Jul 24, 2016 UT: Photometry of SN 2016coj in NGC 4125

Michael Richmond
Jul 24, 2016

On the night of Jul 23/24, 2016, I observed SN 2016coj in NGC4125. I've stopped taking B-band images on a regular basis.

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
• 120-second exposures in VRI

Notes from the night

• no skunk tonight
• very warm: temperature of 27 C soon after sunset
• long wisps of cirrus here and there throughout the run
• waning gibbous Moon didn't seem to have much effect
• twilight flats
• the 120-second images had FWHM 3.0 pixels in VRI (seems to be about the usual)
• near the end of the night, the telescope appeared to be drifting to the North (stars appeared to slide South), opposite to the usual trend. Perhaps the Hour Angle was so large (more than 6 hours) that we entered a different regime in the tracking error.

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 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 9-15 images in each filter, guiding in all filters. I used longer guide exposure times in 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 = 27 C, I determined

• R: 0.584

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  V =   15.149   +/-   0.071  (ens  0.032 zp  0.063)    2457593.60856 
   SN  R =   14.939   +/-   0.097  (ens  0.027 zp  0.093)    2457593.59071 
   SN  I =   14.680   +/-   0.117  (ens  0.039 zp  0.110)    2457593.62544 

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 07/24/2016 by MWR.