PT observation of transit of HAT-4 on UT 2008 Feb 24 = MJD 54520

Michael Richmond
Douglas Tucker
Feb 25, 2008

On the night of UT Feb 24, 2008, the SDSS Photometric Telescope ("PT" for short) took a series of exposures of HAT-4. We saw a full transit under poor conditions; the ingress is too noisy to detect clearly, but we should have decent measurement of the egress.

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 pretty bad for this run. 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 0.0; 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 -- not very good, due to the clouds. The two outliers between differential mags 1.5 and 2.5 are eclipsing binary stars (as are many of the fainter outliers).

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.

The ingress is too noisy to see -- it would be at about 520.810 (see ephemeris below). I estimate by eye the egress to occur at about 520.98.

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
2454520.81 2008  2 24  7 26   2454520.90 2008  2 24  9 41   2454521.00 2008  2 24 11 55

The ephemeris ingress of UT 2008 Feb 20 07:26:00 corresponds to JD 2,454,520.810. The ephemeris egress of UT 2008 Feb 20 11:55:00 corresponds to JD 2,454,520.997, not far from my eyeball estimate.

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, Feb 24, 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
Feb24.77074      4520.77074   4520.77262   0.063  0.004 
Feb24.79228      4520.79228   4520.79416   0.071  0.004 
Feb24.79372      4520.79372   4520.79560   0.065  0.004 

Last modified 02/25/2008 by MWR.