Electronic Telegram No. 5697
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


(40724) 1999 SY_8
     B. Whitehurst and D. Sailing, International Occultation Timing 
Association (IOTA), report the discovery of an apparent satellite of minor 
planet (40724) based on an occultation of the star Tycho-2 1977-01080-1 (which 
has catalogued Gaia magnitudes G = 9.41 and R = 8.63) on 2026 Feb. 10.178 UT.  
The magnitude of (40724) was 16.8 (7.4 magnitudes fainter than the target 
star).  Whitehurst operated three stations (one attended, two unattended) and 
Sailing operated one attended station, with all four stations being located 
north of Tulsa, OK, USA.  The sky-plane configuration diagram showing the 
observed chords (1, 2, 4 and 5) is posted at the following website URL:
http://www.cbat.eps.harvard.edu/iau/cbet/005600/CBET5697_Fig1.png.  At chord 
1, Whitehurst used a 9-cm refractor and observed a 0.467-s occultation, 
corresponding to a chord length of 4.7 km, with the flux during the 
occultation dropping to the limiting magnitude of the recording (approximately
G = 11.7).  At chord 2, Sailing used a 20-cm reflector and observed a 0.446-s 
occultation, corresponding to a chord length of 4.5 km, with the flux during 
the occultation dropping to the limiting magnitude of the recording 
(approximately G = 11.2).  At chord 4, Whitehurst used a 8-cm refractor and 
observed a 0.18-s occultation, corresponding to a chord length of 1.8 km, 
with the flux during the occultation dropping below that of comparison star 
TYC 1977-00663-1 (Gaia magnitudes G = 10.38 and R = 10.02).  At chord 5, 
Whitehurst used a 13-cm refractor and did not record an occultation event. 
All four stations used an Astrid GPS-disciplined CMOS video system.  The 
depths of all occultation drops, including that at chord 4, are sufficiently 
great to exclude a double star being the explanation for all observations.  
The main body is represented as an ellipse fitted to chords 1 and 2 with an 
oblateness consistent with that given in Carry et al. (2024, A.Ap. 687, A38),
with resulting axes of 4.9 x 3.9 km.
     Three Fresnel diffraction models of the system showing different sizes 
of the apparent satellite are posted at the following website URL:
http://www.cbat.eps.harvard.edu/iau/cbet/005600/CBET5697_Fig2.png.  The top 
figure shows the maximum body size of the apparent satellite consistent with 
the light curves for chords 2 and 4; the middle plot shows the minimum size 
consistent with chord 4; and the bottom plot shows the maximum size consistent 
with the light curves for chords 4 and 5.  The observed light curves, together 
with the corresponding light curve derived from the diffraction models, are 
posted at http://www.cbat.eps.harvard.edu/iau/cbet/005600/CBET5697_Fig3.png.  
The plot for chord 2 includes an arrow on the axis indicating the point of 
closest approach to the apparent satellite, the plot for chord 4 has an arrow 
indicating the time of closest approach to the main body, and the plot for 
chord 5 has an arrow indicating the time of closest approach to the apparent
satellite.  The width plus depth of the light drop in chord 4 precludes a 
circular body of any size being centrally located on that chord.  From the 
fitting of the Fresnel models, the apparent satellite has a diameter of (2.1 
+/- 0.3) x (1.5 +/- 0.9) km, with a separation from the main body of 0".086 
+/- 0".004 in p.a. 116 +/- 5 degrees, corresponding to a distance of 8.4 km. 
P. Tanga (GaiaMoons, Observatoire de la Cote d'Azur) advises that the analysis 
of Gaia astrometry in Liberato et al. (2026, A.Ap., submitted; accessible via
URL https://doi.org/10.48550/arXiv.2605.22702) supports the existence of a 
satellite having an orbital period of 7.80 +/- 0.54 hr.
     D. Herald and D. Gault, Trans-Tasman Occultation Alliance; and C. Weber,
European Section of the IOTA, aided in the analysis.  The reader is also 
referred to the end notes on CBET 5679.


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

                         (C) Copyright 2026 CBAT
2026 May 30                      (CBET 5697)              Daniel W. E. Green