Jun 27, 2016 UT: Photometry of SN 2016coj in NGC 4125 through clouds

Michael Richmond
Jun 27, 2016

On the night of Jun 26/27, 2016, I observed SN 2016coj in NGC4125 through thick clouds. I was able to extract some noisy measurements in VRI, but not in B.

The main setup was:

Notes from the night

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:

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)

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 


On this night, I took two series of 30-second dark frames: one set at the start of the run, and one set at the end, about 2 hours later. The CCD-TEMP keyword in the FITS header indicated a constant temperature of about -8 degrees Celsius -- warmer than usual, but that was due to the very warm air temperature (83 F at 11:00 PM!). Despite the constant CCD temperature (if it really WAS constant), the dark current changed considerably over this period:

In addition to this change in bulk properties, there were also a few "hot" pixels which appear only in the early set, and a few different "hot" pixels which appear only in the late set. I decided to use the "late" set of dark images to clean the SN images.

I took sets of 10-20 images in each filter, with no guiding on this night. 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.

On this night, I used "method 3", which means combining the individual images in a passband to make a master image, and then performing the "rotsub" technique. I figured that the very low signal-to-noise in some images would make it difficult to compute the centroid of the galaxy accurately.

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 (note the lack of a value in B-band -- those images suffered the most from clouds, and did not yield a good result)

     filter  mag         mag_uncert                          Julian Date

   SN  V =   13.827   +/-   0.034  (ens  0.028 zp  0.019)    2457566.59363 
   SN  R =   13.625   +/-   0.127  (ens  0.026 zp  0.125)    2457566.58610 
   SN  I =   13.472   +/-   0.111  (ens  0.045 zp  0.102)    2457566.60231 

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.