Dissipative photospheres in gamma-ray bursts

The initial stage of a gamma-ray burst. The core of the star has collapsed deep inside the star. A black hole has formed within the star, and within a few seconds launches a jet of matter away. (Larger image) (Credit: NASA / SkyWorks Digital)
The origin of the emission during the prompt phase in gamma-ray bursts is still a mystery. One suggestion is that the photosphere of the relativistic jet plays an important role. Indeed, recently the Fermi gamma-ray space telescope has made interesting observations of the gamma-ray spectra of several GRBs which show a clear signature of a photospheric emission component.

Another recent development in the field is the realisation that energy dissipation naturally should occur close to the jet photosphere. Such theoretical predictions for kinetic outflow as well as for Poynting flux dominated outflows are confirmed by numerical jet simulations.

In a recent paper, that has just been accepted for publication in the Monthly Notices of the Royal Astronomical Society, we present observational evidence for the onset of such subphotospheric dissipation. This is clearly seen during the prompt phase in the exceptionally bright burst GRB090902B. Initially the main spectral emission component is close to a Planck function, expected for a photosphere. Later this component broadens into a spectral shape that is typical for GRBs. This illustrates that the photosphere emission can have a variety shapes. This is indeed what is expected if the dissipation pattern in the jet changes and gives rise to subphotospheric heating. This we show through numerical simulation of the dissipation processes and argue that the change in spectral behaviour as being due to a decrease in the outflow Lorentz factor. This leads to a substantial part of the kinetic energy being dissipated at optical depth of approximately 10. This causes the change in spectral shape since the photons do not have time to thermalise into a Planck function.

These observations show that the photosphere emission indeed is important in GRBs and can even be a common feature.
The spectral shape of the photospheric emission can have a variety of shapes and not only a Planck function shape.
The identification of the photosphere as cause of the main emission in GRBs provides us a way to study the physics of the relativistic jet. This allows us to learn more about these enigmatic events that are the largest explosions in the Universe.

If you want to read the original paper, check this out: http://arxiv.org/abs/1103.0708

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