New Physical Models of Magnetar Burst Fireballs in Curved Spacetime

Magnetars are the most magnetized neutron stars with magnetic fields of the order of ~ 10¹⁴ - 10¹⁶ G. Magnetar short bursts (0.01-1 s long) are high-energy transients peaking in the hard X-rays (~ 30-50 keV) originating from confined hot plasmas in magnetar magnetospheres. They are of significant interest to current and future high-energy instruments (e.g. NICER, IXPE, STROBE-X, HEX-P) and wide field monitors such as Fermi-GBM and AMEGO-X. Interest in the physics of magnetar short bursts has intensified recently with their association with Fast Radio Bursts. We report on new energy-dependent magnetar burst fireball models accounting for curved spacetime, radiative transport in strong magnetic fields, and photon splitting. This is accomplished via a fully general relativistic ray tracing formalism accounting for local emission anisotropy and emergent spectra at burst photospheres accommodated assuming local thermal equilibrium within flux tubes for different physical descriptions of the photon gas. Radiative transport in the burst photosphere is handled via a suite of magnetized photosphere models, accounting for the polarization state and energy of photons relative to the local cyclotron resonance. The spectro-temporal lags, and transfer functions (familiar within the black hole community) are reported in a magnetar context. Multiple gravitationally-lensed images of burst regions may result depending on location and viewing geometry. Future fitting of bursts in our models could enable geometric constraints on magnetars (e.g. viewing angle and magnetic obliquity) and their bursts, as well as perhaps constraints on magnetar masses and radii.