Aug 21, 2013 UT: UZ Boo and SN 2013ej

Michael Richmond
Aug 21, 2013

On the night of Aug 20/21, 2013, I observed UZ Boo and SN 2013ej in M74. Conditions were good -- no clouds, decent seeing -- BUT the nearly full moon was very bright, making all measurements noisy.

The main setup was:

Notes from the night

UZ Boo fading further

I took a series of images of UZ Boo, using a 150-second exposure time, since UZ Boo has been faint recently. Tonight, the star was about the same as 24 hours ago, at mag V = 16.5 or so. If it doesn't rebrighten, I'll need to increase the exposurure time -- or maybe give up.

The zero-point offset graph shows clouds a slow, steady rise as UZ Boo slowly set.

I measured UZ Boo and many other stars in the field. The bright stars "A", "B" and "D" were somewhat saturated, so I left them out of the ensemble solution. Instead of the star "A", I used the star "E" = UCAC4 561-055129 = AAVSO 000-BBV-649 to shift the instrumental magnitudes to the standard scale. Remember, the measurements are made without a 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.

Because the sky was so bright, I used a smaller aperture to measure the stars than usual: radius of 3 pixels (5.6 arcsec) instead of 4 pixels (7.4 arcsec).

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, probably swamping any variation in UZ Boo itself.

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 at about the same level as last night, near the bottom of its current outburst.

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 21, 2013 UT, 
#    in good but bright 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
Aug21.05148     2456525.55148  2456525.54928  16.521  0.105 
Aug21.05343     2456525.55343  2456525.55123  16.304  0.079 
Aug21.05536     2456525.55536  2456525.55316  16.308  0.080 

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 unguided images -- the sky was so bright that I couldn't hold the guide star reliably. After discarding the bad images, I was left with 5, 9, 9, and 9 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.

The transformation from instrumental to Johnson-Cousins magnitude had a larger uncertainty in the B-band than usual. I checked and saw (as I had several days ago) that star "I", the faintest of the B-band reference stars, was about 0.1 mag away from the other reference stars in zeropoint. I discarded it from the B-band solution.

Results from this morning are:

filter  mag         mag_uncert                          Julian Date

SN  B =   13.734   +/-   0.137  (ens  0.124 zp  0.058)    2456525.69900 
SN  V =   12.787   +/-   0.058  (ens  0.052 zp  0.027)    2456525.69290 
SN  R =   12.429   +/-   0.026  (ens  0.016 zp  0.020)    2456525.68227 
SN  I =   12.211   +/-   0.034  (ens  0.032 zp  0.012)    2456525.70562 

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 08/20/2013 by MWR.