WZ Sge : King of cataclysmics

On 2001 July 23, T. Ohshima discovered that WZ Sge, a prototype of WZ Sge-type dwarf novae, was in a bright outburst. The Kyoto team confirmed the outburst at 9.5mag on July 23 14:30 (UT) (vsnet-campaign 1009). This is the forth outburst, and the previous outbursts were observed in 1913, 1946, and 1978 (vsnet-campaign 1015).

Early superhumps

An observation by M. Reszelski shows the object was faint on July 22.969 (vsnet-campaign 1009).

CCD image during the outburst presented by the Kyoto team

K. Ayani promptly performed spectroscopy and revealed that Balmer lines were in generally absorption. In the spectrum on July 23, there is no helium emission line (vsnet-campaign 1022). The light curve obtained by the Kyoto team detected rapid brightening with a rate of 5 mag/d and birth of short-term modulations (vsnet-campaign 1025). The light curve provided by G. Masi following to the Kyoto run showed clear humps with a large amplitude, and furthermore, it had a longer periodicity compared with its orbital period (vsnet-campaign 1030).

Evolution of early superhumps

On July 24, large early superhumps with amplitudes reaching 0.5mag were reported (vsnet-campaign 1026, 1034). The period of early superhumps was reported to be 0.05662 day (vsnet-campaign 1030). H. Baba reported that optical spectra taken by K. Sadakane using 1.88m Okayama telescope on July 24 show double peak emission lines of He II and C III/N III which are not seen in spectra on July 23. Balmer lines had both absorption and emission components which varied with time (vsnet-campaign 1028).

On July 25, the spectra taken by K. Ayani also show He II and C III/N III emissions (vsnet-campaign 1033). The Kyoto team reported that the early superhumps were similar to those on July 24 during the early July 25 (vsnet-campaign-dn 1007), however their profiles changed from "V"-shaped to "U"-shaped one (vsnet-campaign-dn 1009). G. Masi reported that amplitudes of humps became smaller and the profiles were changing (vsnet-campaign 1032).

On July 26, G. Masi and F. Mallia reported that the profile of humps, particularly secondary maximum, varies with time as observed on July 25 (vsnet-campaign 1042). The decrease of hump amplitudes was confirmed in the light curve presented by the Nyrola team. Their data also show possible feature like QPOs (vsnet-campaign 1043).

On July 27, G. Masi reported that the hump profile became complex. He also reported that the short-term fluctuations became dominant (vsnet-campaign-dn 1019, 1029). T. Kato also reported similar characteristics from the Kyoto observation (vsnet-campaign-dn 1025).

On July 28, contrary to the last tendency, G. Masi reported that the profile became more regular and the period seems to be longer, which may imply that normal superhumps appeared (vsnet-campaign-dn 1030). T. Kato confirmed this longer periodicity, but noticed that such phenomenon was observed in the first night of the current outburst (vsnet-campaign-dn 1035).

On July 29, T. Kato performed period analysis during July 28.8 to 29.7. It yielded a period of 0.05669 d, which is in agreement not with superhump period, but with the orbital period (vsnet-campaign-dn 1043). The hump profile became more irregular in this phase (vsnet-campaign-dn 1052).

CCD image during the outburst presented by G. Masi

Spectrum presented by K. Ayani

Since July 30, the profile of main humps became broader and the secondary hump became stronger (vsnet-campaign-dn 1057). The period analysis of humps showed possible variation of period, however the ordinal superhumps are not apparently seen until August 4 (vsnet-campaign-dn 1066).

Light curve obtained by the VSNET collaboration team

Genuine superhump

Long waited superhumps finally appeared: The secondary humps (preceding to the main peak of the early superhumps) became stronger on August 4 (vsnet-campaign 1066). A longer period of humps were reported, which indicated genuine superhumps were evolving (vsnet-campaign 1068).

On August 5, the evolution of the superhumps were confirmed, and furthermore, their evolution was quite rapidly ongoing (vsnet-campaign 1070, vsnet-campaign 1085). T. Kato determined the period of true superhumps as 0.057157 +/- 0.000077 d (vsnet-campaign 1082).

Once the superhumps were evolved, however the profile showed a rapid variation: G. Masi commented that new secondary humps (following to the main peak of genuine superhumps) appeared since August 6 (vsnet-campaign-dn 1122).

On August 7, G. Masi reported that eclipses were re-occurring in excellent agreement with the ephemeris (vsnet-campaign-dn 1129). On August 8 and 9, the general fading almost stopped and the beat phenomenon of ~4 d modulation was observed (vsnet-campaign-dn 1133, 1188). On August 10, the secondary humps became further stronger and almost equivalent as those on August 9 (vsnet-campaign-dn 1156). The eclipses were weak at that time (vsnet-campaign-dn 1155). T. Kato commented that it may be possible that the unique profile was caused by the somwhow enhanced early superhumps at this stage (vsnet-campaign-dn 1163). On August 11, two humps gradually merged into single-peaked superhumps (vsnet-campaign-dn 1169, 1193). The eclipses were again appeared since August 10 and became deeper on August 11 (vsnet-campaign-dn 1170, 1184). On August 12, the fading was relatively rapid (vsnet-campaign-dn 1204). Eclipses seem to locate at the top of superhumps and looks very shallow (vsnet-campagn-dn 1207). On August 13, superhump profiles became somewhat complicate and a secondary maximum can be seen (vsnet-campaign-dn 1217). The flickerings or QPOs were still seen on the declining branch (vsnet-campaign-dn 1222). Eclipses located just on the ascending branch of superhumps and became more evident (vsnet-campaign-dn 1223). On August 14, the eclipses became deeper and more evident (vsnet-campaign-dn 1229). The hump maxima was not clear and showed complicate feature including two maxima, so called "M"-shaped profile (vsnet-campaign-dn 1233). The fading was almost stopped (vsnet-campaign-dn 1241). On August 15, the first peak was observed to move closer to the second one (vsnet-campaign-dn 1249), and then, made a main peak. A dip around the orbital phase 0.6 was observed (vsnet-campaign-dn 1250). On August 16, sharp eclipses were again emerging (vsnet-campaign 1100). The decline speed became faster again (vsnet-campaign 1099, 1101). E. Kuulkers commented that the only big difference between the current and the last outburst is that the main outburst lasted about 5 days longer during the 1978 outburst (vsnet-campaign-dn 1388).

Rapid decline and re-brightenings

On August 17, the object entered a rapid decline phase. During this phase the humps once became small (vsnet-campaign 1388). In the late phase, new humps were evolving at the orbital phase of 0.35-0.4 (vsnet-campaign 1106, 1107), and the decline speed became slower (vsnet-campaign-dn 1281). On August 18, another large humps were evolved and the fading trend almost stopped (vsnet-campaign 1110). The phase was different from that of August 17, so they are of diffenrent nature. (vsnet-campaign 1108). The amplitude of humps became about 0.35mag and the period was that of superhumps (vsnet-campaign 1109).

The object reached a minimum from the main outburst on early August 20 and showed a rapid re-brightening during the latter half of Aug. 20 (vsnet-campaign 1115). Superhumps were seen and their period during the dip was 0.057003 d, 0.56% longer than the orbital period (vsnet-campaign 1119). Their period was quite stable contrary to those observed during the main superoutburst (vsnet-campaign-dn 1310). Eclipses located at the peak of superhumps during this re-rising phase (vsnet-campaign-dn 1312, 1117). The object returned to 11mag on August 21 (vsnet-campaign-dn 1314, 1315). The amplitude of superhumps was decayed to 0.03-0.04mag with the re-brightening (vsnet-campaign 1121, 1125). After the second peak, the object was again rapidly faded on August 22 (vsnet-campaign 1127). S. Walker reported it faded 0.12mag in 4 hours (vsnet-campaign 1126). The amplitude of humps was growing and the profile became edge-like one (vsnet-campaign 1129). Eclipses disappeared with the fading (vsnet-campaign-dn 1336). During this rapid decline phase, quasi-periodic oscillations (QPOs) were detected (vsnet-campaign-dn 1338). The period was typically 3-5 min and the amplitude was about 0.1mag (vsnet-campaign 1130).

The object showed hints of re-brightening on August 23 (vsnet-campaign 1132). In early August 23, humps (0.3mag) and eclipses (0.1mag depth ) became prominent (vsnet-campaign 1133, 1135), and QPOs were less prominent (vsnet-campaign-dn 1348). The peak of the second re-brightening was observed on August 24. The magnitude was brighter than that seen in the first one (vsnet-campaign 1136). The object then again showed rapid fading. Doule-peaked humps were reported during the onset of this fading (vsnet-campaign-dn 1360). The fading rate was similar to that in the first re-brightening (vsnet-campaign 1137). The bottom of this fading was reported to be 12.0mag, which may be brighter than that in the first one (vsnet-campaign 1139). The humps were less intensive (vsnet-campaign-dn 1368). On August 25, time-series observations showed new humps at phases completely different from the previous superhumps. This phenomenon may be some sort of "late superhumps". The modulations of the period of ~20min were also newly revealed (vsnet-campaign 1140, 1141).

The third re-brightening was then reported on August 26 (vsnet-campaign-dn 1372). There were two humps in one orbital period and eclipses were less prominent (vsnet-campaign-dn 1392). After the peak, a rapid fading was again started (vsnet-campaign 1142). During the rapid fading phase, the QPOs were again observed on August 27 (vsnet-campaign 1143). The fading continued until early August 28 (vsnet-campaign-dn 1394).

The peak brightness of the fourth rebrightening may even surpass those of the three preceding ones. The profile of humps became a "triple wave" (vsnet-campaign 1145). The fourth rebrightening was terminated on August 28. During its fading phase, J. Pietz reported short-term variations with an amplitude of 0.1-0.15mag (vsnet-campaign-dn 1409) and eclipses became sharper and deeper (vsnet-campaign-dn 1420). The QPOs detected by M. Moilanen showed somewhat shorter period of 2-3min than that observed in the previous one (vsnet-campaign-dn 1427). At the bottom, the main hump was observed at phase 0.5 and shallow, slightly broad, eclipses were also present (vsnet-campaign-dn 1431).

A rapid rising to the fifth rebrightening with 0.15mag/hr was reported (vsnet-campaign-dn 1428). During this phase, there were weak periodic signals. The strongest hump signal resided just after the eclipse times. It may be that altough superhumps may have newly grown (vsnet-campaign-dn 1433). At the peak of the fifth rebrightening on August 30, the superhumps and eclipses became weak (vsnet-campaign-dn 1444). The duration of this peak of was a little longer (vsnet-campaign 1147), however faded on August 31 when humps and eclipses were again strong (vsnet-campaign 1150).

The sixth rebrightening was reported on August 31/September 1 (vsnet-campaign-dn 1458). The clear appearance of eclipses and humps at the sixth rebrightening maximum may suggest the presence of a precessing component, which was eclipsed at certain times (vsnet-campaign-dn 1460). The duration of the flat maximum exceeded 0.5 d (vsnet-campaign 1154). The complicated "triple" humps were observed (vsnet-campaign-dn 1463). The phase of eclipses particularly changed to a great extent. At maximum, the phase tended to be earlier than the predicted times. Later during the decline, the eclipses became quite asymmetric (vsnet-campaign-dn 1466). On September 2, the object faded from the sixth rebrightening. The QPOs with 2-5min were detected (vsnet-campaign-dn 1472). The "phase 0.6 dip" was also again observed (vsnet-campaign-dn 1474).

A new rebrightening was observed on September 2/3 (vsnet-campaign-dn 1473). At the top of the seventh rebrightening, eclipses were still seen and humps became weak (vsnet-campaign-dn 1490). It was faded on September 3 and the humps were again stronger (vsnet-campaign 1156).

The onset of the eighth one was reported on September 4 by L. Cook (vsnet-campaign-dn 1485). At the top, no strong QPOs were detected and two dips (not eclipse) were intensive (vsnet-campaign-dn 1496). The object started rapid fading within one day, which means that the period of rebrightening shortened to 1.5 d (vsnet-campaign-dn 1489).

On September 5, J. Pietz's observation detected the 9-th rebrightening (vsnet-campaign-dn 1501). D. Starkey's data showed defined humps and a hard, sharp eclipse during this rebrightening (vsnet-campaign-dn 1502). The main humps occurred earlier than before (vsnet-campaign-dn 1503). The hump profile was similar to that of the 4-th rebrightening, and it may be related to 9-d beat period (vsnet-campaign 1157).

The next 10-th brightening was detected on September 7 by D. Starkey's observations (vsnet-campaign-dn 1511). On the fading from the maximum, humps and eclipses, which were slightly earlier than predicted times, clearly appeared (vsnet-campaign-dn 1518). The rate of the fading seems to become larger (vsnet-campaign-dn 1519). Possible QPOs were reported in the data by the Kyoto team (vsnet-campaign-dn 1522).

The 11-th rebrightening was confirmed on September 9. The humps were seen but, eclipses were not very evident (vsnet-campaign-dn 1528). The phase of humps were different from those in the last rebrightening (vsnet-campaign-dn 1529). On September 10, the object was fading from the 11-th rebrightening. The humps became very apparent (vsnet-campaign-dn 1537), however their profiles were quite different from the previous one. It showed sinusoidal variations and no hump like feature (vsnet-campaign-dn 1538). The fading was ongoing, and the object became faintest since the first rebrightening (vsnet-campaign 1163).

The fading stopped (vsnet-campaign 1165), and the 12-th rebrightening was reported (vsnet-campaign-dn 1545). The object gradually brightened between Sep. 11.5 and 12.5. The rise time of ~1 d is extremely long compared to those of the previous rebrightenings (vsnet-campaign 1166). On this ascending, there were huge humps at phase 0.85. Sharp eclipses and phase 0.5 dip are also seen (vsnet-campaign-dn 1551). After the peak on September 12, the object again faded (vsnet-campaign-dn 1550). On early September 13, the fading continued, and then stopped this trend (vsnet-campaign-dn 1561). At the bottom, the amplitude of humps are reduced (vsnet-campaign-dn 1562). On September 15, the object remained the same magnitude and not in rapid fading or rebrightening (vsnet-campaign-dn 1568).

On all re-brightenings, the maxima of them have strictly followed the smooth extension of the decline of the main superoutburst. The damping nature of the oscillation may be naturally explained at a constant mass-transfer to the progressively shrinking accretion disk (vsnet-campaign-dn 1515).

Post-outburst phase

The data on September 16 showed a regrowth of the hump and broad eclipses, or superposed hump minimum, were also seen (vsnet-campaign-dn 1579). The period (September 14 - 16) was 0.05760(2) d, which is significantly longer than the orbital period, and even strikingly longer than the main superhump period (0.05719-0.05722 d) (vsnet-campaign-dn 1580). The amplitude was further grown to 0.4mag on September 17 (vsnet-campaign-dn 1585, 1588). The phase of superhump maxima changed onto eclipses on September 18 (vsnet-campaign-dn 1593). G. Masi and F. Mallia reported possible QPOs or short-term modulations on September 19 (vsnet-campaign-dn 1596).

The large humps suddenly disappeared on September 20. The light curve then showed low-amplitude complex humps. Eclipses also became less apparent, and seen as only a shallow dip-like feature (vsnet-campaign-dn 1605).

On September 21, the humps again became stronger. Eclipses became weaker (vsnet-campaign-dn 1610). The amplitude of humps were slightly reduced on September 23 (vsnet-campaign-dn 1621). The profile is similar to that on September 18, which supports the shortening of the beat period (vsnet-campaign-dn 1629). On September 24, the humps were complicated and eclipses became less apparent (vsnet-campaign-dn 1628). On September 27, the amplitude of humps was again grown to 0.35-0.6mag (vsnet-campaign-dn 1648). After September 28, the hump amplitude reduced to 0.2mag or more flatter (vsnet-campaign-dn 1656). Since September 29, the humps again became stronger (vsnet-campaign-dn 1664).

T. Kato reported that a combination of moving (late) superhumps and the steady orbital structure seems to partly explain the nightly variation of the hump structure (vsnet-campaign-dn 1643). On October 3, eclipses became prominent and then the amplitude of humps decreased (vsnet-campaign-dn 1679). The emergence of humps (~0.4mag) is reported on October 7 (vsnet-campaign-dn 1686).

The light curve on Oct. 12 show very clear humps (0.3 mag) and narrow, sharp eclipses (vsnet-campaign-dn 1710). On October 14, the hump amplitudes further decreased and their profile became double-peaked. Eclipses became very indistinct (vsnet-campaign-dn 1725). On October 16, the humps again showed regrowth (vsnet-campaign-dn 1731). The light curve on October 17 shows possible eclipses and humps at phase near 0.45. The amplitude of the possible variations was about 0.2mag. The amplitude seems to be smaller on October 19. We can see possible shallow and wide eclipses. The eclipses precede the prediction time on October 22. Two possible dips (or humps?) were seen (vsnet-campaign-dn 1780). The analysis of the Kyoto Oct. 25 data shows a jump (or eclipse-like fading) at phase 0.2 which has not been recorded in the recent light curves (vsnet-campaign-dn 1789). On October 30, the hump amplitude was 0.35 mag, with a general profile resembling singly humped superhump-like one (vsnet-campaign-dn 1816).

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WZ Sge : King of cataclysmics

Early superhumps

Genuine superhump

Rapid decline and re-brightenings

Post-outburst phase

General Information on This Object