PT photometry of transit of TrES-3 on UT 2008 Apr 21 = MJD 54577

Michael Richmond
Douglas Tucker
May 28, 2008

On the night of UT Apr 21, 2008, the SDSS Photometric Telescope ("PT" for short) took a series of exposures of TrES-3. We detected the egress of the transit only (the ingress is too close to the start of the observations). The transit is (as usual with this system) slightly different than the ephemeris predicts.

Notes from the night

This is a chart of the field. TrES-3 orbits the is the bright star indicated by the crosshairs. The star labelled "C" will appear in later graphs.

The host star of TrES-3 has a magnitude V=12.40 according to TrES-3: A Nearby, Massive, Transiting Hot Jupiter in a 31-Hour Orbit .

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 okay. 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.

TrES-3 is the star at differential mag 0.08; it shows a very small excess of scatter than neighboring stars of the same brightness. The "noise floor" in these measurements is about 0.007 mag -- higher than usual for good conditions. I don't know why. Many of the outliers in the plot above are real variable stars (which were removed from the ensemble solution).

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 ingress appears to occur before or just at the start of observations. My eyeball estimate for the end of the transit is 577.79, though note that there seem to be two tracks, and the "low track" would suggest a later time.

An ephemeris grabbed from transitsearch.org predicts for this night

----------------------------------------------------------------------------------------
 Begin Transit Window           PREDICTED CENTRAL TRANSIT     End Transit Window
                                     All Times UT

                               HJD        Year M  D  H  M
2454577.73 2008  4 21  5 27   2454577.77 2008  4 21  6 24   2454577.81 2008  4 21  7 22
----------------------------------------------------------------------------------------

The ephemeris ingress of UT 2008 Apr 21 05:27:00 corresponds to JD 2,454,577.727, which is indeed before the start of measurements. The ephemeris egress of UT 2008 Apr 21 07:22:00 corresponds to JD 2,454,577.807, which is about 24 minutes LATER than the eyeball estimate.

This pattern confirms the indications from earlier nights,

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 TrES-3 made with APO PT, Apr 21, 2008 UT. 
# Each exposure 30 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
Apr21.73611      4577.73611   4577.73771   0.081  0.008 
Apr21.73743      4577.73743   4577.73903   0.053  0.007 
Apr21.73876      4577.73876   4577.74036   0.077  0.007 


Last modified 05/28/2008 by MWR.