PT photometry of transit of XO-2 on UT 2007 Dec 19 = MJD 54453

Michael Richmond
Douglas Tucker
Dec 31, 2007

On the night of UT Dec 19, 2007, the SDSS Photometric Telescope ("PT" for short) took a series of exposures of XO-2. We detected ingress and egress under very good conditions.

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.

Below is a graph of the scatter in differential magnitude versus magnitude in the ensemble solution.

XO-2 is the star at differential mag 0.61; it shows a small excess of scatter over neighboring stars of the same brightness. The "noise floor" in these measurements is about 0.003 mag -- as good as 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 in the green symbols is easy to see, despite the small gap in the middle.

An ephemeris grabbed from predicts for this night

 Begin Transit Window           PREDICTED CENTRAL TRANSIT     End Transit Window
                                     All Times UT

                               HJD        Year M  D  H  M
2454453.74 2007 12 19  5 52   2454453.80 2007 12 19  7 14   2454453.86 2007 12 19  8 37

The ephemeris ingress of UT 2007 Dec 19 05:42:00 corresponds to JD 2,454,453.744, compared to my rough guess of 453.745. The ephemeris egress of UT 2007 Dec 19 08:37:00 corresponds to JD 2,454,453.859, compared to my guess of of 453.860. In short, the agreement between prediction and observation is good.

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, Dec 19, 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
Dec19.70128      4453.70128   4453.70589   0.612  0.003 
Dec19.70266      4453.70266   4453.70727   0.617  0.003 
Dec19.70402      4453.70402   4453.70863   0.609  0.003 

Last modified 12/31/2007 by MWR.