Aug 15, 2013 UT: UZ Boo and SN 2013ej

Michael Richmond
Aug 15, 2013

On the night of Aug 14/15, 2013, I observed UZ Boo and SN 2013ej in M74. Conditions were pretty good.

The main setup was:

• Meade 12-inch telescope
• no focal reducer, so working at f/10
• SBIG ST-8E CCD camera with no filter (UZ Boo) and BVRI filters (SN 2013ej)
• 90-second guided exposures of UZ Boo
• 30-second guided exposures of SN 2013ej (except unguided in B)

Notes from the night

• The sky was mostly clear, though a few light clouds may have been present during the SN observations.
• FWHM was poor: 2.9 to 3.5 pixels.

UZ Boo bright again

I took a series of images of UZ Boo, using a 90-second exposure time, since UZ Boo has been brighter recently. It was bright again tonight, around V = 14.1, so this length provided plenty of signal-to-noise.

The skies were clear tonight, as this plot of zero-point adjustment shows:

I measured UZ Boo and many other stars in the field. The bright stars "A", "B" and "D" were somewhat saturated, so I left them out of the ensemble solution. Instead of the star "A", I used the star "E" = UCAC4 561-055129 = AAVSO 000-BBV-649 to shift the instrumental magnitudes to the standard scale. Remember, the measurements are made without a filter , but I'll shift them to match the V-band scale: 000-BBV-649 has mag V = 14.267 according to the AAVSO chart 12447AMF.

Below are light curves for UZ Boo and several of the unsaturated stars in the field:

Here's a closeup of UZ Boo and another star:

Below are the first few lines of the report I've sent to the AAVSO and VSNet.

# Measurements of UZ_Boo made at RIT Obs, Aug 15, 2013 UT, 
#    in fair conditions, 
#    by Michael Richmond, using 12-inch Meade and SBIG ST-8E CCD. 
# Exposures 90 seconds long, clear 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 7.4 arcseconds. 
#    which has been shifted so UCAC4 561-055129 has mag=14.267 
#    which is its V-band magnitude according to AAVSO. 
# 
# UT_day             JD            HJD        mag    uncert
Aug15.04787     2456519.54787  2456519.54610  14.007  0.012 
Aug15.04918     2456519.54918  2456519.54741  14.027  0.013 
Aug15.05049     2456519.55049  2456519.54872  14.015  0.012 
Aug15.05178     2456519.55178  2456519.55001  14.039  0.012 

• download ASCII data for UZ Boo, UT 2013 Aug 15

SN 2013ej in M74

SN 2013ej is a Type II supernova in the relatively nearby galaxy M74. It was discovered by the KAIT group some time (a week?) before maximum light. Here's a chart showing the galaxy, the SN, and some reference stars:

The reference stars marked above have magnitudes in AAVSO chart 12459CA, as follows:

 letter      B     sigB       V     sigV      R      sigR     I    sigI
  B        13.012  0.019   12.510  0.019    12.154  0.019   11.834  0.019
  F        13.848  0.026   13.065  0.022    12.622  0.025   12.152  0.027   
  H        14.338  0.029   13.692  0.024    13.329  0.029   12.964  0.030
  I        14.832  0.027   13.912  0.023    13.416  0.026   12.939  0.030
  K        15.192  0.034   14.613  0.027    14.275  0.034   13.915  0.036

After discarding the bad images, I was left with 7, 5, 13 and 6 images in B, V, R, and I, respectively.

Using aperture photometry with a radius of 4 pixels (radius of 7.4 arcsec), I measured the instrumental magnitudes of a number of reference stars and the target. 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. I used the AAVSO magnitudes, plus color terms to convert the ensemble instrumental magnitudes to the standard Johnson-Cousins BVRI scale.

In the B-band, the transformation from instrumental to Johnson-Cousins magnitude had a larger uncertainty than usual (0.08 vs. 0.03 mag). I tracked the difference to measurements of the star "K" = 000-BBD-099. It appeared brighter than expected by about 0.15 mag. I saw no obvious outliers or image defects near it. Hmmm. I discarded it from the solution in the B-band only.

Results from this morning are:

filter  mag         mag_uncert                          Julian Date

B =   13.239   +/-   0.032  (ens  0.011 zp  0.030)    2456519.70731 
V =   12.586   +/-   0.026  (ens  0.019 zp  0.018)    2456519.70128 
R =   12.310   +/-   0.028  (ens  0.010 zp  0.026)    2456519.69379 
I =   12.176   +/-   0.035  (ens  0.025 zp  0.024)    2456519.71723 

The uncertainties here are roughly equally distributed between extracting the instrumental magnitudes and transforming the instrumental magnitudes to the standard scale.

Grab the text file below for all the RIT measurements of SN 2013ej. All these values have been recomputed with the new color terms of UT 2013 Aug 05.

• sn2013ej_mags.out

Last modified 08/15/2013 by MWR.