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Astron. Astrophys. 362, 273-280 (2000)

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2. Observation and data reduction

The spectroscopic observation was carried out with the SWS spectrometer (de Graauw et al. 1996) of the ISO satellite on July 31, 1997 with the second fastest speed covering full grating scan from 2.3 to 45 µm (SWS AOT 01, speed 2).

The Off Line Processing (OLP) version 8.6 data were corrected for dark current, up-down scan differences, flat-fielded and flux calibrated by using the Observers SWS Interactive Analysis (OSIA) package. The deglitching and averaging to equidistant spectral points in wavelengths across the scans and detectors was done by using the ISO Spectral Analysis Package (ISAP) package. The spectral resolution for the following analysis is taken to be 300.

The ISOCAM imaging was performed on July 31, 1997 with the CAM camera in mode AOT 01 with the LW3 filter centered at 15 µm. The data was reduced by using the ISOCAM Interactive Analysis (CIA) package. The source is point-like in the ISOCAM image. The flux through the filter LW3 measured by using the aperture photometry method is 14.38 Jy. Because of the photometric uncertainties and the waveband difference between the ISO-CAM LW3 and the IRAS 12 µm filters, the ISOCAM result is consistent with that of IRAS (13.77 Jy at 12 µm). This agreement shows that the object is probably not a large-amplitude variable, which is consistent with the variability index 29 from the IRAS photometry.

The spectra obtained from the OSIA analysis for band 3A, 3C and 3D (from 12.0 to 27.5 µm) are quite noisy and for further analysis only data from the best 6 to 7 detectors (out of 12) are used. The data for bands 3E and 4 (from 27.5 to 45.2 µm) are of poor quality and they are not taken into consideration. Among the reduced SWS data there are small discrepancies between consecutive bands. Namely, the flux levels of bands 2C, 3C and 3D are low in comparison with other bands. In order to correct for deficient flux (about 2 Jy) of band 2C, the spectrum in the same wavelength range as the photometric range of IRAS 12 µm filter is normalized to match with the photometric results from the IRAS PSC. The scaling factor of 1.15 applied to band 2C results in a correct simulated IRAS 12 µm flux and produces smoother transitions between neighbouring bands. At the same time, a factor of 1.245 was applied to the IRAS LRS spectrum in order to match with the IRAS photometric result at 12 µm. Since the SWS data beyond 27.5 micron are not reliable, the procedure of normalization to the IRAS 25 µm band flux cannot be applied. Therefore, bands 3C and 3D were scaled with respect to band 3A by multiplicative factors of 1.25 and 1.15 respectively, just to make the overall shape of the SWS spectrum smooth. Because the flux calibration of SWS observation has an uncertainty of 20-30 %, such corrections seem to be reasonable. The reliable part of the SWS spectrum (bands 1A-3D), after the flux adjustments described above, is displayed in Fig. 1 by a solid line. The dotted line represents the IRAS LRS spectrum multiplied by factor 1.245 and filled squares mark the IRAS PSC fluxes at 12 and 25 µm. As can be seen, the overall agreement between both sets of spectroscopic data is satisfactory.

[FIGURE] Fig. 1. The ISO-SWS spectrum of IRAS 03201+5459 (solid line) together with the IRAS LRS spectrum (dotted line) and the IRAS PSC photometric data (filled squares). C-bearing features are labeled by vertical solid lines, while O-bearing ones are labeled by vertical dotted lines.

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© European Southern Observatory (ESO) 2000

Online publication: October 30, 19100