2. Observations and data reduction
VW Hyi is monitored at optical wavelengths by the American Association of Variable Star Observers (AAVSO). On Sep 23 1998 the optical magnitude of VW Hyi started to decrease. The outburst lasted for 5-6 days and reached a peak magnitude of 9.2. This outburst served as a trigger for a sequence of six observations by BeppoSAX between Sep 24 and Oct 18. As a result we have obtained one X-ray observation during outburst and five observations during quiescence.
Since VW Hyi appears as an on-axis source, the Low Energy Concentrator Spectrometer (LECS, Parmar et al. 1997) source counts are extracted from a circular region with a 35 pixel radius centered at the source. We use the Sep 1997 LECS response matrices centered at the mean raw pixel coordinates (130,124) for the channel-to-energy conversion and to fold the model spectra when fitted to the data. The combined Medium Energy Concentrator Spectrometer (MECS2 and MECS3, Boella et al. 1997) source counts are extracted from a circular region with a 4´ (30 pixel) radius. The September 1997 MECS2 and MECS3 response matrices have been used. These matrices are added together. The background has been subtracted using an annular region with inner and outer radii of 35 and 49.5 pixels for the LECS and 30 and 42.5 pixels for the MECS, around the source region.
We ignore the data of the High Pressure Gas Scintillation Proportional Counter (HPGSPC, Manzo et al. 1997) and the Phoswitch Detection System (PDS, Frontera et al. 1997) since their background subtracted spectra have a very low signal to noise ratio.
The LECS and MECS data products are obtained by running the BeppoSAX Data Analysis System pipeline (Fiore et al. 1999). We rebin the energy channels of all four instruments to of the spectral resolution and require a minimum of 20 counts per energy bin to allow the use of the chi-squared statistic. The total LECS and MECS net exposure times are 82.5 ksec. and 181.4 ksec. respectively. The factor 2.2 between the LECS and MECS exposure times is due to non-operability of the LECS on the daytime side of the earth.
© European Southern Observatory (ESO) 1999
Online publication: September 2, 1999