Alert Notice 682: Observing campaign on Polars

Hello Observers,

One of the PIs on this campaign on polars (AAVSO Alert Notice 682) contacted me to follow up on coverage of the target stars. He is most appreciative of the ongoing coverage of VV Pup, and for the coverage of EP Dra, ST LMi, and V2301 Oph, and gratefully thanks the observers. He sees that there have not been observations of these last three stars for the past couple of weeks or more and wonders if this is due to a visibility issue. He also notes that UZ For has not been observed at all to date.

I know that V2301 Oph (V=15.7) is now an evening twilight target and will soon be out of reach until Spring. And, I realize EP Dra and UZ For are both extremely faint (bright state threshold magnitudes V=18.5 and 18.0, respectively). ST LMi seems reasonably placed (bright state threshold V=15.4).

The PIs are depending on knowing the state of the targets in order to trigger NuSTAR observations. Would it be possible to boost the level of coverage on these these stars? V or CV twice a week is what has been requested (see the notice for more details). If positive observations are not obtainable, a fainter-than observation at or below the bright state threshold magnitude would be very valuable (that may be all that is possible for EP Dra and UZ For).

I understand the weather has been very cloudy in many areas these past weeks, and that the holiday season is ramping up. Any observations you are able make will be extremely valuable and very much appreciated!

Many thanks, and Good observing,

Elizabeth

Dear AAVSO members, 

We wanted to report on our polar observation program (in collaboration with AAVSO) which ends this month. First of all, we want to thank you and all of the AAVSO astronomers for putting in the effort to observe with astonishing consistency all of our polar targets—everyone here is beyond grateful for the data and found the light-curve generator and the VSX super useful. We appreciate the continuing monitoring of our targets while everyone in the country was facing the enormous difficulties during the virus pandemic period.

Below we summarize our polar observation campaign with AAVSO since October 2019 in conjunction with Swift and NuSTAR X-ray telescopes. It is the right time to make a brief report as our observing program with NuSTAR will be terminated by the end of May 2020. In a nutshell, this is how our collaboration with AAVSO worked. We frequently monitored the optical lightcurves of our polar targets obtained by AAVSO members as polars often show good correlations between the optical and X-ray fluxes. We triggered a short Swift/XRT observation once any of our targets became bright in the optical band (below the threshold optical magnitudes we set based on some historical lightcurve data). We understand that polars are variable both over their orbital periods and over months or years. Therefore, we attempted to capture their X-ray bright states during which we can obtain sufficient X-ray counts for NuSTAR observations. The optical magnitude data from AAVSO was the first and crucial step in order to trigger Swift and NuSTAR observations. 

Of the five polar targets listed in Alert Notice 682 (UZ For, VV Pup, ST LMi, V2301 Oph, EP Dra), only ST LMi ultimately ended up being observed by NuSTAR (which, for you own interest, was fitted to a plasma temperature of 9.9 ± 1.3 keV). We are going to fit our state-of-the-art X-ray spectral model (called MCVSPEC) from the accretion columns of magnetic CVs and achieve our ultimate goals of measuring their white dwarf masses accurately. We also triggered Swift ToO’s for both UZ For (January 15, 2020) and VV Pup (May 16, 2020) to see if their high optical brightness at the time correlated with brightness in the X-ray band, but both ToO’s were inconclusive and so we decided not to go forward with NuSTAR observations. As for V2301 Oph and EP Dra, their optical brightness never seemed to peak in such a way that would convince us to conduct a Swift ToO. Owing to these results from this pilot program, we learned valuable lessons about the long-term optical vs X-ray flux correlation as well as the stochastic behaviors of polars; we could make the best use of the AAVSO and X-ray data and optimize our future observations of polars.

Again, we want to thank AAVSO for their relentless and precise observations of our targets, and let you know that we’ve been monitoring with great satisfaction your website daily for the past months. We plan to submit a paper to the Astrophysical Journal by the end of this summer by presenting our polar observation campaign and the latest results from NuSTAR spectral analysis. AAVSO will be acknowledged in our paper and presentations for sure. This is not the end of our endeavor of understanding X-ray spectral properties of polars - we plan to expand our observation program by proposing to observe more polars within a year. We are looking forward to future collaborations with your team.

Best,

Kaya Mori, Chuck Hailey, Gavin Ramsay, Ninad Nirgudkar and Ben Vermette