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 $dN_e /d��_e \propto ��_e^{-2}$, and their synchrotron radiation flux density has a spectral shape $F_��\propto ��^{-1/2}$. 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 $d N_e / d ��_e \propto ��_e^{-2}$ 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 $��$ has a typical value -1 could be due to synchrotron radiation in this moderately fast cooling regime.

Accepted for publication in Nature Physics. This version is the original submitted version. A refereed version (with minor revision) will appear in Nature Physics