Spectral puzzle of the off-axis gamma-ray burst in GW170817

Abstract

ABSTRACT 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 suggest that this spectral and energetic puzzle of GRB170817A is explained if the origin of the off-axis emission arises from the off-centre jet. This emission location is generally created by the product of the rapidly declining (with angle) jet energy and the increasing beaming term. Hence the region of most luminous emission for an off-axis observer is neither the jet core around the primary axis, nor the line of sight at the viewing angle. The off-centre jet could be loaded with baryon or cocoon. The off-axis model predicts that roughly ${\sim } 10{{\ \rm per\ cent}}$ events are brighter at smaller viewing angles than sGRB 170817A, although the exact event rate sensitively depends on uncertainties of the off-centre structure. The model also predicts outliers to Amati relation, providing future tests to reveal the central engine activities.