MAGIC upper limits on the high energy emission from GRBs

During its cycle-1 observation period, between April 2005 and March 2006, the MAGIC telescope was able to observe nine Gamma Ray Burst (GRB) events since their early beginning. Other observations were performed during the following months in the cycle-2 observation period. The observations, with an energy threshold spanning from 80 to 200 GeV, did not reveal any gamma ray emission. The computed upper limits are compatible with a power law extrapolation, where intrinsic fluxes are evaluated taking into account the attenuation due to the scattering in the metagalactic radiation field.


Introduction
The Major Atmospheric Gamma Imaging Cherenkov (MAGIC) telescope [9], located on the Canary Island of La Palma, is currently the largest Imaging Air Cherenkov Telescope (IACT). MAGIC has a 17 m diameter tessellated reflector dish consisting of 964 (0.5 × 0.5m 2 ) diamond-milled aluminium mirrors. In its current configuration, the MAGIC photo-multiplier camera has a trigger region of 2.0 • diameter [5], and a trigger collection area for γ-rays of the order of 10 5 m 2 , which increases further with the zenith angle of observation. Presently, the accessible trigger energy ranges from 50 − 60 GeV (at small zenith angles) up to the TeV region close to the horizon. The MAGIC telescope is focused to 10 km distance -the most likely height at which a 50 GeV γ-ray shower has its shower maximum. The accuracy in reconstructing the direction of incoming γ-rays, the point spread function (PSF), is about 0.1 • , slightly depending on the analysis. In this contribution we report the results of observation of GRBs by the MAGIC telescope during cycle-1. Preliminary results from a selected sample of GRBs of cycle-2 are also included. The techni-cal details of the MAGIC observations are reported in a separate contribution [3].

The Observations
During the fourteen months of observation cycle-1 nine GRB events suitable for analysis were selected. Approximately the same amount of events was also collected during cycle-2, spanning from May 2006 up to May 2007, out of which four were analyzed in a preliminary way. Table 1 summarizes the thirteen GRBs observed by MAGIC up to now with their principal features obtained from the GCN circulars. The majority of the events were triggered by the Swift satellite. GRB050502a and GRB060121 were triggered by Integral and HETE-2 respectively. Apart from GRB060121 and GRB061217, all GRBs observed by MAGIC are long bursts. In two cases, for GRB050713a and GRB050904, the fast repositioning of the telescope allowed the observation of their prompt emission phase [1,3], while the burst was still emitting low-energy γrays. In six cases the early GRB afterglow observation with MAGIC overlapped with the X-ray observation from space, namely for GRB050421, GRB050713a, GRB050904, GRB060206, GRB060825 and GRB061028. During cycle-1, in the case of GRB050421 the X-Ray Telescope (XRT) on board Swift detected two flaring events in the afterglow at T 0 + 110 s and T 0 + 154 s, as well as in GRB050904, where a clear flare was detected at T 0 + 466 s. In these two cases data taking with the MAGIC telescope started at T 0 + 108 s and T 0 + 145 s respectively, the flaring activity in X-rays was therefore covered by the MAGIC observation window. During cycle-2 only in the case of GRB060825 flaring X-ray emission was also covered by the MAGIC observation window.

Analysis and Results
For the reconstruction of γ-events the standard MAGIC analysis software [4] based on the Hillas analysis [8] was used. The reconstructed signal was calibrated and cleaned from spurious backgrounds using an image cleaning algorithm which requires a signal exceeding fixed reference levels, improved with the reconstructed information of the arrival time [6]. Some additional quality cuts based on combinations of Hillas parameters were ap-plied to remove non-physical images from the data. The separation between γ and hadron showers was performed by means of the Random Forest (RF) method, a classification method that combines several parameters describing the shape of the image into a new parameter called hadronness [7]. The energy of the γ-ray was also estimated using the RF approach, yielding a resolution of ∼ 30% at 200 GeV.
Dedicated OFF-data samples were selected for each GRB according to the same observation conditions (zenith angle, discriminator threshold, trigger rate, Moon phase). The cut on hadronness, ensuring at least 90% of efficiency on γ-like events, was optimized using a dedicated Monte Carlo simulation. In the final analysis step a cut on the alpha parameter was chosen. This parameter is related to the direction of the incoming shower, thus it is expected to peak at alpha = 0 • if the telescope points directly at the source, while it is uniformly distributed for background events. The alpha parameter is used to evaluate the significance of the signal. No significant excess of VHE γ-rays could be extracted with the standard analysis approach. In parallel to the analysis of the complete data set a search for short emission periods in different time bins was performed. Also with this approach no significant deviation of the number of excess

Conclusions
We presented the results of thirteen GRB observations performed by MAGIC during the last two years of operation. As a response to the GCN alerts provided by dedicated satellites, MAGIC was able to start the observation during the early afterglow emission phases, while the X-ray emission was still visible. In three cases the MAGIC observation covered flaring activity in X-rays. In two cases the observation started even when the prompt emission in γ-rays was still active. No significant γ-ray emission above ∼ 100 GeV was detected. We derived upper limits for the γ-ray flux between 85 and 1000 GeV. These limits are compatible with a naive extension of the power law spectrum, when the redshift is known, up to hundreds of GeV. For the first time a IACT is able to perform direct rapid observations of the prompt emission phase of GRBs. This is particularly of interest to extend the observational energy range in the so called "Swift era" and for the future experiments as AGILE and GLAST.