UT Jun 10, 2016: Photometry of 2MASS J16211735+4412541 and SN 2016coj in NGC 4125

Michael Richmond
Jun 10, 2016
Jun 21, 2016

On the night of Jun 09/10, 2016, I acquired a set of observations of the cataclysmic variable star 2MASS_J16211735+4412541. I recorded roughly one complete cycle, showing two big dips.

I also acquired images of SN 2016coj in NGC 4125, and some pictures of the Moon while waiting for the sky to grow dark.

The main setup was:

Notes from the night:


2MASS J16211735+4412541

This cataclysmic variable star was recently the subject of several telegrams:

The object is located at



  RA = 16:21:17.35   Dec = +44:12:54.1

A chart of the field is shown below. The size of the chart is about 12 by-12 arcminutes.

Entries for one of the marked comparison stars from the UCAC4 follows:



  star        UCAC4              B          V
----------------------------------------------------
   A       672-059554         13.214      12.191

----------------------------------------------------

 

Conditions were good. The quarter moon set early in the run, and the skies were dark. Measurements of the sky background value show a smooth decrease as the field rose, until dawn came. Just one 20-minute interruption by clouds.

The guider did a good job of keeping the telescope fixed in place. There was a small jump in RA near the end of the run, around image 129 at JD 549.787.

Using aperture photometry with a radius of 4 pixels (radius of 5.7 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.

Sigma-vs-mag plots show that the floor was about 0.004 mag. 2MASS is the big outlier at mag 2.8.

Image adjustment factor shows a brief spike early due to clouds, then a gradual rise as dawn approached.

Here are light curves of the variable and the field stars:

I used the UCAC value for the V-band magnitude of star "A" = UCAC4 672-059554 to shift the ensemble magnitudes to the standard V-band scale -- but remember that these are UNFILTERED measurements.

Here's a closeup on the variable:

You can download my measurements below. A copy of the header of the file is shown to explain the format.

# Measurements of 2MASSJ16211735+4412541 made at RIT Obs, Jun 10, 2016 UT, 
#    in good conditions. 
#    by Michael Richmond, using 12-inch Meade and SBIG-ST9E. 
# Exposures 90 seconds long, no filter. 
# Tabulated times are midexposure (FITS header time - half exposure length) 
#    and accurate only to +/- 1 second (??). 
# 'mag' is a differential magnitude based on ensemble photometry 
#    using a circular aperture of radius 5.7 arcseconds.  
#    which has been shifted so UCAC4 672-059554 has mag=12.191 
#    which is its V-band mag according to UCAC4.  
# 
# UT_day             JD            HJD        mag    uncert
Jun10.12549     2457549.62549  2457549.62766  14.904  0.014 
Jun10.12662     2457549.62662  2457549.62879  14.886  0.013 
Jun10.12778     2457549.62778  2457549.62995  14.903  0.014 


The Moon

Here's a picture of the Moon, taken through the B-band filter with an exposure time of 0.1 seconds. Just fun!


SN 2016coj in NGC 4125

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)

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 10 images per filter of the SN and its host galaxy. I was able to guide in VRI, but not B-band.

I used the rotsub technique to remove the galaxy's light, as best I could. It's not perfect.

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 =   13.199   +/-   0.089  (ens  0.044 zp  0.077)    2457549.58866 
   SN  V =   13.068   +/-   0.045  (ens  0.031 zp  0.032)    2457549.59553 
   SN  R =   13.032   +/-   0.059  (ens  0.024 zp  0.054)    2457549.60074 
   SN  I =   13.400   +/-   0.119  (ens  0.074 zp  0.093)    2457549.60681 


Last modified 6/21/2016 by MWR.