LIGHTCURVE ANALYSIS OF NEA 2009 FG19

Abstract

CCD photometric observations of the near-Earth asteroid 2009 FG19 were made in 2014 September and October to supplement radar observations made at the same time. Analysis of the CCD data from September only found several possible periods, all commensurate with an Earth day. The most likely period was 8.00 ± 0.02 h with an amplitude of 0.80 ± 0.05 mag with an alternate solution of 9.61 ± 0.02 h being possible. The addition of data from October, even though the lightcurve had evolved noticeably, removed the 9.6 hour alias and confirmed the 8-hour solution. There were no obvious signs of non-principle axis rotation (NAPR; tumbling) but that cannot be formally excluded.

CCD photometric observations of the near-Earth asteroid (NEA) 2009 FG19 were made at the request of Lance Benner (private communications) to supplement radar observations being made in 2014 September. The initial observations were made at the Palmer Divide Station at CS3 (CS3-PDS) from 2014 September 21-26. Analysis of the resulting data showed that the period was nearly commensurate with an Earth day. Since the individual observing runs from a single station were too short to resolve the rotational ambiguities, observations were requested of and made by Pray at Sugarloaf Mountain Observatory (SMO). The additional data helped remove some of the alias periods, but not all. It was not until data from early October were added by Pollock et al. using the PROMPT telescope in Chile and the R-COP telescope in Perth, Australia, that a final result was found. Tables I and II give the equipment and observation details.

Table I.

Telescope/cameras used at each location.

Obs	Telescope	Camera
CS3-PDS	0.35-m f/9.6 Schmidt-Cass	SBIG STL-1001E
SMO	0.50-m f/4 Newtonian	SBIG ST-10XME
PROMPT	0.40-m f/10 Schmidt-Cass	Apogee Alta
R-COP	0.35-0 f/10 Schmidt-Cass	ST-10XME

Table II.

Dates of observations from each location. Phase is the phase angle, in degrees, on the first and last dates in the range. The PAB columns are, respectively, the longitude and latitude, also in degrees and on the first and last dates in the range.

Location	2014 mmm dd	Phase	LPAB	BPAB
CS3-PDS	Sep 21-28	93,57	47,37	29,−12
SMO	Sep 27-28	66,60	41,37	−6,−12
R-COP	Oct 6	47	16	−32
PROMPT	Oct 7	47	15	−33

Image processing and measurement by Warner and Pray were done using MPO Canopus (Bdw Publishing). Pollock used Mira for his images. Master flats and darks were applied to the science frames prior to measurements. The MPO Canopus export data sets and raw text from Mira were collected by Warner for period analysis in MPO Canopus, which incorporates the FALC Fourier analysis algorithm developed by Harris (Harris et al., 1989).

For Warner and Pray, conversion to an internal standard system with approximately ±0.05 mag zero point precision was accomplished using the Comp Star Selector in MPO Canopus and the MPOSC3 catalog provided with that software. The magnitudes in the MPOSC3 are based on the 2MASS catalog converted to the BVRcIc system using formulae developed by Warner (2007). This internal calibration works well within a given telescope-camera system. In some cases, a nearly constant offset is required to merge data from one system to another. This was not necessary in this case. The data from Pollock were differential magnitudes only. His two data sets were first aligned to produce a best fit (lowest RMS) in a period search from 5 to 20 hours. The zero point for the combined data set was then adjusted as needed to obtain a minimum RMS when merging with the data from Warner and Pray.

Figure 1 shows a period spectrum produced by the Fourier analysis ranging from 4 to 20 hours when using only the data from Warner and Pray. There are several almost equally likely solutions (minimum RMS value), several of which differ by an integral of half-integral number of rotation over 24 hours. The CS3-PDS data alone could not resolve which of the periods was most likely. The two runs by both stations on Sep 27 and 28 extended the total span of observation by about 3 hours and helped reduce the ambiguities down to two probable periods.

Figure 1.

The period spectrum from 4 to 20 hours for 2009 FG19 using data from Warner and Pray shows several almost equally likely solutions.

Figure 2 shows the favored solution of 8.00 hours. The amplitude of the lightcurve, 0.80 ± 0.05 mag, is made a little more uncertain by the lightcurve not being complete at the first maximum at 0.4 rotation phase. Evidence of the changing shape of the lightcurve is seen around rotation phase 0.3.

Figure 2.

The data for 2009 FG19 phased to the favored period of 8.00 hours.

Figure 3 shows the alternate solution of 9.60 hours, also with a 0.80 mag amplitude. The two periods have an integral ratio of 6:5. Even though Figure 2 shows a significant gap in the lightcurve, it is considered the more likely solution because of the misalignment of the data in Figure 3 near 0.2 rotation phase.

Figure 3.

The data for 2009 FG19 phased to an alternate solution of 9.61 hours.

At the time, Petr Pravec (personal communications) reviewed the data set for indications of tumbling. While nothing certain was found, the possibility could not be formally excluded without having data from stations at other longitudes and/or not showing any significant changes in the lightcurve due to changing viewing aspects.

If nothing else, the initial observations indicating a period of several hours helped the radar team plan their observations, which is a vital service that is easily provided by the backyard astronomy community in addition to astrometry to reduce position uncertainties.

The Second Campaign

The asteroid moved too far south at the end of September for Warner and Pray to try filling in the lightcurve and solving the rotational alias. Pollock had been contacted in September but, due to the northerly location of the asteroid and other circumstances, could not observe the asteroid until early October, when he arranged runs on Oct 6 UT for R-COP in Perth, Australia, and Oct 7 for the PROMPT telescope in Chile.

Using only his data, a period search strongly favored a solution near 8 hours (Figure 4). Since the lightcurve did evolve so much, we used a combined data set that included the observations from Sep 25 through Oct 7. This excluded the observations at the more extreme phase angles of α > 80°, although the range of angles in the subset still exceeded 30 degrees. Figure 5 shows the resulting period spectrum from 4 to 20 hours. The 9.6 hour solution has been completely eliminated, leaving three solutions, each commensurate with an Earth day and differing by one rotation over a 24-hour period.

Figure 4.

The lightcurve for 2009 FG19 using only the data from Pollock et al. and phased to the best fit period of 8.06 hours. The reduced magnitudes are differential values added to an arbitrary constant.

Figure 5.

The period spectrum using a subset of the data ranging from 2014 Sep 25 through Oct 7. The 9.6 hour period is eliminated.

The lightcurves in Figures 4 and 6 used data binned 2x10, meaning any given bin used the average of up to 2 unique data points separated by no more than 10 minutes. No data point was used in more than one bin. Figure 6 appears to verify the lower amplitude of the secondary minimum at about 0.8 rotational phase that was suspected during the initial analysis.

Figure 6.

The lightcurve for 2009 FG19 using the subset of data described in the text and phased to the most likely period of 8.016 hours.

We have adopted the period of 8.016 hours for this paper. The lightcurves for the longer periods are either trimodal (12 hours) or quadramodal (16 hours). The latter is highly symmetrical over the first and second half, which is physically improbable and so represents the double period.

Photometry observations and analysis of near-Earth asteroids are fraught with difficulties. This campaign showed that even some of the more challenging of circumstances – short-lived campaigns and longer, Earth-day commensurate periods – can be overcome through timely response and coordinated efforts.