May 31, 2016 UT: Photometry of SN 2016coj in NGC 4125

Michael Richmond
Jun 21, 2016

On the night of May 30/31, 2016, I observed SN 2016coj in NGC4125 for the first time.

The main setup was:

• Meade 12-inch telescope
• no focal reducer, so working at f/10
• SBIG ST-9E CCD camera with BVRI filter, 1.38 arcsec/pixel
• 30-second unguided exposures

Notes from the night

• a few puffy clouds at sunset, but clear during the night
• no Moon
• FWHM was good, around 2.5-3.0 pixels (3.5 to 4.1 arcsec), except for a few B-band images
• twilight sky flats
• I acquired images of M66, in which SN 2016cok had just been found, but that field turns out to be too complex, and the SN too faint, for me to study

SN 2016coj is a Type Ia supernova in the relatively nearby galaxy NGC4125. It was discovered by the KAIT group some time before maximum light:

• Astronomer's Telegram number 9095 announces the discovery

NGC 4125 RA = 12:08:05.7 Dec = +65:10:30 (J2000)

There are, alas, no really good sources of photometry for the stars marked above. I have created an interim set of magnitudes using the UCAC4 and converting the SDSS r and i magnitudes into Johnson-Cousins R and I via the conversion formulae of Jester et al. (2005). I hope to replace these interim values with better ones at some point.

letter   B     sigB    V     sigV    R     sigR    I    sigI
-------------------------------------------------------------------------
A      10.981 0.004   9.888 0.006   9.247 0.032   8.651 0.036   
B      15.202 0.004  14.130 0.005  13.621 0.032  13.146 0.035  

C      13.320 0.004  12.671 0.006  12.302 0.032  11.955 0.036 
D      15.939 0.003  15.038 0.007  14.370 0.031  13.749 0.034 

F      13.066 0.004  11.663 0.006  10.947 0.032  10.282 0.035  
G      15.047 0.006  14.365 0.006  13.977 0.032  13.613 0.035 
-------------------------------------------------------------------------

I took sets of about ten 30-second unguided images in each filter.

I used the rotsub technique to remove the galaxy's light, as best I could. It's not perfect.

Using aperture photometry with a radius of 3 pixels (radius of 4.1 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 interim reference magnitudes above 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 evening are:

filter       mag         mag_uncert                          Julian Date

   SN  B =   14.015   +/-   0.080  (ens  0.042 zp  0.069)    2457539.63898 
   SN  V =   13.965   +/-   0.049  (ens  0.038 zp  0.031)    2457539.63730 
   SN  R =   13.769   +/-   0.092  (ens  0.040 zp  0.083)    2457539.64112 
   SN  I =   13.824   +/-   0.139  (ens  0.084 zp  0.111)    2457539.63662 

Last modified 06/21/2016 by MWR.