|Proposer||(3360) Bradley Schaefer (email@example.com) obscode: SCK|
|Assigned To||(3663) Dirk Terrell|
|Date Submitted||May 7, 2019|
GET FOUR GOOD ECLIPSE TIMES FOR THE RECURRENT NOVA U SCORPII
My AAVSO Alert Notice #664 asked for continuous monitoring of the recurrent nova U Scorpii, because it is due to erupt anytime now. This has resulted in much activity by AAVSO observers and by professionals writing ToO and regular proposals for various ground-based telescopes and three spacecraft. As a result of the AAVSO observations coming from this Alert Notice, I now have two indirect-and-weak grounds for thinking that the eruption has already happened, perhaps around Thanksgiving 2016 or even around Thanksgiving 2017. This is the nightmare scenario where the entire 60-days of the eruption are invisible around the time of its close solar conjunction. There is nothing that can be done to help this possibility, and the nightmare scenario is that the eruption comes and goes completely-unobserved, and the world of observers keeps a long fruitless vigil, while many useless proposals are written. It is actually very hard to recognize such a nightmare scenario, except by a double length inter-eruption interval many years later. The best of the indirect methods is to see whether the O-C curve for the U Sco eclipses has undergone a sharp kink, with any such pointing to a missed eruption. But right now, I have no good eclipse timings since 2016.5. This proposal is to use the 0.61-m scopes of AAVSOnet to get four 3-4 hour time series centered on the times of mid-eclipse, so as to add good O-C points for 2019. This will provide a confident measure of the current O-C, and this can be compared to my very-well-measured parabolic O-C curve from 2010.4 to 2016.5. If I find good evidence that the O-C curve must have had a sharp kink sometime around 2017, then I will call off the AAVSO Alert, saving much effort. If I find no kink in the O-C curve, then the AAVSO Alert will keep going with redoubled importance. With the urgency of knowing whether the Alert should be continued, I am asking for these four eclipse times to be made before 18 May 2019 (i.e., 11 days from now).
U SCO HISTORY:
U Sco goes off every ten years or so, with the shortest known inter-eruption time being 7.9 years. Schaefer (2005, ApJLett, 621, L53) came up with a way to predict upcoming eruptions of recurrent novae, where the B-band flux provides a measure of how much material has accreted onto the white dwarf, and prior eruption histories calibrates how much mass is required to reach the nova trigger mass. Applied to U Sco, I predicted the next eruption in 2009.3±1.0. In 2008, I started organizing a large international effort to prepare for the upcoming eruption. A key part of this was to get AAVSO observers to nearly-continually monitor U Sco on a hair-trigger waiting for it to go up, all set to notify the world fast. I had some grant money that I funneled to AAVSO HQ for help in serving as a central clearinghouse. B. Harris and S. Dvorak independently discovered U Sco rising in January 2010, and this triggered many spacecraft observations and many observers from the ground. For the next 65 days, we had photometry averaging once every 2.3 minutes, and had seven spacecraft observing at all wavelengths (Schaefer et al. 2010, AJ, 140, 925; Pagnotta et al. 2015, ApJ, 811, 32). This is even now the all-time best observed nova event of any type. We discovered two new classes of phenomena (the inexplicable short flares during the transition phase and the weird dippers late in the tail.).
THE U SCO ERUPTION ROUND ABOUT NOW:
With this, I got two new data dumps, pointing to the eruption already having gone. First, additional data from 2011 and 2012, along with more detailed calculations, point best to an eruption date of 2016 or 2017. Second, MGW provided an ingress and egress for the U Sco eclipse, and this pointed to a sharp kink in the O-C curve. Both of these indirect arguments are weak. The theory model is just a model, with one great success (for U Sco in 2010) and one great failure (for T Pyx in 2011). The MGW eclipse time had huge error bars, so the existence of a kink in the O-C curve is not of high confidence. The AAVSO data shows two large seasonal gaps during which the U Sco eruption could easily be hidden. So this is the nightmare scenario, where an entire U Sco eruption can be lost as being completely inside the solar gap. This likely is what happened in 1927 and 1957. Suddenly, we have the horror story of not knowing whether to call off the AAVSO Alert or whether to redouble the efforts.
What to do? It is actually very hard to realize that an eruption is over and was missed. U Sco does not eject a visible shell, and it would take a long time to appear anyway. The photometry and spectroscopy just after an eruption is the same as just before. In principle, we could wait until 2030 and then declare that an eruption was missed sometime a decade previously. This basic nightmare dilemma was recognized and agonized over back in 2009 and earlier this year. Then and now, the best way to spot a missed eruption is to catch a kink in the O-C curve, but this is only possible a year or three after the eruption is long gone. In the meantime, all we can do is watch and wonder.
Well, maybe U Sco erupted a year or two ago, and we can now spot the kink in the O-C curve? I already have a wonderful O-C curve from 2010.4 to 2016.5, including 59 eclipse times from the Kepler spacecraft in 2014, and this forms a good parabola. So the kink can be spotted with eclipse times from 2017 or 2018 or 2019. The 2019 eclipse times would be most sensitive to any kink, as they would have the largest deviations from the 2010.4-2016.5 parabola. For various reasons (see next section), I have no good eclipse times from 2016.5 to now. So this proposal for AAVSOnet time is to get eclipse times to see whether the U Sco O-C curve has a sharp kink due to a missed eruption.
ALTERNATIVES TO AAVSOnet:
I have tried desperately to get archival light curves from 2017 and 2018 so as to find points near the deepest eclipse, so that I can get an approximate eclipse time for placing on the O-C curve. The AAVSO data base has no useable time series. (The best is the great HBB with a single random point very faint in February of this year, but a single point leaves much too large an uncertainty in the eclipse time to decide the question.) The best that I have is many faint magnitudes from the Catalina Sky Survey, but these are all on the egress, so I only have a limit on the eclipse time, and this goes in the wrong direction to decide anything. The ATLAS survey has many points going to 2017.4, but these have so much scatter as to be useless. Similarly, the CRTS has too much scatter to find any eclipse. The NOAO archives have nothing useable, while the DECam data has no light curve points in eclipse. Arne Henden emails that the APASS and AAVSOnet archival data is too shallow to be helpful. Fred Walter does not have SMARTS Nova Catalog data covering the time interval of interest. The LONEOS, NEAT, Palomar QUEST, LINEAR, and Spacewatch surveys are no longer active. I have examined the 15-days in 2016 and in 2017 where the SOHO LASCO C3 camera shows the position of U Sco passing through solar conjunction with a limiting magnitude of V=8.8. Pan-STARRS data are not publicly available. ZTF will sometime soon make public a batch of 2018 data, of unknown quality and coverage, and this is my only remaining hope. The point of this is to show that I have tried exhaustively to pull up archival data from 2017 or 2018 to get a good point or two on the O-C curve, but that this has largely failed.
But what about getting eclipse times in 2019 from other telescope resources? Well, my AAVSO Alert Notice #664 had an appeal for eclipse timings. But no one followed up, except MGW, who had too small a telescope. (His time series only show the ingress and egress, with nothing useable at the minimum, so the derived eclipse time has greatly too large an uncertainty to provide a confident enough of an answer to call off the worldwide U-Sco-eruption efforts.) At this point it seems as though no one with big enough of a scope is going for it. I tried appealing to Arto Oksanen, but his scope in the south has closed down for a move to Tasmania. I am not a member of any of the Las Cumbres institutions, so I cannot even put in for DD time. This is all too late for putting in any proposal (like at CTIO or KPNO) that could get done before middle 2020. SMARTS is closing down and I don't have any funding source to buy this time anyway. I could ask Dan Reichart for time on the PROMPT scopes at CTIO, but these are too small. All the other observers that I have used in the past for the big USCO2010 campaign (Schaefer et al. 2011, ApJ, 742, 113) have too small a telescope. (These were great for coverage during the eruption and its tail, but inadequate to cover the deep eclipse at minimum.) So I am out of options for how to get the current O-C for U Sco. That is why I am asking for AAVSOnet time.
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Comments on this proposal are closed.