Sep 14, 2007 UT: Photometry of HS 2331+3905

Michael Richmond
Sep 14, 2007

On the night of Sep 13/14, 2007 EDT, Tracy Davis used the RIT Observatory's 12-inch Meade telescope and SBIG ST8 CCD camera to monitor the cataclysmic variable star HS 2331+3905, which was in the early stages of an outburst. This star is a cataclysmic variable similar to WZ Sge. For more information about it, read

The plan:

Notes from the night

This is a chart of the field based on images taken on earlier nights. Click on the chart for a larger version.

The chart has several of the brighter stars in the field labelled with letters, just to keep me straight as I perform the reductions. Some of these stars have good photometry, as mentioned in AAVSO Alert Notice 357 .

label     RA               Dec                 V               B       visual
      23:37:10.49     +39:27:09.7      8.366 (0.014)   9.430 (0.025)   84
      23:31:57.51     +39:19:43.4      9.248 (0.024)   9.969 (0.031)   93
      23:33:05.63     +39:23:11.8      9.572 (0.029)  10.693 (0.059)   96
      23:33:53.74     +38:57:22.6     10.007 (0.042)  11.115 (0.087)  100
      23:34:46.69     +39:16:44.0     10.310 (0.043)  10.961 (0.054)  103
 B    23:33:23.41     +39:17:58.8     10.481 (0.055)  11.167 (0.070)  105
 A    23:34:23.38     +39:15:34.9     10.900 (0.078)  11.393 (0.076)  109

I'll use the star marked "A" to set the zeropoint of my differential magnitudes back onto the standard system, at least roughly.

I measured the instrumental magnitude of each star with aperture photometry, using a radius of 3 pixels = 5.6 arcseconds and sky defined by an annulus around each star. (I tried using a smaller aperture of 2 pixels, but the scatter from mean was smaller with the 3-pixel aperture). 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.

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

HS 2331 is the star with large scatter at differential magnitude 0.5. The brightest two comparison stars, "A" and "B", have a scatter of 0.006 and 0.007 mag, respectively, from their mean values. The star at mag roughly 3.5 with a slightly elevated scatter has a single image with a big cosmic ray -- it isn't intrinsically variable.

Light curves for selected stars (HS 2331 and stars A - G) in the field are shown below. HS 2331, shown by light green crosses near the top, is clearly variable. It has faded a bit since last night, however: It had V-band mag of about 8.7 last night, but was at V-band of about 9.2 tonight.

Here's a closeup of the variation in HS 2331 itself. I've also plotted the measurments of star "A", shifting them by 0.4 magnitudes so that they would fit nicely on the graph. Star "A" has errorbars to show the estimated uncertainty in each measurement.

Note several very sharp changes in the star's brightness. Our exposure time was 10 seconds, and readout time about 7 seconds, so each point on graph is separated by about 17 seconds. For example, at about JD 357.63, the star brightened by about 0.08 magnitudes (eight percent) and then faded back to its original level within about one minute. You can see this feature in our measurements AND in Gianluca Masi's measurements from the Bellatrix Observatory:

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, TYC 3231-533-1, has value 10.900.

Here's the start of the table.

# Measurements of HS 2331+3905 made at RIT Obs, Sep 14, 2007 UT, 
#   made by Tracy Davis. 
# All data taken with 12-inch LX-200 + V filter + SBIG ST-8 CCD 
#    no focal reducer, so at native f/10 
# Each exposure 10 seconds long; tabulated times are midexposure 
#    and accurate only to +/- 1 second. 
# 'mag' is a differential magnitude based on ensemble photometry 
#    which has been shifted so TYC 3231-533-1 mag=10.900 
#    to match value from AAVSO Alert Notice 357. 
# UT day      JD-2,450,000  HJD-2,450,000   mag    uncert
Sep14.06562      4357.56562   4357.56987  11.019  0.010 
Sep14.06587      4357.56587   4357.57012  11.025  0.009 
Sep14.06612      4357.56612   4357.57037  11.019  0.009 
Sep14.06637      4357.56637   4357.57062  11.012  0.008 
Sep14.06662      4357.56662   4357.57087  10.977  0.008 

Last modified 9/14/2007 by MWR.