(53110) 1999 AR7: A NEW NEA BINARY DISCOVERY

Abstract

Analysis of CCD photometric observations of the near-Earth asteroid (53110) 1999 AR7 made in 2015 December show it to be a binary system with a primary period of 2.7375 ± 0.0005 h and orbital period of 31.31 ± 0.02 h. The depth of the secondary mutual event indicates a minimum effective diameter ratio (Ds/Dp) of 0.41 ± 0.02.

The near-Earth asteroid (NEA) (53110) 1999 AR7 was observed as part of the ongoing program at Center for Solar System Studies (CS3) to determine the rotation periods and other photometric characteristics of NEAs and to support radar observations with optical lightcurves. The initial observations were made at the Palmer Divide Station at CS3 (CS3-PDS) starting on 2015 Dec 19. See Tables I and II for equipment observing circumstances.

Table I.

Telescope/cameras used at each location.

OBS	Telescope	Camera
CS3–PDS	0.35–m f/9.6 Schmidt–Cass	STL–1001E

Table II.

Observing circumstances. Phase is the phase angle, in degrees. LPAB and BPAB are, respectively, the phase angle bisector longitude and latitude, also in degrees. For rows with a range of dates, the values are for the first and last date in the range. All values are computed for 0 h UT.

Location	2015	Phase	LPAB	BPAB
CS3-PDS	Dec 19–31	21.3–27.2	101–100	14–25

The observations from Dec 19-21 seemed to indicate an object having a lightcurve with a period of about 33 hours and a shape with pronounced, sharp minimums and possibly even “shoulders” which are significant increases in the slope on the descending part of a minimum and decrease on the ascending part. Such a lightcurve is often considered evidence of a near- or full-contact binary asteroid. The latter would be two lobes of about equal size joined by a narrow bridge.

However, with later observing runs, the long period trend seemed to disappear was replaced by a distinct short period (2-3 hours) lightcurve. Figure 1 shows the unsubtracted lightcurve after combining all sessions. The short period component is very apparent but there are a number of data points well below the average curve. Following the Dec. 27 observing run, a dual-period search was employed using MPO Canopus.

Figure 1.

The complete data set for 1999 AR7 forced to a period between 2.5-3.0 hours but without subtracting a secondary period.

The process involved searching for a period between 2-5 hours without subtracting any data points. This led to an initial solution of about 2.7 hours. The resulting Fourier model curve was subtracted from the data during the search for a second period, which found a strong solution at about 31 hours. That result was subtracted from the original data set and the search for the short period started anew. This process continued until both periods stabilized.

Because of the deep minimums in the long period lightcurve, an 8^th-order fit was used while only a 4^th-order fit was used for the short period. Figures 2 and 3 show the final results. The primary (Figure 2) has a period of 2.7375 ± 0.0005 h and amplitude of 0.10 mag. The latter indicates a nearly spheroidal body. Figure 3 shows the mutual events, i.e., occultations and/or eclipses due to the satellite, and gives an orbital period of 31.31 ± 0.02 h.

Figure 2.

The lightcurve for the primary of 1999 AR7 indicates a nearly spheroidal body.

Figure 3.

The lightcurve for 1999 AR7 after subtracting the one due to the primary clearly shows the mutual events (occultations and/or eclipses) due to the satellite.

The lightcurve in Figure 3 outside the events is essentially flat, indicating that the satellite is nearly spheroidal as well. Otherwise, the periods outside the events might show an upward “bowing.” The dips in the flat sections, most notably at 0.9 rotation phase, are artifacts due to the high harmonic order fit required for the model lightcurve to follow the events all the way to the bottom.

The final result explains why analysis of the first few sessions was led awry: they were capturing the events almost exclusively, which gave a misleading impression of what was happening and of the true nature of the asteroid. When the observation window moved away from the events and so included only the rotation of the primary, the fact of the asteroid being a “simple” binary became more apparent.

The depths of the two events range from about 0.17-0.20 mag. The shallower event allows estimating the effective diameter ratio of the two bodies using

$$D_s/D_p\ge \sqrt{{10}^{0.4\Delta m}-1}$$ (1)

where m is the magnitude drop of the shallower event. This gives

$$D_s/D_p\ge 0.41\pm 0.02$$

where the actual error may be larger.

Note that this is a minimum value because neither event is “flat-bottomed,” which would indicate a total eclipse. Therefore, only partial eclipses were seen and so the value gives a minimum size ratio. Note also that the events are asymmetrical in the sense that they are not 0.5 rotation phase apart. This may indicate an eccentric orbit for the satellite. Additional data at future apparitions will help model the system.