source: trunk/MagicSoft/GRB-Proposal/Monitor.tex@ 9168

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1\section{The Burst Alarm System at La Palma}
2
3{\bf Current status:}
4
5\par
6
7The Burst Alarm System {\it gspot} (Gamma
8Sources Pointing Trigger) is installed and working in La Palma since last Summer.
9It performs a full-time survey of the {\it GRB Coordinates Network} (\g) alerts~\cite{GCN}.
10Different satellite experiments
11send GRB coordinates to the \g which in its turn broadcasts
12the alerts to registered users.
13The Burst Alarm System is composed of a core program which
14manages the monitoring of the \g and the communication with the Central Control (CC).
15It also handles three communication channels to notice the shifters
16about an alert. It is a c-based daemon running 24
17hours a day on the {\it www} machine, our external server, in a
18{\it stand alone} mode. It does not need to be operated and is
19fully automatic. It manages network disconnections
20within the external net and/or the internal one.
21
22
23\subsection{The Connection to the GCN}
24
25The connection to the \g is performed by {\it gspot} through a
26TCP/IP connection to a computer at the Goddard Space Flight Center (GSFC).
27This computer distributes the alerts from the satellite
28experiments through an internet socket connection. {\it gspot}
29acts as a server, while the client, running at the GSFC,
30manages the communication of the GRB data the status.\\
31
32The format of the data distributed via \g depends on the broadcasting satellite
33and on the kind of package. Currently three satellites participate in the GRB survey:
34HETE-2~\cite{HETE}, INTEGRAL~\cite{INTEGRAL} and SWIFT~\cite{SWIFT}.
35The alerts include the UTC, the GRB coordinates (not always), error on coordinates
36(not always) and intensity (photon counts) of the burst.
37The first notices from HETE-2 and INTEGRAL usually do not include the coordinates.
38In few cases only coordinates are distributed in refined notices.
39The \sw alerts are predicted to arrive with coordinates between 30-80 sec after the onset of the burst.
40The error on the coordinates from the BAT detector will be 4 arcmin which is smaller than the size of one
41inner pixel of the \ma camera.\\
42
43In case of alert, {\it gspot} stores the informations and enters into
44an {\bf Alarm State}. The duration of the alarm depends on the following parameters:
45
46\begin{itemize}
47\item {\bf Darkness of the sky}: The Sun has to be below
48the astronomical horizon or have a zenith angle larger than 108$^\circ$.
49\item {\bf Position of GRB}: The GRB equatorial
50coordinates are transformed into local horizontal coordinates.
51The resulting GRB zenith angle has to be smaller than 70$^\circ$. If the Moon is
52shining, the maximal zenith angle is reduced to 65$^\circ$.
53\item {\bf Position of Moon}: The angular
54distance from the GRB to the Moon has to be at least 30$^\circ$. This constant
55value of 30$^\circ$ will change in the future as soon as the camera experts
56will provide a plot of the safe distance from the Moon vs. Moon phase.
57Therefore such dynamical limit for this value will be used.
58\end{itemize}
59
60If one or more of these conditions fail, {\it gspot} enters into a
61{\color[rgb]{0.9,0.75,0.}\bf Yellow Alarm State} (it means the GRB is not observable at the moment).
62In this case the program saves the alert in a list and calculates when the GRB will become observable for \ma.
63At the moment when the criteria listed above are fulfilled for this burst, and the time interval
64after the burst onset is smaller than 5 hours, {\it gspot} enters into \textcolor{red}{\bf Red Alarm State}.
65If all the mentioned conditions are satisfied from the beginning, {\it gspot} enters
66into \textcolor{red}{\bf Red Alarm State} immediately.
67If more than one alert is recived and the burst cannot be observed immediately,
68the alert information are saved in a list.
69The software weights the alerts according the total amount of time in which
70the GRB will be observable, the delay from the onset of GRB observability,
71the intensisty of the burst and the mean GRB zenith angle during its
72period of observability.
73The best candidate is sent to the CC as soon as {\it gspot}
74enters into the \textcolor{red}{\bf Red Alarm state}, i.e. as soon as such
75candidate becomes observable.\\
76
77However, in case of \textcolor{red}{\bf RED Alarm State},
78if the communication with the CC is available then {\it gspot} sends to it
79the GRB equatorial coordinates (RA/DEC J2000).
80For the communication with CC, format defined in~\cite{CONTROL} is used.
81At the same time, shifters and the GRB-MAGIC group are contacted.
82
83\subsection{The Interface to the Central Control}
84
85An interface of {\it gspot} sends all the relevant information to the CC.
86When {\it gspot} is not in alarm state, standard packages are continuously exchanged between CC and {\it gspot}.
87These packages contain the main global status of the two subsystems.
88In case of \textcolor{red}{\bf RED alert}, {\it gspot} starts to send special alert packages to the CC
89containing information about the GRB.
90The exchange of the alert packages continues until:
91
92\begin{itemize}
93\item {\it gspot} receives from the CC the confirmation
94that the alert notice has been received (CC must send back the alert in order
95to perform a cross-check of relevant data);
96\item the \textcolor{red}{\bf RED Alarm state} expires because of the
97missing of one or more of the needed criteria mentioned above;
98\item the alarm state expires after {\bf 5 hours}.
99\end{itemize}
100
101In the case of a \textcolor{red}{\bf RED alert} CC shows a pop-up window
102with all the important alert information received from the Burst Monitor.
103The operator has to confirm the notice by closing the pop-up window.
104He can decide whether to stop the current scheduled observation or to point the GRB coordinates.
105A new button is displayed in the CC allowing to point the telescope directly
106the GRB coordinates.
107
108\subsection{GRB Archive and Emails to the GRB-mailing List}
109
110In case of alert -- even if it does not contain the necessary coordinates -- the
111information is translated into ``human language'' and stored in ASCII files.
112At the same time, an e-mail is sent to the MAGIC GRB-mailing list
113{\it magic\_grb@mppmu.mpg.de}.
114
115\subsection{The GRB Web Page}
116
117The status of the GRB Alert System and relevant informations about the
118current and/or the last alert are displayed on a separate web page.
119The page is hosted at the web server in La Palma and can be accessed under:\\
120
121\qquad \qquad http://www.magic.iac.es/site/grbm/\\
122
123The web page updates itself automatically every 10 seconds. In this way
124the status of the Burst Alarm System can be checked by the shifters and from outside too.
125
126\subsection{The Acoustic Alert}
127
128A further CC-independent acoustic alarm called {\it phava}
129(PHonetic Alarm for Valued Alerts) will be installed
130in La Palma soon. It will provide a loud acoustic signal
131even if the CC is switched off, so that persons in the counting house
132can be noticed about the alert situation. The signal will be on as long as
133{\it gspot} remains in alarm state for a minimum of one minute.
134The device features also a display with the status of the system and the alert.
135
136\subsection{Summary of Alerts Received Until Now}
137
138Since July 15$^{\mathrm{th}}$, 2004, {\it gspot} has been working stably at La Palma.
139It received about 100 alerts from HETE-2 and INTEGRAL, out of which
14019 contained GRB coordinates. Time delays to the onset of the burst
141were of the order of several minutes to tens of minutes. The Burst Monitor can be considered stable
142since November 2004. Since that date we have received the following four significant alerts:\\
143
144\begin{tabular}{lllcccl}
14519th & December & 2004 & 1:44 am & INTEGRAL & ZA $\sim 60^\circ$ & time delay 71 s \\
14628th & January & 2005 & 5:36 am & HETE-2 & ZA $\sim 65^\circ$ & time delay 73 m \\
147\end{tabular}
148
149It's a pitty that weather conditions at La Palma were bad.
150
151\subsection{Experience from SWIFT GRBs until now}
152
153According to the \sw home page~\cite{SWIFT}, the satellite has detected 20 GRBs since mid-December last year.
154The bursts were detected by chance during the commissioning phase. Since February 15$^{\mathrm{th}}$
155the satellite sends burst allerts to the \g in real time. The current sample contains five bursts which could
156have been observed by \ma. \\
157
158\begin{tabular}{lllccl}
15919th & December & 2004 & 1:42 am & ZA $\sim 65^\circ$ & \\
16026th & December & 2004 & 8:34 pm & ZA $\sim 52^\circ$ &\\
16115th & Februar & 2005 & 2:33 am & ZA $\sim 17^\circ$ &\\
1625th & March & 2005 & 8:42 pm & ZA $\sim 40^\circ$ & time delay 40 s \\
1635th & March & 2005 & 10:23 pm & ZA $\sim 70^\circ$ & time delay 80 s \\
164\end{tabular}
165
166In the first three alerts weather conditions in La Palma were bad. In the last two
167a couple of GRBs were detected within two hours by SWIFT. They were observable since
168their own onset and for all the following 5 hours. The weather was good, but unfortunately
169the Telescope was off-service because of the exceptional events occured in La Palma
170during the previous weeks.
171
172\subsection{Comparison between the Satellite Orbits}
173
174Figure~\ref{fig:orbit} shows the orbits of the \sw, \he and \ig satellites.
175\sw and \he satellites are situated in a circular orbit with
17620.6$^\circ$ and 2$^\circ$ inclination, respectively.
177One revolution of \sw and \he satellites lasts about 100\,min.
178\ig satellite has a highly eccentric orbit with a revolution period of three sidereal days around the Earth.
179
180\par
181
182It is difficult to draw strong conclusions from the individual satellite orbits.
183The orientation of satellites FoV is influenced by the scheduled targets.
184However, \sw is the satellite with the largest inclination and overlaps mostly with the FoV of \ma.
185This increases the chance to receive {\bf Red Alerts} from this satellite.
186
187\begin{figure}[htp]
188\centering
189\includegraphics[width=0.6\linewidth]{GCNsatellites.eps}
190\caption{Orbits of \sw (top), \he (center) and \ig (bottom) satellites: dot lines
191show the orbit while drawn lines show the horizon of the Sun. Here, a typical night at
192La Palma is shown. \sw satellite passes over Roque seven times each night.}
193\label{fig:orbit}
194\end{figure}
195
196
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