Jul 31, 2013 UT: Photometry of SN 2013ej in M74

Michael Richmond
Aug 2, 2013

On the night of July 30/31, 2013, I observed SN 2013ej in M74.

The main setup was:

• Meade 12-inch telescope
• no focal reducer, so working at f/10
• SBIG ST-8E CCD camera with BVRI filter
• 30-second guided exposures

Notes from the night

• Cirrus and light cloud were present most of the night
• FWHM was mediocre, around 2.5-3.4 pixels (4.6 to 6.2 arcsec)
• I used domeflats

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

I took 30-second guided images -- expect in B-band: the guide star was too faint for the guider to track. I took 5 images in VRI, 20 images in B. Afterwards, I discarded those images which were trailed or showed a "jump" in the tracking.

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 which I am currently revising -- so please treat these results as preliminary to convert the ensemble instrumental magnitudes to the standard Johnson-Cousins BVRI scale.

Results from this morning are:

filter  mag         mag_uncert                          Julian Date

B =   12.689   +/-   0.019  (ens  0.015 zp  0.012)    2456504.74164 
V =   12.614   +/-   0.019  (ens  0.015 zp  0.012)    2456504.73380 
R =   12.507   +/-   0.027  (ens  0.015 zp  0.022)    2456504.73075 
I =   12.434   +/-   0.054  (ens  0.019 zp  0.051)    2456504.75007 

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

Last modified 08/02/2013 by MWR.