Aug 08, 2005 UT: Photometry of HS 1813+61 and EI Psc

Michael Richmond
Aug 08, 2005
Aug 09, 2005
Aug 10, 2005
Aug 11, 2005

On the night of Aug 07/08, 2005 EDT, Tracy Davis and I used the RIT Observatory's 12-inch Meade telescope and SBIG ST8 CCD camera to monitor two cataclysmic variable stars: HS 1813+61 and EI Psc. The first was a target of the Center for Backyard Astrophysics; see Jochen Pietz' analysis of earlier observations of HS 1813+61 for more details about it. The second, EI Psc, was reported to be in outburst just a few hours before we started work, and we switched to it as soon as it rose above our horizon.

The plan:

Notes from the night

HS 1813+61

We started monitoring HS1813+61 at sunset, but then switched to EI Psc after about 1.3 hours.

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

The best photometry I could find for the stars in this field is simply the USNO B1.0 -- oh, well. The star marked "B" in the chart above is USNO B1.0 1514-0236071. Based on the USNO B1.0 "B" and "R" magnitudes, I calculate a very very rough "V" magnitude via the simple average "V" = (B+R)/2 = 13.77. I'll use that to set the zeropoint of the differential unfiltered measurements; not good, but all I can do for now. I hope someone does good photometry of field stars on the standard scale ...

The star marked "X" on the chart above is USNO-B1.0 1514-0236034. It showed strongs hints of variability during other nights, but tonight was steady within the noise -- see the figures below.

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.

HS 1813+61 is the star near differential mag 1.8, with an elevated scatter.

Light curves for selected stars in the field are shown below. HS 1813+61, shown by light green crosses near the middle, is clearly variable. The black points around differential magnitude 6 are measurements of the star "X". It doesn't show any significant change outside the (large) scatter.

Here's a closeup of the variation in HS 1813+61 itself:

I've looked at the image containing the one bright outlier, and I can't see any obvious defect to cause it.

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 13.77, 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 HS 1813+61 made at RIT Obs, Aug 08, 2005 UT, 
#   made by Michael Richmond and Tracy Davis. 
# All data taken with 12-inch LX-200 + no filter + SBIG ST-8 CCD 
#    no focal reducer, so at native f/10 
# Each exposure 120 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 1514-0236071 has mag=13.77 
#    (a rough guess = (B+R)/2 and the USNO B1.0 magnitudes). 
# 
# UT day      JD-2,450,000  HJD-2,450,000   mag    uncert
Aug08.07124      3590.57124   3590.57173  14.879  0.027 
Aug08.07281      3590.57281   3590.57330  14.894  0.044 
Aug08.07437      3590.57437   3590.57486  14.886  0.032

EI Psc = 1RXS J232953.9+062814

This little-known star is an unusual cataclysmic variable because its period, about 66 minutes in quiescence, is below the so-called "period gap" for CVs. It was discovered only recently, and has been observed well only for one past outburst. You can read about in these articles:

We were alerted to its outburst by this message from Patrick Schmeer: 

Date: Mon, 8 Aug 2005 00:23:00 +0200 (CEST)
From: Patrick Schmeer 
Subject: [vsnet-outburst 6590] EI Psc in outburst

Visual magnitude estimates by P. Schmeer, Bischmisheim, Germany:
PSCEI 20050806.94 <138 Scp
PSCEI 20050807.918 124 Scp

Taichi Kato added a comment that this might be a superoutburst, so it seemed important to check out. We had to wait until about 11 PM local time for the object to rise far enough above our eastern horizon to observe it. Here's a chart based on one of our images: click on the image for a larger version.

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 (but see noted added Aug 11, below): 

                                                             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.

Note added Aug 11, 2005:

The AAVSO has a chart of this field (several, actually) which indicates that the star marked "A" in my picture above has magnitude V = 12.2.

My unfiltered images from UT Aug 08 and Aug 10 show that star "B" is brighter than star "A" by 0.077 +/- 0.001 mag. So, that suggests that one might estimate a V-band magnitude for star "B" of V = 12.2 - 0.077 = 12.122. That is just about 0.50 mag brighter than the value I have adopted in my analysis so far, for the UT Aug 08 and Aug 10 datafiles.

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 "A" (in red) and "B" (in blue). It is undergoing periodic variations with amplitude roughly 0.04 mag, and at the same time, gradually brightening by about 0.15 mag over the course of our 5-hour run.

Just for fun, I tried to phase the data with several periods. Uemura et al. found that the orbital period (from observations during quiescence) was 64.184 minutes, while superhumps during the 2001 superoutburst had a period of 66.774 minutes. I tried phasing our measurements with both those periods; I also tried removing a linear fit to the magnitudes, then phasing with an intermediate period of 65 minutes.

I can't tell which period, if any, fits best; it appears to me that the amplitude of the variation was slowly increasing throughout our run, and the period might have been changing, too.

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 08, 2005 UT, 
#   made by Michael Richmond and Tracy Davis. 
# 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
Aug08.15818      3590.65818   3590.66261  13.123  0.048 
Aug08.15875      3590.65875   3590.66318  13.107  0.048 
Aug08.15929      3590.65929   3590.66372  13.126  0.048

Note added Aug 9, 2005: Greg Bolt writes:

At the end of my run @ 08/Aug, 22:00UT - the superhumps had grown to almost 0.3 magnitude. A preliminary analysis of the full data run which covered about 5.5 cycles gives a period of 0.0489+-0.0010 days.

This is looking like another good superoutburst!

Note added Aug 9, 2005: John Thorstensen writes: 

I also have an updated ephemeris -- the time of blue-to-red
crossing of the secondary (i.e. when the secondary star is 
closest to us) is 

HJD 2452237.65157(8) + 0.04456689(3) E,

where E is an integer.  This is the _orbital_ period, not
superhump.   It's 64.17632 +- 0.00005 minutes.  The period
is accurate enough that the orbital phase can be computed
within about a tenth of a cycle at the present time.

Note added Aug 10, 2005: John Thorstensen writes: 

I found a few more spectra and refined the _orbital_ ephemeris
for EI Psc slightly compared to the message I sent yesterday; 
for the record, it's

HJD of blue-to-red crossing = 

2452295.58854(6) + 0.04456691(2) E, 

where E is an integer.  

The orbital period is  64.17635 +- 0.00003 minutes.

The period is this precise because the cycle count is unambigous
over the whole span of the data, which are from 2001 November to
2004 January.  I may grab a few more spectra next month if it's
returned to quiescence, which should nail the ephemeris down
even better.

Last modified 8/9/2005 by MWR.