Over the years, INTEGRAL has performed several series of
Earth-occultation observations to measure the intensity of
the cosmic X-ray background (CXB). The CXB is the overall
radiation of unresolved active galactic nuclei located at
cosmological distances. The strategy of these observations
was to allow exceptionally the Earth to pass through the
field of view of INTEGRAL. The occultation of the CXB by the
Earth would then result in a decrease of detector counts (see
Picture of the Month of
March 2006 and
November 2013).
In 2013 two Earth-occultation observations were scheduled at
the end of the revolution when the spacecraft approaches the
perigee of its orbit. This was done in July (see Picture of the
Month of
November 2013)
and December.
For the first time INTEGRAL imaged nicely the Earth with the
Optical Monitor Camera (OMC) instrument on UT 17 December 2013 00:22,
just when leaving the Earth disk, between South America and Africa
(see Figure). This was not possible during previous observations
because reflections from the highly illuminated daylight side
saturated the night side, and the few times these reflections were
blocked by the spacecraft the night side was too dark for OMC. This
time the full moon illuminating the clouds over the Atlantic,
combined with a specific geometry (i.e., no Sun illuminated Earth
disk within the OMC field of view, reflections blocked by the spacecraft)
conspired to give us this nice, unexpected image. Although the OMC
camera was not designed for such observations, even in this image
we can distinguish the Earth's atmospheric haze over the disk perimeter.
The blue cross at the centre of the OMC image corresponds approximately
to an Earth location of longitude = 18.5 deg West and latitude = 5.5 deg South,
i.e., some place in the South Atlantic Ocean. The coastlines have been
plotted in green as reference. Kourou should be somewhere at the left of
the image, more or less at the centre on the vertical axis, at the time
when GAIA was being prepared for launch.
Credits: Albert Domingo Garau (Centro de Astrobiología, CAB/CSIC-INTA, INTEGRAL/OMC-team)
