti instability strip of single white dwarfs. The spectral model implies a distance of d = 90 ± 15 pc, and a low value for the distance is supported by the large proper motion of the system, µ = 0.14 yr −1 . The non-detection of molecular bands and the low J, H, and K fluxes of HS 2331+3905 make this object a very likely candidate for a brown-dwarf donor.

FS Aur as a Permanent Superhump System

AbstractWe discovered that the short period cataclysmic variable FS Aur at some epochs shows a photometric period close to the orbital. It exceeds the orbital period by ∽2%, which is a sign of the presence of a permanent superhump in the system. Superhumps tend to appear near short, low amplitude outbursts. We assume that FS Aur possesses a large thermally stable accretion disc and that the outburst may be due to the variable mass transfer rate. This, however, does not alter our previous explanation of yet another, 2.4 times longer than orbital, photometric period of FS Aur, found earlier, and persistently observed in its light curves.

Detection of superhumps in the Z Camelopardalis-type dwarf nova AT Cnc at standstill

We present results of our photometry of the Z Camelopardalis-type dwarf nova AT Cnc. The observational data were obtained during 13 nights in February and March 2003 when AT Cnc was in its long standstill. Two sets of our data reveal brightness variations with quasi-periods of (4.65 ± 0.02) and of (4.74 ± 0.02) h. The semiamplitude of these variations was observed in the range 5-9 mmag, showing changes from night to night. This signal varies in period and in phase on a time-scale of weeks, as is typical of superhumps. A comparison with the orbital period obtained by Nogami et al. (1999) from the radial velocity measurements of AT Cnc, which equals (4.826 ± 0.014) h, shows that this signal can be a negative superhump since the average period of brightness variations is approximately 3% shorter than the orbital period. These results make AT Cnc a permanent superhump system with a rather large orbital period and a large mass ratio. This is also the first detection of superhumps in a Z Camelopardalis-type system. In addition, we have found signs of an unstable signal with a period of about 2.3 h, which may represent the second harmonic of the 4.7-h signal or the second orbital sideband of the wobble frequency of the tilted precessing accretion disc. The average power spectrum of AT Cnc reveals a broad hump at frequencies in the range 0.4-0.7 mHz, which is evidence of quasiperiodic oscillations. The hump crest corresponds to a period of 1700-1800 s, which is remarkably close to 10% of the orbital period. This may indicate that this system contains a magnetic white dwarf creating the tilted accretion disc.

DW Cancri: a magnetic VY Scl star with an orbital period of 86 min

We present the first time-resolved spectroscopic study of the cataclysmic variable DW Cancri. We have determined an orbital period of 86.10 +- 0.05 min, which places the system very close to the observed minimum period of hydrogen-rich cataclysmic variables. This invalidates previous speculations of DW Cnc being either a permanent superhumper below the period minimum or a nova-like variable with an orbital period longer than 3 hours showing quasi-periodic oscillations. The Balmer and HeI lines have double-peaked profiles and exhibit an intense S-wave component moving with the orbital period. Remarkably, the Balmer and HeI radial velocity curves are modulated at two periods: 86.10 +- 0.05 min (orbital) and 38.58 +- 0.02 min. The same short period is found in the equivalent width variations of the single-peaked HeII 4686 line. We also present time-resolved photometry of the system which shows a highly-coherent variation at 38.51 min, consistent with the short spectroscopic period. The large number of similarities with the short-period intermediate polar V1025 Cen lead us to suggest that DW Cnc is another intermediate polar below the period gap, and we tentatively identify the photometric and spectroscopic 38-min signals with the white dwarf spin period. DW Cnc has never been observed to undergo an outburst, but it occasionally exhibits low states \~2 mag fainter than its typical brightness level of V~14.5, resembling the behaviour of the high mass-transfer VY Scl stars.

Kilo-Second Quasi-Periodic Oscillations in the Cataclysmic Variable DW Cancri

Abstract Our photometric monitoring revealed that DW Cnc, which was originally classified as a dwarf nova ($ V=15 \hbox{--} 17.5$), remained at a bright state of $ R_\mathrm{c}=14.68\pm 0.07$ for 61 days. In conjunction with optical spectra lacking a strong He II emission line, we propose that the object is not a dwarf nova, but a non-magnetic nova-like variable. Throughout our monitoring, the object showed strong quasi-periodic oscillations (QPOs) with amplitudes reaching about 0.3 mag. Our period analysis yielded a power spectrum with two peaks of QPOs, whose center periods are $ 37.5\pm 0.1$ and $ 73.4\pm 0.4 \,\mathrm{min}$ and, furthermore, with a significant power in frequencies lower than the QPOs. DW Cnc is a unique cataclysmic variable in which kilo-second QPOs were continuously detected for 61 days. We propose two possible interpretations of DW Cnc: (i) A permanent superhumper below the period minimum of hydrogen-rich cataclysmic variables. (ii) A nova-like variable having an orbital period over 3 hours. In this case, the QPOs may be caused by trapped disk oscillations.

Detection of negative superhumps in a low-mass X-ray binary an end to the long debate on the nature of V1405 Aql (X1916053)

The detection of two similar periodicities (3001 and 3028 s) in the light curve of V 1405 Aql, a low mass X-ray binary (LMXRB), has attracted the attention of many observers. Two basic competing models have been offered for this system. According to the first, V1405 Aql is a triple system. The second model invokes the presence of an accretion disc that precesses in the apsidal plane, suggesting that the shorter period is the orbital period while the longer is a positive superhump. The debate on the nature of V1405 Aql has been continued until very recently. Re-examination of previously published X-ray data reveals an additional periodicity of 2979 s, which is naturally interpreted as a negative superhump. The recently found 4.8-d period is consequently understood as the nodal precession of the disc. This is the first firm detection of negative superhumps and nodal precession in a LMXRB. Our results thus confirm the classification of V1405 Aql as a permanent superhump system. The 14-yr argument on the nature of this intriguing object has thus finally come to an end. We find that the ratio between the negative superhump deficit (over the orbital period) and the positive superhump excess is a function of orbital period in systems that show both types of superhumps. This relation presents some challenge to theory as it fits binaries with different components. We propose that a thickening in the disc rim, which causes increased occultation of the X-ray source, is the mechanism responsible for both types of superhumps in LMXRBs. However, the positive signal is related only to the pronounced dips in the light curve, where the point-like central source is covered up, whereas the morphology of the negative superhump signal appears quite smooth, implying obscuration of a larger X-ray emitting region, possibly the inner accretion disc or a corona. According to our model, superhumps (both in the X-ray and optical regimes) are permitted in high-inclination LMXRBs contrary to the Haswell et al. prediction.

Nova Sagittarii 1998 (V4633 Sgr): a permanent superhump system or an asynchronous polar?

We report the results of observations of V4633 Sgr (Nova Sagittarii 1998) during 1998-2000. Two photometric periodicities were present in the light curve during the three years of observations: a stable one at P=3.014 h, which is probably the orbital period of the underlying binary system, and a second one of lower coherence, approximately 2.5 per cent longer than the former. The latter periodicity may be a permanent superhump, or alternatively, the spin period of the white dwarf in a nearly synchronous magnetic system. A third period, at P=5.06 d, corresponding to the beat between the two periods was probably present in 1999. Our results suggest that a process of mass transfer took place in the binary system since no later than two and a half months after the nova eruption. We derive an interstellar reddening of E(B-V)~0.21 from our spectroscopic measurements and published photometric data, and estimate a distance of d~9 kpc to this nova.

Spectroscopic evidence for the binary nature of AM CVn

We analysed archival spectroscopic data of AM CVn taken with the William Herschel Telescope in 1996. In the literature two orbital periods for AM CVn are proposed. A clear S-wave in the HeI 4471, 4387 and 4143 \AA lines is revealed when the spectra are folded on the 1029 s period. No signature of this S-wave is seen when folded on 1051 s. Doppler tomography of the line profiles shows a clear signature of the hotspot. Using this we can constrain the value of K_2 to lie between 210 and 280 km/s. Our work confirms the binary nature of AM CVn beyond any doubt, establishes 1028.73 s as the true orbital period and supports the interpretation of AM CVn as a permanent superhump system.

Thermal stability and nova cycles in permanent superhump systems

Thermal stability and nova cycles in permanent superhump systems

Superhumps in low-mass X-ray binaries

We propose a mechanism for the superhump modulations observed in optical photometry of at least two black-hole X-ray transients (SXTs). As in extreme mass-ratio cataclysmic variables (CVs), superhumps are assumed to result from the presence of the 3:1 orbital resonance in the accretion disc. This causes the disc to become non-axisymmetric and precess. However, the mechanism for superhump luminosity variations in low-mass X-ray binaries (LMXBs) must differ from that in CVs, where it is attributed to a tidally-driven modulation of the disc's viscous dissipation, varying on the beat between the orbital and disc precession period. By contrast in LMXBs, tidal dissipation in the outer accretion disc is negligible: the optical emission is overwhelmingly dominated by reprocessing of intercepted central X-rays. Thus a different origin for the superhump modulation is required. Recent observations and numerical simulations indicate that in an extreme mass-ratio system the disc area changes on the superhump period. We deduce that the superhumps observed in SXTs arise from a modulation of the reprocessed flux by the changing area. Therefore, unlike the situation in CVs, where the superhump amplitude is inclination-independent, superhumps should be best seen in low-inclination LMXBs, whereas an orbital modulation from the heated face of the secondary star should be more prominent at high inclinations. Modulation at the disc precession period (10 s of days) may indicate disc asymmetries such as warping. We comment on the orbital period determinations of LMXBs, and the possibility and significance of possible permanent superhump LMXBs.

Thermal stability and nova cycles in permanent superhump systems

Archival data on permanent superhump systems are compiled to test the thermal stability of their accretion discs. We find that their discs are almost certainly thermally stable as expected. This result confirms Osaki's suggestion that permanent superhump systems form a new subclass of cataclysmic variables (CVs), with relatively short orbital periods and high mass-transfer rates. We note that if the high accretion rates estimated in permanent superhump systems represent their mean secular values, then their mass-transfer rates cannot be explained by gravitational radiation, therefore, either magnetic braking should be extrapolated to systems below the period gap or they must have mass-transfer cycles. Alternatively, a new mechanism that removes angular momentum from CVs below the gap should be invoked. We suggest applying the nova cycle scenarios offered for systems above the period gap to the short orbital period CVs. Permanent superhumps have been observed in the two non-magnetic ex-novae with binary periods below the gap. Their post-nova magnitudes are brighter than their pre-outburst values. In one case (V1974 Cyg) it has been demonstrated that the pre-nova should have been a regular SU UMa system. Thus, it is the first nova whose accretion disc was observed to change its thermal stability. If the superhumps in this system indicate persistent high mass-transfer rates rather than a temporary change induced by irradiation from the hot post-nova white dwarf, it is the first direct evidence for mass-transfer cycles in CVs. The proposed cycles are driven by the nova eruption.

Mass Transfer Processes in Classical Nova Systems around Their Outburst Events and Beyond

A continuous photometric study of 12 novae was carried out for more than 365 nights during the past 4 years at the Wise Observatory in a motivation to extend our knowledge on these systems. Main results: we found observational evidence for the early presence of the accretion disc in Nova V1974 Cyg 1992 (permanent superhumps) and probably also in Nova V1425 Aql 1995 (classified as an intermediate polar candidate). We also predicted the future of superhumps in classical nova systems, and in particular the permanent superhumps in Nova V1974 Cygni 1992. They will either continue to prevail in its light curve, while the brightness of the nova stays well above its pre-outburst level, or the system will continue to decay and eventually evolve into a regular SU UMa system, with superhumps appearing only during superoutbursts.