CBA Belgium Observatory
Extremely rare outburst of WZ Sge - August 2001 Light curves

WZ Sge is among the most important cataclysmic variables. Therefore it's no surprise that the July 2001 outburst immediately attracted the highest attention of amateur and professional astronomers, who organised world-wide ground-based and space-based observing campaigns. 

The current outburst of WZ Sge was detected by T. Ohshima (Japan) on July 23, 2001 and reported to VSNET by T. Watanabe. We refer to the VSNET web page on WZ Sge for a full account of the outburst history and for further background information.

Below, I present the results obtained at CBA Belgium Observatory during the month of August 2001. You will notice that my participation in the observing campaign of WZ Sge started very late (August 09/10). Reason for this is that I was on holiday during the entire early stage of the outburst, and when I returned on August 3rd, the Belgian weather was not really inclined to cooperate. I'll be absent again between August 13 and 17, but the predictions for the next couple of nights look reasonably good.

The light curve below was obtained on August 9/10, during a 2.8 hours run, using the 0.35-m f/6.3 SCT and ST-7 CCD (unfiltered) of CBA Belgium Observatory. Sky conditions were extremely good initially, but degraded after about 2 hours, and finally clouds prevented further observations. The light curve shows a number of extremely interesting phenomena : next to the genuine superhumps (main peak), there are 'secondary superhumps', as also pointed out by Gianluca Masi, who obtained identical results during his August 09/10 observing session (based on a longer time basis). The observations furthermore confirm Masi's statement about the existence of a 'constant brightness' plateau of about 20 minutes, that appears just after a WZ Sge eclipse.

It will be exciting to see how these features evolve over the coming nights.



The night of August 10/11, 2001 was of moderate quality, with some cloud fields hampering the observing session. Nevertheless, we got a pretty interesting light curve, that is presented below.  There are some striking differences with WZ Sge's behaviour of the night before. First of all, the 'secondary superhump' now is dominating the true superhump, both being almost equal now. More pronounced is also a small hump, that emerges just after an eclipse of WZ Sge. Please note that the depth of the eclipses is very small, in the order of about 0.05 mag. The plateau, that we observed yesterday, seems to have disappeared completely. What suprises will the next night bring ?

One night later, on August 11/12 2001, the light curve again has changed considerably. We got an excellent 5.6 hours run, with only some cirrus clouds, but no interruptions during the entire run. Most obvious in WZ Sge's light curve now is the single peaked superhump, with a slight hint of a 'secondary'  hump-like feature in the lowering ("right end") part of the genuine hump. This feature furthermore becomes more pronounced towards the end of the run. The more pronounced eclipse is present in our light curve too. I have never before seen such a 'dynamic' dwarf nova !
Since I was away from home from August 13 till August 17, my next occasion for observing WZ Sge was on August 17/18 2001. Sky conditions were very good initially, but gradually clouds entered the field, and finally the sky got completely covered. The light curve below therefore shows a number of gaps, but nevertheless indicates the rapid decline phase of the variable (fading rate about 1 mag/d). Next to the large superhumps, one can recognize eclipses in the light curve, as well as some smaller scale modulations. 
The night of August 21/22 2001 was not a particularly favourable photometric night, and also hampered by low clouds. We nevertheless managed to collect a dense time-series, over a period of about 6 hours. The resulting light curve is depicted below. The blue light curve is an averaged profile, whilest the gray curve shows the original data.The figure clearly shows a number of interesting features. First, the rebrightening of WZ Sge, which was still very obvious in the hours preceding our observations, seems to have come to an end. The superhumps are much smaller than in any of our previous observing nights, having an amplitude of 'barely' 0.06 mag. In between superhumps, there are a number of dips. The most pronounced dip (the one immediately preceding the superhump peaks) likely is to be attributed to eclipses.  
5.8 hours of nearly uninterrupted time-series photometry - that is the result of our WZ Sge run during a quite good August 22/23 night at CBA Belgium Observatory (see corresponding light curve below). Most striking is the fast fading trend of the variable, after its recent rebrightening, and the low amplitude of the superhumps. The light curve also has a much 'cleaner' outlook, with almost no dips left anymore. It seems that eclipses, which were still obvious in yesterday night's light curve, now have completely disappeared. It will be interesting to further track WZ Sge over the next nights and to see if another outburst develops, or if the fading trend continues.
WZ Sge's light curve of Aug 23/24, 2001 is presented below. Superhumps with an amplitude of about 0.3 mag and sharp peaks are dominating again, but are clearly interrupted by dip-like phenomena of about 0.1 mag, some of which probably are to be attributed to eclipses. 
Below is the Aug 24/25 light curve of WZ Sge, obtained under very clear and stable skies. The run started at about 19:50 UT and continued for 7.1 hours. I was using 20 sec (unfiltered) exposure times. The resulting  light curve is quite impressive and reveals a quick fading of the variable (0.27 mag in approx. 7 hours, or about 0.9 m/d), clear double-peaked superhumps, and multiple well-pronounced dips, mostly in the declining branch of the superhumps. The eclipses are the deep dips, immediately following the second superhump peak. Interesting to see how the overall profile of the light curve has been changing compared to previous nights.
August 25/26 was again a very clear and stable night, but also very warm, and therefore not perfect for CCD photometry. We nevertheless collected a relatively good time series of unfiltered data during 6 hours, which is graphically depicted below. The fading trend of the variable clearly is continuing (at a rate quite similar to yesterday, i.e. 0.9 m/d), and the complex modulations not only persist, but even increased. It's getting more difficult to distinguish superhump peaks from other modulations and to differentiate eclipses from other dips.
My August 27/28 run was performed under excellent weather conditions, and revealed an intriguing light curve, almost as complex in structure as the one obtained last night. Obvious elements in WZ Sge's light curve are the growth (towards the end of the run) of the humps, and the two smaller humps that exist in between (super)hump peaks. Following other observations posted to VSNET, WZ Sge underwent its 4th rebrightening, immediately following the end of my session.
The August 28/29 light curve (see below) of WZ Sge, also obtained under excellent conditions, seems to be even more spectacular than the Aug 27/28 one. It shows a steep decline in WZ Sge's brightness (about 1.1 mag/d), following the 4th rebrightening of the dwarf nova. Quite spectacular too is the very complex structure of the humps : a lot of them have more or less the same amplitude now (about 0.13 mag), making it very difficult to distinguish superhumps (if still there ?) from other hump structures. I never considered that variable star photometry could show that many interesting variations on such a short time scale (hours !!).
The night of August 29/30 was clear also (interrupted by two clouds fields), and resulted in one of my most impressive WZ Sge .light curves. I collected 714 CCD images over a period of nearly 7.8 hours, between August 29.81 UT and August 30.13 UT. During the initial 4.0 hours of the run, WZ Sge was humping more or less at a constant brightness level, showing again complex modulations. But, it then gradually started to brighten at a slow rate first, suddenly followed by an extra-ordinary increase of 0.8 mag in 2.4 hours (or 0.33 mag per hour !). This became the fifth rebrightening of WZ Sge during its present outburst !
My observing session of WZ Sge on Aug 31 / Sep 01 was a very short one, because of the cloudy sky. I managed to collect data between August 31.81 UT and 31.90 UT. The resulting light curve (see below) shows another very steep brightening of WZ Sge, as predicted by Kato-san in [vsnet-campaign-dn 1456], i.e. 0.41 mag in almost 2.2 hours. This is already the sixth (!) rebrightening of the variable, since the start of the outburst. You will also notice the dips and humps in the rising trend of the variable, but I haven't been able to analyse these any further.
Click here for my September 2001 light curves of WZ Sge


                                                                                                                                      (c) Copyright Tonny Vanmunster, 2001