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Alert Notice 664: Monitoring recurrent nova U Sco to discover its imminent eruption

April 3, 2019: Dr. Bradley E. Schaefer (Louisiana State University) provides the following information:

"Frequent monitoring is requested of the recurrent nova U Scorpii over the next year or so to discover the upcoming very-fast eruption. The predicted outburst is for the year 2020.0±0.7. The eruption is likely within the next year, but it could well go off tonight.  

"Observers of many types are needed before the next outburst:

    • VISUAL OBSERVERS: Observe U Sco nightly, perhaps even two to three times a night, to watch for and discover the eruption. Must be able to check to at least 9th or 10th mag.  

    • CCD/DSLR OBSERVERS: Observe nightly as above, and to get the pre-eruption light curve so as to look for a pre-eruption plateau and to catch the initial rise that will last perhaps 2 hours. V or B filtered images are preferred, but any other standard filters are fine, while CV and CR are adequate if you tell us your comparison stars.

    • CCD OBSERVERS WITH MODERATELY LARGE TELESCOPES: Get well-sampled time series across the minima of U Sco's deep eclipses (in any filter or with no filter). Exact times of eclipses soon before the next eruption are valuable for measuring the sudden orbital period change across the nova event, of high recent interest and controversy. Exposure times of 5 to 15 minutes are likely appropriate.

"Observers should report their observations to the AAVSO using WebObs, and report them immediately if U Sco is seen in outburst (brighter that V~16). Speed is very important! When the outburst has been confirmed, the astronomical community will be notified so that observations may be triggered at observatories worldwide and in space.

"U Sco has had ten known eruptions, which reliably have gone off every 10.6 years on average (with an RMS variation of ±1.6 years), with the last eruption starting in 2010.1 [AAVSO Alert Notice 415]. But AAVSO light curves show that U Sco was substantially brighter than average in the years 2011 and 2012, so the accretion rate was higher than average, so the accumulated material on the surface of the white dwarf is somewhat ahead of the average schedule, so the eruption should happen substantially before the average expectation year of 2020.7. The most likely eruption time is any night now.

"U Sco has a peak magnitude of V=7.5. The rise from near the quiescent brightness to peak is apparently under 6 hours. After the peak, U Sco, the all-time fastest nova, fades very fast (falling to V=10.5 in just 2.6 days). This speed makes the fast alert from AAVSO of prime importance.

"In quiescence, U Sco is around V=18.0 and B=18.4. It is a deep eclipsing system (going down to typically V=19.0) with a full duration near 6 hours. The current ephemeris is

HJD(mid-eclipse)=2455223.9707 + N*1.23057135 - N(N-1)*1.2E-9,

for integer N. (This is only an approximate extrapolation, with the deviations from this ephemeris being the critical science.)"

Observers with AAVSO web accounts may want to create a MyNewsFlash (MNF) subscription on U Sco in order to keep up to date with its status. To use MNF, log in to your account and go to your My Account page, then click on the MyNewsFlash tab. If you do not have an AAVSO account, go to the homepage (https://www.aavso.org), click on the User login button, and follow the instructions to create an account.

Coordinates (2000.0): RA 16 22 30.78  Dec -17 52 42.8

Finder charts with a comparison star sequence for U Sco may be generated using the AAVSO Variable Star Plotter (VSP).

Please report all observations promptly to the AAVSO using the name U SCO. Be sure to include the comparison stars used in making your observations.

AAVSO Forums: This campaign is the topic of the AAVSO Campaigns and Observation Reports forum thread https://www.aavso.org/u-sco-2019-campaign and the Cataclysmic Variables forum thread https://www.aavso.org/u-sco-2019-campaign

This AAVSO Alert Notice was compiled by Elizabeth O. Waagen using material provided by Dr. Bradley E. Schaefer.

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