GRB Polarization

Artist’s illustration of a gamma-ray burst jet. Credit: NASA/Dana Berry.

Seeing inside a cosmic jet

Light can be polarised: its waves can vibrate in a preferred direction. Measuring the polarisation of a gamma-ray burst tells us about the magnetic fields inside its jet and how its gamma rays are produced. Daksha will measure the polarisation of several bright bursts every year.

For astronomers: technical details

The MEPs measure polarisation using Compton scattering between CZT pixels, as demonstrated with AstroSat CZTI. The minimum detectable polarisation is ~30% for a burst of fluence 10⁻⁴ erg cm⁻², and Daksha expects to measure polarisation for at least five GRBs per year. Method and sensitivity: Bala, Mate et al. 2023 (JATIS 9, 048002); science context: Bhalerao et al. 2024, Science with the Daksha High Energy Transients Mission (Experimental Astronomy 57, 23), section “Polarization from GRBs”. All Daksha papers are listed on our Publications page.

What is polarisation?

Light is a wave. In ordinary light, the waves vibrate in all directions at random; in polarised light, they prefer one direction. Polarising sunglasses use this to cut glare: light reflected off water is partly polarised, and the glasses block it.

Gamma rays can be polarised too. How strongly they are polarised, and in which direction, carries information about where and how the light was produced.

What polarisation can tell us about gamma-ray bursts

Scientists have several competing ideas for how a burst’s jet makes its gamma rays. In some, electrons spiral around strong, orderly magnetic fields, which would produce highly polarised light. In others, the magnetic fields are tangled, or the gamma rays come from hot glowing gas, and the polarisation would be weaker. Measuring polarisation together with the burst’s spectrum is one of the cleanest ways to tell these ideas apart.

If the polarisation changes during a burst, it can even reveal how the structure of the jet changes over a few seconds. So far, polarisation has been measured for only a few dozen bursts, many of them by the CZT Imager on India’s AstroSat.

How Daksha measures it

When a gamma ray hits one of Daksha’s CZT detectors, it sometimes bounces off one pixel and lands in a neighbouring one. Polarised gamma rays bounce in some directions more than others, so the pattern of these double hits reveals the polarisation. The same technique has been used successfully on AstroSat. Daksha’s large detector area, spread over many faces pointing in different directions, will give a larger and more uniform set of measurements, with a built-in check against errors.

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