source: trunk/MagicSoft/GRB-Proposal/Introduction.tex@ 6231

\section{Introduction}

\subsection{Observation of GRBs}

The \ma telescope's support structure and mirrors have been designed exceptionally light in order to
react quickly to GRB alerts from satellites. \cite{design} and~\cite{PETRY} set
the objective to turn the telescope to the burst position within 10-30\,sec.
in order to have a fair chance to detect a burst when the prompt $\gamma$--emission is still ongoing.
During the commissioning phase, it could be proven that our goal was reached.
The telescope is able to turn 180\,deg. in azimuth within 20\,sec. and 90\,deg. in zenith within 10\,sec.\\

Very high energy (VHE) GRB observations have the potential to constrain the current GRB models
on both the prompt and extended phases of GRB emission~\cite{HARTMANN,MANNHEIM}.
Models based on both internal and external shocks predict VHE gamma-ray fluences comparable to,
or in certain situations stronger than, the keV-MeV radiation,
with durations ranging from shorter than the keV-MeV burst to extended TeV 
afterglows~\cite{DERMER, PILLA, ZHANG1, RAZZAQUE}.

\par

In many publications, the possibility that more energetic $\gamma$-rays come along with the
(low-energy) GRB, have been explored. Proton-synchrotron emission~\cite{TOTANI} have been suggested
as well as photon-pion production~\cite{WAXMAN,BOETTCHER} and inverse-Compton scattering
in the burst environment~\cite{MESZAROS93,CHIANG,PILLA,ZHANG2,BELOBORODOV}.
Long-term HE $\gamma$-emission from accelerated protons in the forward-shock has been predicted in~\cite{LI}. 
This model predicts GeV inverse Compton emission even one day after the burst.
Even considering pure electron-synchrotron radiation, measurable GeV-emission for a significant 
fraction of GRBs is predicted~\cite{ZHANG2}.\\

GeV-emission in GRBs is particularly sensitive to the Lorentz factor and the photon density of the 
emitting material --
and thus to the distance of the radiating shock from the source -- due to the 
$\gamma \gamma \rightarrow \textrm{e}^+\textrm{e}^-$ absorption in the emission region. 
Direct comparison of the prompt GRB flux at $\sim$\,10\,GeV and $\sim$\,100\,keV may 
allow to determine the magnetic field strength~\cite{ASAF2}.

\par

Several attempts have been made in the past to observe GRBs in the GeV range,
each indicating some excess over background but without stringent evidence.
The only significant detection was performed by \eg which was able to observe seven GRBs 
emitting high energy (HE)
photons in the 100\,MeV to 18\,GeV range~\cite{EGRET, DINGUS1}. The data shows no evidence of a HE cut-off in the GRB spectrum~\cite{DINGUS2}. Recent results indicate that the spectrum of some GRBs contains a very hard, luminous, long-duration component~\cite{GONZALES}.
There have been results suggesting gamma rays beyond the GeV range from the TIBET air shower array
in coincidence with BATSE bursts~\cite{AMENOMORI}, rapid follow-up observations by the
Whipple Air Cherenkov Telescope~\cite{CONNAUGHTON1}, and coincident and monitoring studies by 
HEGRA-AIROBICC~\cite{PADILLA}, Whipple~\cite{CONNAUGHTON2} and the Milagro prototype Milagrito~\cite{MILAGRO}. 
The GRAND array has reported some excess of observed muons during seven BATSE bursts~\cite{GRAND}. 
In this context, especially the publication from the TASC detector on \eg is important~\cite{GONZALES}, 
finding a HE spectral component presumably due to the ultra-relativistic acceleration of hadrons and 
producing a spectral index of $-1$ with no cut-off up to the detector energy limit at 200\,MeV.\\

Concerning estimates of the \ma GRB observability, a study of GRB spectra obtained from the
third and fourth \ba catalogue has been made in~\cite{ICRC,NICOLA}. The spectra were extrapolated to GeV 
energies with a simple continuation of the observed high-energy power law behaviour and the calculated 
fluxes compared with \ma sensitivities. Setting conservative cuts on observation times and significances,
and assuming an energy threshold of 15~GeV, a 5\,$\sigma$-signal rate of $0.5-2$ per year
was obtained for an assumed observation delay between 15 and 60\,sec. and a \ba trigger rate 
($\sim$\,360/year). As the \sw alert rate is about a factor~2 lower including even fainter bursts than 
those observed by \ma, this number will still have to be lowered.

Taking into account the local rate of GRBs estimated in~\cite{GUETTA}, late afterglow emission from a 
few tens of GRBs per year should be observable over the whole sky above our energy threshold.
The model of~\cite{ASAF2} predict delayed GeV-emission that should be significantly detectable by \ma 
in 100\,sec.

\subsection{Observation of XRFs}

While the major energy from the prompt GRBs is emitted in $\gamma$-rays with a peak energy of 200\,keV, 
X-ray flashes (XRFs) are characterized
by peak energies below 50~keV and a dominant X-ray fluence. Because of similar properties, a connection 
between XRFs and GRBs is suggested. 
The most popular theories suggest that XRFs are produced from GRBs observed ''off-axis''.
Alternatively, an increase of the baryon load within the fireball itself or low efficiency shocks could 
produce XRFs.
If there is a connection between XRFs and GRBs, 
they should originate at rather low redshifts (z $<$ 0.6).\\

Gamma-ray satellites react in the same way to XRFs and GRBs. 
In case of a detection the coordinates are distributed
to other observatories (see section 2.1). Only from later analysis the difference can be established.

\par

We include therefore the observation of XRFs by \ma in this proposal.

\subsection{Observation of SGRs}

Soft Gamma Repeaters (SGRs) are believed to be extremely rare strong magnetic neutron stars that 
periodically emit $\gamma$-rays. Only four identified SGRs were discovered in the last 20 years: 
SGR0526-66, SGR1806-20, SGR1900+14, SGR1627-41. 
GRBs and SGRs can be explained within one same gamma jet model where the jet is observed at different 
beam-angles and at different ages.\\

The BAT instrument on the SWIFT satellite triggered on an outburst from SGR1806-20 on 30. January 2005. 
The fluence was about $10^{-5}$\,erg/cm$^2$ in the range between 15 and 350\,keV. 
This event was five orders of magnitude smaller than the giant flare from this source on the 
December 27$^{th}$, 2004. 
If a giant flare from SGR occurs as SGR1806-20, MAGIC would be able to detect the $\gamma$-ray 
emission from the source with 100\,sec. observationes time.





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