CBET 4413

                                                  Electronic Telegram No. 4413
Central Bureau for Astronomical Telegrams
Mailing address:  Hoffman Lab 209; Harvard University;
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e-mail:  cbatiau@eps.harvard.edu (alternate cbat@iau.org)
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Prepared using the Tamkin Foundation Computer Network

S/2017 (113) 1
     P. Maley, International Occultation Timing Association (IOTA) and
Johnson Space Center Astronomical Society, Carefree, AZ, USA; T. Blank, IOTA,
Fountain Hills, AZ, USA; R. Campbell and D. Eisfeldt, Central Texas
Astronomical Society, Waco, TX, USA; W. Thomas, East Valley Astronomy Club,
Florence, AZ, USA; S. Insana, Phoenix Astronomical Society, Phoenix, AZ, USA;
S. Deen, Simi Valley, CA, USA; T. George, IOTA, Scottsdale, AZ, USA; B.
Timerson, IOTA, Rochester, NY, USA; D. Herald, IOTA, Murrumbateman, Australia;
S. Preston, IOTA, Medina, WA, USA; W. J. Merline, Southwest Research
Institute; and J. D. Drummond, Albuquerque, NM, USA, report the discovery of
an apparent satellite of minor planet (113) Amalthea from observations
made of the Tycho Catalogue star 1878-01081-1 (magnitude V = 10.0) during an
occultation by the minor planet (which was at V = 12.9) on 2017 Mar. 14.167
UT.  A total of ten observation sites recorded seven positive events and
three negative events for (113), two of which are attributed to the possible
satellite.  This was a coordinated campaign based on the shadow-path
calculations by Preston (see the following website URL:
     Tabulated below (in order from the northernmost to the southernmost
sites, in relation to the roughly east-west track) are each observer, the
respective observing site along with the distance off the nominal
centerline of prediction, whether an occultation event was seen or not,
the duration of any occultation in seconds, and the telescope aperture
in centimeters:

 Observer  Location           Distance from      Event   Duration   Telescope
                              Predicted Center   Seen?   (seconds)   Aperture
 Deen      Simi Valley, CA    134.3 km north      no       --         15 cm
 Blank     Goldfield, AZ       30.1 km north      no       --          8 cm
 Blank     Florence, AZ        20.0 km north      yes     2.14         8 cm
 Maley     Buckeye, AZ         17.6 km north      yes     3.04         8 cm
 Thomas    Florence, AZ         9.0 km north      yes     5.32        28 cm
 Blank     Florence, AZ         4.5 km south      yes     6.37         8 cm
 Blank     Florence, AZ        11.6 km south      yes     4.70         8 cm
 Insana    Gila Bend, AZ       17.9 km south      no       --          8 cm
 Eisfeldt  Waco, TX            19.4 km south      yes     0.45        20 cm
 Campbell  Clifton, TX         19.5 km south      yes     0.50        61 cm

The five positive chords across the primary minor planet show a projected
ellipse of size about 67 x 36 km.  The two shortest chords, separated from
the primary by the negative chord of Insana, would be consistent with a
satellite of size 6 km or larger.  These two shortest chords result from
separate stations, the same distance off the track, but separated from each
other by 24 km (along-track).  These two shorter chords are coincident on
the sky plane, crossing the presumed satellite.  If the apparent satellite is
indeed centered near the short chords, then on Mar. 14.167 the satellite
would have been located at separation 0".017 (24 km) from the projected
center of the primary minor planet (113) in p.a. 156 degrees.  This is the
first time that two chords have been observed across a previously unknown
minor-planet satellite.  The times and locations of the three critical
chords (Insana, Campbell, Eisfeldt) were triple-checked to minimize the
possibility of spurious results.  The original videos containing the
continuous video recordings were analyzed by George.  A drop in brightness
of 3.1 +/- 0.1 magnitudes, close to that expected, was seen in all of the
positive chords except one; the chord of Campbell showed a smaller drop of
1.6 mag, which is considered spurious due to the non-standard video
compression used.  No double dips or stepped dips, characteristic of a
double star, were seen in any of the light curves.  George also analyzed an
occultation of the same star by (27) Euterpe in 2015, and no such evidence
of a double star was seen in that event.  The possibility of an occultation
of a double star with components of the same magnitude -- with one star
resulting in the northern chords, and the other star resulting in the
(separated) southern two chords -- was examined; in this scenario, all chords
would have a brightness drop of only half (0.7 mag) that predicted.  George
used field stars to estimate the observation-time brightness of the target
star, which was found to be within 0.25 mag of the catalogued magnitude.
Maximum excursion of the asteroid's rotational light curve is about 0.26 mag.
It is estimated that uncertainties in star brightness/color, camera
spectral responses, and rotational variation of the minor planet's brightness
could combine to add a systematic uncertainty in the measured brightness
drop of about 0.6 mag.  Thus, the double-star hypothesis cannot be
definitively ruled out, but this is thought to be unlikely.  No other known
solar-system object was within 100" of (113) at the time of the event.
     To check whether prior imaging had shown any evidence of a satellite,
Merline re-examined adaptive-optics (AO) images of (113) taken by his group at
the Canada-France-Hawaii Telescope and the Keck Telescope in 2000, plus some
archival data from the Very Large Telescope in 2007 (program 079.C-0528,
Principal Investigator P. Rojo).  No definitive evidence of a satellite could
be seen, but the data were taken with older instrumentation and/or during
nights of poor conditions, with a point-spread function (PSF) worse than
standard for these facilities.  Drummond further analyzed these AO data sets
with an independent deconvolution technique and also could not detect a
satellite.  Drummond and Merline have found, by deriving the sub-earth
latitude (-59 deg) on 2017 Mar. 14, from the minor planet's pole position of
Tungalag et al. (2002, Kinematika i Fizika Nebesnykh Tel 18, 508), that the
observed satellite orbit could not be equatorial.  Typical small satellites
around large main-belt asteroids orbit at around 10 primary radii.  If one
assumes that size here, then on Mar. 14 the orbital plane would have been at
a low inclination to the line-of-sight, with the satellite near conjunction.
Assuming a brightness ratio of 100 and this orbit size, and given the PSF
qualities on the AO nights, it is determined that the above-mentioned AO
imaging would have had only a marginal chance of detecting the satellite.

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

                         (C) Copyright 2017 CBAT
2017 July 12                     (CBET 4413)              Daniel W. E. Green

Central Bureau for Astronomical Telegrams