Astron. Astrophys. 356, 1-10 (2000)

2. ISO observations

PKS 2155-304 was observed with ISO between 1996 May 7 and June 8. Two additional observations were performed on 1996 November 23 and 1997 May 15.

The ISO satellite (Kessler et al. 1996) is equipped with a 60 cm Ritchey-Chrétien telescope and has four scientific instruments on board. For the PKS 2155-304 observations both the camera ISOCAM and the photometer ISOPHOT were used.

The 3232 pixel imaging camera ISOCAM (Cesarsky et al. 1996) has two detectors: an InSb CID (Charge Injection Device) for short wavelengths (SW detector; 2.5 - 5.5 µm) and a Si:Ga photoconductor array for longer wavelengths (LW detector; 4 - 17 µm). It is equipped with a set of 21 broad-band filters and a circular variable filter with a higher spectral resolution. The spatial resolution ranges from 1.5" to 12" per pixel.

The photometer ISOPHOT (Lemke et al. 1996) has three subsystems: a photo-polarimeter (PHT-P) (3 - 120 µm), which has 3 detectors, sensitive at different wavelengths, 14 broad-band filters and different apertures, from 5" to 180"; an imaging photometric camera (PHT-C) (50 - 240 µm), with a 33 and a 22 pixel detectors, a field of view of 43.5" 43.5" and 89.4" 89.4" per pixel, respectively, and 11 broad-band filters; two low-resolution grating spectrometers (PHT-S) (2.5 - 5 µm and 6 - 12 µm).

In order to determine the variability characteristics in the infrared band, 15 identical observations were performed in the period between 1996 May 7 and June 8, at 4.0, 14.3, 60, 90 and 170 µm (see Table 1 for the filter characteristics). From May 13 to May 27 ISO observed PKS 2155-304 almost each day. The observing modes (AOTs, Astronomical Observation Templates) were CAM01 (ISOCAM Observer's Manual 1994), in single pointing mode, and PHT22 (ISOPHOT Observer's Manual 1994), in rectangular chopped mode (see Appendix A.2).

Table 1. Characteristics of the filters used with ISOCAM and ISOPHOT.

On 1996 May 27 the source was observed in a large wavelength range (from 2.8 to 200 µm) with 17 different filters in order to determine the infrared spectrum. The same AOTs as before were used, except the observation with the P2_25 filter, for which the PHT03, still in rectangular chopped mode, was used.

On 1997 May 15 two 33 raster scans, centered on PKS 2155-304 (R.A. 21h 58m 52s, Dec -30^o 13´ 32") were performed with the photometric camera PHT-C, at 60 µm and at 180 µm; the distance between two adjacent raster positions was 180", in order to have an almost complete sky coverage of an area of 9´ side. This mapping was performed to search for any structure in the cirrus clouds; a non flat background could compromise a reliable photometry of the source. In this observation the AOT PHT22 was used in staring mode.

The ISOPHOT observation of 1996 May 25 failed because of problems during the instrument activation.

The complete log of the observations is shown in Tables 2 and 3.

Table 2. ISOCAM observation log.

Table 3. ISOPHOT observation log.

© European Southern Observatory (ESO) 2000

Online publication: March 28, 2000
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