Aug 17, 2005 UT: Photometry of EI Psc

Michael Richmond
Aug 17, 2005

• Summary of EI Psc 2005 superoutburst so far ...

On the night of Aug 16/17, 2005 EDT, RIT student George Privon and I used the RIT Observatory's 12-inch Meade telescope and SBIG ST8 CCD camera to monitor the cataclysmic variable star EI Psc (also known as 1RXS J232953.9+062814) as it was near the peak of its outburst. We are acting on behalf of the Center for Backyard Astrophysics, VSNET, and all others who study these very interesting variable stars.

The plan:

• the Meade 12-inch telescope with SBIG ST-8E CCD (no focal reducer); the plate scale is about 1.85 arcsec per pixel
• no filter
• exposure time 30 seconds

Notes from the night

• the night had a poor start: many clouds came overhead during the first two hours. After 1 AM, the clouds disappeared, so the data improved. The graph below shows the zero-point offset for images taken during the night.

  

  You can see that one bunch of thin clouds passed over the field around 599.78 -- you'll notice an increased scatter in the measurements at that time in the light curves below.

You can read about EI Psc in these articles:

• Discovery of a Dwarf Nova Breaking the Standard Sequence of Compact Binary Evolution by Uemura et al., PASJ 54L, 15 (2002)
• Superhump Evolution in the Ultrashort Period Dwarf Nova 1RXS J232953.9+062814 by Uemura et al., PASJ 54, 599 (2002)
• 1RXS J232953.9+062814: A Possible Progenitor of AM CVn Stars by Uemura, RMxAC, 20, 109 (2004)
• The VSNET page on this star (which uses its name 1RXS J232953.9+062814) has lots of good information and charts.
• 1RXS J232953.9+062814: A Dwarf Nova with a 64-minute Orbital Period and a Conspicuous Secondary Star by Thorstensen et al. (to appear in ApJ Letters)
• Short summary of measurements of EI Psc during 2005 outburst, which includes pointers to our measurements.

The stars marked "A" and "B" were used as comparisons in the 2001 superoutburst, according to the Uemura et al. papers. Here is the rather poor information I can find about them:

                                                             USNO B1.0 mags
star  GSC_1.2_ID   USNO B1.0 ID         RA   (J2000) Dec       B        R
----------------------------------------------------------------------------
 A   0591.1689     0964-0601121      23:30:02.7  +06:24:37   13.64?  11.83?

 B   0584.366      0963-0598647      23:29:49.9  +06:22:11   12.4    12.85
----------------------------------------------------------------------------

I have used star "B" to set the zeropoint of my unfiltered measurements. I create a pseudo "V-ish" magnitude by the simple calculation (B+R)/2 based on the USNO B1.0 magnitudes, and end up with "V-band" mag = 12.63.

Yes, I know now that there is a good measurement of V = 12.2 for star "A", so that my differential measurements have a systematic offset of about +0.5 magnitudes. However, I will continue to use this (incorrect) zero-point for my unfiltered measurements for internal consistency.
MWR 8/17/2005

I measured the instrumental magnitude of each star with aperture photometry, using a radius of 4 pixels = 7.5 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.

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

The two brightest stars -- which you can see between HD 221248 and star "A" in the chart above -- are both saturated in our 30-second images. The outlier at differential mag approx 2.0 is EI Psc. The formal scatter for the brightest unsaturated stars, "A" and "B", is about 0.004 mag.

Light curves for selected stars in the field are shown below. EI Psc, shown by light green crosses near the middle, is clearly variable.

Here's a closeup of the variation in EI Psc itself, with the comparison stars "C" (in purple) and "D" (in aqua). The amplitude of the variations is smaller than it was a week ago: only about 0.1 mag peak-to-peak. Because the data is so noisy, I show also in black a version of the measurements smoothed with a uniform boxcar of width 5 points.

It appears that EI Psc was fading slightly near the end of our run.

I've made a table of the measurements themselves, with three different flavors of time. The differential unfiltered magnitudes from the ensemble solution has been shifted so that star "B" in my chart, has value 12.63, matching a very rough guess at its V-band magnitude, based on B and R mags in the USNO B1.0 catalog.

Here's the start of the table.

# Measurements of EI Psc made at RIT Obs, Aug 17, 2005 UT, 
#   made by Michael Richmond and George Privon. 
# All data taken with 12-inch LX-200 + no filter + SBIG ST-8 CCD 
#    no focal reducer, so at native f/10 
# Each exposure 30 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 USNOB1.0 0963-0598647 has mag=12.63 
#    (a rough guess = (B+R)/2 and the USNO B1.0 magnitudes). 
# 
# UT day      JD-2,450,000  HJD-2,450,000   mag    uncert
Aug17.15182      3599.65182   3599.65675  14.511  0.028 
Aug17.15236      3599.65236   3599.65729  14.586  0.027 
Aug17.15291      3599.65291   3599.65784  14.531  0.032 

• download the entire table (plain ASCII)

Last modified 8/17/2005 by MWR.