Dec 29, 2013 UT: SN 2013ej in M74 (the last night)

Michael Richmond
Jan 16, 2014

On the night of Dec 28/29, 2013, I observed SN 2013ej in M74, for what would turn out to be the last time (sniff, sob). Conditions were fair: clear at the start of the night, but some small changes in transparency (+/- 0.2 mag) during the observations.

The main setup was:

Notes from the night


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, though the guide star was occasionally lost. After discarding the bad images, I was left with 24, 23, and 28 images in V, R, and I, respectively.

I re-centered the field after taking 6 good images, which left the southernmost portion of the image somewhat underexposured compared to the rest. That underexposed region does not come close to the SN, but it does almost touch star I, one of the reference stars.

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.

The SN is now so faint that I've stopped making measurements in B. I coadded (using a median technique) all the good images in each filter to create a single image with higher signal-to-noise. I now measure the SN in co-added images only. 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 

Sep 29   UT                      -0.015     0.019     0.010     0.015 

Oct 02   UT                       0.026    -0.006    -0.002    -0.020 

Oct 03   UT                       0.015    -0.018     0.005     0.002 

Oct 08   UT                      -0.004     0.004     0.002     0.004 

Oct 09   UT                       0.001     0.011    -0.002    -0.011 

Oct 12   UT                       0.005     0.007    -0.004     0.014 

Oct 13   UT                       0.054     0.016    -0.003    -0.003 

Oct 24   UT                       0.056    -0.008    -0.010     0.027 

Oct 29   UT                                -0.016    -0.013    -0.021

Nov 03   UT                                -0.024     0.019     0.005

Nov 11   UT                                +0.154     0.009     0.020

Nov 20   UT                                 0.090    -0.012     0.028 

Nov 21   UT                                           0.041     0.085 


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

Results from this evening are:


filter  mag         mag_uncert                          Julian Date

V =   17.089   +/-   0.217  (ens  0.216 zp  0.021)    2456655.49267 
R =   16.144   +/-   0.109  (ens  0.109 zp  0.009)    2456655.47490 
I =   16.132   +/-   0.196  (ens  0.195 zp  0.019)    2456655.51409 



The uncertainties here are dominated by extracting the instrumental magnitudes.

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.

The final stage of the light curve is noisy, but it appears consistent with an exponential decay (which looks like a straight line in this magnitude-versus-time diagram).


Last modified 1/16/2014 by MWR.