Astrophysics

Magnetic fields in γ-ray bursts

Observations of a high degree of polarization in the immediate optical afterglow of a γ-ray burst indicate that these powerful cosmic explosions carry large-scale, ordered magnetic fields. See Letter p.119

Naturally occurring magnetic fields protect life on Earth from energetic cosmic rays but are relatively weak and barely noticeable. However, for astronomical objects, magnetic fields can have a dynamically important, and often dominant, role, especially for gravitationally collapsed objects such as neutron stars and accreting black holes. On page 119 of this issue, Mundell et al.1 report the possible observation of an ordered magnetic field that plays a significant part in a cosmic explosion known as a γ-ray burst.

As a black hole gravitationally pulls matter in, magnetic fields, which are frozen into the accreting plasma owing to the plasma's high conductivity, are compressed and thus amplified. Compressed magnetic fields can produce spectacular astrophysical phenomena because of a key ingredient: rotation. Collapsed objects, as well as the material that they accrete, typically rotate rapidly. The combination of rotation and compressed magnetic fields leads to the astronomical realization of the Faraday wheel — an electric generator of constant electrical polarity — that produces large currents and voltages2.

This is how powerful astrophysical outflows such as γ-ray bursts (GRBs) are produced: material accreting onto a rotating black hole brings with it the magnetic field and sets up a mega-version of the Faraday wheel3,4. The resulting plasma outflow may reach extremely high (relativistic) velocities and, in the case of minutes-long GRBs, carry energy comparable to the total energy that the Sun will radiate over its multibillion-year lifetime5. In addition, hoop stresses produced by the large-scale helical magnetic field that permeates the plasma outflow can collimate the outflow into a narrow beam, with an opening angle of only a few degrees.

As a GRB outflow interacts with the surrounding medium, two shocks are launched: a forward shock into the external medium and a reverse shock into the ejecta. Observations of the emission from the reverse shock, whose frequency typically falls in the optical range, can probe the properties of the ejecta and the Faraday-wheel model. Testing this model requires verification that the outflow carries a large-scale, ordered magnetic field (assuming that the field extends well into the outflow).

In their study, Mundell et al. report the possible detection of just such a magnetic field, in a GRB dubbed GRB 120308A, through observations of polarized, early optical emission from a reverse shock in the GRB. Medium-sized optical telescopes, such as the 2-metre Liverpool Telescope used in the present study, can slew to the position of the burst within minutes of receiving the alert that the burst has occurred, and detect a typically faint optical afterglow. But in the case reported here, the optical telescope was also equipped with a purpose-built polarimeter, called RINGO2, that could detect the preferred orientation, or polarization, of the afterglow's oscillating electric field. Polarization indicates that the process that produced the afterglow is sensitive to a particular direction in the emitting plasma outflow.

Mundell and colleagues measured a linear polarization content — how much the electric field vibrates in a fixed plane — of 28% for the optical emission, with the angle of polarization remaining stable. This high value presumably comes from the reverse shock in the ejecta. It is close to the maximum degree of polarization that can be produced by synchrotron-radiation-emitting electrons in a relativistically expanding outflow carrying a large-scale, ordered magnetic field.

“This result is likely to contribute to the heated debate on the nature of γ-ray bursts.”

This result is likely to contribute to the heated debate on the nature of GRBs. Despite decades of intensive research, we are still not clear about the basic, high-energy emission mechanism in GRBs, with the competing models being synchrotron emission from electrons and Compton scattering of photons by electrons. Previous claims6 of high polarization values in GRBs, which were based on observations made in the γ-ray energy regime, were inconclusive7 because polarization measurements are difficult to perform at high energies and subject to large uncertainties. By contrast, optical polarization, such as that obtained by the authors, can be measured with much higher certainty. Mundell and colleagues' detection of a high degree of polarization in the optical afterglow both confirms the Faraday-wheel model of launching powerful astrophysical outflows and argues in favour of synchrotron radiation being the dominant high-energy emission process.

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Correspondence to Maxim Lyutikov.

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Lyutikov, M. Magnetic fields in γ-ray bursts. Nature 504, 92–93 (2013). https://doi.org/10.1038/504092a

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