2 Observation

 

The main ISOCAM technical characteristics are presented in (Cesarsky 1996). The LW channel of ISOCAM operates between 4 and 18 $\mu$m. A lens wheel allows the selection of the field of view per pixel (1.5, 3, 6 and 12 arcsecs), and a filter wheel the selection of the spectral band pass (10 broad band filters and two Continuously Variable Filters). The detector is a 500 $\mu$m thick crystal made of gallium doped silicon photo-conductor hybridized by Indium bumps. The $32\times 32$ square pixels are defined with a pitch of 100 $\mu$m.

A given observation (characterized by a given configuration (lens, filter, integration time)) is presented as a collection of $32\times 32$ images (called readouts) gathered together in a data cube. Three different observational modes were available with ISOCAM (Siebenmorgen et al. 1997): 1) single pointing, raster mode, 2) beam switching and 3) spectrophotometry. The observations presented in this paper were obtained in raster mode where many readouts are put together to build a mosaic of the observed object (called the sky image). The scanning strategy is made in such a way that each readout taken on the sky has some overlap with its neighbors (see Fig. 1).

To illustrate the data reduction procedures, we use two ISOCAM observations of the gamma-ray burst GRB 970402 first observed by BeppoSAX (Feroci et al. 1997) on April 2.93 UT 1997. The coordinates (J2000) of the burst were $\alpha = 14^{\rm h}50^{\rm m}16^{\rm s}$, $\delta=-69^\circ19\hbox{$.\mkern-4mu^\prime$ }9$ with an error circle of radius 3'. Target-of-opportunity ISO observations (ISOCAM and ISOPHOT) were requested by Castro-Tirado et al. (1998) to detect a transient infrared emission of this burst. Unfortunately, the observations did not show such emission. Nevertheless, these particular observations are of great interest for us since the same field was observed twice, subsequently within the same orbit, in exactly the same configuration, to look for a decrease in the GRB emission. The observational strategy of the observations is sketched in Fig. 1 and described in Table 1. Comparing the result of our processing on both observations (GRB1 and GRB2 in the following) allows us to validate the method.

Figure 1: Raster observing strategy. The crosses indicate the positions on the sky of the camera center. The first (solid line) and the sixteenth (dash line) positions of the camera are shown as square. The arrow shows the scanning direction