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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 verylimited, 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 thecircum-burst medium as well as the initial Lorentz factor of the outflow. Thetemporal behavior of the high energy gamma-ray emission is consistent with theforward shock synchrotron radiation model and the circum-burst medium likelyhas a constant-density profile. The Lorentz factor is about a few hundred,similar to other bright GRBs.

High energy emission of GRB 130821A: constraining the density profile of the circum-burst medium as well as the initial Lorentz factor of the outflow