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Short gamma-ray bursts from binary neutron star mergers in globular clusters


Observations by the Swift gamma-ray-burst (GRB) mission located short GRBs in (or near) elliptical galaxies, that are no longer active in star formation. This suggested that short GRBs are produced when neutron stars (NSs) merge with other NSs or with black holes (BHs). However, the spatial offset of some short GRBs from their host galaxies is not consistent with double-neutron-star (DNS) systems formed from massive binary stars, which appear to remain in galactic disks. Instead, short GRBs may arise from NS mergers in compact binary systems that are naturally produced in globular clusters, in which extreme densities of very old stars can create and exchange compact binaries efficiently. Here we present a simple scaling from the DNS binary observed in the globular cluster M15 in our own Galaxy to the numbers expected for globular clusters around galaxies generally. We present numerical simulations that demonstrate that DNS production in globular clusters may account for 10–30% of the observed short GRBs. The much more numerous DNS merger rates predicted for galactic disks suggests their associated short GRBs are significantly more beamed, perhaps by the aligned spins and greater magnetic field of their secondary NSs.

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Figure 1: Cross-sections calculated for an incoming NS to exchange into a cluster compact binary composed of a NS and a secondary with masses indicated.
Figure 2: Distribution of binary period versus eccentricity for DNS systems formed by NS exchange into LMXBs with 0.4M secondaries.

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J.G. thanks N. Gehrels for an initial discussion of the first short GRB observed with Swift. This work was supported in part for J.G. by NASA grant NNG04GK33G and for S.M. by NASA grant NNG04GL50G. S.P.Z. is supported in part by the Royal Netherlands Academy of Arts and Sciences (KNAW) and the Netherlands Advanced School for Astronomy (NOVA). The authors thank Ed van den Heuvel for comments.

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Correspondence to Jonathan Grindlay.

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Grindlay, J., Zwart, S. & McMillan, S. Short gamma-ray bursts from binary neutron star mergers in globular clusters. Nature Phys 2, 116–119 (2006).

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