2

burst (for example, GRB 940217, GRB 030329 and GRB
080319B) and these can be used to constrain the models.
With such a hope, we present in this work an overview
of the theoretical studies of high energy emission from
GRBs.
The structure of this review is as follows. We first dis-
cuss the observational aspects of high energy emission of
GRBs and afterglows in Section II, and then the physi-
cal processes in Section III. We discuss the high energy
emission processes in GRBs and afterglows, the interpre-
tations of available high energy observations and possible
progresses in the next decade in Sections IV-VI, respec-
tively.
II. OBSERVATIONS
We begin with a short review of the observations of
high energy emission from GRBs and afterglows. We
divide this section into four parts, beginning with an in-
troduction of the detectors. We then have a short discus-
sion of the cosmic absorption of high energy -y-rays, that
plays a crucial role in the detection prospects above 50
GeV. We continue with the prompt high energy emission
the GRB itself but in energies above 20MeV and proper-
ties of high energy afterglows.
A. Detectors
Space   telescopes.     Among    space  telescopes
dedicated  to   high   energy  7-ray    astrophysics,
three  are particularly interesting  for GRB   peo-
ple,  including  the  Energetic  Gamma    Ray   Ex-
periment   Telescope   (EGRET)     onboard   CGRO
(see    http://heasarc.gsfc.nasa.gov/docs/cgro/egret/),
Gamma-Ray Imaging Detector (GRID) onboard AGILE
(see http://agile.rm.iasf.cnr.it/), and the upcoming LAT
onboard GLAST satellite. EGRET and GRID have a
similar peak effective area ~ 1000 cm2. That of LAT,
however, is much larger. We compare them in Table I.
With a higher sensitivity, LAT is expect to detect high
energy photons from GRBs much more frequently than
both EGRET and GRID. The Burst Monitor (GBM)
onboard GLAST is sensitive to X-rays and gamma
rays with energies between 8 keV and 25 MeV. The
combination of GBM and LAT provides a powerful
tool for studying gamma-ray bursts, particularly for
time-resolved spectral studies over a very broad energy
band.
The effective area of the LAT as a function of photon
energy is shown in the upper panel of Fig.1. For photons
below 100 MeV, the effective area of LAT is small, which
limits the detection prospect of the MeV photons. GBM
won't help in this aspect because of its rather small area
~ 126 cm2. The high energy photons are much less nu-
merous than the keV-MeV photons because of the large
energy each photon carries. So even in the most opti-

On-Axis Effective Area vs. True Energy I

s6ooo
E10004
<4000
2000

U    102       10       104       10
Energy (MeV)
E                     -   -        Effective Area
104  -   -             -  -
10  -       -
10 A
44 deg
t      ~       57 deg
10      102     103     1o4     E0 V
E [GeV]
FIG. 1: Upper panel: On-axis effective areas of LAT (from
http: //www-glast. slac. stanford. edu/software/IS /glast _ lat _ performance.1h
Lower panel: Effective area of MAGIC for three different
zenith angles (from [29]).
mistic extreme cases, the number of high energy photons
detected by GLAST-like satellites is not expected to be
much more than    103.
Ground-based    telescopes.   Ground-based high
energy  detectors have very    large effective  areas
S   104 - 105 m2 but work in the energy range
of tens GeV    to 100 TeV. There are two kinds
of Cherenkov   telescopes:  water Cherenkov    tele-
scopes like Milagro   (http://www.lanl.gov/milagro/)
and    atmospheric   Cherenkov    telescopes,  such
as   MAGIC,     H.E.S.S.,  Whipple,    Cangaroo-III
(http://wwwmagic.mppmu.mpg.de/links/)           and
VERITAS      (http://veritas.sao.arizona.edu/),  and
their   advanced    generations,  like    MAGIC-II
(http://wwwmagic.mppmu.mpg.de/introduction/magic2.
html) and H.E.S.S.-II.
Milagro is a TeV gamma-ray detector locating in
northern New Mexico operating in the energy band > 100
GeV. It uses the water Cherenkov technique to detect ex-

-            ______ lass A (Standard)
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