Collaborative Research: RUI: Short Gamma-Ray Bursts Arising From Misaligned Structured Jets in the Dawn of Gravitational Wave Astronomy

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The detection of both gravitational waves (GWs) from a neutron star (NS) merger event and its electromagnetic (EM) counterpart marks a new era in multi-messenger astronomy. The detection of the faint short gamma-ray burst (GRB), GRB170817A, the EM counterpart of a gravitational wave event provides the most conclusive evidence that NS mergers are indeed the progenitors of short GRBs. The need to fully understand the EM signals from GRBs is even more pressing after this discovery. A research collaboration between California State University, Sacramento, Purdue University, and Northwestern University will investigate short GRBs and how they are produced by jets, or beamed outflows of ionized matter, which have geometrical structure. When such jets are misaligned with respect to the direction from the GRB to earth, they are observed to be under-luminous, or relatively faint, as was seen for GRB170817A. The researchers will use relativistic magneto-hydrodynamical (MHD) simulations to fully explain current and future observations of EM counterparts from GW signals. An important component of this proposed research is training both graduate and undergraduate students, who will be encouraged to conduct original research projects. The research team will also broaden participation in science and public literacy with a series of public lectures at the soon-to-be opened Sacramento State planetarium. They will also continue outreach efforts at the three campuses, including public solar viewing parties, engaging grade 6-12 students with hands-on activities and assisting high school educators to design hands-on classroom demonstrations.The proposal team will expand on its correct prediction that the most likely electromagnetic signal from a neutron star merger event would be a faint gamma-ray pulse from a misaligned GRB jet. Based on preliminary MHD simulations of a short GRB jet, the team found that a jet is naturally expected to have some structure, where the jet's properties vary smoothly with angle, such that the luminous core is surrounded by a slower, fainter sheath, consistent with observations of GRB170817A. The researchers will carry out large-scale relativistic MHD simulations of short GRB jets in realistic ambient gas profiles and realistic initial black hole conditions in order to determine the structure of the jet. Through these simulations, they will follow the jet from the launching region, through the confining ambient gas and the break out distance, where its slower sheath forms, all the way to the transparency region where the gamma-ray signal is expected to be produced. The work will be complemented by analytical studies of jet collimation by a confining medium as the jet breaks out from it. The team will calculate the expected signal from a jet pointed at an arbitrary angle to the observer for a broad range of prompt emission models and from the accompanying long-term afterglow emission.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.