Gamma-ray Burst Polarization Research Articles

Polarization in Gamma-Ray Bursts Produced by Pinch Discharge

Large-voltage and high-temperature plasma columns produced by pinch discharge can generate gamma-ray flashes with energy spectra and spectral evolution consistent with that observed in gamma-ray bursts (GRBs). The inverse Compton scattering (ICS) during the discharge process can produce high linear polarization. The calculation indicates that the observed polarization depends on the angle between the observer's line-of-sight to the GRB and the direction of the pinch discharge, but only weakly depends on observed gamma-ray energy.

Polarization of Gamma-Ray Bursts by Scattering off Relativistically Moving Material: Compton Sailing and High Polarization

The polarization of gamma-ray emission scattered off the baryon-rich material that collimates a gamma-ray burst (GRB) fireball and the coasting speed of the irradiated matter are calculated numerically for different geometries of the radiation source and the collimating wall. It is shown that when the scattering material is Compton sailing, the direction of maximum polarization is quite generally well within the beaming cone of scattered radiation. As a result, the probability for observing bright highly polarized GRBs increases considerably, provided the Lorentz factor of the coasting matter is not well below 30, and the scattered radiation is highly polarized even when the beam thickness is large compared to 1/Γ. It is suggested that a correlation between polarization and intensity could provide clues as to whether energy flows from matter to photons or the reverse.

Compton drag as a mechanism for very high linear polarization in gamma-ray bursts

The recent claim by Coburn & Boggs to have detected a very high degree of linear polarization in the prompt emission of GRB 021206 has stimulated interest in how much polarization could arise in gamma-ray bursts (GRBs) from synchrotron emission. Alternatively, as Shaviv & Dar have shown, GRB polarization could be produced by inverse Compton scattering in the point-source limit. We discuss polarization from a fireball that upscatters a soft radiation field. We show that, after the proper angular integration, the residual polarization can be large, in some cases approaching the point-source limit. We discuss the probability of realizing the geometrical conditions in which a large polarization is obtained showing that, for a particularly bright burst such as GRB 021206, the detection of polarization at the first attempt in the Compton drag scenario is not unlikely.

Polarization of Gamma-Ray Bursts via Scattering off a Relativistic Sheath

Gamma-ray burst fireballs could be ensheathed by a relativistic, baryon-enriched outflow that would in selected directions scatter a polarized beam of gamma rays. The strong polarization recently announced by RHESSI is attainable by the scattering mechanism, although not most of the time. The scattering mechanism, when acting in addition to the synchrotron mechanism, could circumvent some of the predictions implied by a purely synchrotron origin for the polarization. The time profile of the scattered component could differ from that of the primary gamma rays.

Linear Polarization in Gamma-Ray Bursts: The Case for an Ordered Magnetic Field

Linear polarization at the level of $\sim 1-3%$ has by now been measured in several gamma-ray burst afterglows. Whereas the degree of polarization, $P$, was found to vary in some sources, the position angle, $\theta_p$, was roughly constant in all cases. Until now, the polarization has been commonly attributed to synchrotron radiation from a jet with a tangled magnetic field that is viewed somewhat off axis. However, this model predicts either a peak in $P$ or a $90^\circ$ change in $\theta_p$ around the ``jet break'' time in the lightcurve, for which there has so far been no observational confirmation. We propose an alternative interpretation, wherein the polarization is attributed, at least in part, to a large-scale, ordered magnetic field in the ambient medium. The ordered component may dominate the polarization even if the total emissivity is dominated by a tangled field generated by postshock turbulence. In this picture, $\theta_p$ is roughly constant because of the uniformity of the field, whereas $P$ varies as a result of changes in the ratio of the ordered-to-random mean-squared field amplitudes. We point out that variable afterglow light curves should be accompanied by a variable polarization. The radiation from the original ejecta, which includes the prompt gamma-ray emission and the emission from the reverse shock (the `optical flash' and `radio flare'), could potentially exhibit a high degree of polarization (up to $\sim 60%$) induced by an ordered transverse magnetic field advected from the central source.