PT photometry of transit of XO-1 on UT 2008 May 30 = MJD 54616

Michael Richmond
Douglas Tucker
June 23, 2008

On the night of UT May 30, 2008, the SDSS Photometric Telescope ("PT" for short) took a series of exposures of XO-1. We observed the ingress only under good conditions, which shows a time slightly earlier than predicted. These are our first measurements of the XO-1 system, I believe.

Notes from the night

This is a chart of the field. XO-1 is the bright star indicated by the crosshairs. The labelled stars will appear in later analysis.

The host star of XO-1 has a magnitude V=11.19 according to A Transiting Planet of a Sun-like Star.

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 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-1 is the star at differential mag 0.70; it ought to show a small excess of scatter over neighboring stars of the same brightness, and does. The "noise floor" in these measurements is about 0.003 mag -- good 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.

My eyeball sees the ingress clearly at 616.86, but the egress must occur after the measurements.

Justin's notes indicate that the expected time of ingress is 616.872, which is about 17 minutes later than my estimate from the data. The egress time is predicted to be 616.992, which is indeed long after the end of the measurements.

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-1 made with APO PT, May 30, 2008 UT. 
# Each exposure 45 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
May30.82088      4616.82088   4616.82461   0.674  0.003 
May30.82253      4616.82253   4616.82626   0.681  0.003 
May30.82400      4616.82400   4616.82773   0.683  0.003 

Last modified 06/23/2008 by MWR.