Jun 19, 2016 UT: Photometry of SN 2016coj in NGC 4125 and HH UMa

Michael Richmond
Jun 19, 2016

On the night of Jun 18/19, 2016, I observed SN 2016coj in NGC4125. I also made a short run of images of the variable star HH UMa.

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

cirrus over much of the sky, though it was thin
waxing gibbous Moon was pretty bright
FWHM was 3.2 - 3.6 pixels (4.4 to 5.0 arcsec)
twilight sky flats
the HH UMa data was taken through a lot of air, and suffered from some trailing.

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

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)

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 10-20 images in each filter, attempting to guide. I discarded any trailed 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.

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.

Results from this evening are:


     filter  mag         mag_uncert                          Julian Date

   SN  B =   13.891   +/-   0.080  (ens  0.041 zp  0.068)    2457558.61076 
   SN  V =   13.478   +/-   0.053  (ens  0.024 zp  0.047)    2457558.60170 
   SN  R =   13.611   +/-   0.077  (ens  0.037 zp  0.067)    2457558.59465 
   SN  I =   13.961   +/-   0.118  (ens  0.080 zp  0.086)    2457558.62396

Note the 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 06/19/2016 by MWR.

Jun 19, 2016 UT: Photometry of SN 2016coj in NGC 4125 and HH UMa

Michael Richmond Jun 19, 2016

Michael Richmond
Jun 19, 2016