UT Jun 04, 2024: Photometry of SS Cyg

Michael Richmond
Jun 04, 2024

On the night of Jun 03/04, 2024, under fair-to-good conditions, I acquired images of the cataclysmic variable star SS Cyg. This star has just (Jun 02) started its outburst (yay!), and the operators of the XRISM satellite would like to start observing it as soon as it brightens. The AAVSO has posted Alert 858 asking observers to monitor it frequently.

I acquired images in B and V passbands for about 3.5 hours. The star has brightened to about V = 8.9, but looks pretty steady over my run. Its color has moved to the blue, from about (B-V) = 0.5 in quiesence to (B-V) = 0.0 tonight.

SS Cyg

This cataclysmic variable brightens by about 4 magnitudes every two months or so, from mag 12 to about mag 8.

These observations involved:

• Meade 12-inch telescope
• no focal reducer, so working at f/10
• ASI 6200MM CMOS camera
• B-filter 30-sec, V-filter 20-sec, then 12-sec, images (I decreased the exposure time at JD 2460465.82 as a precaution against saturation)
• images binned 4x4
• no guiding, due to intermittant clouds in first half
• focus position on 12-inch:
  • in V: 0.761 yields FWHM 2.7 at T = 67/19

Notes from the night:

• broken clouds for first half of the night, but cleared nicely for the second half
• very thin crescent Moon visible at dawn
• camera at T = -16 C
• air temperature in the dome dropped from T = 70/21 at start of run to T = 62/16 at the end. Very humid the entire time -- the smooth concrete floor squeaked
• no sign of skunks

The picture below shows an image of the field of SS Cyg from May 30/31. The field of view is about 22 arcminutes across.

I've marked the location of several comparison stars, with magnitudes and names taken from the AAVSO's chart.

  star       name                  B          V         
------------------------------------------------------
      A     000-BCP-235          9.872      8.556
      B     000-BCP-198         10.162      9.974
--------------------------------------------------------------------------

 

When the target is centered, the finder TV shows this field:

Here's the sky background over the course of the run. Note several episodes of clouds in the first half.

The FWHM improved slightly over the run.

The graph below shows changes in the photometric zeropoint of an ensemble solution of the imstrumental magnitudes over the course of the run. The jump near the end marks the switch from 20-second exposures to 12-second exposures in V-band.

Using aperture photometry with a radius of 5 pixels in V filter (binned 4x4, each pixel is 1.052 arcsec, so a radius of 5.3 arcsec), and 7 pixels in B filter (binned 4x4, each pixel is 1.052 arcsec, so a 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.

Sigma-vs-mag plots show that the floor in V-band was about 0.006 mag, which is so-so. I blame the early clouds.

The measurements show a nearly steady brightness over 3.5 hours, with only very small variations on roughly hourly timescales. The object may be close to the peak of its outburst.

You can download my measurements below. A copy of the header of the file is shown to explain the format. First, the V-band data.

# Measurements of SS_Cyg made at RIT Obs, UT 2024 Jun 4, 
#    in fair-to-good conditions, 
#    by Michael Richmond, 
#    using Meade 12-inch LX200 and ASI 6200MM. 
# Exposures 20 or 12 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 5 pix = 5.2 arcseconds.  
#    which has been shifted so AAVSO 000-BCP-198 has mag=9.794 
#    which is its V-band magnitude according to AAVSO.  
# 
# UT_day             JD            HJD        mag    uncert
Jun04.21337     2460465.71337  2460465.71295   8.721  0.005 
Jun04.21418     2460465.71418  2460465.71376   8.719  0.005 
Jun04.21499     2460465.71499  2460465.71457   8.730  0.005 

• download ASCII text file, UT 2024 Jun 04, SS Cyg V-band from RIT Obs

Now, the B-band data.

# Measurements of SS_Cyg made at RIT Obs, UT 2024 Jun 4, 
#    in fair-to-good conditions, 
#    by Michael Richmond, 
#    using Meade 12-inch LX200 and ASI 6200MM. 
# Exposures 30 seconds long, B 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 pix = 7.2 arcseconds.  
#    which has been shifted so AAVSO 000-BCP-198 has mag=10.162 
#    which is its B-band magnitude according to AAVSO.  
# 
# UT_day             JD            HJD        mag    uncert
Jun04.20888     2460465.70888  2460465.70846   8.793 99.000 
Jun04.20970     2460465.70970  2460465.70928   8.779 99.000 
Jun04.21295     2460465.71295  2460465.71253   8.760 99.000 

• download ASCII text file, UT 2024 Jun 04, SS Cyg B-band from RIT Obs

I've submitted these measurements to the AAVSO.