Spectral Puzzle of the Off-Axis Gamma-Ray Burst in GW170817

Understand

Gravitational waves from a merger of two neutron stars (NSs) were discovered for the first time in GW170817, together with diverse electromagnetic counterparts, providing a direct clue to the origin of short gamma-ray bursts (sGRBs).

• The associated sGRB 170817A was much fainter than typical, suggesting off-axis emission from a relativistic jet.
• However the observed prompt spectrum is inconsistent with the spectral (Amati) relation and causes the compactness problem in the simplest off-axis model.
• We focus on this spectral puzzle and suggest a possible key that the off-axis emission generally comes from the off-center jet, neither the jet core nor the line-of-sight jet, if the jet structure is exponentially faint outward as inferred from observations.