Aug 04, 2005 UT: Photometry of HS 1813+61

Michael Richmond
Aug 04, 2005

On the night of Aug 03/04, 2005 EDT, Tracy Davis and I used the RIT Observatory's 12-inch Meade telescope and SBIG ST8 CCD camera to monitor the cataclysmic variable star HS 1813+61. This target of the Center for Backyard Astrophysics has a period of about 203 minutes; see Jochen Pietz' analysis of earlier observations of HS 1813+61 for more details.

The plan:

Notes from the night

This is a chart of the field based on images taken on this night. 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 our observing run tonight -- see the figures below.

I measured the instrumental magnitude of each star with aperture photometry, using a radius of 3 pixels = 5.6 arcseconds (smaller than usual to reduce the very high sky noise), 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 3.3, with an elevated scatter. Star "X" is the outlier near differential mag 4.9. The outliers at the bright end are stars which were saturated -- some of these should be fine on good nights, but were pushed into the non-linear portion of the chip by the high sky value.

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 5 are measurements of the star "X". As you can see, its gradual increase in brightness is very different from the behavior of the similarly faint star "Y", so I think it may be a variable -- perhaps an contact eclipsing binary.

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

The large scatter hides any fine detail (especially near the ends of the observing run), but there is clearly some slow, rolling variation.

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, which is a rough, rough guess at its V-band magnitude, based on the B and R measurements in USNO B1.0 catalog.

Here's the start of the table.

# Measurements of HS 1813+61 made at RIT Obs, Aug 4, 2005 UT, 
#   made by Michael Richmond and Tracy Davis in poor conditions. 
# 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
Aug04.07340      3586.57340   3586.57391  15.154  0.031 
Aug04.07499      3586.57499   3586.57550  15.147  0.032 
Aug04.07814      3586.57814   3586.57865  15.061  0.034 
Aug04.08132      3586.58132   3586.58183  15.070  0.037 

Last modified 8/6/2005 by MWR.