4.1.2. VHE Observations and Results

The MAGIC telescopes started the follow-up of GRB 160821B with a very short delay of 24 s after *T*<sup>0</sup> and continued observations for about 4 h [145]. The observations were performed with a relatively high Night Sky Background (NSB) (2–8 times brighter than in dark nights) due to the presence of the Moon, and in mid-high zenithal angle conditions (from 34◦ to 55◦ ). Unfortunately, the first ∼1.7 h of the data were strongly affected by clouds. As a result, dedicated and optimized software configurations were used. A more stringent image cleaning with respect to dark conditions was applied to take into account the spurious contribution coming from the high NSB. The analysis required cuts optimized on the Crab Nebula and on Mrk421 observed in similar conditions, and correction factors for the low atmospheric transmission, calculated thanks to the LIDAR facility [155].

The pre-trial analysis showed the presence of a 3.1*σ* (2.9*σ* post-trial<sup>4</sup> ) significant excess at the GRB position provided by Swift-XRT (see Figure 10). The flux has been estimated for energies above 0.5 TeV assuming a power-law spectrum with photon index *α* = −2. In the first 1.7 h, where data taking was affected by bad atmospheric transmission, only flux upper limits could be derived. This time window has been divided into two time intervals (24–1216 s and 1258–6098 s) and the derived upper limits are, respectively, 1.1 <sup>×</sup> <sup>10</sup>−<sup>11</sup> cm−<sup>2</sup> s <sup>−</sup><sup>1</sup> and 5.4 <sup>×</sup> <sup>10</sup>−<sup>12</sup> cm−<sup>2</sup> s −1 . In the subsequent 2.2 h (6134–14,130 s) assuming that the signal is real, a flux value could be calculated and is 9.9 <sup>±</sup> 4.8 <sup>×</sup> <sup>10</sup>−<sup>13</sup> cm−<sup>2</sup> s −1 . For the same time interval, a flux upper limit has also been estimated and gives 3.0 <sup>×</sup> <sup>10</sup>−<sup>12</sup> cm−<sup>2</sup> s −1 . All the mentioned fluxes are shown in Figure 11 (red symbols) and refer to the observed values, i.e., without correcting for EBL absorption. Upper limits have been calculated at a 95% confidence level following the prescriptions of [156]. The low (∼3*σ*) significance estimated did not allow us to obtain an unfolded spectrum. As a result, in the SED (Figure 12) the reconstructed flux in the third bin (6134–14,130 s) over the energy range 0.5–5 TeV is represented as an error box. The statistical error on the photon flux has been taken into account, while, for simplicity, the systematic error for the assumed spectral index was neglected. The flux inferred by MAGIC observations in the 0.5–5 TeV energy range would imply a TeV luminosity at least 5 times larger (when de-absorbed by EBL) than the luminosity emitted in the X-ray band.

**Figure 10.** GRB 160821B: angular distance distribution *θ* <sup>2</sup> between the nominal source position and the reconstructed event arrival directions. The gray histogram represents the background events, while the black points with blue crosses are the *γ*-like events. The vertical dashed line marks the *θ* 2 cut value and defines the region in which excess events and signal significance are calculated. From [145]. ©AAS. Reproduced with permission.
