We discuss here the BeppoSAX observation of GRS 1915+105
started on April 19, 1998 at 11:33:19 UT and ended on April 20, 1998
at 20:14:52 UT (Figure 1).
Out of the entire ~ 120
ks observation,
some time intervals were sorted out where the source variability
is less pronounced. These are the six orbits starting from 3x104
s after the beginning of the observation.
Data from the LECS, MECS and PDS instruments have been used
in the intervals 0.1-4, 1.7-10 and 15-150 keV, respectively.
We assumed an optically thick, neutral, and geometrically thin
Keplerian accretion disk with solar elemental abundances.
Spectral fits are performed with the XSPEC v.11 software package.
The basic ingredient to model the continuum is a powerlaw with
cutoff (cutoffpl).
As a model for the locally emitted fluorescent and reflection features
we initially considered numerically computed spectra including
the Fe line and the
underlying reflected continuum, derived from Monte Carlo computations
(Matt et al. 1991) and processed through our additive routine
kerrspec, thus with a fully GR treatment.
After having verified, however, that the model
is quite insensitive to the Compton-reflected continuum component
(the latter's normalization tends to be very small)
and that the results are almost insensitive to GR corrections,
to save computational time we
adopted a simple, non-relativistic model for this component
(pexrav in XSPEC), and used kerrspec
only to fit the line profile.
Since, looking at preliminary fit results, the dependence of the disc
emissivity on the radius results to be rather flat,
we assumed a phenomenological dependence ~ R
-2.
Applying these ingredients on MECS and PDS data only,
Rin stays always above 6m (cp. results
in Table 1; Martocchia et al. 2001).
The iron line parameters do not significantly
change with more refined assumptions for the continuum, for instance
accounting for LECS data and including a multicolor blackbody component
(diskpn), which is expected since
we are dealing with a galactic BHC. However, the disc thermal emission is
not very strong. Furthermore, the estimated disc
temperature comes out unrealistically high (kT ~ 3.3
keV),
while the BH mass comes out far too small from a physical point of
view. These facts indicate that the standard interpretation of the
continuum is not fully appropriate here.
However, because of the distorted profile (cp. Figure 2, which
shows the line residuals in the fifth interval of the observation),
the fit with a relativistic line is always
better than with a gaussian. We therefore infer that the relativistic
solution for the iron line, besides being the most natural physical
explanation for the very broad line observed, seems not to depend from
the choice of the continuum model.