On the night of UT Jan 01, 2008, the SDSS Photometric Telescope ("PT" for short) took a series of exposures of XO-2. We clearly detected a full transit at the time predicted by the ephemeris.
Notes from the night
This is a chart of the field. XO-2 is the bright star indicated by the crosshairs. The labelled stars will appear in later analysis.
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 was clear. 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. The very small range of image adjustment factor, just 0.015 mag over a period of 4.8 hours, is the smallest I've seen in PT dataset. The transparency was nearly constant!
Below is a graph of the scatter in differential magnitude versus magnitude in the ensemble solution.
XO-2 is the star at differential mag 0.6 with a small excess of scatter over neighboring stars of the same brightness. The "noise floor" in these measurements is about 0.002 mag -- the best I've seen for 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. I estimate by eye the ingress to start at about 66.82 and end at about 66.93.
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
2454466.82 2008 1 1 7 46 2454466.88 2008 1 1 9 9 2454466.94 2008 1 1 10 31
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The ephemeris ingress of UT 2008 Jan 01 07:46:00 corresponds to JD 2,454,466.824. The ephemeris egress of UT 2008 Jan 01 10:31:00 corresponds to JD 2,454,466.938. These times agree nicely with my quick eye estimates.
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, Jan 1, 2008 UT. # Each exposure 50 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 Jan01.77695 4466.77695 4466.78187 0.610 0.002 Jan01.77834 4466.77834 4466.78326 0.607 0.002 Jan01.77973 4466.77973 4466.78465 0.607 0.002
Last modified 01/26/2008 by MWR.