On April 18, 2025 UT, I observed an occultation of the 11.9-magnitude star (mv) UCAC4 379-071051 in the constellation Libra by the 17.7-magnitude asteroid (mv , at the time of observation) 2392 Jonathan Murray.
As you can see in the light curve below, I observed a two-dip event. Since the magnitude drops are unequal, that rules out an asteroid satellite.
The magnitude drop when the fainter star was occulted was 0.44. When the brighter star was occulted, the magnitude drop only goes down as far as the limiting magnitude of my sky in the images, which was around 14th magnitude (Tucson is not as dark as it once was). Using the magnitude drop when the fainter component was being covered by the asteroid (the only reliable magnitude drop we have), and knowing the magnitudes of the two components must sum to 11.9 magnitude, we can calculate that the magnitude of the two components are:
Primary Component (1st star occulted): 12.34
Secondary Component (2nd star occulted): 13.09
Occultation analysis expert Dave Gault (Australia) used my light curve and knowledge of the asteroid’s size and motion (and all the other factors that need to be considered) to provide the following preliminary double star solutions (there was not enough information for a single solution):
Double Star Solution #1
Separation = 36.9 mas
Position Angle = 103.6°
Double Star Solution #2
Separation = 37.0 mas
Position Angle = 113.8°
These two solutions are quite close to one another. Averaging the two separations, we get 36.95 milliarcseconds. Gaia DR3 indicates that the parallax of this star is 1.323 milliarcseconds. That’s a distance of almost 756 parsecs or 2,500 light years. At that distance, these two stars have an apparent separation (in the plane of the sky, so a minimum) of 28 AU, or a little less than the distance between the Sun and Neptune in our own solar system. This is not unreasonable for a true binary star system, but, of course, the fainter star could be many light years further away than the brighter star (which is presumably the component Gaia measured in determining the parallax). In that case, this would just be a chance alignment of two stars at different distances but not physically associated with one another.
High-resolution spectroscopic observations of UCAC4 379-071051 over time could determine whether or not this is a true binary star system. Astrometric measurements over time with whatever supercedes Gaia (likely) or analysis of occultations of this star by other asteroids (unlikely) could also determine whether or not these stars are a true binary star system.
Confounding factors in the double star solutions include (1) We don’t know the exact size of asteroid 2392 Jonathan Murray (Neowise gives a diameter of 6.5 miles); (2) We don’t yet know the shape or orientation (at event time) of this asteroid (my occultation observation was the first time this asteroid has been observed to occult a star); (3) We don’t know the orientation of my single observation chord (what part of the asteroid crossed the two stars).
As you can see, there’s a lot to consider in trying to interpret this atypical (though not all that unusual) “double dip” occultation event. Of course, the very first thing we did was to rule out any terrestrial cause of the second smaller dip (clouds, for example), and we had to also rule out any equipment anomalies that could have caused the second smaller dip (CCD anomalies, for example). After convincing ourselves that this was a real event, we proceeded with the analysis. A big thank you to Dave Gault and Dave Herald in Australia for their work and expertise in analyzing this data!
