Jun 18, 2016 UT: Photometry of SN 2016coj in NGC 4125 (and PG1633 and PG1657 fields)

Michael Richmond
Jun 19, 2016

On the night of Jun 17/18, 2016, I observed SN 2016coj in NGC4125. I also took images of the Landolt calibration fields PG1633 and PG1657.

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 exposures in V, R; 60 sec guided in I; 30-sec unguided in B

Notes from the night

• sky was clear during observations of NGC 4125 but clouds came later during my observations of the calibration fields
• waxing gibbous Moon was pretty bright
• FWHM was 3.2 - 3.6 pixels (4.4 to 5.0 arcsec)
• twilight sky flats
• failed to guide in B-band. Rats.

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 10-20 images in each filter, attempting to guide. As often happens, I couldn't guide in B-band. I used a longer exposure time (60 sec) for I-band, as the SN is relatively fainter in it. I discarded any trailed images.

As explained in the notes to Jun 14, 2016, I used the "rotsub" technique to remove the galaxy's light at the position of the SN.

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 =   13.817   +/-   0.130  (ens  0.056 zp  0.117)    2457557.62344 
   SN  V =   13.394   +/-   0.060  (ens  0.025 zp  0.055)    2457557.60029 
   SN  R =   13.515   +/-   0.079  (ens  0.038 zp  0.069)    2457557.61277 
   SN  I =   13.856   +/-   0.131  (ens  0.100 zp  0.085)    2457557.61112 

Note the much larger uncertainty in I-band, due mostly to the difficulty of subtracting the galaxy's light.

Below is a preliminary light curve, based on RIT Observatory measurements.

Last modified 06/17/2016 by MWR.