Alert Notice 828: V844 Her monitoring requested

July 11, 2023

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Dr. Peter Garnavich (University of Notre Dame) requests AAVSO observers' assistance in monitoring the SU UMa-type cataclysmic variable V844 Her in a dual-purpose campaign. He writes:

"V844 Her shows a 29 minute periodicity during outbursts that may be the rotation period of a magnetic white dwarf. We have two goals:
1 -    Be alerted to an outburst to trigger X-ray observations from Swift
2 -    Request AAVSO observers to obtain time series CCD observations to confirm the periodicity.

"The 29-minute period was not seen by TESS during quiescence, and it appears only during super-outburst. Super-outbursts occur approximately once per year and last for two weeks.  It is important to trigger x-ray observations early in the burst and have Swift visits over the two weeks of the outburst.

"The 29-minute oscillation has been marginally detected in only one archival observation in the AAVSO database. It requires long times series (multiple hours) and multiple nights of observation."

The range of V844 Her is ~12-17 V (12.3-18.0 CV in VSX). There is not always a significant difference in brightness between normal outbursts, which last about 2 days, and superoutbursts, which last about 15 days. Once an outburst is detected, the star will be between 12th and 14th mag.

Two categories of observations are requested:
- Beginning immediately, nightly monitoring to continue the light curve and watch for a superoutburst. For these observations, CCD, CMOS/DSLR, and visual observations are requested.

- During a superoutburst, time series CCD observations (not DSLR or visual) to confirm the periodicity. Long CCD time series lasting many hours with a cadence of between 5s to 30s are best. Because it is not possible to tell immediately whether an outburst is a normal one or a superoutburst, begin time series when an outburst occurs.

Continue observations until the superoutburst is over and the star has returned to minimum. If the outburst was a normal one, continue nightly observations until a superoutburst has occurred and has concluded.

Dr. Garnavich continues: "Data can be taken unfiltered (calibrated to CV or CR) or in standard B,V,R filters. Signal to noise ratio per exposure should be greater than 20 (magnitude uncertainty of 0.05 mag or smaller). Unfiltered is best to maximize the signal-to-noise. Times-series observations need to cover multiple hours as the periodicity is 29 minutes long."

Spectroscopy is not requested for this campaign.

Coordinates (2000.0):  R.A. 16 25 01.75  Dec. +39 09 26.4  (from VSX entry for V844 Her)

Finder charts for V844 Her with comparison stars may be created using the AAVSO Variable Star Plotter (VSP).

Please submit your observations to the AAVSO  using the name V844 HER as soon after you make them as possible, especially if you see V844 Her brighter than magnitude 14.0 V.

This AAVSO Alert Notice was compiled by Elizabeth O. Waagen using information provided by Dr. Garnavich.

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