PT photometry of transit of TrES-3 on UT 2008 Mar 31 = MJD 54556

Michael Richmond
Douglas Tucker
May 2, 2008

On the night of UT Mar 31, 2008, the SDSS Photometric Telescope ("PT" for short) took a series of exposures of TrES-3. We detected an entire transit, although with little to spare at egress. The transit is (as usual with this system) briefer 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 so-so. 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 1.38; it shows a very small excess of scatter than neighboring stars of the same brightness. The "noise floor" in these measurements is about 0.005 mag -- not very good for the PT. 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. The lowest three light curves are eclipsing binary stars; one shows no features during this run.

In this closeup, I have shifted the data for two comparison stars to move them closer in magnitude to the target.

There appears to be a "preliminary dip" before the real ingress; I don't know if it's real or not, but similar features have appeared in the past.... My eyeball estimate for the start of the transit is 556.84, and my estimate for the end is 554.90.

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
2454556.83 2008  3 31  7 52   2454556.87 2008  3 31  8 50   2454556.91 2008  3 31  9 47

The ephemeris ingress of UT 2008 Mar 31 07:52:00 corresponds to JD 2,454,556.828, which is about 17 minutes EARLIER than the observed ingress. The ephemeris egress of UT 2008 Mar 31 09:47:00 corresponds to JD 2,454,556.908, which is about 12 minutes LATER than the observed egress.

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, Mar 31, 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
Mar31.77641      4556.77641   4556.77709   1.366  0.006 
Mar31.77774      4556.77774   4556.77842   1.372  0.006 
Mar31.77907      4556.77907   4556.77975   1.375  0.006 

Last modified 05/02/2008 by MWR.