A Double Star "discovery" and an unexplained profile for (2875) Largerkvist
On 2018 Sept 23, a routine asteroid appulse (see Note-1) was being monitored by two UK observers who were both in the predicted occultation shadow. They were separated by 11 Km along the direction of motion and 5km at a normal.
The asteroid was (2875) Lagerkvist (diameter 10km) and was predicted to occult UCAC4 461_131617 (v 11.2) at 0204 UT with a few seconds uncertainty in mid time. Since the asteroid was magnitude 16 a total extinction of the 11th magnitude star was expected with a probability of 5% due to uncertainty in the orbit. The result is reported on Euraster.net . The equipment used by the observers is summarised in Notes 2 and 3, and their locations in Notes 4 and 5.
T. Haymes: The observation was unusal. There was roughly a 30% flux drop of duration 0.6s +/- 0.1s (15 frames), or about 0.4 magnitudes and no dissappearance. The event mid time was 2204h 20.2s UT
P. Tickner: Reported a dip in his light curve at lower S/n owing to dew formation on the SCT corrector plate. The light drop was not seen initially but on closer inspection a drop of similar duration (0.55s) was identified, and interestingly was at a mid time of 2204hr 18.5s UT, that is 1.7 sec earlier than Haymes. (Should have been later )
It was concluded (disscusion on PLANOCCULT forum) that the asteroid occulted a minor component of the star and this was a double-star "discovery". When reporting this to the forum, several observers remarked that similar step events have been recorded involving different stars in the past (example), and in this example of UCAC4 461_131617, the Gaia catalogue had indicated a less than ideal profile. So there seems to be prior evidence of an unresolved multiple system. Looking at the Hubble GSC catalogue used by SkyMap, the star GSC 0585-0490 (= UCAC4 461_131617 ) is given two positions. So the step event is most likely caused by unresolved multiplicy, the asteroid having occulted the companion. The component separation is estimated to be 0.2 +/- 0.1" arc based on the spacial resolution of Gaia.
The difference in mid time of our two observers is not yet explained. With the shaddow direction from East to West, a diffence of a few seconds is expected, but Haymes should see the event first. The order is reversed so there musts be a 2 x1.7 sec displacement in one of the observers GPS clocks. This i would suggest is unlikly given the accuracy of these GPS.
One possible source of absolute time error is in P. Tickner's set up which uses a GPS-USB time source (AiryLab"TimeBox") and USB3 camera. While this has been tested at 100 fps with good results, there is a small delay ( 10s of ms ) in the W7 Operating system compared to Analoge video-GPS. This does not account for the large difference in our observations of these occultation chords.
If not caused by an absolute timing error, then some physical reason with the asteroid is suggested, perhaps a double system or very unusal shape.
Conclusion: Further measurment needed
The Occult Watcher display of observations for (2875). The green ticks are the positive chords
Tangra light curve from T Haymes' observation in AAV format
AOTA analysis of the light curve by T Haymes, processing courtesy of Eric Frappa (Euraster.net)
Proposed "hypothesis" to explain the observations
Published light curve (See Note-6)
Measured by Tim Haymes: Period is 3 hrs ( probably half period ) with 0.7 magnitude amplitude.
Note-1: Prediction provided by John Talbot, Abingdon, UK using Occult
4 software. Details from the UKOCL
feed to OccultWatcher application.