[vsnet-alert 25489] Re: SS Cyg: standstill

[Brian Skiff]

     Since this event is on-going and occurring on a genuinely bright target, would Kato-san or others suggest the most useful observations to be made?  I think it would be useful to be a little prescriptive about what is to be done with these very many cv outbursts.  It seems that the opportunity exists in this case to do something more use diagnostic than simple unfiltered photometry (i.e. the usual thing).  Would having, say, U, V, I, or Sloan z (= Paschen decrement) time-series photometry be helpful?  Time-resolved H-alpha spectroscopy?  Helium lines?  Polarization?  Spectra in the near-IR?


\Brian



On Mar 2, 2021, at 5:42 PM, Taichi Kato <tkato at kusastro.kyoto-u.ac.jp> wrote:

Re: SS Cyg: standstill

> Akazawa-san and Kasai-san reported their latest observations.
> The standstill in SS Cyg seems to be accompanied by oscillatory
> variations even at this moment.  Their timescale is around a week.
> It would be important to see if SS Cyg enters Z Cam-type standstill
> with constant luminosity or repeats oscillations.

  It has been documented that some standstills are
associated with a hint of mini-outbursts occurring
within the standstill at the normal interval
frequency (Szkody and Mattei 1984, PASP 96, 988).
So, the presence of low-amplitude oscillations
may not be a surprise.

  It would be noteworthy that the outburst preceding
this standstill had a "shoulder" in the light curve
at the end of 2020 Dec., particularly evident in
Akawaza-san's light curve.  This phenomenon indicates
that the disk radius reached the tidal truncation
radius during this outburst.  The angular momentum
of the disk was maximal during this outburst.
The present standstill appears (to me) to be a result
of the insufficient removal of the disk mass and angular
momentum during this outburst (SS Cyg-type -- currently
an ironical term -- outbursts reaching the tidal truncation
radius are usually long ones, but this one lacked
the flat-topped portion).

  The presence of oscillations during the standstill
somewhat reminds me of IW And stars.  The present
phenomenon might provide a clue in understanding
the IW And-type phenomenon and SS Cyg-type phenomenon
as a whole.