From: Rich Townsend <townsend@astro.wisc.edu>
Subject: Re: sigma Ori E
Date: March 2, 2009 11:37:45 PM GMT
To: Jason Rowe <jasonfrowe@gmail.com>
Cc: "Anthony F.J. Moffat" <moffat@astro.umontreal.ca>

Thanks again. Interesting -- this star isn't known to be variable, yet shows a clear-cut sinusoidal light curve.

One more question: do you know whether the observations were in direct or fabry mode?

cheers,

Rich

Jason Rowe wrote:
Hi Rich,
Here's the ID, RA, DEC, mag and SpType for the first target:
HD37525     05 39 01.492      -02 38 56.37     6,92    B5V
I'll sort out the first of the subrasters soon and send that information along.
Jason
Rich Townsend wrote:
Hi Tony --

Thanks for the info!

One quick, further question: could you let me know the identity of star 01?

Many thanks again,

cheers,

Rich

Jason Rowe wrote:
Hi Rich,

Sorry for the delay.  We are days away from a possible Kepler launch, so I've had trouble keeping up with correspondence.

I've had a look at the light curves.  Star 03 appears to be saturated, hence the errors have no correlation with the scatter seen.

For sigma Ori E (star 02), the scatter appears to be mostly noise.  The errors in the data files are shot noise only.  Most of the scatter left over is most likely due to intrapixel gain variations.  This will be proxied by the X,Y centroid of the star.  The pointing performance of the satellite repeats with the orbital period, which adds an additional correlation with the stray light.  This makes untangling artifacts in the lightcurves difficult.

Hope this helps.
Jason



Rich Townsend wrote:
Hi Jason --

Do you have any further info about my questions below -- specifically, the identity of the other stars in the data files, and the origin of the scatter in the sigma Ori E light curve? (I'm copying Tony, in case he might also be able to help).

Many thanks,

Rich

Rich Townsend wrote:
Hi Jason --

It's been quite a while since you emailed me with the sigma Ori E data; finally, with teaching out of the way, I'm getting around to looking at the data. First things first, can you provide me with the IDs of the other stars in the dataset? I've looked at star 01, and the light curve looks nice and sinusoidal -- I'm wondering whether this is real, or an instrumental artifact. If real, the period (~5 days) is consistent with a slowly-pulsating B star.

Also, I'm wondering whether the ~0.01mag scatter seen in the light curve of sigma Ori E (star 02) is real, or again an artifact -- certainly, the error bars on the measurements (~0.001mag) seem to be much smaller, suggesting that this is a real signal.

Many thanks,

Rich

Jason Rowe wrote:
Dear Rich Townsend,

I am forwarding you MOST photometry for sigma Ori E.

The attached archive contains photometry for 6 stars label 01-06. The
second star (02) is sigma Ori E.   I have to double check the IDs for
the other stars as the observation log appears to be incorrect (there
are 5 entries, but photometry for 6 stars!) .  Regardless, the other
stars are mostly likely B stars with apparent magnitudes from V=6.2 to
7.6, thus comparable in brightness to sigma Ori E.

The file format is:

510  format(F13.8,1X,2(F9.6,1X),F8.2,1X,2(F6.2,1X),2(F5.3,1X),F6.3,
   .     1X,F7.3,1X,F9.2,2(1X,F9.2),1X,F8.3,1X,F6.2,1X,F8.4)


Column 1: HJD-2451545.0
Column 2: instrumental magnitude (zero point removed)
Column 3: photometric error (bias/darks are incorrect, so this is
probably overestimated)
Column 4: Sky value ADU/pixel
Column 5: x co-ordinate (pixel)
Column 6: y co-ordinate (pixel)
Column 7: x PSF width
Column 8: y PSF width
Column 9: xy PSF width
Column 10: Temperature Board reading (good indicator of long term trends)
Column 11: magnetic field strength (based on model) good indicator of
SAA passage
Column 12: total flux within photometric aperture
Column 13: Star flux within photometric aperature (for recovering zero point)
Column 14: Number of pixels for aperture (thus $13=$12-$4*$14)
Column 15: integration time
Column 17: Standard Deviation of Sky Pixels (was for finding crosstalk)


You are probably only interested in the first 3 columns.

Couple tips:

1. Stray light varies with a period of 101 minutes (or 14.2 c/d)  and
is modulated by the day-night cycle of the Earth (in particular,
bright areas such as the S. Pole).  Thus, frequencies related to 14.2
c/d +/- 1,2,3.. c/d should be consider instrumental.  Same applies to
frequencies at 1,2,3 c/d.  I've tried my best to remove instrumental
effects, but there is a bit remaining.

2.  When the stray light peaks, photometry degrades, thus I highly
recommend weighting the data by the photometric error (Poisson) in
column 3 for any analysis.

Finally, if you have any questions about the data feel free to ask!

Clear Skies,
Jason









