Jun 20, 2016 UT: Photometry of SN 2016coj in NGC 4125 (and PG1657+078)

Michael Richmond
Jun 21, 2016

On the night of Jun 19/20, 2016, I observed SN 2016coj in NGC4125. I also took some images of the calibration field PG1657+078, but they suffered from the bright moon.

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 BVRI

Notes from the night

• sky was clear overhead, though cirrus was approaching from the west when I quit around midnight
• almost full Moon was bright, and its light shone on the front corrector plate of the telescope when I pointed to PG1657
• FWHM was 2.8 - 3.5 pixels (3.9 to 4.8 arcsec)
• twilight sky flats
• I tried to compute color terms from the images of PG1657+078, but found mediocre results. I'll try again when the Moon is gone.
• a pair of students visited around 10 PM

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)

The AAVSO sequence team kindly provided photometry for stars near this object. You can see their full photometric sequence on their website. Below, I show only the members of that sequence which fall into my very small field of view -- taken from AAVSO sequence X16266F.

letter   B     sigB    V     sigV    R     sigR    I    sigI
------------------------------------------------------------------------
B      15.198 0.086  14.133 0.052  13.627 0.116  13.155 0.156 

C      13.317 0.093  12.673 0.058  12.316 0.121  11.980 0.161 

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

I took sets of 10-20 images in each filter, attempting to guide. 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 =   13.996   +/-   0.064  (ens  0.050 zp  0.040)    2457559.61730 
   SN  V =   13.454   +/-   0.063  (ens  0.029 zp  0.056)    2457559.60648 
   SN  R =   13.530   +/-   0.114  (ens  0.030 zp  0.110)    2457559.59631 
   SN  I =   13.774   +/-   0.152  (ens  0.059 zp  0.140)    2457559.63261 

Note the 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 06/21/2016 by MWR.