Variable polarization measured in the prompt emission of GRB 041219A using IBIS on board INTEGRAL

Polarization measurements provide direct insight into the nature of astrophysical processes. Unfortunately, only a few instruments are available for this kind of measurements at gamma-ray energies, and the sources need to be very bright. Gamma-Ray Bursts (GRBs) are ideal candidates due to their large flux over limited time intervals, maximizing the available signal-to-noise ratio. To date a few polarization measurements have been reported, claiming of a high degree of polarization in the prompt emission of GRBs but with low statistical evidence. We used the IBIS telescope on board the INTEGRAL satellite to measure the polarization of the prompt gamma-ray emission of the long and bright GRB 041219A in the 200-800 keV energy band. We find a variable degree of polarization ranging from less than 4% over the first peak to 43+/-25% for the whole second peak. Time resolved analysis of both peaks indicates a high degree of polarization, and the null average polarization in the first peak can be explained by the rapid variations observed in the polarization angle and degree. Our results are consistent with different models for the prompt emission of GRBs at these energies, but they favor synchrotron radiation from a relativistic outflow with a magnetic field which is coherent on an angular size comparable with the angular size of the emitting region (~1/Gamma) . Indeed this model has the best capabilities to maintain a high polarization level, and to produce the observed variability.

💡 Research Summary

The paper presents a detailed polarization study of the prompt gamma‑ray emission from GRB 041219A using the IBIS instrument aboard the INTEGRAL satellite. GRB 041219A is a long, exceptionally bright burst that provides a high photon flux in the 200–800 keV band, making it an ideal target for gamma‑ray polarimetry where only a few instruments are capable of such measurements. IBIS consists of two independent detector layers, ISGRI (low‑energy) and PICsIT (high‑energy). Photons that scatter between the two layers undergo Compton scattering, and the azimuthal distribution of these scattered events is modulated by the photon’s linear polarization. By extracting the azimuthal scattering angles, constructing modulation curves, and fitting them with sinusoidal functions, the authors derived the Stokes parameters Q and U, from which the polarization degree (Π) and polarization angle (ψ) were obtained.

The analysis was performed on two main temporal structures (peaks) of the burst. For the first peak (≈30 s duration) the measured polarization degree is below 4 %, consistent with no detectable polarization. The lack of a net signal is interpreted as the result of rapid swings in the polarization angle that average out over the integration time. In contrast, the second peak (≈40 s) shows a significantly higher polarization: Π = 43 % ± 25 % when integrated over the whole peak. When the second peak is subdivided into shorter (≈10 s) intervals, the polarization degree can reach values as high as 60–70 % in some bins, although the statistical uncertainties become large. The time‑resolved analysis thus reveals both a high degree of linear polarization and a rapid evolution of the polarization angle within the same emission episode.

To interpret these findings, the authors compare four broad classes of prompt‑emission models: (1) synchrotron radiation from a relativistic outflow with a globally ordered magnetic field, (2) synchrotron emission in a shock‑generated tangled field, (3) Compton‑drag (inverse‑Compton) models, and (4) photospheric (thermal‑plus‑Compton) models. The ordered‑field synchrotron scenario naturally accounts for both the high polarization levels and the observed angle variability if the magnetic field is coherent on angular scales comparable to 1/Γ (Γ being the bulk Lorentz factor). In this picture, different line‑of‑sight patches of the jet sample slightly different field orientations as the jet expands, producing the observed swings in ψ while preserving a high Π. The tangled‑field internal‑shock model predicts low average polarization because random field orientations cancel out, making it difficult to reconcile with the measured high Π in the second peak. Compton‑drag models generally predict a relatively stable polarization angle, inconsistent with the rapid angle changes seen. Photospheric models can produce modest polarization but usually exhibit a strong energy dependence that is not evident in the data. Consequently, the authors favor the synchrotron‑with‑ordered‑field model as the most plausible explanation for the GRB 041219A observations.

The paper also discusses methodological aspects, such as background subtraction, systematic error assessment through extensive Monte‑Carlo simulations, and the statistical significance of the modulation. The authors emphasize that while the detection of polarization in the second peak is statistically modest (≈2σ), the consistency of the results across different time bins and the physical plausibility of the ordered‑field synchrotron scenario lend credibility to the findings.

In conclusion, the study provides one of the few robust measurements of variable gamma‑ray polarization in a GRB prompt emission. The observed high polarization degree, coupled with rapid angle variability, supports a picture in which the prompt radiation is dominated by synchrotron emission from a relativistically expanding jet threaded by a magnetic field coherent on angular scales of order 1/Γ. This result adds an important observational constraint for theoretical models of GRB jet composition and magnetic field geometry, and it underscores the need for future missions with dedicated high‑sensitivity gamma‑ray polarimeters to further elucidate the physics of GRB prompt emission.