Aug 18, 2005 UT: Photometry of EI Psc (short run, poor conditions)

Michael Richmond
Aug 18, 2005

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

On the night of Aug 17/18, 2005 EDT, 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 increased to 90 seconds, since EI Psc has faded recently

Notes from the night

• the night was awful: constant clouds which just became worse. I gave up after about two hours. The graph below shows the zero-point offset for images taken during the night.

  

• both of the reasonably bright stars "A" and "B" (see charts below) which I have been using to find the photometric zero-point on previous nights were saturated on this night (due to the very bright sky background caused by moonlight on clouds). There may be a small error in the relative photometry compared to other nights

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

And yes, on this particular night (UT Aug 18), stars "A" and "B" were both slightly saturated.
MWR 8/18/2005

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.

Note how large the scatter is for all stars compared to the scatter on good nights. I excluded "A" and "B" from full participation in the photometric ensemble, which explains their very large scatter. EI Psc is the star with slightly elevated scatter around differential mag 2.9.

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

Here's a closeup of the variation in EI Psc itself, with the comparison stars "C" (in purple), "D" (in aqua), and "F" (dark blue). I can't really say anything about any variations of EI Psc within the run; the sharp decline in the final few points may be real, but that is about all I would believe.

What is clear is that EI Psc is much fainter than it was just 24 hours ago. As our analysis of the UT Aug 17 data shows, EI Psc was about 0.2 mag fainter than star "C" (purple) then; it is about 1.2 mag fainter than "C" now.

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. Remember, because the comparison star "B" was slightly saturated tonight, this offset might be bad.

Here's the start of the table.

# Measurements of EI Psc made in poor conditions at RIT Obs, Aug 18, 2005 UT, 
#   made by Michael Richmond.
# 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). 
#    Warning: this star may have been slightly saturated!    
# 
# UT day      JD-2,450,000  HJD-2,450,000   mag    uncert
Aug18.11457      3600.61457   3600.61955  15.486  0.063 
Aug18.11705      3600.61705   3600.62203  15.560  0.058 
Aug18.11828      3600.61828   3600.62326  15.503  0.058 

• download the entire table (plain ASCII)

Last modified 8/18/2005 by MWR.