Cyclotron Resonant Scattering in the Spectrum of GB870303
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**Session 15 -- Gamma Ray Bursts**
*Display presentation, Monday, 9, 1995, 9:20am - 6:30pm*

## [15.11] Cyclotron Resonant Scattering in the Spectrum of GB870303

*P.~E.~Freeman, D.~Q.~Lamb (U.~Chicago), J.~C.~L.~Wang (U.~Maryland), I.~Wasserman, E.~E.~Salpeter (Cornell U.), E.~E.~Fenimore (LANL), T.~Murakami, J.~Nishimura (ISAS), A.~Yoshida (IPCR), I.~Kondo (SIT)*
We use Monte Carlo radiation-transfer calculations to fit dips seen at
$\approx$ 20 keV in the 4 s spectrum S1, and at $\approx$ 20 and 40 keV
in the 9 s spectrum S2, of the *Ginga*
data for GB870303.
These lines are thought to be due to cyclotron scattering in a cool
scattering layer ($T \sim$ 5 keV) near the surface
of a strongly magnetic ($B \approx$ 10$^{12}$~G) neutron star.
Previous theoretical Monte Carlo calculations of
cyclotron scattering lines (Wang *et al.*
1989)
assumed a plane-parallel geometry with
a constant $B$-field
perpendicular to a slab with electron column density $N_e$.
This assumption is only valid if the
line-forming region is a small area at the magnetic pole.
We generalize this approach by letting
the $B$-field to lie at an arbitrary angle $\Psi$ relative
to the slab normal, which allows us to explore line-emitting regions covering
an arbitrary fraction of the neutron star surface.
For several plane-parallel slab geometries with the continuum source either
below or within the slab,
we calculate best-fit values and confidence intervals for $B$, $\Psi$,
$N_e$, and the viewing angles of the observer relative to the slab normal
($\theta$,$\phi$), for both GB870303 S1 and S2.
The confidence intervals allow us to place limits on the size and location
of the emitting region; also, analysis of the difference in fit parameters
between S1 and S2 allows us to place constraints on the rotation period of
the neutron star.
\bigskip

This research has been supported in part by NASA Grants NGT-50778,
NAGW-830, NAGW-1284, and NAG5-1454.

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