Jun 14, 2016 UT: Photometry of SN 2016coj in NGC 4125

Michael Richmond
Jun 21, 2016

On the night of Jun 13/14, 2016, I observed SN 2016coj in NGC4125.

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 guided (VRI) and unguided (B) exposures

Notes from the night

• wispy cirrus at sunset, looked pretty clear later
• FWHM ranged from good to bad
• twilight sky flats
• the filterwheel had some problems early, while I was preparing for flats. I fixed it by unplugging and re-plugging all three cables going into the ST9 camera and its filterwheel.

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 ten to twenty 30-second images in each filter. I discarded any trailed images.

On this night, I switched the order of filters from BVRI to RVIB -- I think that works better as the sky darkens.

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 4 pixels (radius of 5.5 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 =   13.371   +/-   0.095  (ens  0.037 zp  0.087)    2457553.59891 
   SN  V =   13.154   +/-   0.045  (ens  0.038 zp  0.024)    2457553.58560 
   SN  R =   13.188   +/-   0.069  (ens  0.049 zp  0.049)    2457553.59188 
   SN  I =   13.665   +/-   0.109  (ens  0.077 zp  0.077)    2457553.59180 

Last modified 06/21/2016 by MWR.