Jul 19, 2016 UT: Photometry of SN 2016coj in NGC 4125

Michael Richmond
Jul 19, 2016

On the night of Jul 18/19, 2016, I observed SN 2016coj in NGC4125.

The main setup was:

Notes from the night

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:

Here's a chart showing the galaxy, the SN, and some reference stars; the chart is about 12 x 12 arcminutes.

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 X16288FJI. Note that star "K" is so faint that I may not detect it clearly in B-band.

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 

J      15.607 0.109  14.956 0.065  14.603 0.136  14.271 0.182  

K      16.573 0.123  15.975 0.082  15.547 0.174  15.147 0.231  

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

I used dark frames taken after all the SN 2016coj images.

I took sets of at least 20 images in each filter, guiding in all filters. I used longer guide exposure times in B and I. I discarded any trailed images.

I used focus settings based on relative positions measured for a bright star earlier. At a temperature of T = 24 C, I determined

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.

On this night, I used "method 2", which means performing "rotsub" on each individual image, then combining all the resulting images in a passband to make a "master rotsub" image.

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.

Note that in the graph below, I combine data calibrated with UCAC4 photometry (first few weeks) with recent data calibrated with AAVSO photometry. That's inconsistent, and I'll re-compute all magnitudes later. Note further that I use only 2 AAVSO stars (B and C) for calibration, for consistency with earlier measurements; I'll use additional stars in my final calculations.


     filter  mag         mag_uncert                          Julian Date

   SN  B =   15.997   +/-   0.078  (ens  0.077 zp  0.015)    2457588.60752
   SN  V =   15.009   +/-   0.059  (ens  0.045 zp  0.038)    2457588.59690
   SN  R =   14.751   +/-   0.087  (ens  0.032 zp  0.081)    2457588.60556
   SN  I =   14.464   +/-   0.107  (ens  0.046 zp  0.097)    2457588.61910

Below is a preliminary light curve, based on RIT Observatory measurements. I also show measurements of SN 2011fe in M101, an ordinary type Ia supernova, shifted arbitrarily. The apparent increase in brightness in VRI (which is smaller than the 1-sigma errorbars) is very likely not real, just an indication of the difficulty of measuring the brightness of a faint object immersed in the galaxy's background.


Last modified 07/17/2016 by MWR.