Jun 15, 2016 UT: Photometry of SN 2016coj in NGC 4125 (and PG1633+099)

Michael Richmond
Jun 17, 2016

On the night of Jun 14/15, 2016, I observed SN 2016coj in NGC4125.

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

Notes from the night

• a few isolated clouds were present early, and thin cirrus later in the evening
• waxing gibbous Moon was pretty bright
• FWHM was mediocre, around 3.8-4.2 pixels (5.3 to 5.8 arcsec)
• twilight sky flats
• I was able to guide on SN field in BVRI. Rah!
• I acquired images of the Landolt calibration field PG 1633+099 after NGC 4125, and used them to estimate the color terms for my equipment

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)

There are, alas, no really good sources of photometry for the stars marked above. I have created an interim set of magnitudes using the UCAC4 and converting the SDSS r and i magnitudes into Johnson-Cousins R and I via the conversion formulae of Jester et al. (2005). I hope to replace these interim values with better ones at some point.

letter   B     sigB    V     sigV    R     sigR    I    sigI
-------------------------------------------------------------------------
A      10.981 0.004   9.888 0.006   9.247 0.032   8.651 0.036   
B      15.202 0.004  14.130 0.005  13.621 0.032  13.146 0.035  

C      13.320 0.004  12.671 0.006  12.302 0.032  11.955 0.036 
D      15.939 0.003  15.038 0.007  14.370 0.031  13.749 0.034 

F      13.066 0.004  11.663 0.006  10.947 0.032  10.282 0.035  
G      15.047 0.006  14.365 0.006  13.977 0.032  13.613 0.035 
-------------------------------------------------------------------------

I took sets of about ten 30-second guided images in each filter. The B-band and I-band images require longer guider exposure times (because this guider looks through the filter), and the guiding wasn't as good in those passbands. I discarded any trailed images.

As you can see, the supernova is immersed in the galaxy's light; this causes problems in measuring the brightness of the SN, especially in the I-band. The proper way to remove the galaxy's light would be to subtract a properly scaled template image of the galaxy, but I don't have one. Therefore, I tried a little trick:

• make a copy of each image
• determine the center of the galaxy
• rotate the image by 180 degrees around this center
• subtract the rotated copy from the original image, yielding what I'll call the rotsub image
• measure the brightness of stars using aperture photometry in the rotsub image

It certainly isn't a perfect solution, since this galaxy is NOT perfectly symmetric, but it's easy to do and does remove much of the galaxy's light. Compare the original of one V-band image

to the rotsub version

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.

Results from this evening are:

filter  mag         mag_uncert                          Julian Date

   SN  B =   13.419   +/-   0.065  (ens  0.038 zp  0.053)    2457554.60169 
   SN  V =   13.229   +/-   0.048  (ens  0.036 zp  0.032)    2457554.58630 
   SN  R =   13.294   +/-   0.091  (ens  0.068 zp  0.060)    2457554.58664 
   SN  I =   13.875   +/-   0.171  (ens  0.151 zp  0.079)    2457554.59450 

Note the much larger uncertainty in I-band, due mostly to the difficulty of subtracting the galaxy's light.

Below is a preliminary light curve, based on RIT Observatory measurements.

Last modified 06/17/2016 by MWR.