Disk Plus Halo Models of Gamma-Ray Burst Sources
Smith, I. A., & Lamb, D. Q.
We show that, in principle, approximately 70% of the gamma-ray bursts observed
by BATSE can come from local (< 1 kpc), galactic disk neutron stars, with the
rest in an extended galactic halo.
We consider three possible forms for the distribution of the
galactic halo sources: a Gaussian halo, an exponential halo, and a
standard ``dark matter'' halo.
We find that for the Gaussian halo the fraction of bursts that can come
from the galactic disk can be approximately 2/3, close to the maximum
possible value; for exponential and dark matter halos,
the fraction can be approx 1/2 and approx 1/5 respectively.
As examples, we consider two particular disk-halo combinations.
In both, we use an exponential disk with scaleheight z_0, and BATSE can
detect disk sources to a distance D_d = (2/3) z_0.
In the first example, we combine this with a Gaussian halo whose peak
is at R_s = 25 kpc, and sigma = 38 kpc.
The Earth is displaced R_0 = 8.5 kpc from the Galactic Center,
and BATSE can detect halo sources to a distance D_h = 200 kpc.
We take 66.8% of the sources to be in the disk.
For the second example, we use a ``dark matter'' halo that has a ``core
radius'' R_c = 22.5 kpc, R_0 = 8.5 kpc, and D_h = 135.0 kpc;
20.2% of the sources are taken to be in the disk.
In each case, the values of < V/V_max >, < sin^2b >, < cos theta >,
and the C_max/C_min distribution are all easily consistent with the
BATSE observations.
Dividing the bursts into three, equal-sized groups of the
brightest, intermediate, and faintest, there is little difference in the
values of < sin^2b > and < cos theta >, agreeing with the BATSE observations.
The disk sources have luminosities of the order of 10^{36-37} ergs s^-1,
while those in the halo have luminosities 10^{41-42} ergs s^-1.
The brightest observed bursts must come from the halo;
this suggests that the bursts that exhibit cyclotron
features come from halo sources.
Status:
1993, ApJ, 410, L23.
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