Jul 20, 2009 UT: Photometry of cataclysmic variable SDSS 150241+333424

Michael Richmond
Jul 20, 2009

On the night of July 19/20, 2009, I observed the cataclysmic variable star SDSS150241+333424, which is currently in outburst.

The setup was:

Notes from the night

This is a chart of the field taken from UT Jul 10's unfiltered measurements.

The chart has several of the brighter stars in the field labelled with letters, just to keep me straight as I perform the reductions. Star A is in the Tycho-2 catalog, with measurements

my label                        RA        Dec            Bt       Vt
  A  Tycho-2 2565-1118-1     15:03:02.00 +33:37:37     13.021   11.874

Just to the east of this chart is the bright star HD 133431 = BD +34 2598 = HIP 73657, at V = 8.4.

I measured the instrumental magnitude of each star with aperture photometry, using a radius of 4 pixels = 5.6 arcseconds and sky defined by an annulus around each star. 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.

One output of the ensemble solution is the value of the zero-point of each frame relative to the others. In the graph below, I plot this zero-point as a function of time. Note the scatter after JD 32.6, as thin clouds pass through.

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

The floor of this diagram corresponds to a scatter of only about 0.003 mag, very good. However, the target is so faint, at differential magnitude 5.2, that its scatter is barely larger than the typical noise.

Light curves for selected stars (SDSS150241 and stars A - D) in the field are shown below. The target is shown by light green crosses.

Here's a closeup of the variation in SDSS150241 and a few comparison stars.

Just for fun, here's a comparison of the light curve of the star on all the nights I've observed it so far. I've shifted the measurements on the time axis by hand so that the minima roughly coincide.

I've made a table of the measurements themselves, with three different flavors of time. The differential magnitudes from the ensemble solution have been shifted so that star "A" in my chart, Tycho-2 2565-1118-1, has value 11.874. Note the extra uncertainty in time for tonight's dataset, due to the coadding of three consecutive images.

Here's the start of the table.

# Measurements of SDSS150241+333424 made at RIT Obs, Jul 20, 2009 UT, 
#    by Michael Richmond, using 12-inch Meade LX-200 and SBIG ST-8E CCD. 
# Exposures 3x30 coadded = 90 seconds long, no filter. 
# Tabulated times are midexposure (FITS header time - half exposure length) 
#    and accurate only to +/- 5 seconds (??). 
# 'mag' is a differential magnitude based on ensemble photometry 
#    using a circular aperture of radius 5.6 arcseconds. 
#    which has been shifted so TYC 2565-1118-1 has Vt=11.874 
#    to match value from Tycho-2 catalogue. 
# UT_day             JD            HJD        mag    uncert
Jul20.06763     2455032.56763  2455032.56777  17.256  0.155 
Jul20.06953     2455032.56953  2455032.56967  17.396  0.173 
Jul20.07113     2455032.57113  2455032.57127  17.007  0.115 

Last modified 7/20/2009 by MWR.