PT photometry of transit of XO-2 on UT 2008 Jan 01 = MJD 54466

Michael Richmond
Douglas Tucker
Jan 26, 2008

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 predicts for this night

 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

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.