High Energy Gamma-Ray Emission from Gamma-Ray Bursts - Before GLAST Page: 4 of 25
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TABLE II: Coefficients for the Baseline IBL (upper low) and
Fast Evolution IBL Fits (lower low). The parametric approx-
imation holds for 10-2 < T < 102 and e < 2 TeV for all
redshifts but also up to 10 TeV for redshifts z 1 (from
-47. 92 14
provide a significant source sample for studies of IBL as
a function of look-back time .
C. Observation of high energy prompt emission
EGRET had detected more than 30 GRBs with high
energy photon emission [17, 18, 19, 20, 21, 22, 23]. Some
of these > 30 MeV photons are simultaneous with the
keV-MeV emission, i.e, they are prompt high energy
GRB 940217 is a good example . This burst
was detected by the Compton Telescope (COMPTEL),
EGRET, Burst And Transient Source (BATSE) and
Ulysses. The blue line in Fig.3 is the Ulysses 25-150
keV light curve. The prompt soft -y-ray emission was
clearly visible in a timescale of ~ 180 s (i.e, phase-1).
Simultaneously, 10 photons ranging from 40 MeV to 3.4
GeV were recorded. The count rate of these photons is
much higher than that of phase-2, the high energy after-
Milagrito observation of GRB 970417A at energies
above ~ 0.1 TeV hinted at a distinct higher-energy com-
ponent (at 3o- level), but lacked energy resolution to pro-
vide a spectrum . The excess had a chance probability
of 2.8 x 10-- of being a fluctuation of the background (one
detection in 54 bursts). Milagro observed more than 50
GRBs but got null results. The Milagro 99% confidence
upper limit on the 0.2-20 TeV fluence ranges from 10-7
to 10-3 erg cm-2 . Null results at lower energies are
also reported by MAGIC observations .
The null results at > 100 GeV are not surprising be-
cause of the huge absorption of such high energy --rays
by the fireball (see eq. (50) below) and by the diffusive in-
frared background, as already mentioned in section II B.
D. Observation of high energy afterglow
In several GRBs, photons above 30 MeV have arrived
after the end of the prompt keV-MeV emission. These
are classified as high energy afterglow. It is very in-
teresting to note that the afterglow emission has been
first detected in this energy range rather than in X-
ray/optical/radio bands even though in low energy bands
the photons are more abundant by a factor of 10 - 106.
The two best known high energy afterglows are those
associated with GRB 940217 and GRB 941017.
GRB 940217: The prompt soft and hard --ray emis-
sion has been described in the last subsection. As the
25-150 keV emission ceased, the hard -y-ray emission
did not and lasted longer than 5400 s, including an 18
GeV photon that arrived about an hour after the trigger
(see the green circles in phase-2 of Fig.3). In total 18
high energy photons have been detected. The total num-
ber could have been higher (probably around 100) if the
source was not occulted by the earth for ~ 3700 s after
the burst. Note that the 18 GeV photon was observed
after the satellite came out from the earth occultation.
This high energy afterglow is also characterized by: (a)
The count rate of high energy photons seemed to be con-
stant; (b) Except of one photon with an extremely high
energy ~ 18 GeV, the energy of the others is nearly a
constant, i.e., ~ 100 MeV.
GRB 941017: GRB941017 is one of highest fluence
bursts observed by BATSE in its 9-yr lifetime. Ninety
per cent of the flux observed by BATSE occurred in a
time interval of 77 s. The high-energy component car-
ried at least 3 times more energy than the lower energy
component and it lasted about 3 times longer . As
shown in Fig.4, there are two additional amazing obser-
vation facts: (1) While the soft -y-ray emission became
weaker and weaker and disappeared, both the spectrum
and the flux of the hard -y-ray emission (up to an energy
> 200 MeV) were almost constant over a timescale of ~
200 s; (2) The spectrum of the high energy emission com-
ponent is rather hard, F~ a vA where v is the observed
frequency of the photon. The peak energy of the hard
component is likely to be above 200 MeV.
Some ground-based Cherenkov detectors have been
used to observe the VHE afterglow emission of GRBs.
After several years' of search, no evidence for afterglow
photons at such high energies has been found [29, 41, 42].
III. PHYSICAL PROCESSES
We focus on relativistic collisionless shocks. Magnetic
energy dissipation, for example via magnetic reconnec-
tion, can also accelerate the particles and then give rise
to prompt [43, 44, 45, 461 and afterglow emission [47, 481.
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Fan, Yi-Zhong & Piran, Tsvi. High Energy Gamma-Ray Emission from Gamma-Ray Bursts - Before GLAST, article, November 29, 2011; United States. (digital.library.unt.edu/ark:/67531/metadc831854/m1/4/: accessed October 17, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.