Electronic Telegram No. 5750
Central Bureau for Astronomical Telegrams
Mailing address:  Hoffman Lab 209; Harvard University;
 20 Oxford St.; Cambridge, MA  02138; U.S.A.
e-mail:  cbatiau@eps.harvard.edu (alternate cbat@iau.org)
URL http://www.cbat.eps.harvard.edu/index.html
Prepared using the Tamkin Foundation Computer Network


(42773) 1998 UN_15
     P. Maley, International Occultation Timing Association (IOTA) and NASA 
Johnson Space Center Astronomical Society, reports the discovery of an 
apparent satellite of minor planet (42773) based on an occultation of the star 
TYC 0027-00264-1 (which has catalogued Gaia magnitudes G = 10.9 and R = 10.5) 
on 2026 Sept. 22.310 UT.  The magnitude of (42773) was 17.9 (which was 7.0 mag 
fainter than the target star).  Maley observed from Scottsdale, AZ, USA, using 
a 20-cm telescope (+ Watec WAT-910HX video camera).  The recording showed two 
consecutive occultations of durations 0.40 and 1.12 s, separated by 1.64 s, 
corresponding to chord lengths of 3.2 and 9.1 km, with a distance of 13.0 km 
between the chords.  The flux for both occultations dropped to the limiting 
magnitude of the recording (approximately G = 14.0 and R = 13.4).  The depths 
of these drops (3.1 magnitude in G; 2.9 magnitude in R) exclude a double star 
being the explanation for the light curve.
     There are no satellite-determined diameters for this minor planet.  As 
the data of Carry et al. (2024, A.Ap, 687, A38) indicate, the minor planet is 
likely to be near-spherical; it was modeled as a circular body with a diameter 
of 8.9 km, with the albedo 0.045 derived from its absolute magnitude of 14.23.
The satellite was modeled as an elliptical body with possible axes ranging 
from 3.6 x 1.0 km to 5.0 x 2.5 km.  The separation of the satellite was 
0".0159 in p.a. 248 +/- 2 degrees, with a sky-plane separation of 19.2 km.
     L. Liberato (GaiaMoons, Observatoire de la Cote d'Azur) advises that an 
analysis of this minor planet in the Gaia FPR astrometry (Liberato et al. 
2026, A.Ap. 711, A299) detected a period of 29.5 hours with low confidence.  
Though the signal measured is not statistically reliable, the Gaia data still 
hint at the presence of a companion around this object, with the period being 
consistent with the physical parameters derived from this occultation.  The 
sky-plane configuration diagram showing the observed occultation chords is 
posted at http://www.cbat.eps.harvard.edu/iau/cbet/005700/CBET5750_Fig1.png.
The observed and modeled light curves, together with the Fresnel diffraction 
model for the main body and satellite, are posted at the following website
URL:  http://www.cbat.eps.harvard.edu/iau/cbet/005700/CBET5750_Fig2.png.
     D. Herald and D. Gault, Trans-Tasman Occultation Alliance; and C. Weber, 
European Section of the IOTA, aided in the analysis.  (See also Herald et al.
2026, Minor Planet Bull. 53-4, 219.)


NOTE: These 'Central Bureau Electronic Telegrams' are sometimes
      superseded by text appearing later in the printed IAU Circulars.

                         (C) Copyright 2026 CBAT
2026 September 28                (CBET 5750)              Daniel W. E. Green