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Astron. Astrophys. 331, 519-523 (1998)

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2. Observation and data reduction

The BeppoSAX X-ray observatory (Boella et al. 1997a), a major programme of the Italian Space Agency with participation of the Netherlands Agency for Aereospace Programs, was launched on April 30th 1996 from Cape Canaveral in Florida. The payload instruments are characterized by a wide spectral coverage and consist of four co-aligned Narrow Field Instruments (NFI) plus two Wide Field Cameras perpendicular to the axis of the NFI and looking in opposite directions.

The NFI include a Low Energy Concentrator Spectrometer (LECS; Parmar et al. 1997), three Medium Energy Concentrator Spectrometers (MECS; Boella et al. 1997b), a High Pressure Gas Scintillation Proportional Counter (HPGSPC; Manzo et al. 1997), and a Phoswich Detector System (PDS; Frontera et al. 1997). LECS and MECS are instruments with imaging capabilities (angular resolution [FORMULA] 1[FORMULA]) and operate in the 0.1-10 keV and 1.5-10 keV spectral bands respectively. In the overlapping energy interval the effective area of the LECS is about one third of the three MECS (which is around 150 cm2 at 6 keV). HPGSPC and PDS are instruments functioning via rocking collimators and cover the 4-120 keV and 15-300 keV energy bands respectively. In the overlapping energy band PDS is the most sensitive (by a factor [FORMULA]) while HPGSPC is optimized for spectral resolution.

BeppoSAX NFI pointed at NGC 7674 from November 25th to November 26th 1996 for about [FORMULA] s of effective observing time. The on-source time for the LECS was [FORMULA] 40 ksec since this instrument is switched on only during satellite night time, while the on-source time for PDS and HPGSPC was [FORMULA] 50% of the effective time due to rocking collimator functioning.

LECS and MECS good time intervals were selected imposing the minimum elevation angle above Earth's limb to be [FORMULA] and the instrument to be in its nominal configuration 1. Given the weak 2-10 keV source flux (see next section) LECS and MECS spectra were extracted from a 2[FORMULA] radius region around the centroid of the source and the appropriate response matrices were generated. The same region was used to extract background spectra from blank sky observation files. The use of local instead of blank sky backgrounds did not make significant difference.

The PDS consists of four phoswich units. They operate in collimator mode, with two of them pointing to the source while the other two point [FORMULA] away. The two pairs switch on and off source every 96 seconds. The net source spectra have been obtained by subtracting the 'off' to the 'on' counts. PDS source data have been multiplied by a factor 1.42 to account for the mismatch in the absolute normalization with the MECS (Matt et al. 1997, and references therein). Leaving this number free to vary would only slightly change the best fit parameters, without affecting the main results of the present analysis. The net source count rate in the PDS was [FORMULA] cts/s between 13 and 60 keV, which gives a significant detection also after conservative subtraction of the systematic residuals which are currently evaluated [FORMULA] cts/s in the 13-200 keV band (Guainazzi and Matteuzzi 1997).

The net source count rate (in units of [FORMULA] cts/s) was [FORMULA] in the LECS (0.1-9 keV), [FORMULA] in the three MECS (2-10 keV), while it was below detection limit in the HPGSPC. Neither the soft ([FORMULA] 0.1-2 keV) nor the hard (2-10 keV) energy band data show significant variability. Statistical [FORMULA] tests against constancy gave a probability greater than 25%, thus consistent with a constant source. The spectra were thus extracted from the overall observing period and were rebinned to have a minimum of 20 counts in each channel.

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© European Southern Observatory (ESO) 1998

Online publication: February 16, 1998