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

\section{Proposed Observation Strategies}

A rough estimate of the needed observation time for GRBs derives
from the claimed GRB observation frequency of about 150-200 GRBs/year by the \sw
collaboration~\cite{SWIFT} and the results of the studies on the \ma duty-cycle
made by Nicola Galante~\cite{NICOLA}.
Taking into account the calculated duty-cycle of about 10\% and a time intervall of 5 hours
from the onset of the GRB, we should be able to point about 1--2 GRB/month.

\par

The duty-cycle studies are based on real weather data from the year 2002 taking the following criteria:

\begin{itemize}
\item maximum wind speeds of 10m/s
\item maximum humidity of 80\%
\item darkness at astronomical horizon
\end{itemize}

In these duty-cycle studies also full-moon nights were considered (requiring
a minimum angular distance between the GRB and the moon of 30$^\circ$).

\par

The duty-cycle in~\cite{NICOLA} will be increased by taking into account that \ma should also observe the afterglow emission of an burst that occured up to 5 hours before the start of the shift. Different GRB models predict delayed prompt GeV emission as well as acceleration of photons during the afterglows up to the threshold energy of \ma (for more details see chapter 5).

The afterglow observation is equivalent to an increase of the duty-cycle of about 6 days per month.

\subsection{GRB observations in case of moon shine}

{\it gspot} allows only GRBs with an angular distance of $> 30^\circ$ from the moon.
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 a movement. In principle
a fast moon-flash shouldn't damage the PMTs, but the behaviour
of the camera and the Camera Control {\it La Guagua} must
be tested. On the other hand, if such test conclude that it is not safe
to get even a short flash from the moon, the possibility
to implement a new feature into the Steering System must be considered
which follow a path around the moon while slewing.

\par

There was a shift observing the Crab-Nebula with half-moon at La Palma in December 2004.
The experience was that the nominal HV 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\%.
It is therefore mandatory that the shifters keep the camera in fully operational conditions with high-voltages switched on from the beginning of a half-moon night until the end. This includes periods where no other half-moon observations are scheduled. If no other data can be taken during the this periond, the telescope shuld be pointed in the north direction, close to the zenith. This increase the probability to overlap with the FOV of the satellites.

\par

Because of higher background with moon-light, we suggest to decrease the maximun zenith angle from
$\theta_{max} = 70^\circ$ to $\theta_{max} = 65^\circ$.

\subsection{Active Mirror Control behaviour}

To reduce the time before starting the observation, the use of the look-up tables (LUTs) is necessary.
Once generated, the {\it AMC} will use the LUTs and automaticaly focus the panels for a given telescope position. The {\it CC} should send the burst coordinates to the {\it Drive} and the {\it AMC} software in the same time. In this way the panels could be focussed already during the telescope movement.

\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 followed by a calibration run is taken.
\item During the data runs, interlaced calibration events are taken at 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 valuable time.

\subsection{Determine the maximum zenith angle}

We determine the maximum zenith angle for GRB observations by requiring that the overwhelming majority of possible GRBs will have an in principle observable spectrum. Figure~\ref{fig:grh}
shows the gamma-ray horizon (GRH) as computed in~\cite{KNEISKE,SALOMON}. The GRH is defined as the
gamma-ray energy at which a part of $1/e$ of a hypothesied mono-energetic flux gets 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 entirely 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 a current energy threshold of 50\,GeV for \ma at a zenith angle of
$\theta = 0$\footnote{As this proposal is going to be reviewed in a couple of months, improvements of the energy threshold will be taken into account then.}. According to~\cite{ecl}, 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 5.6\,TeV at $\theta = 80^\circ$,
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 $z = 0.1$ at $\theta = 70^\circ$ and $z = 0.2$ at $\theta = 65^\circ$.
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}

We propose to analyse the GRB data at the following day in order to tell whether a follow-up observation during the next night is useful. We think that a limit of 3\,$\sigma$ significance should be enough to start such a follow-up observation of the same place. This follow-up observation can then be used in two ways:

\begin{itemize}
\item In case of a repeated outbursts for a longer time period of direct observation.
\item In the other case for having off-data at exactly the same sky location.
\end{itemize}

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