On the night of UT Nov 07, 2007, the SDSS Photometric Telescope ("PT" for short) took a series of exposures of XO-2. We detected an entire transit, at the time predicted by the ephemeris.
Notes from the night
This is a chart of the field. XO-2 orbits the is the bright star indicated by the crosshairs. The similar star just south of it has a common proper motion. The star labelled "B" will appear in later graphs.
The host star of XO-2 has a magnitude V=11.18 according to XO-2b: Transiting Hot Jupiter in a Metal-rich Common Proper Motion Binary .
Following the procedures outlined by Kent Honeycutt's article on inhomogeneous ensemble photometry, I used all stars available in each image to define a reference frame, and measured each star against this frame. You can find the software package used to do the ensemble photometry online; it's free!
The night had a few thin clouds. The graph below shows the amount by which instrumental magnitudes from each image needed to be shifted to match the ensemble reference. On a clear night, this graph would show a straight horizontal line.
Below is a graph of the scatter in differential magnitude versus magnitude in the ensemble solution.
XO-2 is the star at differential mag 0.62; it shows a small excess of scatter over neighboring stars of the same brightness. The "noise floor" in these measurements is about 0.002 mag -- some of the best of the APO transit work.
Below are the light curves for the target (green symbols) and four comparison stars in the field.
In this closeup, I have shifted the data for two comparison stars to move them closer in magnitude to the target.
The transit is obvious.
An ephemeris grabbed from transitsearch.org predicts for this night
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Begin Transit Window PREDICTED CENTRAL TRANSIT End Transit Window
All Times UT
HJD Year M D H M
2454411.89 2007 11 7 9 23 2454411.95 2007 11 7 10 46 2454412.01 2007 11 7 12 8
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The ephemeris ingress of UT 2007 Nov 07 09:23:00 corresponds to JD 2,454,411.891, which agrees with my estimate by eye from the light curve of 411.895. The ephemeris egress of UT 2007 Nov 07 12:08:00 corresponds to JD 2,454,412.006, which also agrees with my eye estimate of 412.005.
You can grab the measurements for your own analysis. Below is a table with three flavors of time, plus the differential magnitude of the target and an estimate of the uncertainty in each measurement. I show the first few lines of the file to give you an idea of its format.
# Measurements of XO-2 made with APO PT, Nov 7, 2007 UT. # Each exposure 35 seconds long in SDSS i-band; # Tabulated times are midexposure (FITS header time - half exposure length) # and accurate only to +/- 1 second (??). # 'mag' is a differential magnitude based on ensemble photometry # using a circular aperture of radius 5.25 arcseconds. # # UT day JD-2,450,000 HJD-2,450,000 mag uncert Nov07.86171 4411.86171 4411.86384 0.604 0.003 Nov07.86309 4411.86309 4411.86522 0.607 0.003 Nov07.86444 4411.86444 4411.86657 0.607 0.003
Last modified 11/13/2007 by MWR.