source: trunk/MagicSoft/GRB-Proposal/Introduction.tex@ 6813

\section{Introduction}

\subsection{Observation of GRBs}

The support structure and mirrors of the \ma telescope were designed to be exceptionally light in order to
react quickly to GRB alerts from satellites. The aim was to turn the telescope toward the burst position
within 30\,s~\cite{design,PETRY},
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 the goal was achieved.
The telescope is able to turn $180^\circ$ in azimuth within 20\,s and $90^\circ$ in zenith within 10\,s.\\

Very high energy (VHE) GRB observations have the potential to constrain the current GRB models
on both the prompt and the extended phase of GRB emission~\cite{HARTMANN,MANNHEIM}.
Models based on either internal or 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

Possible causes range from proton-synchrotron emission~\cite{TOTANI} to
photon-pion production~\cite{WAXMAN,BOETTCHER} and inverse-Compton scattering
in the burst environment~\cite{MESZAROS93,CHIANG,PILLA,ZHANG2,BELOBORODOV}.
A long-term high energy (HE) $\gamma$-emission can come from accelerated protons in the forward-shock, as predicted in~\cite{LI}.
This model predicts GeV inverse Compton emission up to one day after the burst.
Even considering pure electron-synchrotron radiation, measurable GeV-emission for a significant
fraction of GRBs is predicted~\cite{ZHANG2}.

\par

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}.\\


Several attempts were made in the past to observe GRBs in the GeV range,
each indicating some excess over background but without stringent evidence.
The only significant detections were performed by \eg, that was able to observe seven GRBs emitting HE photons with energies between 100\,MeV and 18\,GeV~\cite{EGRET, DINGUS1}. The data shows 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 TASC detector energy limit at 200\,MeV~\cite{DINGUS2, GONZALES}. 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}.\\

To estimate the observability of GRB by \ma, sources of the
third and fourth \ba catalogue were studied~\cite{ICRC,NICOLA}. Their spectra were extended to GeV 
energies with a simple power-law and using the observed high-energy spectral index: the extrapolated fluxes
were at last 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\,s 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 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} predicts delayed GeV-emission that should be significantly detectable by \ma
in 100\,s.

\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. 
Some theories~\cite{DADO} suggest that XRFs are produced from GRBs observed ''off-axis''.
Alternatively, an increase of the baryon load within the fireball itself~\cite{HUANG} or low efficiency 
shocks~\cite{BARRAUD} could produce XRFs.
If there is a connection between XRFs and GRBs, they should originate at rather low redshifts ($z < 0.6$)
because otherwise, the XRF energies would not fit into the observed correlation 
between GRB peak energy and isotropic energy release~\cite{LEVAN}.

\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 the same gamma jet model where the jet is observed at different
beam-angles and different times~\cite{FARGION}.\\

The BAT instrument on the SWIFT satellite triggered on an outburst from SGR1806-20 on January $30^{\mathrm{th}}$, 2005.
The fluence was about $10^{-5}\mathrm{erg}\cdot\mathrm{cm}^{-2}$ 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$^{\mathrm{th}}$, 2004~\cite{GCN3002}.
MAGIC has enough sensitivity for observing events with fluences bigger than $2.5 \times 10^{-2}\mathrm{erg}\cdot\mathrm{cm}^{-2}\mathrm{s}^{-1}$ at 100\,keV, when a spectral index of $-2.0$ and 100\,s of observation time are assumed.
Therefore if an SGR as the giant flare of SGR1806-20 occurs, MAGIC would be able to detect its $\gamma$-ray emission.\\

Gamma-ray satellites react in the same way to XRFs, SGRs 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. We include therefore the observation of XRFs and SGRs by \ma in our proposal.


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