Journal Article

The effects of <i>r</i>-process heating on fallback accretion in compact object mergers

B. D. Metzger, A. Arcones, E. Quataert and G. Martínez-Pinedo

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 402, issue 4, pages 2771-2777
Published in print March 2010 | ISSN: 0035-8711
Published online March 2010 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2009.16107.x
The effects of r-process heating on fallback accretion in compact object mergers

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We explore the effects of r-process nucleosynthesis on fallback accretion in neutron star (NS)–NS and black hole–NS mergers, and the resulting implications for short-duration gamma-ray bursts (GRBs). Though dynamically important, the energy released during the r-process is not yet taken into account in merger simulations. We use a nuclear reaction network to calculate the heating (due to β decays and nuclear fission) experienced by material on the marginally bound orbits nominally responsible for late-time fallback. Since matter with longer orbital periods torb experiences lower densities, for longer periods of time, the total r-process heating rises rapidly with torb, such that material with torb≳ 1 s can become completely unbound. Thus, r-process heating fundamentally changes the canonical prediction of an uninterrupted power-law decline in the fallback rate at late times. When the time-scale for r-process to complete is ≳1 s, the heating produces a complete cut-off in fallback accretion after ∼1 s; if robust, this would imply that fallback accretion cannot explain the late-time X-ray flaring observed following some short GRBs. However, for a narrow, but physically plausible, range of parameters, fallback accretion can resume after ∼10 s, despite having been strongly suppressed for ∼1–10 s after the merger. This suggests the intriguing possibility that the gap observed between the prompt and extended emission in short GRBs is a manifestation of r-process heating.

Keywords: nuclear reactions, nucleosynthesis, abundances; gamma-rays: bursts

Journal Article.  5281 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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