# Jul 05, 2009 UT: Photometry of cataclysmic variable RXJ1643+34 = V1084 Her

#### Michael Richmond Jul 5, 2009

On the night of July 4/5, 2009, 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 placed near the meridian at sunset.

The setup was:

• Meade LX-200 12-inch telescope (the dome helped prevent bright moonlight from striking telescope aperture)
• no focal reducer, so working at f/10
• SBIG ST-8E CCD camera with no filter
• 15-second unguided exposures

Notes from the night

• The conditions were good: clear and relatively cool. The gibbous moon made the sky bright, however.
• The FWHM was pretty steady at 2.4 pixels
• RXJ1643 was approximately V=12.4 during this run.

This is a chart of the field taken from the DSS.

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 photometry provided by Mickaelian et al., A&A 381, 894 (2002):

```my
label                           RA        Dec            B        V
----------------------------------------------------------------------------
A  USNOB1.0 1240-0246555   16:43:40.59 +34:03:02     13.51    12.97
B  TYC 2585-1631-1         16:43:27.81 +34:02:07     13.20    12.16
----------------------------------------------------------------------------

```

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. This was a good night: note the small change over the 4.5 hours of observation, only 0.12 mag in zeropoint.

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. That's very good! RXJ1643 appears at differential magnitude 1.6; its scatter of 0.06 mag is much larger than that of other bright stars. showing that it is a variable.

Light curves for selected stars (RXJ1643 and stars A - D) in the field are shown below. The target is shown by light green crosses. I've shifted star "C" (pink squares) a bit to separate it from star "A" (red plus signs).

Here's a closeup of the variation in RXJ1643 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, USNOB1.0 1240-0246555, has value 12.97.

Here's the start of the table.

```# Measurements of RXJ1643+34 made at RIT Obs, Jul 5, 2009 UT,
#    by Michael Richmond, using 12-inch Meade LX-200 and SBIG ST-8E CCD.
# Each exposure 15 seconds long with no 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 USNOB1.0 1240-0246555 has V=12.97
#    to match value from Mickaelian et al., A&A 381, 894 (2002)
#
# UT_day             JD            HJD        mag    uncert
Jul05.07905     2455017.57905  2455017.58153  12.477  0.010
Jul05.07933     2455017.57933  2455017.58181  12.482  0.010
Jul05.07961     2455017.57961  2455017.58209  12.459  0.010

```