Sep 27, 2013 UT: SN 2013ej in M74

Michael Richmond
Sep 29, 2013

On the night of Sep 26/27, 2013, RIT physics major Brandon Doyle and I observed SN 2013ej in M74. Conditions were good at first, but low lake-effect clouds moved in from the north as we were trying to gather images of the SN.

The main setup was:

• Meade 12-inch telescope
• no focal reducer, so working at f/10
• SBIG ST-8E CCD camera with BVRI filters
• 30-second BVRI exposures of SN 2013ej

Notes from the night

• Weather was good at first, but then clouds started to approach. We took a few images only in VRI, then a bunch in B, and managed to squeeze in a few more VRI before the clouds ruined the party.
• Moon not much of an issue.
• FWHM was very good: a few images had FWHM less than 2 pixels.

SN 2013ej in M74

SN 2013ej is a Type II supernova in the relatively nearby galaxy M74. It was discovered by the KAIT group about one 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

I took 30-second guided images in VRI, and on this night managed to guide in B-band, too. After discarding the bad images, I was left with 8, 5, 5, and 9 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 addition to extracting instrumental magnitudes from each individual image of the SN, I also co-added (using a median technique) all the good images in each filter to create a single image with higher signal-to-noise. Below is a comparison of the results derived from individual images to those derived from the median image:

 median - individual               B         V         R         I
-------------------------------------------------------------------------
Sep 11   UT                      -0.098    -0.016    -0.003    -0.002

Sep 18   UT                       0.059    -0.005     0.009    -0.013 

Sep 19   UT                      -0.078     0.015    -0.007     0.003 

Sep 25   UT                       0.019     0.006     0.009     0.006 

Sep 27   UT                       0.073    -0.022    -0.005    -0.015 

-------------------------------------------------------------------------

I will report the results based on the median images below.

Results from this evening are:

filter  mag         mag_uncert                          Julian Date

B =   14.993   +/-   0.056  (ens  0.048 zp  0.029)    2456562.61615 
V =   13.570   +/-   0.041  (ens  0.020 zp  0.036)    2456562.61816 
R =   13.006   +/-   0.016  (ens  0.014 zp  0.007)    2456562.61509 
I =   12.691   +/-   0.026  (ens  0.019 zp  0.018)    2456562.62406 

The uncertainties here are (except for B) 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 09/29/2013 by MWR.