Electronic Telegram No. 5544
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


(148358) 2000 SY_18
     D. Herald (Murrumbateman, Australia), on behlaf of the Trans Tasman
Occultation Alliance (TTOA), reports that P. C. Stuart (Clear Lake Shores, TX,
USA) of the International Occultation Timing Association has found an apparent
satellite of minor planet (148358), based on an occultation of the 10.5-mag
star TYC 5519-00010-1 on 2025 Mar. 11.1854 UT.  Stuart recorded two
consecutive occultations of 0.20 and 0.10 s, separated by 0.39 s, using a
20-cm Schmidt-Cassegrain reflector (+ Watec 910BD video camera), with a
recording limiting magnitude of V = 12.6.  The resulting chords were computed
to be 1.8 and 0.9 km in length, separated by 3.7 km.  The flux from the minor
planet, which was 8 magnitudes fainter than the star, was too small to affect
the interpretation of the observation.  To exclude a double-star explanation,
the flux drops of the two events were determined using comparison stars; if
the components of a double star were occulted in turn, the sum of the drops
would equal the flux of the star as a whole.  For the main body, the flux
drop was 85 percent (2.1 mag) in V; for the satellite, the flux drop was 65
percent (1.1 mag) in V.  The combined flux drop from the two occultations of
150 percent is significantly greater than the flux of the star, thereby
excluding the two drops from being caused by different components of a double
star.  The flux drop for the satellite was smaller than the drop for the main
body as a result of Fresnel diffraction on the smaller body.  The main body's
1.8-km chord aligns with its expected diameter from its absolute magnitude of
H = 16.01, assuming an albedo of 0.20.  An elongate shape cannot account for
the observed separation.  The sky-plane separation is 0".0047 at p.a. 102
degrees.
     The light curve is posted at website URL
http://www.cbat.eps.harvard.edu/iau/cbet/005500/CBET5544_Fig1.png, and a
configuration diagram (with the star moving right to left) is posted at URL
http://www.cbat.eps.harvard.edu/iau/cbet/005500/CBET5544_Fig2.png.  Herald and
D. Gault (TTOA) contributed to the analysis.


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      superseded by text appearing later in the printed IAU Circulars.

                         (C) Copyright 2025 CBAT
2025 April 26                    (CBET 5544)              Daniel W. E. Green