Unique components of the WZ Sge-type dwarf nova SDSS J080434.20+510349.2

Detailed photometrical monitoring of the cataclysmic variable SDSS J080434.20+510349.2 began at the Crimean Astrophysical Observatory (Ukraine) and the Apache Point Observatory (United States) before the 2006 outburst, continued during the outburst, as well as the following two years. We established the unique nature of the primary and secondary components of the binary. We performed a comprehensive study of the white dwarf’s pulsations over the course of five months, two years after the 2006 outburst. It is shown that the most stable pulsations are equal to or double a period of 12.6 min. On the basis of all the available observations, more precise values for the orbital and the superhump periods were found to be 0.0590048(3) days and 0.059729(4) days, respectively. Our estimation of the mass of the secondary component lies in the range of solar mass from 0.037 to 0.087. This confirms the previous suggestion that the secondary component is most probably a brown dwarf.     

The dwarf nova WX Cet: a clone of WZ Sge or a pretender?

We present charge-coupled device (CCD) photometry of WX Cet in quiescence. Apart from the flickering which is characteristic to cataclysmic binaries, our data also reveal the periodic modulation of the brightness of WX Cet with a period of 0.058 27±0.000 02, with further restrictions on it. This period is derived from our data alone, but it agrees, within errors, with the spectroscopic period of Thorstensen et al. Hence the most likely spectroscopic and photometric periods are identical and correspond to the orbital motion. Our data were obtained during two observational seasons in 1990 and 1998. In the former season we observed what appears to be the ordinary orbital hump. However, in 1998 we observed both single- and double-hump orbital modulation. Several authors have noted the similarities between WX Cet and WZ Sge: the occurrence of rare, large-amplitude, long-lasting superoutbursts with superhump modulation, and the slow rate of decline. Both stars have similar, extremely short orbital periods. We recorded further similarities: the orbital modulation of brightness, with switching between single- and double-hump light curves. Patterson noticed that superhump excesses in WX Cet and WZ Sge are different in that they may fall on different evolutionary branches (pre-period minimum versus post-period minimum). We demonstrate that the masses of their white dwarfs differ by a factor of two.     

Analysis of the oscillations in HST observations of the quiescent SU UMa type dwarf nova WZ Sagittae

An analysis of the UV oscillations in WZ Sge is presented, in which we obtain the oscillation amplitude spectra. We find a strong 27.9-s oscillation in our Hubble Space Telescope ( HST ) UV and zeroth-order light curves as well as weaker oscillations at 28.4 s in the UV and 29.1 s in the zeroth order. We find that the main oscillation amplitude spectrum can be fitted with static white dwarf spectra of about 17 000 K, an accretion hotspot of only a few 100 K hotter than the underlying white dwarf temperature or a variety of cool (<14 500 K) white dwarf pulsation amplitude spectra. A pulsating white dwarf can also explain the very blue colour of oscillations of different periods previously found in the optical. Comparing our results with those of Welsh et al., we see that the amplitude spectra of the main oscillations in WZ Sge measured with different periods in data sets from different epochs are similar to each other. Our results raise questions about using the magnetically accreting rotating white dwarf model to explain the oscillations. We suggest that the pulsating white dwarf model is still a viable explanation for the oscillations in WZ Sge.     

Features of the orbital variability in the brightness of the WZ Sge type dwarf nova V1108 Her

Results are presented from photometric studies of the dwarf nova V1108 Her conducted at the primary focus of the 2.6-m G. A. Shajn Telescope at the Crimean Astrophysical Observatory during June-July 2008, 4 years after the 2004 outburst. An orbital period of 0.05672(4) days is found for the system. An analysis of observations made earlier during the 2004 outburst reveals an orbital signal which indicates that V1108 Her is an eclipsing system. The mass ratio of the secondary component to the white dwarf is estimated to be q = 0.068, which makes it highly likely that the secondary component of this system is a brown dwarf. The orbital light curves indicate a complex structure for the accretion disk whose radius has reached a 2:1 resonance. An explanation is suggested for a quasi-periodic modulation in the brightness at 1/4 of the orbital period observed in V1108 Her and other WZ Sge systems.     

A dwarf nova in Taurus

In searchs for flare stars in the vicinity of the Pleiades cluster, three flares were detected in 1970,1972, and 1977 in a star with the coordinates α ₁₉₅₀ = 3 ^h 48 ^m ·9, δ _l950 = 25‡15’.8. The star’s brightness at a minimum is >21 ^m ·5. The star was tentatively assigned to the U Gem type. To confirm this assumption, we examined photographic plates for the period of 1947–1987. As a result, we found 12 more flares. The average recurrence time based on the 1963–1977 observations is about 330 days, and the maximum flare amplitude is >6 ^m .Narrow superflares and steady-state flares lasting over 40 days have been observed in the star. The results show that this is a dwarf nova of the UGSU subtype. Translated from Astrofizika, Vol. 42, No. 1, pp. 47–52, January–March, 1999.     

The 1978 outburst of the recurrent nova WZ Sagittae

We give the results of photographic observations of the recurrent nova WZ Sge during its 1978 outburst, between December 5 and 25, using the double astrograph of Beijing Observatory. We discuss the observed properties and calculate the total energy released during the outburst to be about 1.07 (+40) erg. We estimate that the next outburst will occur around the year 2011.     

Unsolved problems of dwarf nova outbursts

Four problems are discussed. (1) Model light curves show significant increase of the disk luminosity during quiescence, an effect which is not present in the observed light curves. It is suggested that the slope of the lower branch of the Σ−T_e relation should be significantly decreased. (2) The width – P_orb relation for narrow outbursts is well reproduced with model data for α_hot=0.2. The bimodal distribution of outburst durations and, in particular, the origin of wide outbursts and the nature of their width – P_orb relation, require explanation. (3) It is suggested that problems with the thermal-tidal instability (TTI) model for superoutbursts might be solved by a hybrid model, combining the TTI model with the irradiation-enhanced mass-transfer model. A strong argument in favour of irradiation is provided by the ratio of the irradiating flux to the intrinsic flux of the secondary component, which turns out to be very large in the case of dwarf novae showing superoutbursts, with U Gem being a borderline case. (4) Characteristic time-scales observed during dwarf nova outbursts depend on the viscous time-scale, allowing an empirical determination of α. Three independent determinations, based on the rates of decline following outburst maximum, the UV delay observed during rising light, and the widths of outbursts, give consistently α_hot≈0.2. It should be added, however, that those time-scales depend also strongly on the radius of the disk. In this context it is disturbing to note that the observed disk radii appear to be smaller than those resulting from model calculations.     

The evolution of nova V5558 Sgr during the decline stage

We report the spectroscopic follow-up of the nova V5558 Sgr carried out in 2008 and 2009 at the Loiano Observatory, Italy, during the decline stage, as a part of an ongoing monitoring campaign. The spectra suggest that V5558 Sgr has entered the nebular stage, as shown by the presence of high ionization lines, and confirm that it is very similar to the slow nova V723 Cas, as previously suggested. We have estimated the decline time by three magnitudes, 170 ± 2 days, typical of slow novae. We have revised the estimation of the absolute magnitude at maximum (?6.3 to ?5.9) and the distance (1.3–1.6 kpc) of V5558 Sgr, in agreement with previous results. The revised white dwarf mass (0.58–0.63 M_⊙) confirms that V5558 Sgr is a critical system whose mass is close to the lower limit to trigger the nova outburst.     

Multifrequency behayiour of the dwarf nova SS Cygni

Several years of coordinated multifrequency observations of the dwarf nova SS Cygni have been performed in order to study the accretion behaviour of this system in particular, and in general to clarify this crucial problem for the cataclysmic variables.From IUE measurements, together with those from optical and IR ground based telescopes, we have found orbital modulations in the continuum and in the equivalent widths of the emission lines. During quiescent phases the behaviour of SS Cygni are dependent on the type of the preceding outburst (long-type or short-type).In this paper we will discuss the physical implications one can derive from these results.Paper presented at the 11th European Regional Astronomical Meetings of the IAU on New Windows to the Universe, held 3–8 July, 1989, Tenerife, Canary Islands, Spain.Based on the observations performed with IUE (VILSPA) satellite, and with the telescopes of the Loiano and Roque de los Muchachos Observatories.     

High-speed photometry of the dwarf nova SS Cyg

P. K. Shternberg State Astronomical Institute. Translated from Astrofizika, Vol. 29, No. 3, pp. 433–439, November–December, 1988.     

Photographic observations of the dwarf nova SS UMi

Observations on 4180 sky patrol plates of Sonneberg Observatory showed that this star belongs to the SS Cygni subclass of cataclysmic variables with cycle length between about 30 and 48 days. Possible orbital periods are discussed.     

Echelle-mepsicron time-resolved spectroscopy of the dwarf nova SS Cygni

High dispersion time-resolved spectrograms of the dwarf nova SS Cygni, obtained with the Echelle-Mepsicron system, show double peaked emission lines with a complex profile. The intensity of the H line appears to be modulated by the orbital period. Radial velocity measurements of the wings of H and of the absorption line system of the late-type star yield semiamplitude values of K_em=101±6 km s^–1 and K_ab=151±7 km s^–1, respectively. Radial velocity measurements of the blue and red peaks and of the central absorption of H reveal a synchronous movement with the broad wings, although there is some evidence of a narrow component probably associated with a hot spot in the disk or a chromospheric emission line from the secondary star. The H modulation, the double profile and recently discovered UBV light variations support an inclination angle i 50°. The masses of the primary are M_p=0.60 M and M_s=0.40 M, respectively. A detailed analysis of the absorption lines reveals a spectral type of K2V.Paper presented at the IAU Colloquium No. 93 Cataclysmic Variables. Recent Multi-Frequency Observations and Theoretical Developments, held at Dr. Remeis-Sternwarte Bamberg, F.R.G., 16–19 June, 1986.     

A TiO study of the dwarf nova IP Pegasi

We present red spectra in the region ∼ λ 7000–8300 Å of the eclipsing dwarf nova IP Peg, with simultaneous narrow-band photometry centred at 7322 Å. We show that by placing a second star on the slit we can correct for the telluric absorption bands which have hitherto made the TiO features from the secondary star unusable. We use these TiO features to carry out a radial velocity study of the secondary star, and find this gives an improvement in the signal-to-noise ratio of a factor of 2 compared with using the Na  i doublet. In contrast with previous results, we find no apparent ellipticity in the radial velocity curve. As a result we revise the semi-amplitude to K ₂=331.3±5.8 km s⁻¹, and thus the primary and secondary star masses to 1.05^-0.07_+0.14 M⊙ and 0.33^-0.05_+0.14 M⊙ respectively. Although this is the lowest mass yet derived for the secondary star, it is still overmassive for its observed spectral type. However, the revised mass and radius bring IP Peg into line with other cataclysmic variables in the mass–radius–period relationships.
By fitting the phase-resolved spectra around the TiO bands to a mean spectrum, we attempt to isolate the light curve of the secondary star. The resulting light curve has marked deviations from the expected ellipsoidal shape. The largest difference is at phase 0.5, and can be explained as an eclipse of the secondary star by the disc, indicating that the disc is optically thick when viewed at high inclination angles.     

Analysis of observations of the dwarf nova pegasi 2010

Analysis of photometric and spectroscopic observations of GSC 02197-00886 at the outburst maximum (on May 8, 2010) and at the stage of relaxation towards the quiescent (on August 4, 2010) was performed. Radiation of an optically thick accretion disc with a hot boundary layer dominates the spectra, which are consistent with the spectra of a WZ Sge-type dwarf novae. In the relaxation phase, an optically thin accretion disc with radiation in the HI and HeI emission lines is observed against the background of the absorption spectrum of a white dwarf. The parameters of GSC 02197-00886, which were determined by combining the radial velocities of the components with the assumption that the secondary component is close to mainsequence stars, differ significantly from the parameters that characterize other WZ Sge-type systems. We hypothesize that the secondary component was excited in the course of the outburst and experienced long-lasting relaxation towards the main-sequence state.     

Orbital period of the dwarf nova HL Canis Majoris

We present a small sample of time-resolved optical spectroscopy of the dwarf nova HL CMa during an outburst state. By combining radial velocity measurements with published data we show that the previously quoted value is not the only candidate for the orbital period of this system. We reduce the significance of daily aliasing but cannot distinguish between two periods at 0.2146±0.0004 and 0.2212±0.0005 d. We show that the low-excitation emission lines are composites from an accretion disc and the companion star, and that high-excitation emission originates in the disc or outflowing material associated with the accreting white dwarf.     

The unusual 2006 dwarf nova outburst of GK Persei

The 2006 outburst of GK Persei differed significantly at optical and ultraviolet (UV) wavelengths from typical outbursts of this object. We present multiwavelength (X-ray, UV and optical) Swift and AAVSO data, giving unprecedented broad-band coverage of the outburst, allowing us to follow the evolution of the longer-than-normal 2006 outburst across these wavelengths. In the optical and UV we see a triple-peaked morphology with maximum brightness ∼1.5 mag lower than in previous years. In contrast, the peak hard X-ray flux is the same as in previous outbursts. We resolve this dichotomy by demonstrating that the hard X-ray flux only accounts for a small fraction of the total energy liberated during accretion, and interpret the optical/UV outburst profile as arising from a series of heating and cooling waves traversing the disc, caused by its variable density profile.