Jul 26, 2016 UT: Photometry of SN 2016coj in NGC 4125

Michael Richmond
Jul 26, 2016

On the night of Jul 25/26, 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

• very warm: temperature of 29 C soon after sunset
• a few low moisture clouds to S, not in target's area; humid
• no Moon
• twilight flats
• the 120-second images had FWHM 2.8 pixels in VRI (somewhat better than usual)
• I ended with R-band flats, as usual, and did not change the filter during sync'ing and focusing and target acquisition. Then I started with R-band images of NGC 4125, and there were no flatfield residuals -- good! Don't give the filterwheel a chance to mis-align itself

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 10 images in each filter, guiding in all filters. The guiding worked fine with an exposure time of 6 seconds in R, V and I-band -- unusual, but welcome. I discarded any trailed images.

I used focus settings based on relative positions measured for a bright star earlier. At a temperature of T = 25 C, I determined

• R: 0.583

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 =   15.162   +/-   0.031  (ens  0.031 zp  0.005)    2457595.60265 
   SN  R =   15.025   +/-   0.083  (ens  0.027 zp  0.078)    2457595.58699 
   SN  I =   14.740   +/-   0.100  (ens  0.033 zp  0.095)    2457595.62068 

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 07/26/2016 by MWR.