Alert Notice 834 - V844 Her in outburst and photometry needed

October 3, 2023

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Further to AAVSO Alert Notice 828, Dr. Peter Garnavich (Notre Dame University) requests heightened coverage of the SU UMa-type cataclysmic variable V844 Her, which has gone into outburst, both to confirm the variable's periodicity and in support of Target of Opportunity (TOO) observations scheduled with NASA's Gehrels Swift X-ray satellite.

Dr. Garnavich reports that, thanks to the detection of the outburst of V844 Her by AAVSO observers, his request for the Swift TOO observations was approved, and he extends his appreciation.

One of the goals is to confirm the 29-minute period. Dr. Garnavich writes: "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. 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. During outbursts, the star will be between 12th and 14th mag.

Time series CCD observations (not DSLR or visual) are requested beginning immediately 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.

CMOS/DSLR and visual observations are also requested to continue to continue to help build the light curve throughout the outburst, and for a few weeks after the star has returned to minimum to give context to the outburst.

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."

The most recent observations in the AAVSO International Database show V844 Her on 2023 Oct. 3.1945 UT at CV magnitude 12.637 +/- 0.025 (R. Stephens, Rancho Cucamonga, CA) and on Oct. 2.9007 UT at V magnitude 12.4576  +/-0.0314 (I. Peretto, Cornedo, Italy).

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.

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

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