SU UMa-type Dwarf Novae Research Articles

Physical parameter eclipse mapping of the quiescent disc in V2051 Ophiuchi

We analyse simultaneous UBVR quiescent light curves of the cataclysmic variable V2051 Oph using the Physical Parameter Eclipse Mapping (PPEM) method in order to map the gas temperature and surface density of the disc for the first time. The disc appears optically thick in the central regions, and gradually becomes optically thin towards the disc edge or shows a more and more dominating temperature inversion in the disc chromosphere. The gas temperatures in the disc range from about 13 500 K near the white dwarf to about 6000 K at the disc edge. The intermediate part of the disc has temperatures of 9000 to 6500 K. The quiescent disc (chromosphere) shows a prominent bright region with temperatures of 10 500 K around the impact region of the stream from the secondary with an extension towards smaller azimuths. The disc has a size of 0.53±0.03Rℒ1 and a mass accretion rate ℳ˙=1015 to 1017 g s−1. The light curves must include an uneclipsed component, a hot chromosphere and/or a disc wind. The PPEM method allows us to determine a new distance of 146±20 pc, compatible with previous rough estimates. For the white dwarf we then reconstruct a temperature of 19 600 K, if the lower hemisphere of the white dwarf is occulted by the disc. We suggest that the accretion disc is a sandwich of a cool, optically thick central disc with hot chromospheric layers on both sides, as was suggested for HT Cas. This chromosphere is the origin of the emission lines. We find that although V2051 Oph is very similar to the SU UMa-type dwarf novae HT Cas, OY Car and Z Cha, there must be a substantial difference in order to explain its unique light curve. The reason for the difference could be a higher mass transfer rate caused by the more massive secondary, and/or a small but significant magnetic field of the white dwarf, just strong enough to disrupt the innermost disc.

Discovery and photometric study of the new in-the-gap SU UMa Dwarf Nova Var 73 Dra

We present the discovery and the first investigation of the new dwarf nova Var 73 Dra. We carried out monitoring and time-resolved CCD observations in 2001 August-October and detected several normal outbursts with a recurrence time of about 7-8 days and one slightly brighter superoutburst with a plateau phase that lasted 13 days. Our photometry during the superoutburst clearly shows the presence of superhumps with the period 0.0954 (0:0001) day (although the one-day alias period, 0.1053 d, could not be ruled out). So, the observed features allow us to classify Var 73 Dra as a new SU UMa-type dwarf nova within the well-known gap of orbital period distribution of cataclysmic variables.

On the Origin of Early Superhumps in WZ Sge-Type Stars

Abstract The origin of early superhumps, which are double-wave periodic modulations seen only during the earliest stage of a WZ Sge-type outburst, has not been well understood. Based on a recent discovery of two-armed arch-like patterns on Doppler tomograms in conjunction with early superhumps, we propose a new interpretation on the origin of early superhumps, following a new interpretation by Smak (2001, Acta Astron., 51, 295) and Ogilvie (2002, MNRAS, 330, 937) of the two-armed pattern seen in IP Peg. If we consider irradiation of the elevated surface of the accretion disk caused by vertical tidal deformation, we can consistently explain the observed features on Doppler tomograms and photometric waves at the same time. We interpret that a combination of low mass-ratios ($ q$) and low mass-transfer rates, necessary to give rise to these tidal effects, discriminate WZ Sge-type stars from other SU UMa-type dwarf novae. Based on recent fluid calculations, such an effect would be observable in higher $ q$ systems. We interpret that RZ Leo is an example of such objects.

Optical Observations of XTE J1118 + 480 during the 2000 Outburst

Abstract We report on photometric and spectroscopic observations of a possible halo black-hole X-ray nova, XTE J1118$ +$480 ($ =$ KV UMa) during outburst. Our photometric monitoring during the main outburst revealed that the optical maximum as well as the onset of the outburst precede those in the X-ray region. This indicates that the event was an “outside-in” type outburst and that its optical flux was dominated by viscous heating, itself, and not the effect of X-ray irradiation. Based on these results, we suggest an outburst scenario analogous to superoutbursts in SU UMa-type dwarf novae. This scenario predicts a superhump phenomenon, which we indeed detected throughout the outburst. We determined its period to be $ 0.170529\pm 0.000006 \,\mathrm{d}$, which is slightly longer than the orbital periods suggested from spectroscopic observations. We have furthermore revealed the first evidence of a continuous period decreasing in X-ray novae. The most prominent feature in our optical spectrum is a double-peak He II 4686Å emission line having an asymmetric profile with an outstanding blue side peak. Using a Doppler mapping method, we found that the He II emission originates from the accretion disk, which particularly concentrates on the hot spot. The time that the blue peak becomes strongest corresponds to a superhump peak. This implies that we see an elongated side of an eccentric disk at that time and, hence, it may cause an asymmetric emission profile. Substituting the observed fractional superhump excess for a theoretically expected relation between it and the mass ratio, we estimate that the black-hole mass is larger than $9.5 \,{{{M}_{\odot}}}$. XTE J1118$ +$480 thus has a large mass of a compact object compared with the typical black-hole X-ray novae.

IR Com: Deeply Eclipsing Dwarf Nova Below the Period Gap —A Twin of HT Cas?

Abstract We observed an X-ray selected, deeply eclipsing cataclysmic variable IR Com ($=\mathrm{S}~10932$). We detected an outburst occurring on 1996 January 1. The light curve of the outburst closely resembled that of a normal outburst of an SU UMa-type dwarf nova, rather than that of an intermediate polar. Time-resolved photometry during the outburst showed that eclipses became systematically deeper and narrower as the outburst faded. Full-orbit light curves in quiescence showed little evidence of orbital humps or asymmetry of the eclipses. In addition to the presence of high–low transitions in quiescence, the overall behavior of the outbursts and the characteristics of the eclipse profiles suggest that IR Com can be best understood as a twin of HT Cas, a famous eclipsing SU UMa-type dwarf nova with a number of peculiarities.

Photometry of the 1991 Superoutburst of EF Pegasi: Super-Quasi-Periodic Oscillations with Rapidly Decaying Periods

Abstract We observed the 1991 October outburst of EF Peg. Prominent superhumps with a period of 0.08705(1)d were observed, qualifying EF Peg as being a long-period SU UMa-type dwarf nova. The superhump period showed a monotonic decrease during the superoutburst, which makes a contrast to the virtually zero period change observed during the 1997 superoutburst of the same object. Large-amplitude and highly coherent quasi-periodic oscillations (super–QPOs) were observed on October 18, when superhumps were still growing in amplitude. Most strikingly, the QPOs showed a rapid decrease in the period from 18 min to 6.8 min within the 3.2-hr observing run. Such a rapid change in the period has not been observed in any class of QPOs in cataclysmic variables. We propose a hypothesis that the rapid decrease of the QPO period reflects a rapid removal of the angular momentum from an orbiting blob in the accretion disk, via viscosity in a turbulent disk. A brief comparison is given with the QPOs in X-ray binaries, some of which are known to show a similar rapid decrease in the periods.

V344 Lyr: an unusual large-amplitude SU UMa-type dwarf nova with a short supercycle

We studied the large-amplitude SU UMa-type dwarf nova V344 Lyr. A combination of our observations and reports to VSNET has yielded an extensive coverage of outbursts between 1994 July and 2001 June. The analysis of this data showed a mean supercycle length of 109.6 d. This value is one of the smallest among known SU UMa-type dwarf novae (except unusual ER UMa-type dwarf novae). The outburst amplitude of V344 Lyr (~5.5 mag) is found to be much larger than those (3.5 +/- 0.3 mag) of SU UMa-type dwarf novae with similar supercycle lengths. Such a deviation of the amplitude in V344 Lyr is difficult to explain by the inclination effect. The extreme outburst parameters of V344 Lyr would require an additional mechanism to effectively reduce the quiescent luminosity or to increase the outburst frequency.

1998 Superoutburst of the Large-Amplitude SU UMa-Type Dwarf Nova WX Ceti

Abstract We observed the 1998 November superoutburst of WX Cet, a dwarf nova originally proposed as a WZ Sge-like system. The observation established that WX Cet is an SU UMa-type dwarf nova with a mean superhump period of 0.05949(1)d, which is 2.1% longer than the reported orbital period. The lack of early superhumps at the earliest stage of the superoutburst, the rapid development of usual superhumps, and the possible rapid decay of late superhumps seem to support that WX Cet is a fairly normal large-amplitude SU UMa-type dwarf nova, rather than a WZ Sge-type dwarf nova with a number of peculiarities. However, a period increase of superhumps at a rate $\dot{P}/P = +8.5\pm 1.0 \times 10^{-5}$ was observed, which is one of the largest $\dot{P}/P$ ever observed in SU UMa-type dwarf novae. A linear decline of light, with a rate of $0.10 \,\mathrm{mag} \,\mathrm{d}^{-1}$, was observed in the post-superoutburst stage. This may be an exemplification of the decay of the viscosity in the accretion disk after the termination of a superoutburst, mechanism of which is proposed to explain a variety of post-superoutburst phenomena in some SU UMa-type dwarf novae.

Photometric Observation of a New in-the-Gap SU UMa-Type Dwarf Nova V 725 Aquilae during the 1999 Superoutburst

Abstract We report our photometric observation of the 1999 September outburst of a dwarf nova, V725 Aql. Our detection of superhumps established that it belongs to SUUMa stars. By performing a period analysis of the superhumps, we revealed a long superhump period of $0.09909 \pm 0.00015\,\mathrm{d}$, which implies that its orbital period is well over 2 hr. The outburst light curve is typical for a superoutburst, though, after the superoutburst, the object experienced a re-brightening which can be seen in some SUUMa stars with short orbital periods. The 1999 superoutburst occurred about 1650 d after the last possible superoutburst in 1995. This indicates the supercycle of V725 Aql to be long, over 1000 d. This long supercycle implies a low mass-transfer rate of this star.

Extreme Ultraviolet ExplorerObservations of OY Carinae in Superoutburst

The Extreme Ultraviolet Explorer (EUVE) satellite was used for three days beginning on 1997 March 26.96 UT to obtain photometric and spectroscopic observations of the eclipsing SU UMa-type dwarf nova OY Carinae in superoutburst. Because of the longer time on source (143 ks), the larger number of eclipses observed (17), and the higher count rate in the detector (0.5-2.2 counts s -1), we are able to significantly strengthen previous reports that there is little or no eclipse by the secondary of the EUV emission region of OY Car in superoutburst. The mean EUVE spectrum extends from 70 to 190 A and contains broad (FWHM ≈ 1 A) emission lines of N V, O V-O VI, Ne V-Ne VII, Mg IV-Mg VI, Fe VI-Fe VIII, and possibly Fe XXIII. Good fits of the observed spectrum are obtained with a model (similar to that of Seyfert 2 galaxies) wherein radiation from the boundary layer and accretion disk is scattered into the line of sight by the system's photoionized accretion disk wind. It is possible to trade off continuum luminosity for wind optical depth, but reasonable models have a boundary layer temperature Tbl ≈ 90-130 kK and a boundary layer and accretion disk luminosity Lbl = Ldisk 4 × 1034 ergs s-1 ≈ 10 L☉ corresponding to a mass accretion rate a 10-8 M☉ yr-1; an absorbing column density NH ≈ 1.6-3.5 × 1019 cm-2; and a wind mass-loss rate w 10-10 M☉ yr-1 ≈ 0.01a. Because radiation pressure alone falls an order of magnitude short of driving such a wind, magnetic forces must also play a role in driving the wind of OY Car in superoutburst.

Discovery of a New SU UMa-Type Dwarf Nova, V 1028 Cygni

Abstract V-band high-speed CCD photometry of V 1028 Cyg revealed that hump structures existed in the early stage of its 1995 July outburst. We obtained 0.06154 (± 0.00002) d as the best estimated period of superhumps. We confirmed that this dwarf nova belongs to the SU UMa stars. A re-brightening occurred after the superoutburst, and may be associated with a normal outburst. An analysis of the timings of superhump maxima showed clear evidence for an increase of the superhump period (Pdot = Ṗ/P) with 7.54 × 10−6. Only a few stars (AL Com, T Leo, SW UMa, etc.) are reported to have such an increase of the superhump period. At the end of the superoutburst, we detected that the amplitude of the superhumps increased considerably. Both of these properties are not typical features of ordinary SU UMa stars. The connection between the WZ Sge-type and the SU UMa-type dwarf novae is briefly discussed.

Outburst Characteristics in the Dwarf Nova SU Ursae Majoris

ABSTRACTThe 1973–1999 V‐band light curve from AAVSO visual estimates and RoboScope CCD exposures is examined to study the systematic properties of normal outbursts and superoutbursts in this prototype of the SU UMa–type dwarf novae. A number of the outbursts’ correlations previously reported in VW Hyi and in SU UMa are not present in these data. Several lines of evidence suggest that, during this 26 year interval, variations in the mass‐transfer rate served to obscure some of the expected correlations. The M ˙ variation apparently caused a nearly complete cessation of outbursting in 1980–1983. We do find a correlation of the time since the last superoutburst with the mean brightness in the interval since the last superoutburst, supporting the notion that superoutbursts are a disk‐initiated phenomenon that occurs when the gas remaining in the disk after each normal outburst has accumulated to some critical value.

Discovery of a New Deeply Eclipsing SU UMa-Type Dwarf Nova, IY UMa (= TmzV 85)

We discovered a new deeply eclipsing SU UMa-type dwarf nova, IY UMa, which experienced a superoutburst in 2000 January. Our monitoring revealed two distinct outbursts, which suggest a superoutburst interval of ∼ 800 d, or its half, and an outburst amplitude of 5.4 mag. From time-series photometry during the superoutburst, we determined a superhump and orbital period of 0.07588 d and 0.0739132 d, respectively.

The 1996 Superoutburst of the New SU UMa–Type Dwarf Nova HS Virginis

ABSTRACTWe report long‐term and time‐resolved photometric observations of the dwarf nova HS Vir (=PG 1341−079). Time‐resolved photometry during the bright outburst in 1996 March revealed superhumps with a period of 0.08059 ± 0.00003 days, which first firmly confirmed HS Vir as a genuine member of SU UMa–type dwarf novae. HS Vir is rather unusual in the smallness (2.7–3.5 mag for brightest superoutbursts) of outburst amplitudes. The marked diversity inferred from the comparison with SX LMi may reflect different mechanisms responsible for the smallness of outburst amplitudes in these dwarf novae.

Superhumps and Accretion Disk Precession in TT Arietis

We have been conducting a long-term (1988-1998) photometric study of the nova-like variable TT Arietis. The main periodic signal in the star's light curve normally occurs at a period that varies but averages ~0.1329 days, which is about 3.5% shorter than the orbital period of the binary. In 1997, this signal disappeared and was replaced by a stronger signal 8.5% longer than the orbital period. This new wave strongly resembles thesuperhumps commonly seen in SU UMa-type dwarf novae during superoutburst. In superhump parlance, we could say that a negative superhump was replaced by a positive superhump (P > Porb${r orb}$ -->$t SUBgt {r orb}t/SUBgt $ -->). This could signify the development of an eccentric instability in the accretion disk. The two superhumps probably signify two types of disk precession: apsidal advance and nodal regression. TT Ari is an excellent candidate for observational studies that probe the origin of superhumps.

“Huruhata's Variable” Revealed: The WZ Sge-Type Dwarf Nova EG Cancri

Time-resolved CCD photometric observations of a dwarf nova EG Cnc were carried out during an outburst which occurred after a 19-year quiescent interval, since 1977 November. Our observations between 1996 December 2 and 14 revealed this outburst to be a definite superoutburst, eventually enabling a definite classification of this object as an SU UMa-type dwarf nova. This outburst was characterized by its long duration of more than two weeks and a large amplitude of at least 6.7 mag. The object showed two distinct types of superhumps: double-peaked modulations attributable to “early superhumps” with a period of 0.0582±0.00025 d in the earlier stage of the current outburst, followed by fully grown typical superhumps with a period of 0.0603±0.00006 d. We regard the period of 0.0582 d as being the orbital period of the object with a 3.7% superhump excess ratio. These characteristics suggest a strong resemblance to an extreme rare class of dwarf novae including WZ Sge, HV Vir, and AL Com with some other similarities between EG Cnc and the WZ Sge-type dwarf novae. The existence of double-peaked “early superhumps” in the early stage of superoutbursts may be a new defining criterion for the WZ Sge-type dwarf novae.

Two New SU UMa-Type Dwarf Novae with Superhump Periods of Over Two Hours: GX Cassiopeiae and V419 Lyrae

We succeeded in detecting superhumps in GX Cas and V419 Lyr during the 1994 long outburst and the 1995 outburst, respectively, and identified those stars as SU UMa stars. A period analysis gives 0.09297 (± 0.00005) d as the best-estimated superhump period of GX Cas, which is the third-longest one following TU Men and NY Ser. This superhump period suggests GX Cas to be located at the upper edge of the period gap of dwarf novae. The superhump period of V419 Lyr is estimated to be close to GX Cas, though the short coverage of superhumps prevented us from determining an accurate value. The recurrence cycle of the superoutburst (supercycle) and the normal outburst of GX Cas is estimated to be 386 d and 50–70 d, respectively, based on the VSNET data archive. This supercycle predicts that the next superoutburst in GX Cas will occur around 1998 November 10. Detailed observations of these stars may give important information concerning the study of dwarf-nova evolution as well as the criterion of the tidal instability.

CCD Photometry of SW Ursae Majoris during the 1996 Superoutburst

Differential CCD photometry of the 1996 April superoutburst of SW UMa was performed both at the Nicholas Copernicus Observatory and at the Ouda Station, Kyoto University. Although superhumps with a period of 0.05818 (± 0.00002) d and possible small-amplitude quasi periodic oscillations (QPOs) were detected, we failed to find the so-called super-QPOs. SW UMa has been proved to be the fourth SU UMa-type dwarf nova, following AL Com, V1028 Cyg, and HV Vir, in which an analysis of the superhump maxima timings revealed an increase in the superhump period.

Detection of Superhumps During the Outburst of BC UMA

BC UMa was observed during its rare outburst in 1994. The superhumps with period 0.0619 day are clearly apparent, accompanied by secondary superhumps. This confirms the classification of BC UMa as an SU UMa type dwarf nova. The maximum brightness is 12.5 mag, which is about six magnitude brighterthan quiescence phase, and it makes BC UMa be classified as a TOAD (Tremendous Outburst Amplitude Dwarf-nova). The superoutburst’s magnitude was fainter than the preceeding normal outburst in September 1990. This faint superoutburst is uncommon among SU UMa stars and also among TOADs. The period analysis of the lightcurve taken after the end of the outburst yields the period of 0.0618 day which is shows that late superhumps is in operation.

ORBITAL PERIODS FOR THE SU UMA-TYPE DWARF NOVAE UV PERSEI, VY AQUARII, AND V1504 CYGNI

ABSTRACT We present mean spectra of three SU-UMa type dwarf novae and report orbital periods based on radial velocities taken near minimum light. For UV Per we find Porb = 0.06489 ± 0.00011 d (= 93.44 ± 0.16 min); for V1504 Cygni, 0.06951 ± 0.00005 (= 100.10 ± 0.07 min), and for VY Aqr, 0.06309 ± 0.00004 d (= 90.85 ± 0.06 min). The period for VY Aqr is slightly shorter than the period determined by Augusteijn from velocities during outburst, and is more consistent with expectations based on the superhump period. For UV Per we also present timeseries photometry obtained on two nights. A weak modulation may be present at a period near, but formally distinct from, the orbital period. A magnitude sequence is presented.