Electromagnetic transients from radioactive decays accompanying short gamma-ray bursts
Final Report Abstract
Neutron star mergers are one of the main, virtually guaranteed, sources of gravitational waves (GWs). Moreover, they are thought to trigger short gamma-ray bursts and the research of the last decade has confirmed that they are excellent candidates for the production site of at least a very substantial part of the heaviest (”r-process”) nuclei. One of our main goals in this project was to further explore the heavy element formation in neutron star mergers. We wanted to know in particular how sensitive the resulting abundance patterns are to the parameters of the merging binary system and to the incompletely understood physics of extremely neutron-rich nuclei. In a first set of calculations we systematically explored the dependence on the parameters of the merging binary systems. Confirming earlier results, we found that the ”dynamic ejecta” produce essentially only very heavy nuclei with nucleon numbers A > 130. Interestingly, the abundance pattern shows an enormous robustness with respect to the merging binary system: essentially all binary systems produce the same pattern. This agrees very well with the observation of metal-poor stars which also show a very robust pattern for the heaviest r-process elements. We have further explored how sensitive the resulting abundance pattern is to variations in nuclear physics. We found that in particular that the region after the second r-process peak is heavily impacted by the fission fragment distribution and we also explored how β-decay rates impact on the exact position of the so-called ”platinum peak” (A ≈ 195). Going beyond the original goal, we have begun to explore neutrino-driven winds as an additional nucleosynthesis source. We found that such winds complement the ejecta with lighter (A < 130) r-process material and actually in very substantial amounts. The second major goal was to explore which electromagnetic (EM) transients go along with merging neutron star systems. Such transients are crucial for pinpointing the sky locations of gravitational wave events. We explored radio transients that appear several months after a merger event when the ejecta dissipate their kinetic energy in the ambient medium. The major focus, however, was on transients that are powered by the radioactive decay of freshly synthesised heavy elements. We found that such transients peak approximately one week after the merger in the near-infrared. Very shortly after our result appeared on the web-archive, such radioactive transients were actually observed for the first time, with properties close to our predictions. The PI of this proposal was asked by the Nature Magazine to place this discovery in a broader context (S. Rosswog, Nature News and Views, Nature 500, 2013, 535). We have further explored similar EM transients that are caused by the neutrino-driven winds. These winds peak even faster (hours after the merger), they are bluer and more luminous than those from dynamic ejecta. Such transients will be pivotal for pinpointing the sky localisations of future GW sources.
Publications
- On the astrophysical robustness of the neutron star merger r-process. MNRAS 426, 1940 (2012)
Rosswog, Arcones, Winteler
(See online at https://doi.org/10.1111/j.1365-2966.2012.21859.x) - The electromagnetic signals of compact binary mergers. MNRAS 430, 2121 (2013)
Piran, Nakar, Rosswog
(See online at https://doi.org/10.1093/mnras/stt037) - The multimessenger picture of compact object encounters: binary mergers versus dynamical collisions. MNRAS 430, 2585 (2013)
Rosswog, Piran, Nakar
(See online at https://doi.org/10.1093/mnras/sts708) - Neutrino-driven winds from neutron star merger remnants. A& A 568, 11 (2014)
Perego et al.
(See online at https://doi.org/10.1093/mnras/stu1352) - The long-term evolution of neutron star merger remnants I. The impact of r-process nucleosynthesis. MNRAS 439, 744 (2014)
Rosswog et al.
(See online at https://doi.org/10.1093/mnras/stt2502) - The long-term evolution of neutron star merger remnants II. Radioactively powered transients. MNRAS 439, 757 (2014)
Grossman et al.
(See online at https://doi.org/10.1093/mnras/stt2503)
