Sep 11, 2013 UT: SN 2013ej in M74

Michael Richmond
Sep 12, 2013

On the night of Sep 10/11, 2013, I observed SN 2013ej in M74. Conditions were good, with clear skies and no moon.

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 unusual: very warm (81 F/27 C) at sunset. No clouds, but the sky looked a bit hazy.
• FWHM was fair: about 3.0 pixels in VRI, 3.5 in B.
• Using a new method to extract instrumental magnitudes: co-adding images before measuring. See notes below.

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, but unguided images in B. After discarding the bad images, I was left with 17, 16, 17, and 10 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.

I tried something new for this night's data: 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

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

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

Results from this evening are:

filter  mag         mag_uncert                          Julian Date

B =   14.642   +/-   0.094  (ens  0.043 zp  0.083)    2456546.67444 
V =   13.302   +/-   0.028  (ens  0.017 zp  0.022)    2456546.67751 
R =   12.824   +/-   0.027  (ens  0.013 zp  0.024)    2456546.67374 
I =   12.542   +/-   0.051  (ens  0.020 zp  0.047)    2456546.67376 

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 09/12/2013 by MWR.