Astron. Astrophys. 355, 885-890 (2000)

2. Observations

Cen A was observed with the LWS grating () as part of the LWS consortium's guaranteed time extragalactic programme. A full grating observation (43 - 196.7 µm) was taken of the nucleus at the centre of the dust lane and a series of line observations were taken at two positions in the SE and NW regions of the dust lane. A short [CII] 157 µm line observation was taken off-source at position #4 (see Table 1) to estimate the Galactic emission near the source. Position #1 was intended to provide a deeper integration coincident with position #2, but was accidently offset.

Table 1. Observation log.
Notes:
Offsets w.r.t. 13h 25m 27.6s -43d 01 ´ 08.6 " J2000

Fig. 1. Cen A digital sky survey image overlaid with the LWS beam positions

A series of half-second integration ramps were taken at each grating position with four samples per resolution element (m m and m m). The total integration time per resolution element and per pointing were: position #1 88 s for the [OIII] 52 µm and 34 s for the [NIII] 57 µm; position #2 (the centre), 30 s for the range 43-196 µm; positions NW and SE (2 point raster map) 22 s for the the [OI] 63 µm, 14 s for the [OIII] 88 µm, 12 s for the [NII] 122 µm, 28 s for the [OI] 145 µm and 12 s for the [CII] 158 µm; position #4 12 s for the [CII] 158 µm.

The data were processed with RAL pipeline 7 and analysed using the LIA and ISAP packages. The LWS flux calibration and relative spectral response function (RSRF) were derived from observations of Uranus (Swinyard et al. 1998). The full grating spectrum at the centre enabled us to estimate the relative flux uncertainty between individual detectors arising from uncertainties in the relative responsivity and the dark-current subtraction. The offsets between the detectors (excluding detector SW1) was %. The [OIII] 88 µm line on detectors SW5 and LW1 had a 15% systematic uncertainty and the [CII] line on detectors LW3 and LW4 had a 10% systematic uncertainty. We therefore adopt a relative flux uncertainty of 15%. Because we only took spectra of individual lines at the NW and SE positions there is no corresponding overlap in wavelength coverage at these positions. One indicator of relative flux uncertainty is a discrete step down in flux, of 25%, at 125 µm at the SE position. The relative flux uncertainty is assumed to be 25% at these positions.

The absolute flux calibration w.r.t. Uranus for point like objects observed on axis is better than 15% (Swinyard et al. 1998). However, extended sources give rise either to channel fringes or to a spectrum that is not a smooth function of wavelength. This is still a calibration issue. For example, in Fig. 2, detectors SW5, LW1, LW2 have slopes that differ from those of their neighbours in the overlap region. This may account for the continuum shape, which is discussed in Sect. 3.1. The LWS beam profile is known to be asymmetric and is still under investigation. We therefore adopt a value for the FWHM of 70 " at all wavelengths, believing that a more sophisticated treatment would not significantly affect our conclusions. We also note that there is good cross calibration between the ISO-LWS results and the Far-infrared Imaging Fabry-Perot Interferometer (FIFI) (Madden et al. 1995); the [CII] peak fluxes agree to within 10%.

Fig. 2. LWS spectrum of the central region of the dust lane (dashed line is the blackbody fit)

© European Southern Observatory (ESO) 2000

Online publication: March 21, 2000
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