SPEX User's Manual - SRON

56 Spectral Models
3. set=3: for R V = 5.5.
Warning: For any instrument where the extraction region has a size comparable to the size of the dust
scattering halo, this model should not be used, as the scattered X-rays will fall within the exctraction
region. Take care when fitting data from different instruments simultaneously.
Warning: This model only calculates the dust scattering, not the dust absorption
The parameters of the model are:
nh - Hydrogen column density in 10 28 m −2 . Default value: 10 −4 (corresponding to 10 24 m −2 , a typical
value at low Galactic latitudes).
f - The covering factor of the absorber. Default value: 1 (full covering)
set - The set of cross sections being used. See table above.
3.10 Etau: simple transmission model
This model calculates the transmission T(E) between energies E1 and E 2 for a simple (often unphysical!)
case:
T(E) = e −τ(E) , (3.12)
with the optical depth τ(E) given by:
τ(E) = τ 0 E a . (3.13)
In addition, we put here T ≡ 1 for E < E 1 and E > E 2 , where E 1 and E 2 are adjustable parameters.
This allows the user for example to mimick an edge. Note however, that in most circumstances there are
more physical models present in SPEX that contain realistic transmissions of edges! If you do not want
or need edges, simply keep E 1 and E 2 at their default values.
Note that τ 0 should be non-negative. For a > 0 the spectrum has a high-energy cut-off, for a < 0 it has
a low-energy cut-off, and for a = 0 the transmission is flat. The larger the value of a, the sharper the
cut-off is.
The model can be used a s a basis for more complicated continuum absorption models. For example, if
the optical depth is given as a polynomial in the photon energy E, say for example τ = 2 + 3E + 4E 2 ,
one may define three etau components, with τ 0 values of 2, 3, and 4, and indices a of 0, 1 and 2. This is
because of the mathematical identity e −(τ1+τ2) = e −τ1 × e −τ2 .
The parameters of the model are:
tau0 - Optical depth τ 0 at E = 1 keV. Default value: 1.
a - The index a defined above. Default value: 1.
e1 - Lower energy E 1 (keV). Default value: 10 −20 .
e2 - Upper energy E 2 (keV). Default value: 10 20 .
f - The covering factor of the absorber. Default value: 1 (full covering)
3.11 Euve: EUVE absorption model
This model calculates the transmission of gas with cosmic abundances as published first by Rumph et al.
([1994]). It is a widely used model for the transimission at wavelenghts covered by the EUVE satellite
(λ > 70 Å). As it these wavelengths metals are relatively unimportant, it only takes into account hydrogen
and helium, but for these elements it takes into account resonances. However it is not recommended to
use the model for harder X-rays, due to the neglect of metals such as oxygen etc.
Otherwise the user is advised to use the ”absm” model or the ”hot” model with low temperature in
SPEX, which gives the transmission of a slab in collisional ionisation equilibrium.
The parameters of the model are:
nh - Hydrogen column density in 10 28 m −2 . Default value: 10 −4 (corresponding to 10 24 m −2 , a typical
value at low Galactic latitudes).