George A. McCarthy | The Royal Astronomical Society

Career Stage

Student (undergraduate)

Poster Abstract

Gamma ray bursts are among the most energetic emission events in the known universe, in which a vast amount of electromagnetic energy is released. Correct interpretation of the light we observe from gamma-ray bursts is essential to understanding the processes which drive them. The light observed shortly after the initial burst, known as an afterglow, is in general reasonably well fit by models of synchrotron radiation, produced by relativistic electrons gyrating in magnetic fields. However, from such a model we derive that the electrons within the material shocked by the burst are often able to cool not only by synchrotron emission but by other processes too. Specifically, through the synchrotron self-Compton (SSC) process electrons cool by up-scattering their own synchrotron radiation. Including the effect of SSC cooling is expected to significantly change the underlying electron energy distribution and hence the observed spectrum. The amount that we expect SSC cooling to affect the observed spectrum is quantified by the Compton Y-parameter and by finding Y we can find the corrected observed spectrum. The electron scattering probability changes with the energy of the participating electron and photon and therefore so does Y. I present Y with and without an energy dependent scattering cross section by solving analytical equations and through iterative integration. I compare the GRB parameters derived from using each Y.

Plain text summary

We show that by including the Klein-Nishina corrections, the physical parameters derived from gamma-ray burst (GRB) observations are different to those found if the effect is ignored. This is quantified by the Y parameter which tells us how important synchrotron-self Compton up-scattering is in effecting the electron population's ability to cool. We show this by giving an example data set, and in the coming weeks we will apply this correction to other GRBs. The first figure shows a typical GRB observed data set alongside a synchrotron fit. The next figure shows that the cross section decreases as the energy of the incoming photon increases. Figure.3 shows that the Y parameter can be much smaller than the classical case if a full Klein-Nishina cross section is considered. Figure.4 shows observational data alongside a parameter fit made excluding any detailed consideration of the Y parameter. We show here that by including a detailed, time-dependent Y parameter, regardless of the correct cross sections, the derived physical parameters change. We then show an example of a set of parameters which give very different cross sections for which the KN corrected lightcurve is very different to the Thomson lightcurve.

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Poster Title

The effect of the electron’s size on the observed light from astrophysical shocks.