Asteroid 285263 (1998 QE2) Photometry: Rotation Light Curves
Webmaster: Bruce L. Gary.  Latest update: 2013.07.27, 00.5 UT

Asteroid 285263 (1998 QE2) will pass within ~ 15 times the distance to the moon on 2013 May 31. It's diameter is estimated to be ~ 2.7 km and will be as bright as V-mag ~ 11.0.  Observability shifts from southern hemisphere to northern when it crosses the celestial equator on 2013 June 5. A rotation light curve was obtained by Kevin Hills during 2013 May 02 - 10; the shape was sinusoidal, the period was 5.281 hours and the semi-amplitude was ~ 90 mmag. 

I began photometric monitoring on May 24, using a 11-inch aperture Celestron telescope at my Southern Arizona observatory (Hereford Arizona Observatory, HAO, G95).  All observations to date are with a r'-band filter and magnitude calibrations for each star field are based on APASS magnitudes (in the UCAC4 catalog). My FOV is 22x14 'arc (changed to 18 x 12 'arc June 2, when I removed a focal reducer lens), so each night requires several FOV moves. This means that a different set of APASS stars are used for calibration for each of the half dozen FOV's per observing session. Joining the several LC segments for an observing session therefore can require magnitude adjustments for each FOV.  Based on the internal consistency of APASS star calibrations (RMS ~ 25 mmag for N ~ 15) the FOV adjustments should be accurate to better than ~ 15 mmag. Since the asteroid's variation with rotation is much larger, of order 200 mmag, it should be possible to combine FOV LC segments to derive an entire observing session's LC.

On May 27 I sent an e-mail (to Benner and Warner) with tentative results for a light curve pattern that repeated with an interval (that I associated with the asteroid's period) of 4.73 hours. I suggested that the asteroid might be tumbling because of the difference in apparent brightness periodicity between Kevin Hills' observations and mine, 3 weeks later.  Subsequent observing sessions show the same periodicity (P = 4.76 hours).

Related Links

    Nightly & Individual Light Curves 
    Hardware, Observing and Processing Procedure
    Mosaic Flattening Equation (MFE) method for flat fielding (when the standard flat field is severely affected by reflected light) 

Summary of Results To Date

This asteroid appears to be rotating with a period of 4.764 0.020 hours. This result is based on 6 observing sessions (another 9 remain to be processed). Observations made in early May by Kevin Hills have been analyzed a couple times by Brian Warner, yielding periods of P = 5.281 hours and 2.3736 0.0003 hours.  My data exhibits a repeating pattern with an interval of 4.764 hours. I have no explanation for these discrepancies.

The easiest way to show that my LCs exhibit a repeating pattern with an interval of 4.764 hours is to present a LC for observing session longer than this interval and show how data after the first 4.764 hours exhibits the same shape as the earlier data. Here's one example:


Figure 1.
June 7 LC segments (corresponding to 6 FOV placements) are plotted on the same graph (without mag offset adjustments). There's an uncanny shape resemblance of the end (8.0 - 11.1 UT) to the beginning (3.0 - 6.5 UT), consistent with a periodicity of ~ 4.7 hours.


Figure 2: Same data, folded so that data after 8.0 UT appears at a time 4.764 hours earlier, showing that the shapes overlap. (A slow fading trend is apparently occurring during the 8-hour observing session).

The same treatment can be applied to the long May 31 observing session.


Figure 3. May 31 observing segments (corresponding to 7 FOV placements) are plotted versus the 6-hour observing session.


Figure 4. This is a phase fold version of the same data, showing shape agreement (using P = 4.72 hours).

If P = 4.724 hours then there are 5.080 rotations per day, and the LC pattern should be approximately the same from one night to the next, with a shift of ~ 0.38 hours to an earlier time on successive nights. Or, going backward one night should show a 0.38 hour shift of the same pattern to a later time. The sharp minimum at 3.55 UT should repeat on the previous date at 3.59 UT. The next figure shows that this indeed occurs.

If the LC shape has a repeating period of ~ 4.7 hours then all observing sessions should show the same LC shape when phase-folded with this period. The next graph illustrates that this is the case for the six observing sessions that have been processed.


Figure 4. Phase fold of data from 19 observing sessions between May 24 and Jun 21, using arbitrary magnitude offsets for presentation clarity. The observing sessions with overlapping phase use different symbols where there is overlap.  

The pattern is the typical asteroid one of 2-peaks and 2 valleys. Notice that for the earliest data (at top) the deeper minimum is at ~ 3.0 hours in the figure, whereas a week later the deeper minimum is the one at 0.5 hours.

I tentatively conclude that the LC shape has a shape that repeats at intervals of ~ 4.7 hours. 


Figure 5. Phase-folded magnitudes for June 9 using 4.75 hour period, showing possible "mutual event" (either a transit or an occultation of the secondary). 




References
 
    JPL Press Release, 2013 May 30, http://www.jpl.nasa.gov/news/news.php?release=2013-182

Related Links

    Description of hardware, image processing and photometry analysis procedures
    Display of individual LCs


____________________________________________________________________

WebMaster: Bruce L. GaryNothing on this web page is copyrighted. This site opened:  2013 May 29