Gamma-ray Bursts

About Gamma-ray Bursts

Gamma-ray Bursts (GRBs) are highly energetic events of extragalactic origin observed as episodes of bright gamma-ray emission over periods of seconds to minutes. GRBs occur randomly in time and are isotropic in the sky. This prompt emission is followed by an afterglow period spanning the full electromagnetic spectrum lasting anywhere from days to months. GRBs originate from the formation of compact objects such as neutron stars and black holes. There are two types of GRBs, short and long, distinguished by the time to achieve 90% of their prompt emission. Long GRBs are believed to be large stars undergoing a special kind of core collapse, whereas short GRBs are believed to be mergers of compact objects from neutron-star/neutron-star or black-hole/neutron-star mergers. In either type of GRB, the formation of a compact object creates the central engine that powers a relativistic jet generating the prompt and afterglow emission. Most GRBs emit a spectrum fit with an empirical model made from two smoothly joined power laws. However, each GRB is a unique event with a highly variable lightcurve (flux over time) and spectra.

The ABEX GRB investigation directly addresses goals in the NASA astrophysics roadmap of revealing the extremes of nature through the study of black holes, accretion disks, and powerful jets. The accretion disk near the central engine fuels a powerful relativistic jet. Central engines are typically black holes with extremely high energy densities, an ideal environment to study high-energy particle physics. ABEX’s science goals address the 2020 astrophysics decadal survey’s report on compact objects and energetic phenomena, addressing B-Q2 and B-Q3 through the study of GRB jets. This is also a synergistic effort with question B-DA through transformational observations in joint detections of gravitational waves and GRBs, such as seen in GRB 170817A. ABEX will provide a new energy domain in the prompt X-ray emission, characterizing spectral features which can resolve tension between theoretical emission models explaining energy dissipation mechanisms in astrophysical jets.

ABEX will resolve key questions in GRBs by distinguishing emission models (internal shock or photospheric) through observations of their unique spectral features which exist in an unobserved energy domain: the prompt X-ray emission.