PT measurements of transit of HAT-4 on UT 2008 Jun 10 = MJD 54627

Michael Richmond
Douglas Tucker
Jun 27, 2008

On the night of UT Jun 10, 2008, the SDSS Photometric Telescope ("PT" for short) took a series of exposures of HAT-4. The conditions were good, and we detected the ingress. The egress occurred long after our measurements ended.

Notes from the night

This is a chart of the field. HAT-4 is the bright star indicated by the crosshairs. The labelled stars will appear in later analysis. The star marked by two line segments turns out to be just too bright to use as a comparison with the 40-second exposure time.

As a side note, there are several eclipsing binary stars in this field. I had to mark them as "variable" before running the photometric solution below.

The host star of HAT-4 has a magnitude V=11.20 according to HAT-P-4b: A Metal-rich Low-Density Transiting Hot Jupiter .

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 apparently fair. 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.

HAT-4 is the star near differential mag 1.12; it shows an excess of scatter over neighboring stars of the same brightness. The "noise floor" in these measurements is about 0.004 mag -- not bad.

Below are the light curves for the target (green symbols) and some 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.

I estimate by eye the ingress to occur at about 627.79; the first "dip" in the light of the target appears in the comparison star B as well, and is therefore some systematic error in the ensemble photometry. I see no egress in this dataset.

Justin's notes for this event indicate that the ingress was predicted to occur at 627.795, in good agreement with my eyeball estimate. The egress was predicted to occur at 627.970, long after our run ended.

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 HAT-4 made with APO PT, Jun 10, 2008 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
Jun10.75700      4627.75700   4627.75946   1.179  0.005 
Jun10.75843      4627.75843   4627.76089   1.185  0.004 
Jun10.75984      4627.75984   4627.76230   1.189  0.003 

Last modified 06/27/2008 by MWR.