4.6.2. VHE Observations and Results

Final results from VHE data analysis of GRB 201216C have not been published yet. Preliminary information reported here have been released in [228,229]. MAGIC observations and data taking of GRB 201216C started with a delay of 56 s after the Swift-BAT trigger time. The observation lasted for 2.2 h and was performed in optimal atmospheric condition and in absence of the moon. The zenith angle ranged from 37◦ to 68◦ . The low level of night sky background was also allowed to retain the low energy events and therefore obtain a low energy threshold compared to the other GRBs observed. To keep as many low-energy events as possible, an image cleaning method to extract dimmer Cherenkov showers initiated by gamma rays than the standard method was adopted.

The signal significance was calculated to be 6.0*σ* pre-trial (5.9*σ* post-trial<sup>7</sup> ) for the first 20 min of observation (see Figure 27). A preliminary time-integrated spectrum for the first 20 min of observation was produced. Due to the strong absorption effect by EBL, a very steep power-law decay was found for the observed spectrum, especially for the events with energies higher than a few hundred of GeV. The intrinsic spectrum, corrected for the EBL absorption, was found to be consistent with a flat single power-law until 200 GeV above, in which no significant spectral points have been derived. A preliminary light curve in the time interval 56 s–2.2 h was also calculated. After 50 min, only upper limits on the emitted flux have been derived, as no significant emission was found after this time. The preliminary results are consistent with a monotonically decaying light curve fitted with a power-law.

**Figure 27.** *θ* <sup>2</sup> angular distance distribution between reconstructed event arrival directions and nominal source position for GRB 201216C. The gray histogram represents background events while the black point with blue crosses are the *γ*-like events. The vertical dashed line describes the *θ* 2 cut value and defines the region in which excess events and signal significance are calculated. From [229].
