[vsnet-chat 5543] Re: [AAVSO-DIS] How common is substantial, very fastvariability?

["Berto Monard" <LAGMonar@csir.co.za>]

Hi,

my excuse is that I don't know much about the actual phenomena and it's
good that our leading professionals, like Prof J Patterson, elaborate on
this. 

From an observer's point it seems that polars in general might often,
if not always, show this rapid fluctuating behaviour as measurements at
CBA Pretoria on RX J1313-3259 in July this year showed. Actually, the
phenomenon, as explained by Joe Patterson, ought to be working always in
accreting polars. It is that active (outbursting) polars give more
fireworks by means of increased amplitudes and more of it. 

One of the interesting bits of the present active state of UW Pic is
that the main oscillation period now seems to be around 0.043d while it
was measured to be more than double that in 1994, presumably during the
calm (if that is the right word for such an object) state. 
One could speculate that an (at present) increased transfer rate is
forcing a second (or further) polar column to strike, to become more
visible or to just show up somehow. Perhaps there is an equivalent
phenomenon working comparable to superhumps in disc accreting systems,
just to cause the observed additional period difference.  
As I said: speculation, perhaps imagination... the prerogative of
amateurs!!

Let me mention that the present measured amplitude of the main
oscillation in UW Pic is about 1.4 CR (magnitude), while that of the
faster oscillations might be less than half that. Sub-ten-second series
photometry will give this amplitude more precisely. A 1m telescope is
sufficient for that.   

But then, what is fast and what is large amplitude? 
The observed fast oscillations (hinted at by measurements at the minute
level for UW Pic and at the ten second level for V4641 Sgr) were around
0.5 magnitude. How fast and/or chaotic they actually are and why, who
knows?

Regards,

Berto Monard / MLF
Bronberg Observatory / CBA Pretoria

>>> Joe Patterson <jop@astro.columbia.edu> 09/26/02 06:11PM >>>
> a couple sources reported on VSNet which showed
> extremely rapid & large optical variations (V4641 Sgr back in May,
and now
> UW Pic; timescales of order 1hr or less, and variations up to a
magnitude).
> How common is this sort of rapid variability?  and what sort of
sources
> exhibit it?   I'm less interested in regular variables with very
short
> periods, than in sources which show this sort of behavior only
occasionally.
> Any references to examples, or even better to general discussions,
would be
> greatly appreciated!

Dear Michael and anyone interested,

I'll take a crack at that.

Most compact accreting binaries accrete through a disk, where the
residence time is days to years.  So you really can't get fast,
violent
up-and-down behavior.  A few physical mechanisms (thermonuclear runaway
on
a white dwarf, a "gate" instability in the disk) can give fast rises,
but
the decay is always slow since a large area is heated and has to cool.
Slowly.

So when observations show such large fast variations, it's
interesting!
We like to make up excuses for why individual stars do it.  Magnetic
binaries have some faster timescales available to them, since much of
their radiation is not thermal (electrons spiralling around magnetic
field
lines radiate their energy away immediately).  The AM Her stars are
the champs here.  Black holes have a huge
disruption of the disk near the Schwarzschild radius, so accretion
near
there there can be very fast indeed (msec, for stellar-mass sized
bhs).
Those are somewhat popular excuses... with a few slight variants
offered
to suit the individual stars.  GX339-4 is the best example that comes
to
mind, but there are many.

My impression is that the number of examples is still so small (at
least
near optical wavelengths) that it's best to regard this as a wide-open
subject.  There is really no "standard model"... just some more or
less
plausible excuses.  It's fun to make up the excuses, but even better
to
gather enough data to find out eventually what's really happening.

   Joe Patterson

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