Alert Notice 841: VY Scl coverage needed now for HST ToO observations

Notes: HST will not be able to observe VY Scl at this time, so the observations have been canceled. However, the data being obtained by you continue to be extremely valuable to the astronomers, and continuing coverage is requested until after VY Scl has returned to its bright state. Thank you!  -  Elizabeth O. Waagen, January 31, 2024

HST was not able to observe VY Scl in November. It may be rescheduled for December/January so please continue good coverage of it until after it has returned to its bright state.  -  Elizabeth O. Waagen, December 5, 2023

November 13, 2023

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Further to Alert Notice 754, Dr. Anna Francesca Pala (European Space Agency, ESAC, Spain) informs us that Hubble Space Telescope Target of Opportunity (ToO) observations have been triggered for VY Scl, the prototype of its class of cataclysmic variables. The observations will take place

2023 November 17 02:24:16 - 04:55:41 UT

Dr. Pala writes: "VY Scl [systems] are typically characterised by high average accretion rates which keep the disc in a stable hot state, equivalent to a permanent outburst. Occasionally the accretion rate drastically drops (“low state”), unveiling the white dwarf. These low states provide a window in which we can directly observe the underlying accreting white dwarf and measure its temperature, mass and accretion rate. For this reason, we have to make absolutely sure that the HST observations are obtained during a low state, when the white dwarf is the dominant source of light in the ultraviolet. Moreover, this observing strategy will protect the detectors of the Cosmic Origin Spectrograph against exposure to too much light which, during high states, could result in their permanent damages."

Beginning immediately, multiple observations in V per night are requested. A minimum S/N of 20 is recommended. VY Scl is presently ~18th magnitude, so if this S/N is not possible, switch to CV (unfiltered, reduced to V zeropoint), but V-filter observations should be the priority whenever possible. Please keep exposure times below 900 seconds if possible.

Dr. Pala also indicates that time-series observations would be very useful. If they are made, an uninterrupted light curve in a single filter (preferably V or B) would be better than alternating with different filters.

DSLR observations, particularly if they are transformed to V, will be useful and are welcome. Visual observations will also be very valuable.

Please continue coverage until after VY Scl has returned to its high state.

Because of the faintness of VY Scl, spectroscopy would likely be extremely difficult or impossible, and so is not requested at this time.

VY Scl has a range of 11.8 - 18.6 V. The most recent observations of it in the AAVSO International Database show it in its low state at <17.1 visual magnitude on 2023 Nov. 13.4993 UT (R. Stubbings, Tetoora Road, Vic, Australia) and at 18.584 V +/- 0.192 on Nov. 9.02509 UT (J. DeYoung, Palmyra, VA).

Coordinates (2000.0): R.A. 23 29 00.47  Dec. -29 46 46.0 (from VSX entry for VY Scl)

Charts with comparison star for VY Scl maybe created using the AAVSO Variable Star Plotter (VSP).

Please submit observations to the AAVSO International Database as soon as possible after making them and using the name VY SCL.

This AAVSO Alert Notice was compiled by Elizabeth O. Waagen using material supplied by Dr. Pala.


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