Fast cooling synchrotron radiation in a decaying magnetic field and $\gamma$-ray burst emission mechanism
Synchrotron radiation of relativistic electrons is an important radiation mechanism in many astrophysical sources. In the sources where the synchrotron cooling time scale $t_c$ is shorter than the dynamical time scale $t_{dyn}$, electrons are cooled down below the minimum injection energy. It has be...
Saved in:
Main Authors | , |
---|---|
Format | Journal Article |
Language | English |
Published |
11.03.2013
|
Subjects | |
Online Access | Get full text |
Cover
Loading…
Summary: | Synchrotron radiation of relativistic electrons is an important radiation
mechanism in many astrophysical sources. In the sources where the synchrotron
cooling time scale $t_c$ is shorter than the dynamical time scale $t_{dyn}$,
electrons are cooled down below the minimum injection energy. It has been
believed that such "fast cooling" electrons have an energy distribution $dN_e
/d\gamma_e \propto \gamma_e^{-2}$, and their synchrotron radiation flux density
has a spectral shape $F_\nu \propto \nu^{-1/2}$. On the other hand, in a
transient expanding astrophysical source, such as a gamma-ray burst (GRB), the
magnetic field strength in the emission region continuously decreases with
radius. Here we study such a system, and find that in a certain parameter
regime, the fast cooling electrons can have a harder energy spectrum, and the
standard $d N_e / d \gamma_e \propto \gamma_e^{-2}$ spectrum is achieved only
in the deep fast cooling regime when $t_c \ll t_{dyn}$. We apply this new
physical regime to GRBs, and suggest that the GRB prompt emission spectra whose
low-energy photon index $\alpha$ has a typical value -1 could be due to
synchrotron radiation in this moderately fast cooling regime. |
---|---|
DOI: | 10.48550/arxiv.1303.2704 |