Astron. Astrophys. 363, 863-868 (2000)

2. Observation and data reduction

BeppoSAX (Boella et al. 1997) observed Mrk 766 on 17-18 May, 1997 as part of a Core Program devoted to a spectral survey of bright Seyfert 1 galaxies. The exposure time was 79 ksec for the MECS, the HPGSPC and the PDS and 42 ksec for the LECS. We discuss here data from the LECS, MECS and PDS only, because the source is too faint for the HPGSPC to be profitably used.

Spectra and light curves were extracted from the imaging instruments within circles of 8' (LECS) and 4' (MECS) radii centred on the source. Background spectra taken in the same regions from blank sky observations have been used for subtraction. Regarding the PDS, background subtracted spectra automatically generated at the SAX Scientific Data Centre (SDC) have been used. Fixed Rise Time thresholds have been adopted.

Time-averaged count rates are: 0.27650.0026 (LECS, 0.1-10 keV); 0.26240.019 (MECS, 1.5-10 keV, 2 units ¹); 0.28630.0038 (PDS, 15-200 keV). The 2-10 keV mean flux (obtained with model 5 in Table 1 and Table 2) is 2.0510^-11 erg cm^-2 s^-1.

Table 1. Best fit parameters for the first half of the observation

Table 2. Best fit parameters for the second half of the observation

In the field of view of the PDS there are two possible confusing sources, both of them BL Lac objects. At an angular distance of about 20 arcminutes there is ON 325. The source is clearly visible in the MECS, with a count rate of about 1/20 that of Mrk 766. The faintness of the source and its off-axis position do not permit a detailed spectral analysis, but the MECS spectrum appears to be rather steep, and therefore its contribution to the PDS is likely to be very small. More serious is the case of the other source, 2A 1219+305 (PG 1218+304), a member of the Piccinotti sample (Piccinotti et al. 1982) with a mean 2-10 keV flux of 3 erg cm^-2 s^-1 and a 2.5 detection with BATSE (Malizia et al. 1999). The source is about 44' arcminutes away from Mrk 766, i.e. outside the MECS field of view, and it is therefore not possible to estimate its contribution to the PDS, which may be substantial. For this reason we preferred to exclude the PDS from the spectral fitting, using the data a posteriori only as an upper limit to the hard X-ray emission of Mrk 766.

© European Southern Observatory (ESO) 2000

Online publication: December 5, 2000
helpdesk@link.springer.de