Gamma-ray bursts: 3rd Huntsville symposium. AIP Conference Proceedings
If gamma-ray bursts originate from a galactic source population, then at some level a galactic pattern must exist in their locations. Expected patterns for galactic sources are a concentration towards the galactic center, measured by the mean dipole moment of the locations towards the center, cosθ, where the θi are the angles between the burst locations and the galactic center, or a concentration towards the galactic plane, measured by the mean quadrupole moment about the plane, sin2 b− 1/3 , where the bi are the galactic latitudes of the locations (23,2). To date, neither pattern has been found in the BATSE data: the values cosθ = 0.011 ± 0.017 and sin2 b − 1/3 = 0.002 ± 0.009 (these values have been corrected for BATSE’s nonuniform sky exposure) for the 1122 bursts of the 3B catalog are both consistent with zero and thus with isotropy (21). The dominant uncertainty in these values is due to the ﬁnite sample size (4). What galactic signatures could be hidden under these uncertainties? The tight limits on the quadrupole moment, in conjunction with the fall-oﬀ in the number of faint sources, rule out a disk origin for the majority of the sources (20,9,3,4). Galactic models which remain under discussion either consist of a extended halo or of multiple components. A halo consistent with the data must be much larger than the solar galactocentric distance of R◦ = 8.5 kpc so that the dipole moment will be suﬃciently small. We can determine the typical scale by considering a very simple halo: a galactocentric shell of radius Rshell. Such a shell has a dipole moment (13): cosθ =2/3 R◦/Rshell. Using the dipole moment of the 3B catalog (above), we obtain a 2σ lower-limit for Rshell of 120 kpc. Any GRBs inside of this radius will have to be balanced with sources located farther away. In the remainder of this paper we compare galactic models which have pub-lished moments with the observed moments of the 3B catalog. The procedures and the models are discussed in greater detail in an earlier work, which used a smaller sample of GRBs (4).
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