PT photometry of transit of XO-2 on UT 2007 Nov 07 = MJD 54411

Michael Richmond
Douglas Tucker
Nov 13, 2007

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 predicts for this night

 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

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.