Observations with INTEGRAL consist of a series of pointings, each
lasting a few ksec. During this time, an image representing the shadow
of the coded mask is formed on the detector plane. The position of the
observed source in the sky can then be reconstructed by means of a
cross-correlation. INTEGRAL also collects data during the periods of
time between one pointing and the next. During these 'slews', the
satellite's pointing direction changes continuously preventing the
analysis with the standard cross-correlation technique.
The work on the slew survey makes use of a back projection approach to
analyse slew data. Each detected photon must have passed through one of
the apertures in the coded mask. The knowledge of the instrument
geometry and the coded mask are used to trace back each detection to all
the regions of the sky that the corresponding photon could have come
from. By tracing a great number of photons from different positions in
the detector, the location of the source is reconstructed. Depending on
the speed of the pointing axis, it is estimated that sources brighter
than 50 to 70 mCrab can be observed in a single slew.
The analysis of slew data obtained by INTEGRAL will significantly
contribute to the study of hard X-ray transient sources and the
long-term monitoring of transient activity of persistent sources. Slew
data will extend the coverage of regions of the sky that aren't
regularly observed and provide additional data points for long-term
light curves. Access to the slew data collected to this point in time
will provide a total of about 30 Ms (about 150 orbits) of extra data.
Image, left: Movie of significance images from 58 slew observations from
INTEGRAL Revolution 80. The pointing axis of the telescope was following
a dithering pattern around the position of black-hole high-mass X-ray
binary Cyg X-1. The varying intensity visible in the bottom panel does
not (only) represent the intrinsic variability of the source, but is due
to the different significances of the detection for science windows of
different duration. A full calibration of the technique is under
development and will provide normalised fluxes. Ghost images caused by
the repeating nature of the coded mask appear around the physical source.
Image, top right: Slew exposure map for the whole IBIS/ISGRI data set.
Image, bottom right: Mosaic image of the 58 science windows used in the
movie. The X-ray binaries Cyg X-3 and EXO 2030+375 are also visible in
the stacked image.
