LIGHTCURVE ANALYSIS OF NEA (154244) 2002 KL6: A POTENTIAL NEW BINARY ASTEROID

Abstract

Analysis of CCD photometric observations of near-Earth asteroid (154244) 2002 KL6 indicate that it may be a binary system. The presumed primary has a synodic rotation period of 4.60869 ± 0.00005 h and lightcurve amplitude of 0.65 ± 0.03 mag. The presumed satellite has an orbital period of 24.05 ± 0.02 h and maximum lightcurve amplitude of 0.07 mag. The secondary lightcurve showed no mutual events and seems to indicate that the satellite’s rotation is tidally locked to its orbital period.

CCD photometric observations of the near-Earth asteroid (154244) 2002 KL6 were conducted from 2016 June 10–27. Table I gives the telescope size and dates of observations for each of the observers.

Table I.

List of telescopes used and dates of observations for each observer.

Obs	Telescope	2016 June
Warner	0.30–m	10–17, 20–22, 27
Benishek	0.35–m	15, 22, 23
Ferrero	0.30–m	23, 26
Skiff	0.70–m	14–15

Each observer used MPO Canopus to process the raw images with dark and flat field frames and then to perform differential photometry. Up to five solar colored comparison stars were used each night to help minimize errors due to color differences between the asteroid and comparison stars. Warner, Benishek, and Ferrero used V magnitudes from the MPOSC3 catalog supplied with MPO Canopus. This catalog is based on the 2MASS catalog (http://www.ipac.caltech.edu/2mass) but with magnitudes converted from J-K to BVRI using formulae developed by Warner (2007). The nightly zero points for both catalogs have been found to be generally consistent to about ± 0.05 mag or better, but on occasion are as large as 0.1 mag.

Skiff used Sloan ŕ magnitudes from the CMC-15 catalog. Merging his data with those from the other observers required a small zero point offset to minimize the RMS error in the Fourier analysis. Fortunately, the large amplitude of the primary lightcurve made matching all the data sets a relatively easy and precise task.

Period analysis was done by Warner using MPO Canopus, which incorporates the FALC Fourier analysis algorithm developed by Harris (Harris et al., 1989). Even with zero point adjustments, a single period solution did not seem to give the best possible fit. The dual period feature in MPO Canopus was used first to find a likely dominant period. To search for a second period, the Fourier curve of the dominant period was subtracted from the dataset before the search began.

The period spectrum favored a solution at 24.05 ± 0.02 h and amplitude of 0.07 mag. This solution was confirmed by searching about the half period, which produced a monomodal solution with much smaller gaps in the coverage. However, an alternate solution of 16.05 ± 0.02 h cannot be formally excluded.

Previous results in the asteroid lightcurve database (LCDB; Warner et al., 2009) agree with the period for the presumed primary: Galad et al. (2010; 4.6063 h) and Koehn et al. (2014, 4.6081 h). Neither reported indications of a satellite. It’s worth noting that the observations in 2016 were at phase angle bisector longitudes (see Harris et al., 1984) roughly 90° from the earlier observations and that the lightcurve in 2016 had the lowest amplitude by 0.2–0.5 mag. This may indicate that the viewing geometry in 2016 was “just so” and allowed seeing signs of a satellite.

Since there were no obvious mutual events (occultations and/or eclipses) seen in the secondary lightcurve and the amplitude of the bimodal solution is so small, it is not appropriate to claim that the asteroid is a binary, only that it is a suspected binary.

Given the presumed orbital period, a single station has little or no hope of confirming the existence of a satellite and so a well-organized campaign with observers at widely-separated longitudes will be required at future apparitions to determine the true nature of the asteroid. Unfortunately, the next time the asteroid will again be V < 14 mag is not until 2023 August.