On the night of May 30/31, 2024, under very good conditions, I acquired images of the cataclysmic variable star SS Cyg. This star is expected to enter an outburst some time this summer, 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 a bit more than 3 hours. The star showed small rumblings around its quiescent magnitude, so no outburst yet.
I also determined the position of the offset guide camera from the main camera's field of view this evening.
This cataclysmic variable brightens by about 4 magnitudes every two months or so, from mag 12 to about mag 8.
These observations involved:
Notes from the night:
The picture below shows an image of the field of SS Cyg from this evening. 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
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A 000-BCP-235 9.872 8.556
B 000-BCP-198 10.342 9.974
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Here's the sky background over the course of the run. No evidence for clouds.
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.
Using aperture photometry with a radius of 7 pixels in clear 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 0.004 mag, which is good. SS Cyg is the obvious outlier at diff mag = 3.
The change in zeropoint shows a smooth decrease as the target rose higher in the sky.
The measurements show semi-regular variations of amplitude 0.2 mag and timescale of 30-60 minutes. The object is still in the quiescent phase, close to V = 12.
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 May 31, # in very good conditions, # by Michael Richmond, # using Meade 12-inch LX200 and ASI 6200MM. # Exposures 30 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 pix = 7.2 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 May31.22330 2460461.72330 2460461.72265 11.624 0.007 May31.22434 2460461.72434 2460461.72369 11.611 0.007 May31.22538 2460461.72538 2460461.72473 11.584 0.008
Now, the B-band data.
# Measurements of SS_Cyg made at RIT Obs, UT 2024 May 31, # in very 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 May31.22382 2460461.72382 2460461.72317 12.116 0.012 May31.22591 2460461.72591 2460461.72526 12.077 0.012 May31.22800 2460461.72800 2460461.72735 11.935 0.010
I've submitted these measurements to the AAVSO.