Jul 06, 2016 UT: Photometry of SN 2016coj in NGC 4125 and false alarm for ASASSN-16gu

Michael Richmond
Jul 06, 2016

On the night of Jul 05/06, 2016, I observed SN 2016coj in NGC4125. When I heard that there might be a new, young supernova in the nearby galaxy NGC 4725, I took some images to confirm ... but later found out that it was just a minor planet. Rats.

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 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  


In the continuing saga of the dark current, I again found that dark images taken immediately after twilight sky flats -- even if no flatfield images were saturated -- showed higher counts than dark images taken at other times.

I also noticed that some target frames of NGC 4125 show "ghost" images of Jupiter, on which I sync'd the telescope and focused some 20 minutes earlier. Sigh. I must be very careful not to expose important portions of the CCD to high light levels; or, if I do, I must allow some 30-60 minutes to pass so that the excess charge can gradually dissipate.

I took sets of 15-20 images in each filter, guiding in VRI, but not B. I used longer guide exposure times in I. I discarded any trailed images. I checked the focus before taking I-band images, but found no obvious evidence for a change.

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.

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  B =   15.795   +/-   0.077  (ens  0.072 zp  0.027)    2457575.61031 
   SN  V =   14.506   +/-   0.036  (ens  0.034 zp  0.012)    2457575.60143 
   SN  R =   14.240   +/-   0.093  (ens  0.032 zp  0.088)    2457575.59432 
   SN  I =   13.862   +/-   0.097  (ens  0.039 zp  0.088)    2457575.63190 

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.

False alarm for ASASSN-16gu

During the day, I noticed that Astronomer's Telegram 9211 announced that ASASSN-16gu might be a young supernova in NGC 4725. Since that galaxy is relatively nearby, only about 13 Mpc away, this would be an unusual chance to see a supernova so close.

So, after acquiring images of SN 2016coj, I turned the telescope to NGC 4725 and took a series of images in BVRI. During the night, I couldn't tell if the images included any object at the reported position; that's not surprising, as the reported magnitude was about mag=17.

The next day, I had time to reduce the raw images and co-add 9 of the 30-second R-band frames. Compare below (on the left) a chart from the Digitized Sky Survey, with a pair of thin lines indicating the approximate position of ASASSN-16gu, and (on the right) my image from last night, the combination of 9 30-second R-band images.

Hooray! I did detect it!

However, this next morning, I found a message in my mailbox: Astronomer's Telegram 9214 explains that the object ASASSN-16gu was, in fact, the minor planet Makemake. D'oh!

Last night, while I was checking the pointing of the telescope, I used Stellarium to make a chart of the region. I recall that when I zoomed in on NGC 4725, I saw in blue letters the word "Makemake" floating nearby. At the time, I simply thought "Huh -- curious coincidence," but now I see that it was not a coincidence.

Oh, well. Better luck next time.

Last modified 07/06/2016 by MWR.