source: trunk/MagicSoft/GRB-Proposal/Strategies.tex@ 6123

\section{Proposed Observation Strategies}

First of all let's consider how many observations are we going to do.\\

A rough estimation of the time consume due to GRB observation comes out
from the claimed GRB observation by SWIFT, of about 150-200 GRBs/year, and
the results on the studies on the MAGIC duty-cycle made by
Nicola Galante \cite{GALANTE} and Satoko Mizobuchi \cite{SATOKO}.
Considering a MAGIC duty-cycle of about 10\% and a tolerance of 5 hours
to point the GRB, we should be able to point about 1-2 GRB/month. 
Such duty-cycle studies, made before MAGIC started its observations,
are reliable as long as weather constraints that were considered
(~maximum wind speed of 10 m/s, maximum humidity of 80\% and
darkness at astronomical horizon~) revealed similar to the real ones that
are affecting MAGIC's observation time. In this duty-cycle study
also full moon night are considered useful (~just requiring
a minimum angular distance of the GRB from the moon of 30$^\circ$~),
while 3-4 nights per month are actually skipped because of full moon,
but this reduction of the real duty-cycle is about compensated
by the tolerance of 5 hours for considering the alert 
(~5 hours more before the beginning of the night useful
for getting GRB's alerts are equivalent to an increase 
of the duty-cycle of about 6 days per month~). Actually 
observation's interruptions due to technical tasks are
not considered here. \\

All this discussion tells us that, excluding from our
considerations interruptions of the observing time due to
technical tasks, MAGIC should employ 1-2 nights per month
in GRB observations. This means that we must do as much
as possible to observe them EVERY time that a useful
alert occurs.

\subsection{What to do with the AMC ? }

\ldots {\bf MARKUS G. } \ldots

\subsection{What to do with moon shine ? }

The telescope's slewing in case of a GRB alert will be done
without closing the camera lids, so that the camera could be
flashed by the moon during such movement. In principle
a fast moon-flash shouldn't damage the PMTs, but the behaviour
of the camera and of the Camera Control {\it guagua} must
be tested. On the other hand,, if such test concludes that it is not safe
at all to get even a short flash from the moon, the possibility
to implement a new feature into the Steering System which
follow a different path while slewing must be considered.
\par
There was a shift observing the Crab-Nebula with half-moon at La Palma in December 2004. 
The experience was that the nominal High-Voltages could be maintained and gave no 
currents higher than 2\,$\mu$A. This means that moon-periods can be used for GRB-observations 
without fundamental modifications except for full-moon periods. We want to stress that 
these periods increase the chances to catch GRBs by 80\%, even if full-moon observations are excluded 
\cite{NICOLA}. 
It is therefore mandatory that the shifters keep the camera in fully operational conditions with high-voltages 
already switched on from the beginning of a half-moon night until the end. 
\par
Because the background is higher with moon-light, we want to decrease then the maximun zenith angle from 
$\theta^{max} = 70^\circ$ to $\theta^{max} = 65^\circ$. 

\subsection{Calibration }

For ordinary source observation, the calibration is currently performed in the following way: 
\begin{itemize}
\item At the beginning of the source observation, a dedicated pedestal run following by a calibration run is 
taken.
\item During the data runs, interlaced calibration events are taken with a rate of 50\,Hz.
\end{itemize}

We would like to continue taking the interlaced calibration events when a GRB
alert is launched, but leave out the pedestal and calibration run in order not to loose valueable time.

\subsection{Determine the maximum zenith angle}

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}[htp]
\centering
\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 sufficiently small in order to neglect the very difficult observations 
beyond these limits.

\subsection{In case of follow-up: Next steps}

Analysis during day:
\par
If some significance is seen, observe the same position next night to get some OFF-data.