Radiation Mechanism of Gamma-Ray Bursts Revealed

UNLV astrophysicist Bing Zhang.
May. 8, 2014

Gamma-ray bursts (GRBs) are the most violent explosions in the universe since the Big Bang. Despite years of observations and theoretical modeling, the exact mechanism that produces intense gamma rays from these events is unknown.

In a paper recently published in Nature Physics, two scientists reported a breakthrough in understanding GRB emission.

“Regarding the physical mechanism of GRBs, the questions have been ‘what’, ‘where,’ and ‘how,’” said Bing Zhang, a UNLV astrophysicist and corresponding author of the paper. “Our research helps answer the how and offers hints to the what and where as well.”

The difficulty in identifying the correct radiation mechanism of GRBs (the “how” question) is in the observed spectra of these events. The spectral index below the peak energy where most energy is released has a mysterious shape, which is at odds with the two leading mechanisms proposed to interpret GRBs.

One mechanism invokes quasi-thermal emission from the “photosphere” of the GRB jet, from which photons trapped within the jet are set free and escape. The spectral shape of this model is too “hard” (too many high-energy photons) compared with the data. The other mechanism is “synchrotron radiation,” emission from electrons gyrating in strong magnetic fields near the speed of light. In the GRB environment, due to the strong magnetic field in the emission region, electrons are expected to rapidly lose energy, so they are in the so-called “fast cooling” regime. The emission spectrum in this regime, however, is too “soft” (too many low-energy photons) as compared with the data.

“In the field of GRBs, the radiation mechanism has been a subject of debate for many years,” explained Z. Lucas Uhm, a postdoc fellow at Kavli Insitute for Astronomy and Astrophysics, Peking University, China, and first author of the paper. “This is because neither proposed mechanism can interpret the observations satisfactorily. Our work suggests that synchrotron radiation can interpret the data well, which makes an important step to settle down this debate.”

According to Zhang, the trick was introducing a very important physical ingredient in the model, which was previously ignored. “Since the jet is streaming in space and expanding, the magnetic field strength in the emission region should continuously decrease with time. When this process is taken into account, one naturally gets the spectra as observed in GRBs.”

Zhang said the theory invokes a new regime for synchrotron radiation cooling that has not been studied before. This same theory can find applications in several other astrophysical environments as well.

“To make the model work, the GRB emission region has to be at a relatively large distance from the central engine, and the GRB composition is likely magnetically dominated,” said Zhang. “These are the implications for understanding the “where” and “what” questions from our modeling.”

Though more data from all electromagnetic wavelengths are needed to better understand the GRB mechanism, the next step is to apply the theory to directly model the gamma-ray data collected from NASA’s Fermi Gamma-Ray Telescope.

About the Study
The paper, “Fast-cooling synchrotron radiation in a decaying magnetic field and γ-ray burst emission mechanism,” was recently published in the journal Nature Physics. Authors included Zhang and former UNLV visiting scholar Z. Lucas Uhm.