Prompt GRB polarization from non-axisymmetric jets

Ramandeep Gill, Jonathan Granot

Research output: Contribution to journalArticlepeer-review

Abstract

Time-resolved linear polarization (П) measurements of the prompt gamma-ray burst emission can reveal its dominant radiation mechanism. A widely considered mechanism is synchrotron radiation, for which linear polarization can be used to probe the jet’s magnetic-field structure, and in turn its composition. In axisymmetric jet models, the polarization angle (PA) can only change by 90, as П temporarily vanishes. However, some time-resolved measurements find a continuously changing PA, which requires the flow to be non-axisymmetric in at least one out of its emissivity, bulk Lorentz factor, or magnetic field. Here, we consider synchrotron emission in non-axisymmetric jets, from an ultrarelativistic thin shell, comprising multiple radially expanding mini-jets (MJs) or emissivity patches within the global jet, that yield a continuously changing PA. We explore a wide variety of possibilities with emission consisting of a single pulse or multiple overlapping pulses, presenting time-resolved and integrated polarization from different magnetic field configurations and jet angular structures. We find that emission from multiple incoherent MJs/patches reduces the net polarization due to partial cancellation in the Stokes plane. When these contain a large-scale ordered field in the plane transverse to the radial direction, П always starts near maximal and then declines over the single pulse or shows multiple highly polarized peaks due to multiple pulses. Observing П < 40 per cent (15 per cent) integrated over one (several) pulse(s) will instead favour a shock-produced small-scale field either ordered in the radial direction or tangled in the plane transverse to it.

Original languageEnglish
Pages (from-to)12178-12195
Number of pages18
JournalMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY
Volume527
Issue number4
DOIs
StatePublished - 1 Feb 2024

Keywords

  • gamma-ray burst: general
  • magnetic fields
  • polarization
  • radiation mechanisms: non-thermal
  • relativistic processes

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science

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