June 24, 2015 UT: V404 Cyg goes crazy

Michael Richmond
June 24, 2015

On the night of June 23/24, 2015, I observed the cataclysmic variable star V404 Cyg.

This time, I used a "V" filter, rather than the "I" filter used on two earlier nights. The star went crazy tonight, dropping by 3.5 magnitudes in an hour and a half, then recovering by 2 magnitudes in the next hour! Just look at it! (click on the image to start animation)

The main setup was:

Notes from the night

V404 Cyg

This object, originally noted as Nova Cyg 1938, is a Low-Mass X-ray Binary (LMXB) which is thought to consist of a black hole of roughly 10 solar masses and an ordinary star of about 0.6 solar masses. A good summary can be found at AAVSO Alert Notice 520 ; one paper from many, picked at random, is Characterizing the quiescent X-ray variability of the black hole low-mass X-ray binary V404 Cyg, which suggests that a jet is responsible for most of the radiation emitted during quiescence. The binary has an orbital period of about 6.47 days.

Here's a chart of the field of V404 Cyg, which is at

      RA = 20 24 03.83     Dec = +33 52 02.2     (J2000)

The chart is about 12x12 arcminutes.

Among the labelled stars are

  B    TYC 2693-1473-1  

  F    UCAC4 620-101865    

The star "F" has a V magnitude of 12.815 +/- 0.014 according to AAVSO sequence 15094IVF, and an Ic magnitude of 11.967 +/- 0.018 according to AAVSO sequence 15088BFS,

Below is a graph showing the sky brightness as a function of time during the observing run. The smooth curve indicates lack of clouds.

Below is a graph showing the FWHM as a function of time during the observing run. I didn't refocus during this half-night.

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 plot: V404 Cyg is the big, big, BIG outlier -- its standard deviation from the mean is almost 0.9 mag!

Image adjustment factor shows no evidence for clouds, and good guiding.

The target, shown in green, shows a crazy variation over the course of the run.

Here's a closeup of V404 Cyg by itself.

I used the AAVSO's value for the V-band magnitude of star "F" = UCAC4 620-101865 to shift the ensemble magnitudes to the standard V-band scale. You can download my measurements below. A copy of the header of the file is shown to explain the format.

# Measurements of V404_Cyg made at RIT Obs, Jun 24, 2015 UT, 
#    in excellent conditions, 
#    by Michael Richmond, using 12-inch Meade and SBIG ST-9E CCD. 
# Exposures 20 seconds long, V 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 7.4 arcseconds.  
#    which has been shifted so UCAC4 620-101865 has mag=12.815 
#    which is its V-band mag according to AAVSO chart 15094IVF. 
# UT_day             JD            HJD        mag    uncert
Jun24.20876     2457197.70876  2457197.71118  11.356  0.008 
Jun24.21115     2457197.71115  2457197.71357  11.373  0.008 
Jun24.21148     2457197.71148  2457197.71390  11.374  0.008 

  • V404Cyg V-band data, UT Jun 24, from RIT Obs

  • Last modified 6/24/2015 by MWR.