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Gamma-ray bursts (GRBs) are the brightest explosions in the universe. In just a few seconds, they can outshine all the stars in their host galaxy combined, releasing more energy than our sun will emit over its entire lifetime. Yet, despite decades of observations, key mysteries remain unsolved: What powers the prompt emission? What is the jet's structure? And what is it made of?

Our study of GRB 180427A, combining observations from NASA's Fermi satellite and India's AstroSat-CZTI, provides new evidence that photons seen during the prompt phase can arise from two distinct emission sites within the relativistic jet. Each carries its own spectral and polarization signature, revealing a more complex picture of GRB jets than a single emission zone. The work is in The Astrophysical Journal.

Two pulses, two origins

GRB 180427A was detected on April 27, 2018, by Fermi-GBM, which provided wide-band spectral coverage, and simultaneously by AstroSat's Cadmium Zinc Telluride Imager (CZTI), one of the few instruments capable of measuring hard X-ray polarization.

The burst showed two bright pulses, separated by about five seconds. Spectral analysis revealed that these pulses were very different in character. The first pulse contained a strong thermal (blackbody) component, consistent with photons emerging from the jet's photosphere—the surface where it becomes transparent. The second pulse was dominated by non-thermal emission, best described by a cutoff power-law spectrum, typical of radiation from farther out in the jet.

This contrast suggested that the two pulses originated from physically distinct regions.

Unraveling the polarization of each burst component

To measure how strongly each part of the burst's spectrum was polarized, we built a simulation framework wherein the virtual model of the AstroSat satellite was illuminated with simulated burst light, varying the polarization strength while using the direction measured from observations. By comparing the simulated detector signals with the real data, we identified the polarization combinations that reproduced the observed behavior.

From this comparison, we estimate polarization fractions (PF) of about 25%–40% for the blackbody component and 30%–60% for the power-law-with-exponential-cutoff component, showing that the two emission sites carry distinct polarization signatures. This novel method for constraining the polarization fraction was used for the first time.

What polarization reveals

The burst's two pulses tell different stories. The first pulse is moderately polarized (about 25%–40%), which fits with the thermal, photospheric emission. The polarized photospheric emission suggests that the burst is viewed off-jet axis. The second pulse is more strongly polarized (about 30%–60%), suggesting non-thermal processes such as synchrotron radiation from tangled magnetic fields or inverse-Compton scattering.

Importantly, the polarization direction shifts by roughly 60° between the pulses, linking this change to the transition from thermal to non-thermal emission, and tells us that these two emissions have different polarization directions.

Insights into the jet's shape and composition

Our measurements do more than identify two different emission sites—they also give clues about the jet's overall shape, its composition and how we are viewing it. The observance of the combination of steep low energy spectral slopes similar to that of a blackbody and fairly strong polarization suggests a jet with fairly sharp edges, rather than a jet with softer edges with gradual change in brightness and speed away from the jet axis.

The spectro-polarimetric information further suggests that we are viewing the jet off its axis and near to the jet's edge. Finally, the strong thermal contribution, nearly 20%–60% to the total observed spectrum, points to a jet that is baryon dominated. In other words, there is no significant burst energy in large scale magnetic fields across the jet.

Why this matters—and what comes next

By combining what the burst's light is made of (its spectrum) with how that light's electric field is oriented (its polarization), we were able to peek inside the jet and see that the prompt gamma ray emission from the relativistic jet from the GRB didn't come from a single emission zone but from at least two separate regions—one thermal (hot, photospheric) and one non-thermal (region above the photosphere).

Measuring polarization is difficult, but it gives unique clues about the jet's shape, what it's made of and how it is viewed. A comprehensive understanding is made possible because Fermi provided wide spectral coverage while AstroSat-CZTI supplied the polarimetric information.

Looking ahead, doing such joint spectral and polarization studies on more bursts—and with next-generation polarimeters such as COSI, POLAR-2, and Daksha—will let astronomers map GRB jets in much greater detail and finally test which emission models really power these cosmic explosions.

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