Index: /trunk/MagicSoft/GRB-Proposal/Strategies.tex
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--- /trunk/MagicSoft/GRB-Proposal/Strategies.tex	(revision 6104)
+++ /trunk/MagicSoft/GRB-Proposal/Strategies.tex	(revision 6105)
@@ -76,12 +76,33 @@
 We determine the maximum zenith angle by requiring that the overwhelming majority of 
 possible GRBs will yield an in principle observable spectrum. Figure~\ref{fig:grh} 
+shows the gamma-ray horizon (GRH) as computed in~\cite{KNEISKE}. The GRH is defined as the 
+gamma-ray energy at which a part of $1/e$ of a hypothiszed mono-energetic flux is absorbed after 
+travelling a distance of $d$, expressed in redshift $z$ from the earth. One can see that at typical 
+GRB distances of $z=1$, all gamma-rays above 100\,GeV get absorbed before they reach the earth. 
+\par
+Even the closest GRB with known redshift ever observed, GRB030329~\cite{GRB030329}, lies at a redshift 
+of $z=0.1685$. In this case, gamma-rays above 200\,GeV get absorbed.
 
-
-\begin{figure}
+\begin{figure}[htp]
 \centering
-\includegraphics[width=0.99\linewidth]{f4.eps}
-\caption{}
+\includegraphics[width=0.85\linewidth]{f4.eps}
+\caption{Gamma Ray Horizon, as derived in~\cite{KNEISKE}}
 \label{fig:grh}
 \end{figure}
+
+\par
+We assume now an energy threshold of 50\,GeV for MAGIC at a zenith angle of $\theta = 0$. According 
+to~\cite{eckart}, the energy threshold of a Cherenkov telescope scales with zenith angle like:
+
+\begin{equation}
+E^{thr}(\theta) = E^{thr}(0) \cdot \cos(\theta)^{-2.7}
+\label{eq:ethrvszenith}
+\end{equation}
+
+Eq.~\ref{eq:ethrvszenith} leads to an energy threshold of about 900\,GeV at $\theta = 70^\circ$ and 
+500\,GeV at $\theta = 65^\circ$. Inserting these results into the GRH (figure~\ref{fig:grh}), one gets 
+a maximal observable GRB distance of $z = 0.1$ and $z = 0.2$, respectively. We think that the probability for 
+GRBs to occur at these distances is suffiently small in order to neglect the very difficult observations 
+beyond these limits.
 
 \subsection{In case of follow-up: Next steps}