Electronic Telegram No. 5476
Central Bureau for Astronomical Telegrams
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2016 PO_296
     S. Deen (Simi Valley, CA, USA) reports his discovery of an apparent
satellite orbiting the transneptunian object 2016 PO_296, which itself was
initially discovered as a pair of unlinked tracklets by the COIAS project (a
Japanese-citizen science project to search through Subaru Strategic Program
data taken by the Subaru telescope between 2016 and 2019 for minor planets.
Deen has been following up their various identified slow-moving objects in an
attempt to identify new TNOs, with 2016 PO_296 being one of those designated
as a result of this work (cf. MPEC 2024-M59).  On 2016 Sept. 5.4 UT, during
exceptionally good seeing (0".4) at Subaru, 2016 PO_296 appeared distinctly
elongated in a consistent direction, perpendicular to its 0".03/min motion
towards p.a. 250 degrees in three 200-s i-band images taken over the course of
95 minutes.  In each image, there appears to be two components:  a brighter
component of i magnitude roughly 24.2 (corresponding approximately to absolute
magnitude H = 6.2), and a fainter component at p.a. 340 degrees located
approximately 0".4 +/- 0".1 away from the primary at magnitude i = 24.4
(correpsonding approximately to H = 6.4).  The ellipticity of the 0".4 (FWHM)
seeing in each image is 0.01, 0.08, and 0.19, respectively, with no
surrounding stars displaying similar elongation to 2016 PO_296 in any of the
three images.  At the time of the 2016 Sept. 5 image, 2016 PO_296 was 66.4 AU
from the earth, resulting in a projected separation of 19000 +/- 5000 km.  The
other five Subaru dates (2016 July 7, 2016 Aug. 1 and 9, 2017 June 27, and
2019 Sept. 27) do not show convincing elongation but also have generally worse
seeing (0".5, 0".7, 0".7-1".1, 0".45, and 0".6-0".8, respectively).  A single
incidental 480-s R-band image from the Very Large Telescope (+ VIMOS) on 2005
June 6 taken in 1".0 seeing shows a possible secondary at p.a. 250 degrees
(separation 0".75 +/- 0".1), but the low signal-to-noise ratio and poorer
seeing in the detection makes this doubtful.  As a result of only a single
confident night of detection, Deen is cautiously reporting this as a candidate
satellite, but with some confidence.  If this is confirmed, it would mark the
second-widest-separation binary system among so-called known "scattered-disk"
objects after (136199) Eris and its satellite Dysnomia.


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

                         (C) Copyright 2024 CBAT
2024 November 18                 (CBET 5476)              Daniel W. E. Green