From jop at astro.columbia.edu Tue Jul 10 18:07:16 2001 From: jop at astro.columbia.edu (Joe Patterson) Date: Tue, 10 Jul 2001 18:07:16 -0400 (EDT) Subject: (cba:news) New stars for July Message-ID: Dear CBAers, There must have been some cloudy nights on Mauna Kea, because Jonathan has been tinkering with the CBA website, and... voila! ... a new feature likely to be of much interest. Namely, a chronological listing of the 50 most recent data sets submitted to cba-data. This means that you can see who is observing a particular star and when -- and if you work hard you can bask in all the glory, hahaha. It updates every 15 minutes, and really hates to be idle. New stars... well it's time to end the vigil for V2400 Oph, on which Bob Rea has been observing up a storm. We have 30 nights now and two longitudes (NZ and SA, latter courtesy of Darragh O'Donoghue), that's enough for a 2001 study. Exit V2400. V1494 Aql... a star of extraordinary interest, but a hiatus is probably advisable. As Alon mentioned a few weeks ago, the waveform is much different from last year, and now resembles some of the "supersoft" binaries, like V Sge. Since it erupted 1.7 yrs ago, this is probably because it IS a supersoft binary right now - heated by a freshly erupted white dwarf still sitting there at 200,000 K. A continuous record is desirable, but we have a well-defined light curve now, and should wait 1-2 months for it to evolve further. This star will continue to delight for years to come! The two most interesting southern stars right are newcomers on this year's stage... V442 Oph (1732-16). We carried out a good observing campaign in 1995, before we had coverage at multiple longitudes. High time to do it right! Very well placed for observation too. V1223 Sgr (1855-31). We've only obtained scattered coverage over the years - this is the year, and season, to do it right. AND FOR BOREALITES... V795 Herculis is the all-star of the season, proudly displaying its new superhumps for all the (northern) world to see. Let's keep going for another month! Plenty of room for new observers, as the grizzled veterans are slipping off-line to various locations in observer-space (telescope repair, movies, the beach, who knows, maybe even golf). And V442 Oph. Very nice to get some USA (or African, or South American?) coverage to complement the expected high dose of Kiwi data. joe From jop at astro.columbia.edu Tue Jul 24 08:46:36 2001 From: jop at astro.columbia.edu (Joe Patterson) Date: Tue, 24 Jul 2001 08:46:36 -0400 (EDT) Subject: (cba:news) WZ Sge! Message-ID: Goodness gracious, the little guy has gone off again! All scopes, all longitudes, latitudes, platitudes, attitudes, beatitudes... fire away on WZ Sge as long as possible as soon as possible. It's about mag 8.5, so use short integrations to avoid saturation. (Don't forget about this! - it can nail you if your saturation test is first carried out through cloud, then the sky clears...) Wow. More when I get to work! joe From jop at astro.columbia.edu Thu Jul 26 14:55:32 2001 From: jop at astro.columbia.edu (Joe Patterson) Date: Thu, 26 Jul 2001 14:55:32 -0400 (EDT) Subject: (cba:news) WZ Sge in eruption Message-ID: Dear CBAers, Well, WZ Sge has been bright for 4 days now, and we're getting some good light curves (Masi, Skillman, Allen). We could use a *lot* more coverage, though. It's the brightest and nearest dwarf nova, and has chosen to erupt at a perfect time - when it's crossing the meridian near midnight. It may teach us a great deal about dwarf nova superoutbursts by the time the eruption is finally over. In 1978, the star erupted in December, a very awkward time for Sagitta. So fire away on WZ! You'll want to use a *short* integration time and study the images for possible saturation, since it's still very bright at about 8.7. Mainly we want long light curves, our usual thing. Most people are using GSC 1621:1830 as a comp star, but you'll want to change when the star gets a lot fainter. At +17 deg, it's somewhat accessible to the southerners too, and since they're at a critical longitude, I hope they can get some data too. Lemme know if you'd like me to send the running light curve. Many big scopes are slewing around towards WZ Sge these days too. In fact Chandra is making two observations in the next 2 days (earliest is tonight). Here are the times: July 27 244-850 UT July 29 1649-1840 UT Simultaneous optical coverage will be extra valuable of course! Happy observing! joe From jop at astro.columbia.edu Mon Jul 30 09:30:38 2001 From: jop at astro.columbia.edu (Joe Patterson) Date: Mon, 30 Jul 2001 09:30:38 -0400 (EDT) Subject: (cba:news) WZ Sge, and its period(s?) (fwd) Message-ID: Dear CBAers, I just sent this note to Gianluca Masi, and it seemed like it might be of interest to others. So far it appears to me that WZ Sge is bumping merrily along at its orbital period, as in 1978. The flickering amplitude is *remarkably* high for a dwarf nova in outburst - higher than any other I know (most outbursting DN just sit there like light bulbs). From the 1978 analogy, a transition to a longer period might perhaps begin somewhere around August 3+-4. I'm going to Germany for a CV meeting on Saturday, and I guess I'll be out of the email loop for about a week - probably able to read email, maybe able to answer, but probably not able to do any analysis of data till I get back. If you'd like to receive swaths of WZ Sge data, please let me know. I'll write again before I leave for sure. joe Dear Gianluca, Thanks for all that data - what a great week you are having! Let's see, I'm pretty sure I have all the data through JD 119.9 now, a total of 5.5 days. From my inspection I don't see any definite evidence for period changes. Naturally there could be some, but it's not big enough for me to measure credibly. One thing you might want to consider... Period-measuring procedures are notoriously inaccurate in the presence of an additional noise source of comparable amplitude. The way to get around this is to accumulate many cycles, i.e. densely packed data over a long baseline. One night is definitely too little for any secure conclusion, and two is borderline - three is a lot better. Some people use a "Rule of Ten" - i.e. you need 10 cycles sampled for the properties of the periodic process to be decently measurable. Of course it depends on the size of the noise ... you wouldn't have to be as conservative if the flickering is small, and you might need as many as 20 cycles if the flickering were, say, bigger than the periodic process. I've noticed another Rule of Ten in my experience with CVs. Maybe I should call it the Rule of Point Ten. It's this: the error of a frequency measurement is about 0.10/N cycles per day, where N is the the duration of the time-series in days. I've been impressed with how universally this applies to real data. For WZ Sge, it implies an error of ~2 min in a single-night measurement, or ~25 s if a two-night (consecutive) time series is available. This error is larger by a factor of 10-20 than some of the errors being quoted now in internet postings. On the other hand, some people derive errors from common statistical notions, like "half-width at half-maximum". This latter prescription always gives errors that are too big, by about a factor of 2. At the risk of sounding silly, I'll reduce the latter point to another Rule of Ten: that the actual 1-sigma error in frequency is the half-width of the power-spectrum peak 10% of the way down from maximum. (Or: half-width at 90% of max). Works pretty damn well. (The reason we have to invoke "rules of thumb" etc., rather than just learn from statistics books, is that we always make these measurements in the presence of noise sources - usually flickering - of unknown properties.) Good luck pursuing WZ Sge - so far you've been the world leader in getting these great light curves, I'm sure you can take a little rest ... but not too much! Oh, one last Rule of Ten. For this advice I hereby rule that you now owe me ten million dollars. At an offshore bank, please, so I don't have to pay those bothersome taxes. joe