1 | \section{Introduction}
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2 |
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3 | \subsection{Observation of GRBs}
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4 |
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5 | The support structure and mirrors of the \ma telescope were designed to be exceptionally light in order to
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6 | react quickly to GRB alerts from satellites. The aim was to turn the telescope toward the burst position
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7 | within 30\,s~\cite{design,PETRY},
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8 | in order to have a fair chance to detect a burst when the prompt $\gamma$--emission is still ongoing.
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9 | During the commissioning phase, it could be proven that the goal was achieved.
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10 | The telescope is able to turn $180^\circ$ in azimuth within 20\,s and $90^\circ$ in zenith within 10\,s.\\
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11 |
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12 | Very high energy (VHE) GRB observations have the potential to constrain the current GRB models
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13 | on both the prompt and the extended phase of GRB emission~\cite{HARTMANN,MANNHEIM}.
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14 | Models based on either internal or external shocks predict VHE gamma-ray fluences comparable to,
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15 | or in certain situations stronger than, the keV-MeV radiation,
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16 | with durations ranging from shorter than the keV-MeV burst to extended TeV
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17 | afterglows~\cite{DERMER, PILLA, ZHANG1, RAZZAQUE}.
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18 |
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19 | \par
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20 |
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21 | Many publications foresee that high-energy $\gamma$-rays can come along with the
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22 | (low-energy) GRB. Possible causes range from proton-synchrotron emission~\cite{TOTANI} to
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23 | photon-pion production~\cite{WAXMAN,BOETTCHER} to inverse-Compton scattering
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24 | in the burst environment~\cite{MESZAROS93,CHIANG,PILLA,ZHANG2,BELOBORODOV}.
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25 | A long-term high energy (HE) $\gamma$-emission can come from accelerated protons in the
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26 | forward-shock, as predicted in~\cite{LI}.
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27 | This model predicts GeV inverse Compton emission even one day after the burst.
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28 | Even considering pure electron-synchrotron radiation, measurable GeV-emission for a significant
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29 | fraction of GRBs is predicted~\cite{ZHANG2}.\\
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30 |
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31 | GeV-emission in GRBs is particularly sensitive to the Lorentz factor and the photon density of the
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32 | emitting material --
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33 | and thus to the distance of the radiating shock from the source -- due to the
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34 | $\gamma \gamma \rightarrow \textrm{e}^+\textrm{e}^-$ absorption in the emission region.
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35 | Direct comparison of the prompt GRB flux at $\sim$\,10\,GeV and $\sim$\,100\,keV may
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36 | allow to determine the magnetic field strength~\cite{ASAF2}.
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37 |
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38 | \par
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39 |
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40 | Several attempts were made in the past to observe GRBs in the GeV range,
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41 | each indicating some excess over background but without stringent evidence.
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42 | The only significant detection was performed by \eg, that was able to observe seven GRBs
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43 | emitting HE photons with energies between 100\,MeV and 18\,GeV~\cite{EGRET, DINGUS1}.
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44 | The data shows no evidence of a HE cut-off in the GRB spectrum~\cite{DINGUS2}.
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45 | Recent results indicate that the spectrum of some GRBs contains a very hard, luminous, long-duration component~\cite{GONZALES}.
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46 | There have been results suggesting gamma rays beyond the GeV range from the TIBET air shower array
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47 | in coincidence with BATSE bursts~\cite{AMENOMORI}, rapid follow-up observations by the
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48 | Whipple Air Cherenkov Telescope~\cite{CONNAUGHTON1}, and coincident and monitoring studies by
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49 | HEGRA-AIROBICC~\cite{PADILLA}, Whipple~\cite{CONNAUGHTON2} and the Milagro prototype Milagrito~\cite{MILAGRO}.
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50 | The GRAND array has reported some excess of observed muons during seven BATSE bursts~\cite{GRAND}.
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51 | In this context, especially the publication from the TASC detector on \eg is important~\cite{GONZALES},
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52 | finding a HE spectral component presumably due to the ultra-relativistic acceleration of hadrons and
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53 | producing a spectral index of $-1$ with no cut-off up to the detector energy limit at 200\,MeV.\\
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54 |
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55 | To estimate the observability of GRB by \ma, sources of the
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56 | third and fourth \ba catalogue were studied~\cite{ICRC,NICOLA}. Their spectra were extended to GeV
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57 | energies with a simple power-law and using the observed high-energy spectral index: the extrapolated fluxes
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58 | were at last compared with \ma sensitivities. Setting conservative cuts on observation times and significances,
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59 | and assuming an energy threshold of 15~GeV, a 5\,$\sigma$-signal rate of $0.5-2$ per year
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60 | was obtained for an assumed observation delay between 15 and 60\,s and a \ba trigger rate
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61 | ($\sim$\,360/year). As the \sw alert rate is about a factor~2 lower, including even fainter bursts than
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62 | those observed by \ma, this number still have to be lowered.
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63 |
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64 | Taking into account the local rate of GRBs estimated in~\cite{GUETTA}, late afterglow emission from a
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65 | few tens of GRBs per year should be observable over the whole sky above our energy threshold.
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66 | The model of~\cite{ASAF2} predicts delayed GeV-emission that should be significantly detectable by \ma
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67 | in 100\,s.
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68 |
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69 | \subsection{Observation of XRFs}
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70 |
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71 | While the major energy from the prompt GRBs is emitted in $\gamma$-rays with a peak energy of 200\,keV,
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72 | X-ray flashes (XRFs) are characterized
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73 | by peak energies below 50~keV and a dominant X-ray fluence. Because of similar properties, a connection
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74 | between XRFs and GRBs is suggested.
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75 | Some theories~\cite{DADO} suggest that XRFs are produced from GRBs observed ''off-axis''.
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76 | Alternatively, an increase of the baryon load within the fireball itself~\cite{HUANG} or low efficiency
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77 | shocks~\cite{BARRAUD} could produce XRFs.
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78 | If there is a connection between XRFs and GRBs, they should originate at rather low redshifts ($z < 0.6$)
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79 | because otherwise, the XRF energies would not fit into the observed correlation
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80 | between GRB peak energy and isotropic energy release~\cite{LEVAN}. \\
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81 |
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82 | Gamma-ray satellites react in the same way to XRFs and GRBs.
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83 | In case of a detection the coordinates are distributed
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84 | to other observatories (see section 2.1). Only from later analysis the difference can be established.
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85 |
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86 | \par
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87 |
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88 | We include therefore the observation of XRFs by \ma in this proposal.
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89 |
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90 | \subsection{Observation of SGRs}
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91 |
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92 | Soft Gamma Repeaters (SGRs) are believed to be extremely rare strong magnetic neutron stars that
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93 | periodically emit $\gamma$-rays. Only four identified SGRs were discovered in the last 20 years:
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94 | SGR0526-66, SGR1806-20, SGR1900+14, SGR1627-41.
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95 | GRBs and SGRs can be explained within the same gamma jet model where the jet is observed at different
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96 | beam-angles and different times~\cite{FARGION}.\\
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97 |
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98 | The BAT instrument on the SWIFT satellite triggered on an outburst from SGR1806-20 on January $30^{\mathrm{th}}$, 2005.
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99 | The fluence was about $10^{-5}$\,erg/cm$^2$ in the range between 15 and 350\,keV.
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100 | This event was five orders of magnitude smaller than the giant flare from this source on the
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101 | December 27$^{\mathrm{th}}$, 2004~\cite{GCN3002}.
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102 | 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.
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103 | Therefore if an SGR as the giant flare of SGR1806-20 occurs, MAGIC would be able to detect its $\gamma$-ray emission.
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104 |
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105 | %%% Local Variables:
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106 | %%% mode: latex
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107 | %%% TeX-master: "GRB_proposal_2005"
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108 | %%% End:
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