PT photometry of transit of XO-2 on UT 2008 Jan 30 = MJD 54495

Michael Richmond
Douglas Tucker
Feb 23, 2008

On the night of UT Jan 30, 2008, the SDSS Photometric Telescope ("PT" for short) took a series of exposures of XO-2. The ingress data are noisy due to clouds, but the egress is very well measured.

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 partly cloudy. 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 gap in the data is due to a brief interruption to observe another field.

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 -- so-so for APO measurements.

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.

I estimate by eye the ingress to start at about 495.60, though it's hard to say exactly due to the noise. The egress occurs at 495.71 under better conditions.

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
2454495.60 2008  1 30  2 21   2454495.66 2008  1 30  3 43   2454495.71 2008  1 30  5  6

The ephemeris ingress of UT 2008 Jan 30 02:21:00 corresponds to JD 2,454,495.600. The ephemeris egress of UT 2008 Jan 30 05:06:00 corresponds to JD 2,454,495.713. Both agree with my eyeball 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 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
Jan30.58101      4495.58101   4495.58570   0.605  0.003 
Jan30.58241      4495.58241   4495.58710   0.611  0.003 
Jan30.58380      4495.58380   4495.58849   0.608  0.003 

Last modified 02/23/2008 by MWR.