[Aavso-photometry] Opinion needed

[arne]

Jim Roe wrote:
> I was taking a time series of CoRoT-2 hoping to catch a transit of its 
> planet.  Clouds seemed to have cut off the onset and finish of the 
> transit  ( :-( ) but in between the data seemed fairly smooth - except 
> for the nearly 15 minute brightening shown in the light curve at
> 
> <http://jamesroe.com/CoRoT2-20080812.JPG>
> 
> Remember, the transit was going on during all of this data.
> 
> The target is the data represented by squares and two comp stars (in 
> ensemble) are the diamonds.  The comp curve closest to the target is 
> only 52 arcsecs away from the target, while the other is some 8 arcmin 
> away (presumably negating potential flat field problems with the 
> intermittent clouds?).
> 
> I can't see any obvious changes in the images involved.  Can anyone 
> suggest what might have happened?  Is it worth following up in some manner?
> 
So the brightening is of the CoRoT-2 parent star, which I presume
was centered on your field?  I see a couple of points around hour 3.75
that are also a tenth of a magnitude off, and some other deviations
that are quite large if you are trying to detect an exoplanet transit.
Granted, having 8 datapoints that are all a tenth high is pretty
convincing evidence that something was going on.

Some possibilities:
exoplanet crossed a star spot, which effectively brightens the parent
star.  However, a tenth of a magnitude is highly unlikely.

A flare on the parent star.  This light curve does not look like a flare.

The parent star crossing a bright pixel on your CCD that is not properly
calibrated out.  This can happen if your field is drifting on the CCD.

My guess is an instrumental effect, but it would require inspection
of the images to see what might have happened.  One thing to do is to
look at the image shape (at least fwhm, but preferable a contour map
or else a radial plot) of the target and of the two comps to see if
anything was peculiar.
Arne