Aug 29, 2013 UT: The last night of UZ Boo (probably), and SN 2013ej in M74

Michael Richmond
Aug 29, 2013

On the night of Aug 28/29, 2013, I observed UZ Boo and SN 2013ej in M74. Conditions were fair -- the skies were clear, and there was no moon, BUT it was very humid. There was almost a fog at ground level.

The main setup was:

Notes from the night

UZ Boo so faint that I'm going to stop observing it

I took a series of images of UZ Boo, using a 150-second exposure time.

The zero-point offset graph shows mostly a steady rise as UZ Boo slowly sets.

I measured UZ Boo and many other stars in the field, using an aperture 3 pixels (5.6 arcsec) in radius. Since I used a clear filter, the bright stars "A", "B" and "D" were saturated, and were not included in the ensemble solution. I used the star "E" = UCAC4 561-055129 = AAVSO 000-BBV-649 to shift the instrumental magnitudes to the standard scale. These measurements are made with a "clear" filter, but I'll shift them to match the V-band scale: 000-BBV-649 has mag V = 14.267 according to the AAVSO chart 12447AMF.

Below are light curves for UZ Boo and several of the unsaturated stars in the field.

Here's a closeup of UZ Boo and stars of similar faintness. The noise is very large, so that it very likely swamps any instrinsic variation in UZ Boo.

The variable star has entered a phase in its evolution in which it fades by about 2 mag, then rebrightens, then fades again, then rebrightens, etc. My measurements at the RIT Observatory catch UZ Boo at several places in this complicated evolution; tonight, it was so faint that I think it's heading back to quiescence.

Below are the first few lines of the report I've sent to the AAVSO and VSNet.

# Measurements of UZ_Boo made at RIT Obs, Aug 29, 2013 UT, 
#    in fair conditions, 
#    by Michael Richmond, using 12-inch Meade and SBIG ST-8E CCD. 
# Exposures 150 seconds long, clear 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 5.6 arcseconds.  
#    which has been shifted so UCAC4 561-055129 has mag=14.267 
#    which is its V-band magnitude according to AAVSO.  
# UT_day             JD            HJD        mag    uncert
Aug29.04397     2456533.54397  2456533.54123  17.653  0.164 
Aug29.04711     2456533.54711  2456533.54437  17.878  0.169 
Aug29.05075     2456533.55075  2456533.54801  17.821  0.159 

SN 2013ej in M74

SN 2013ej is a Type II supernova in the relatively nearby galaxy M74. It was discovered by the KAIT group about one week before maximum light. Here's a chart showing the galaxy, the SN, and some reference stars:

The reference stars marked above have magnitudes in AAVSO chart 12459CA, as follows:

 letter      B     sigB       V     sigV      R      sigR     I    sigI
  B        13.012  0.019   12.510  0.019    12.154  0.019   11.834  0.019
  F        13.848  0.026   13.065  0.022    12.622  0.025   12.152  0.027   
  H        14.338  0.029   13.692  0.024    13.329  0.029   12.964  0.030
  I        14.832  0.027   13.912  0.023    13.416  0.026   12.939  0.030
  K        15.192  0.034   14.613  0.027    14.275  0.034   13.915  0.036

I took 30-second guided images in VRI, but the B-band is so insensitive that, as usual, the guider couldn't hold the guide star reliably. After discarding the bad images, I was left with 7, 8, 7, and 10 images in B, V, R, and I, respectively.

Using aperture photometry with a radius of 4 pixels (radius of 7.4 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 AAVSO magnitudes, plus color terms to convert the ensemble instrumental magnitudes to the standard Johnson-Cousins BVRI scale.

Results from this evening are:

filter  mag         mag_uncert                          Julian Date

B =   14.109   +/-   0.085  (ens  0.050 zp  0.068)    2456533.68319 
V =   13.026   +/-   0.045  (ens  0.034 zp  0.029)    2456533.67713 
R =   12.574   +/-   0.023  (ens  0.022 zp  0.005)    2456533.67107 
I =   12.340   +/-   0.028  (ens  0.023 zp  0.016)    2456533.66303 

The uncertainties here are roughly equally distributed between extracting the instrumental magnitudes and transforming the instrumental magnitudes to the standard scale.

Grab the text file below for all the RIT measurements of SN 2013ej. All these values have been recomputed with the new color terms of UT 2013 Aug 05.

Last modified 08/29/2013 by MWR.