(939) Isberga: a binary asteroid
Observed: 21, 22 Oct, 12, 13, 15 Nov 2011
Michel Bonnardeau
23 Oct 2011
Updated 13, 15, 16, 18 Nov 2011 (more data, smoothing)
Abstract
Light curves for this binary asteroid are presented, showing the spin modulation and eclipses.
Introduction
(939) Isberga is an asteroid with an orbital period around the Sun of 3.37yr. It was discovered as having a spin with a period of 2.9hr and a companion (so it is a binary system) orbiting with a period of 26.8hr (Molnar et al (2008)).
Observations
The observations were carried out with a 203mm f/6.3 SC telescope, a Rc filter and a SBIG ST7E camera (KAF401E CCD). The exposure duration for each image is 200s (except for the first images of each session: 60s). The magnitudes of the asteroid are measured by differential photometry.
Session 21 Oct 2011
95 usable images where obtained.
For the differential photometry, the comparison star is GSC 620-1142 with
the magnitude Rc=12.650 (computed from the r' magnitude of the CMC14,
the 2MASS magnitudes, and the transformations formulae of Bilir et al
(2007) and of Smith et al (2002)). The check star is GSC 620-993 and is
then observed with the magnitude Rc=13.834+/-0.019.
The resulting light curve, showing the modulation due to the spin/rotation
of the asteroid:
Red: the magnitudes of Isberga, blue: of the check star with an offset
of -0.3mag. The error bars are +/- the 1-sigma statistical uncertainties.
For the asteroid, the dates are corrected for the light travel time to
the observer.
Session 22 Oct 2011
108 usable images where obtained.
The comparison star is GSC 620-880 with the magnitude Rc=12.154 (computed
as above). The check star is GSC 620-1262, observed with the magnitude
Rc=13.200+/-0.013.
The resulting light curve, showing an eclipse at the beginning of the
session:
The magnitudes of the check star have an offset of +0.6mag. There is
a primary eclipse at the beginning of the session.
Session 12 Nov 2011
95 usable images were obtained.
The comparison star is GSC 615-1135 with the magnitude Rc=11.426 (computed
as above). The check star is GSC 616-745, observed with the magnitude
Rc=12.717+/-0.011.
The resulting light curve, showing an eclipse around 878.45:
The magnitudes of the check star have an offset of +1.8mag. Full Moon
at 50°. There is a primary eclipse around 878.45.
Session 13 Nov 2011
50 usable images were obtained.
The comparison star is again GSC 615-1135. The check star is GSC 615-1143, observed with the magnitude
Rc=12.831+/-0.010.
The resulting light curve:
The magnitudes of the check star have an offset of +1.5mag.
Session 15 Nov 2011
68 usable images were obtained.
The comparison star is GSC 615-1129 with the magnitude Rc=12.064 (computed
as above). The check star is GSC 615-1197, observed with the magnitude
Rc=12.841+/-0.010.
The resulting light curve:
The magnitudes of the check star have an offset of +1.8mag. There
is a secondary eclipse at the beginning of the session.
Phase analysis
From my data, spanning 189 rotations, the following ephemeris may be
derived for the modulation minima:
TJD(n) = T + P*n (truncated JD, corrected for the light travel time)
with T = 856.337
and P = 2.9168 hr.
Sessions 21-22 Oct 2011
The resulting phase plot for 21-22 Oct 2011, excluding the eclipse at
the beginning of Session 22 Oct:
The data for the Session 22 Oct are offset by -0.1mag (because of the
different comparison star and, may be, the different geometry of the Sun-asteroid-Earth
system).
The residuals of the light curve at the eclipse of 22 Oct 2011 is
obtained by smoothing the above phase plot (over 3 measurements) and substracting
this smooth curve from the light curve. The result is:
Sessions 12-15 Nov 2011
The resulting phase plot for 12-13 Nov 2011, excluding the eclipse
of Session 12 Nov:
The residuals of the light curve at the eclipses of 12 and 15 Nov 2011
minus the above phase plot and its smoothing is:
Astronomical note
An animation showing Isberga in the Solar System is available from the JPL.
References
Bilir S., Ak S., Karaali S., Cabrera-Lavers A., Chonis T.S., Gaskell C.M. (2007) MNRAS arXiv/astro-ph:0711.4356v1.
Molnar L.A., Haegert M.J., Beaumont C.N. et al (2008) MPBu 35 9.
Smith J.A., Tucker D.L., Kent S. et al (2002) AJ 123 2121.
Technical notes
Telescope and camera configuration.
Computer and software configuration.
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