Jun 24, 2016 UT: Photometry of SN 2016coj in NGC 4125 (and PG1633 and PG1657)

Michael Richmond
Jun 24, 2016

On the night of Jun 23/24, 2016, I observed SN 2016coj in NGC4125 under nice conditions. I also acquired images of PG1633 and PG1657 fields for calibration.

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 nice and clear -- overall, a beautiful night
• the past-full Moon was below the horizon for all the SN 2016coj observations, but rose for the PG1657
• FWHM was about 3 pixel (4.2 arcsec), though the R-band images were better, at 2.4 pixel = 3.3 arcsec.
• twilight sky flats

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, 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.

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 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 =   14.569   +/-   0.056  (ens  0.031 zp  0.047)    2457563.60817 
   SN  V =   13.636   +/-   0.018  (ens  0.018 zp  0.001)    2457563.59741 
   SN  R =   13.669   +/-   0.081  (ens  0.018 zp  0.079)    2457563.63184 
   SN  I =   13.675   +/-   0.102  (ens  0.030 zp  0.097)    2457563.62124 

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 06/23/2016 by MWR.