An Extremely Bright Slow-rising Afterglow from an Off-axis Jet in GRB 260310A

We present a multiwavelength study of GRB 260310A, a nearby long-duration gamma-ray burst (GRB) at z ≃ 0.153 associated with a broad-lined Type Ic supernova. Despite its modest prompt gamma-ray output, E γ ,iso ≃ 3.5 × 10 50 erg, GRB 260310A exhibits one of the brightest afterglows ever observed in the X-ray, optical, and radio bands. Its apparent brightness is not its only remarkable feature. The optical afterglow displays a delayed onset, characterized by a slow rising phase, with slope α ≈ −1, and a late peak at ∼0.1 day. We argue that the combination of weak prompt emission, hard peak energy, and late afterglow onset is naturally explained by a GRB jet viewed off-axis. The radio spectral energy distributions are consistent with synchrotron radiation and indicate the presence of both reverse- shock (RS) and forward-shock (FS) components, thus providing a first test of RS models in an off-axis geometry. The X-ray afterglow displays a prominent rebrightening, monitored for up to ∼68 days with no evidence of spectral evolution. A low level of linear polarization, Π ≈ 1.7%, is measured at 15 GHz at T 0 + 55 days and suggests that, at these late times, the FS is the dominant emission component from radio to X-rays. This late-time rebrightening represents a critical test for the two-component jet model. If interpreted as the emergence of a narrow jet core viewed further off-axis, it would imply extreme luminosities and energetics for an on-axis observer.