Observational detection of eclipses of J5 Amalthea by the Galilean satellites

¹ Armagh Observatory, College Hill, Armagh BT61 9DG, Northern Ireland, UK
e-mail: aac@arm.ac.uk
² Faulkes Telescope Project School of Physics and Astronomy, Cardiff University, Queens Buildings, 5 The Parade, Cardiff CF24 3AA, UK
³ Department of Physics and Astronomy, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK
⁴ Division of Earth, Space and Environment, University of Glamorgan, Pontypridd CF37 1DL, UK
⁵ Las Cumbres Observatory Global Telescope, 6740 Cortona Dr. Ste. 102, Goleta, CA 93117, USA
⁶ Department of Physics, University of California, Santa Barbara, CA 93106, USA

Received: 19 April 2010
Accepted: 24 June 2010

Abstract

Aims. We carried out observations of the small jovian satellite Amalthea (J5) as it was being eclipsed by the Galilean satellites near the 2009 equinox of Jupiter in order to apply the technique of mutual event photometry to the astrometric determination of this satellite’s position.

Methods. The observations were carried out during the period 06/2009−09/2009 from the island of Maui, Hawaii and Siding Spring, Australia with the 2m Faulkes Telescopes North and South respectively. We observed in the near-infrared part of the spectrum using a PanStarrs-Z filter with Jupiter near the edge of the field in order to mitigate against the glare from the planet. Frames were acquired at rates  > 1/min during eclipse times predicted using recent JPL ephemerides for the satellites. Following subtraction of the sky background from these frames, differential aperture photometry was carried out on Amalthea and a nearby field star.

Results. We have obtained three lightcurves which show a clear drop in the flux from Amalthea, indicating that an eclipse took place as predicted. These were model-fitted to yield best estimates of the time of maximum flux drop and the impact parameter. These are consistent with Amalthea’s ephemeris but indicate that Amalthea is slightly ahead of, and closer to Jupiter than, its predicted position by approximately half the ephemeris uncertainty in these directions. We argue that a ground-based campaign of higher-cadence photometry accurate at the 5% level or better during the next season of eclipses in 2014-15 should yield positions to within 0″̣05 and affect a corresponding improvement in Amalthea’s ephemeris.