PT photometry of transit of WASP-1 on UT 2007 Nov 27 = MJD 54431

Michael Richmond
Dec 2, 2007

On the night of UT Nov 27, 2007, the SDSS Photometric Telescope ("PT" for short) took a series of exposures of WASP-1. We detected the egress only, under poor conditions; the time of egress agrees roughly with that predicted by the ephemeris.

Notes from the night

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

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 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.

Below is a graph of the scatter in differential magnitude versus magnitude in the ensemble solution.

WASP-1 is the star at differential mag 0.77; it shows a small excess of scatter over neighboring stars of the same brightness. The "noise floor" in these measurements is about 0.004 mag -- decent 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.

The transit in the green symbols is obvious, but it is hard to determine the exact time of egress due to the large scatter at the end of the run.

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
2454431.54 2007 11 27  1  3   2454431.63 2007 11 27  3  1   2454431.71 2007 11 27  4 59

The ephemeris egress of UT 2007 Nov 27 04:59:00 corresponds to JD 2,454,431.708, which agrees with my rough guess of 431.70.

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 WASP-1 made with APO PT, Nov 27, 2007 UT. 
# Each exposure 40 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
Nov27.60447      4431.60447   4431.60800   0.780  0.004 
Nov27.60590      4431.60590   4431.60943   0.775  0.004 
Nov27.60732      4431.60732   4431.61085   0.777  0.004 
Nov27.60883      4431.60883   4431.61236   0.773  0.004 

Last modified 12/02/2007 by MWR.