PT photometry of transit of XO-2 on UT 2008 Mar 30 = MJD 54555

Michael Richmond
Douglas Tucker
May 2, 2008

On the night of UT Mar 30, 2008, the SDSS Photometric Telescope ("PT" for short) took a series of exposures of XO-2. We clearly detected the ingress only at (close to) 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 mostly 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. Note the clouds coming at the end of the observations.

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 0.004 mag -- only fair for PT work. Most of the outliers in the plot above are stars near the edges of the frame, or with close companions.

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 555.770.

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
2454555.76 2008  3 30  6 17   2454555.82 2008  3 30  7 40   2454555.88 2008  3 30  9  3

The ephemeris ingress of UT 2008 Mar 30 06:17:00 corresponds to JD 2,454,555.762. This is about 12 minutes earlier than the start of the dip which I see in the measurements .... but I'm not sure if my eyeball estimate is accurate.

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, Mar 30, 2008 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
Mar30.72611      4555.72611   4555.72695   0.600  0.004 
Mar30.72748      4555.72748   4555.72832   0.602  0.004 
Mar30.72885      4555.72885   4555.72969   0.597  0.004 

Last modified 05/02/2008 by MWR.