From time series CCD observations of a fixed and large star field that contained the binary trans-Neptunian Object ( 90482 ) Orcus ( formerly 2004 DW ) , taken during a period of 33 days , we have been able to derive high-precision relative astrometry and photometry of the Orcus system with respect to background stars .
The right ascension residuals of an orbital fit to the astrometric data revealed a periodicity of 9.7 \pm 0.3 days , which is what one would expect to be induced by the known Orcus companion ( Vanth ) .
The residuals are also correlated with the theoretical positions of the satellite with regard to the primary .
We therefore have revealed the presence of Orcus ’ satellite in our astrometric measurements , although the residuals in declination did not show the expected variations .
The oscillation in the residuals is caused by the photocenter motion of the combined Orcus plus satellite system around the barycenter along an orbital revolution of the satellite .
The photocenter motion is much larger than the motion of Orcus around the barycenter , and we show here that detecting some binaries through a carefully devised astrometric technique might be feasible with telescopes of moderate size .
We discuss the prospects for using the technique to find new binary Transneptunian Objects ( TNOs ) and to study already known binary systems with uncertain orbital periods .
We also analyzed the system ’ s mid-term photometry in order to determine whether the rotation could be tidally locked to the satellite ’ s orbital period .
We found that a photometric variability of 9.7 \pm 0.3 days is clear in our data , and is nearly coincident with the orbital period of the satellite .
We believe this variability might be induced by the satellite ’ s rotation .
In our photometry there is also a slight hint for an additional very small variability in the 10 hr range that was already reported in the literature .
This short-term variability would indicate that the primary is not tidally locked and therefore the system would not have reached a double synchronous state .
Implications for the basic physical properties of the primary and its satellite are discussed .
From angular momentum considerations we suspect that the Orcus satellite might have formed from a rotational fission .
This requires that the mass of the satellite would be around 0.09 times that of the primary , close to the value that one derives by using an albedo of 0.12 for the satellite and assuming equal densities for both the primary and secondary .