May 31, 2013 UT: Photometry of cataclysmic variable RXJ1643+34 = V1084 Her

Michael Richmond
May 31, 2013

On the night of May 30/31, 2013, summer student Laura Maher and I observed the cataclysmic variable star RXJ1643+34 = V1084 Her. Joe Patterson of the Center for Backyard Astrophysics suggested that members of the CBA observe this star during the summer. The object is at RA = 16:43:45.70 Dec = +34:02:40, so it is well placed for measurements all night long.

The setup was:

Notes from the night

This is a chart of the field taken from last night's data:

The chart has several of the brighter stars in the field labelled with letters, just to keep me straight as I perform the reductions. Photometry of star A provided by Mickaelian et al., A&A 381, 894 (2002) and by the UCAC4:

my                                                Mickaelian        UCAC4
label                    RA        Dec            B        V       B      V
A  UCAC4 621-054484   16:43:40.59 +34:03:02     13.51    12.97    13.47  12.96

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. 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 outliers near the end of the run -- there was a period of heavier clouds, then a slight clearing before the end of the run.

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 0.010 mag, which is so-so. V1084 Her appears at differential magnitude 0.3; its scatter of 0.07 mag is much larger than that of other bright stars, showing that it is a variable.

Light curves for selected stars (V1084 Her 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 V1084 Her and a few comparison stars.

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, UCAC4 621-054484, has value 12.96.

Here's the start of the table.

# Measurements of V1084Her made at RIT Obs, May 31, 2013 UT, 
#    by Michael Richmond and Laura Maher, 
#    using 12-inch Meade LX-200 and SBIG ST-8E CCD. 
# Each exposure 30 seconds long with V filter. 
# 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 5.6 arcseconds. 
#    which has been shifted so UCAC4 621-054484 has V=12.96 
# UT_day             JD            HJD        mag    uncert
May31.09535     2456443.59535  2456443.59862  12.531  0.013 
May31.09581     2456443.59581  2456443.59908  12.521  0.013 
May31.09628     2456443.59628  2456443.59955  12.529  0.013 

Last modified 5/31/2013 by MWR.