  GRB 130821A was detected by Fermi-GBM/LAT, Konus-Wind, SPI-ACS/INTEGRAL,
RHESSI and Mars Odyssey-HEND. Although the data of GRB 130821A are very
limited, we show in this work that the high energy gamma-ray emission (i.e.,
above 100 MeV) alone imposes tight constraint on the density profile of the
circum-burst medium as well as the initial Lorentz factor of the outflow. The
temporal behavior of the high energy gamma-ray emission is consistent with the
forward shock synchrotron radiation model and the circum-burst medium likely
has a constant-density profile. The Lorentz factor is about a few hundred,
similar to other bright GRBs.
