2. Spectral model
The spectral modelling is done using a series of "slabs" which are set up to represent hot gas emission regions and cold gas absorbing layers (Sidher et al. 1996, see also Pietz et al. 1998 and references therein for similar analyses). In this work the furthestmost slab defines the extragalactic component of the SXRB, which is taken to have a power law source spectrum of spectral index 1.4. The next two slabs contain a hot gas component, which we associate with the Galactic halo, and the cold absorbing Galactic disk (see Sidher et al. 1996 for justification of this). The closest slab to us then contains the hot gas component within the local cavity plus a small amount of intermixed absorbing material. The radiation transfer function through each slab is
where I i(E ) is the incident radiation entering the shell from the outside, E is the photon energy, (E ) is the energy dependant absorption cross-section, n e is the electron density, n H is the H I column density, T is the temperature and l is the depth of the shell. P (E,T ) is the radiative power loss function for a plasma at a temperature T . In this work the Landini & Monsignori Fossi plasma code has been used to model the emission from the hot gas components in the halo and the local cavity (Landini & Monsignori Fossi 1990). The H I absorption column in the cold gas of the disk has been derived for each pointing direction using the Stark catalogue (Stark et al. 1992; For a comparison with the more recent Leiden/Dwingeloo 21-cm line survey (Hartmann & Burton 1997) see the discussion in Sect. 3.1). The absorption cross-section as a function of energy is taken from Morrison & McCammon (1983). To economise on computing time the local cavity hot gas temperature is kept fixed at log T = 5.9 with an intermixed column density of 6.6 . 1018 cm-2 (see Juda et al. 1991 and Lieu et al. 1992). During the fitting procedure the emission measure (EM) for both the local cavity and the halo component are treated as free variables. In addition the temperature of the halo component is allowed to vary. For each combination of parameters the spectrum arriving at the telescope aperture is modelled with a 1 eV resolution between 50 eV and 3 keV. This modelled spectrum is then convolved with the ROSAT PSPC energy response function and the predicted spectrum is binned at 50 eV intervals. The predicted binned spectrum is then compared with the observed spectrum over the entire PSPC energy range using standard .
© European Southern Observatory (ESO) 1999
Online publication: March 10, 1999