PT photometry of a transit of WASP-1 on UT 2007 Oct 10 = MJD 54383

Michael Richmond
Douglas Tucker
Oct 12, 2007

On the night of UT Oct 10, 2007, the SDSS Photometric Telescope ("PT" for short) took a series of exposures of the field around WASP-1 . Unfortunately, bad weather ruined the observations.

Notes from the night

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

The host star of WASP-1 has a magnitude V=11.79, according to WASP-1b and WASP-2b: Two new transiting exoplanets detected with SuperWASP and SOPHIE.

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 very cloudy -- and here's evidence. 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. I marked all stars brighter than differential magnitude 2.0, as "saturated," so that they would not influence the ensemble solution.

WASP-1 is the star at differential mag 1.34; it doesn't show any more scatter than neighboring stars of the same brightness. The "noise floor" in these measurements is about 0.006 mag -- high for PT measurements -- due to the clouds in the later set of observations.

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 star D to move them closer in magnitude to the target.

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
2454383.67 2007 10 10  3 57   2454383.75 2007 10 10  5 55   2454383.83 2007 10 10  7 53

The ingress of UT 2007 Oct 10 03:57:00 corresponds to JD 2,454,383.665, which is long before the PT measurements start. Our first set of measurements are therefore within the full transit. The ephemeris predicts an end to the transit at UT 2007 Oct 10 07:53:00 = JD 2,454,383.8285, which is in the middle of the second, cloudy section of measurements. We can't say anything about this transit, alas.

You can grab the measurements for your own analysis. Below is a table with three flavors of time, plus the differential magnitude of WASP-1 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 WASP-1 made with APO PT, Oct 10, 2007 UT. 
# Each exposure 40 seconds long in SDSS i-band; 
# Tabulated times are midexposure 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
Oct10.71424      4383.71424   4383.71937   1.343  0.009 
Oct10.71567      4383.71567   4383.72080   1.336  0.009 

Warning: the times in this datafile have a small systematic error. I assumed the FITS header time was the shutter open time, so added half the exposure time to it. The FITS header time is instead roughly the shutter CLOSING time. My calculated times are therefore later than correct by about the duration of one exposure.

Last modified 10/12/2007 by MWR.