MAGIC TELESCOPE OBSERVATIONS OF GAMMA-RAY BURSTS

1. Deciphering the Ancient Universe with Gamma-Ray Bursts · Kyoto, Japan, April 2010 MAGIC TELESCOPE OBSERVATIONS OF GAMMA-RAY BURSTS M. Garczarczyk1, A. Antonelli2, D. Bastieri3, J. Becerra-González1, A. Carosi2, S. Covino4, A. Dominguez-Diaz5,M. Gaug1, A. La Barbera2, F. Longo6, V. Scapin7 and S. Spiro2 on behalf of the MAGIC collaboration* 1. Instituto Astrofísica de Canarias, Tenerife, Spain 4. Osservatorio Astr. di Brera and INAF, Merate, Italy 7. Universitá di Udine and INFN, Udine, Italy 2. Osservatorio Astr. di Roma and INAF, Rome, Italy 5. Instituto de Astrofísica de Andalucía, Granada, Spain * Complete member list: wwwmagic.mppmu.mpg.de 3. Universitá di Padova and INFN – Padova, Italy6. Universitá di Trieste and INFN, Trieste, Italy Abstract MAGIC is built to perform observations of prompt and early afterglow emission from Gamma-Ray Bursts (GRBs) above 25 GeV. The instrument is designed to have the lowest possible energy threshold among the ground based gamma ray detectors and the fastest reaction time to alerts distributed over the GRB Coordinates Network (GCN). The MAGIC-I telescope observed 57 GRBs during the first six years. In non of the cases Very High Energy (VHE) gamma ray emission above the threshold energy could be detected. The telescope has undergone several major improvements in sensitivity and repositioning performance. The biggest improvement in sensitivity was achieved with the installation of the second MAGIC-II telescope. MAGIC is the only telescope fast and sensitive enough to extend the observational energy range of satellite detectors, while GRB prompt and early afterglow emission is still ongoing. Introduction GRBs are known to emit high energy radiation up to the detector sensitivity limits of space born detectors. At these energies gamma ray emitting sources have to lie at small redshifts to be detectable, given the absorption of gamma rays by the extragalactic background light. Typical GRBs are located at redshifts around and higher than z=1, making it mandatory to lower the energy threshold of ground based experiments well below 100 GeV. This goal was achieved with MAGIC [1] using an upgraded sum-trigger system and demonstrated with the detection of the Crab pulsar at 25 GeV [2]. Consequently, the new trigger system is nowadays also used for most GRB observations carried out by the MAGIC-I telescope. Observation criteria Due to the small field of view (3.5o) MAGIC is obliged to follow GRB alerts provided by GCN. The alerts are received directly over TCP/IP internet socket connection and are validated with the observation criteria: • Zenith angle of the Sun >103° (includes twilight 103° - 108°) • Angular distance to the Moon > 30° • Zenith angle of the GRB < 60°, in case of Moon shine < 55° If these conditions are fulfilled the GRB candidate is observed within the time of maximum 4h starting from the GRB onset. Due to possible large localization error from the FERMI GBM detector the above criteria are extended with: • In addition for FERMI GBM/LAT alerts: • Error on coordinates < 2° • Significance > 100 • Hardness ratio < 1 If no updated coordinates with error < 1.5° are received the observation stops already after 1h. GRB observations have highest priority in MAGIC. In the case of an observable GRB event the ongoing observation is stopped immediately and the new source is automatically followed. As the redshift of the source is normally only known a few hours to days later, one is obliged to observe all candidates, although a later redshift measurement can classify the observation as useless. As the energy threshold of an telescope as MAGIC depends sensitively on the observation zenith angle, most GRBs are observed with higher thresholds than the lowest possible. In the following the interpretation of the results from one selected GRB are presented. The most updated list of GRB candidates observed by MAGIC can be followed at: http://mojorojo.magic.iac.es/grb/ The image above shows the MAGIC telescopes located on 2200 m a.s.l. at the Observatorio del Roque de Los Muchachos on the canary island of La Palma. Each telescope is composed of 236 m2 light weight Al- and glass sandwich structured mirror segments. The dish is made of carbon fiber tubes, which further reduce the total weight of the telescope. An active mirror control system corrects automatically dish deformations at varying elevation angles and keeps the overall focus at the optimum. The light weight design allows the telescope to slew very fast to any position on the sky. Repositioning to the opposite location of the sky, e.g. 180° in azimuth, is reached within 17 s. The cameras are equipped with 0.1° diameter photomultipliers whose photon sensitivity provides a peak in the quantum efficiency above 30% at 400 nm. The large mirror area, high quantum efficiency photomultipliers and sophisticated trigger and readout electronics provides MAGIC with an energy threshold well below 100 GeV. GRB080430 This burst become an interesting target for us to model the expected VHE component and compare it with the results from our observation as: (1) it was observed by many detectors covering a wide range of energy and time and (2) the relative moderate redshift of z=0.758 allows gamma rays in the energy range between 80-125 GeV to reach our detector [3]. The figure above shows the predictions of the SSC model for different time delays from the burst onset and different absorption models by the extragalactic background light. Upper limits obtained from the MAGIC-I telescope observation are also shown. The result indicate the importance of short observation delays to the burst onset, as the flux is changing by an order of magnitude within tens of minutes. As a matter of fact, this burst was an average event in terms of energetics. More energetic GRBs are relatively common and due to the positive dependence on the isotropic energy of an GRB, much higher fluxes than the presented can be foreseen. References [1] See: http://wwwmagic.mppmu.mpg.de [2] E. Aliu et al., Science 322, 1221, 2008 [3] J. Aleksić et al., subumitted to A&A, 2010