Afterglow Polarization from Off-axis Gamma-Ray Burst Jets

Abstract

As we further our studies on gamma-ray bursts (GRBs), via both theoretical models and observational tools, more and more options begin to open for exploration of its physical properties. As GRBs are transient events primarily dominated by synchrotron radiation, it is expected that the synchrotron photons emitted by GRBs should present some degree of polarization throughout the evolution of the burst. Whereas observing this polarization can still be challenging due to the constraints on observational tools, especially for short GRBs, it is paramount that the groundwork is laid for the day we have abundant data. In this work, we present a polarization model linked with an off-axis spreading top-hat jet synchrotron scenario in a stratified environment with a density profile n(r) ∝ r −k . We present this model's expected temporal polarization evolution for a realistic set of afterglow parameters constrained within the values observed in the GRB literature for four degrees of stratification k = 0, 1, 1.5, and 2 and two magnetic field configurations with high extreme anisotropy. We apply this model and predict polarization from a set of GRBs exhibiting off-axis afterglow emission. In particular, for GRB 170817A, we use the available polarimetric upper limits to rule out the possibility of an extremely anisotropic configuration for the magnetic field.

Afterglow Polarizations in a Stratified Medium with Effect of the Equal Arrival Time Surface

“…For a large q with q > 1, the only PD bump in the PD curve will move to a late observational time and its peak value will increase with an increase of q value. This trend is consistent with the case for a two-dimensional random magnetic field in the emission region of Rossi et al (2004) and Pedreira et al (2022).…”

“…Afterglow polarizations considering both the reverse-shock and the forward-shock emission were investigated by Lan et al (2016). Recently, afterglow polarizations of an off-axis tophat jet with lateral expansion in a stratified medium were discussed (Pedreira et al 2022). However, they did not include the EATS effect in their treatment, which might be important for off-axis detections (Huang et al 2007).…”

The IXPE View of GRB 221009A

“…Our model is constructed so that the PD zero-point between the two bumps is at ≈1 day, to match the estimated jet break time (D'Avanzo et al 2022). We derive the expected PD by integrating the intensity and polarization of the comoving volume elements of the jet over the equal arrival time surfaces (Sari 1999;Granot & Knigl 2003;Shimoda & Toma 2021;Pedreira et al 2022).…”