2 Fast follow-up observations

In order to make fast follow-up observations of GRBs possible, the MAGIC Telescope will have an interface (probably a socket connection) to the GRB Coordinate Network GCN (Barthelmy et al. 1994 and these proceedings) and a secondary ground station for direct communication with HETE II (Ricker et al., these proceedings). HETE II is a dedicated GRB research satellite which is expected to be launched in 2000. It will provide approx. 30 burst positions per year with accuracies better than 10 arcmin. These notifications are expected to arrive with a delay of less than 5 s. The MAGIC Telescope is specially designed to have low inertia such that the telescope can be positioned on any point in the sky within less than 30 s.

In case of a notification, a fast check of the observability of the GRB location will be performed. If the decision is positive, the present observations will be stopped immediately and a special fast drive will position the telescope on the GRB which will then be observed for the remaining night-time and probably also the following night in order to detect possible delayed emission.

Taking into account a decision time of 5 s at our site, we expect the reaction time between the actual start of the burst and the start of the follow-up observation to be $30\pm10$ s. The MAGIC Telescope will thus be able to perform observations of non-delayed emission of all bursts with durations above 30 s. This is the position of the rightmost peak in the BATSE burst duration (T₉₀) distribution (see e.g. Kouveliotou et al. 1995) and corresponds to a fraction $\approx$ 33% of all bursts which trigger BATSE on the 64 ms time scale.

The telescope will be built on the Canary Islands (Tenerife or La Palma). Cherenkov telescopes can only observe during the night. Observations will be possible up to zenith angles of $\approx 80^\circ$,i.e. about 40% of the total sky ($4\pi$) will be accessible. Assuming 30% of the nights to have bad weather and taking into account that the presence of the moon can prevent observations of certain positions, we arrive at a duty cycle of $\approx$ 10%.

The effective field of view of the photo-sensor camera of the MAGIC Telescope is 1.6$^\circ$ in diameter. We will therefore be able to safely observe any of the positions provided by HETE II and also some from the GCN and expect $\approx$ 5 serious immediate follow-up observations per year. For delayed emission (time-window of the order of an hour up to several days) this number will be larger.