Strong Lensing in Magnetar Burst Fireballs: Transfer Functions

Magnetar short bursts are 0.01-1 s high-energy transients peaking in the hard X-rays. Thousands of bursts have been detected over the past decade, and their relevance has broadened recently from associations of some high energy bursts with bright radio bursts in SGR 1935+2154. Observational evidence strongly favors a fixed and low altitude emission locale for Comptonized fireballs confined within magnetic flux tubes as the emitting volume for magnetar short bursts. At such low altitudes, gravitational lensing can be important and a second lensed image can result for bursts behind the neutron star for the observer. This lensing can imprint not only flux changes, but time delays and temporal-spectropolarimetric variation of bursts at particular rotational phases because of QED photon splitting. For fixed active regions and flux tubes, X-ray and gamma-ray instruments with high temporal resolution (such as NICER, NuSTAR, eXTP, STROBE-X, HEX-P, Fermi-GBM and AMEGO-X) could enable constraints fundamental parameters of magnetars via spectral, timing, and polarization of bright bursts which occur behind the star for observers. We detail the development of the first 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 via Monte Carlo sampling of a multicolor blackbody within flux tubes. Radiative transport in the burst photosphere is handled via a suite of magnetized atmosphere models, accounting for the polarization state and energy of photons relative to the local cyclotron resonance. The spectrotemporal lags, and transfer functions (familiar within the black hole community) are developed for the first time in a magnetar context. The outcome of such studies will enable geometry constraints on magnetars and their bursts, as well as perhaps measurements of magnetar masses and radii.