Jun 23, 2016 UT: Photometry of SN 2016coj in NGC 4125

Michael Richmond
Jun 23, 2016

On the night of Jun 22/23, 2016, I observed SN 2016coj in NGC4125. I processed the data in several different methods -- there's a brief discussion near the end of the page.

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 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, guiding with success in all filters. I used longer guide exposure times (12 sec) in B and I. I discarded any trailed images, and those with very high background levels from early in the evening. When switching from BVR to I, I re-focused the telescope. That seemed to help I-band, and also a subsequent set of R-band 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.

However, this time, I tried three different methods to extract the instrumental magnitudes from the images:

  1. run "rotsub" on each individual image, extract magnitudes from each resulting rotated-and-subtracted image, combine those via the ensemble technique
  2. run "rotsub" on each individual image, combine all the rotated-and-subtracted images in each passband to create a "master", extract magnitudes from this master
  3. combine all the cleaned images in each passband to create a "master", run "rotsub" on the master, extract magnitudes from this rotated-and-subtracted master

The results were not very different, but I did note that the zero-point calibration was better for methods 2 and 3 than for method 1. The final SN magnitudes generated by these methods varied at most by about 0.04 mag, and in most cases by much less; methods 2 and 3 were nearly identical except for I-band.

In the future, when I re-analyze all the data, I'll probably choose method 2 or 3. But for now, I'll continue using method 1, which has been my default since the start.

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.

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.

Results from this evening are:

     filter  mag         mag_uncert                          Julian Date

   SN  B =   14.411   +/-   0.071  (ens  0.051 zp  0.050)    2457562.59758 
   SN  V =   13.617   +/-   0.056  (ens  0.020 zp  0.052)    2457562.59023 
   SN  R =   13.618   +/-   0.118  (ens  0.033 zp  0.113)    2457562.60095 
   SN  I =   13.723   +/-   0.144  (ens  0.066 zp  0.129)    2457562.61036 

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.

Last modified 06/22/2016 by MWR.