4.2.2. VHE Observations and Results

The H.E.S.S. telescopes followed-up the event for ∼2 h starting from 10.1 h, revealing the presence of a source with a 5.3*σ* pre-trial significance (5.0*σ* post-trial<sup>6</sup> ). The observation was performed in standard dark and good weather conditions with a mean zenith angle of 31.5◦ . Another observation was performed under similar conditions 18 days after the previous one with results consistent with background events. The inferred flux at ∼11 h and the flux upper limit at 18 d are shown in Figure 14 (red symbols).

The observed spectrum in the 0.1–0.44 TeV energy band has been fitted both with a power-law (panel on the left in Figure 15) and with a power-law with a cutoff *Fint* = *F*0,*int E E*0 −*γint* , to describe an intrinsic power-law spectrum affected by the EBL attenuation *e* −*τ*(*E*,*z*) (see Figure 15, panel on the right). With reference to the second fit, the analysis returns a photon index *γint* = 1.6 ± 1.2 (statistical) ±0.4 (systematic) and a flux normalization *<sup>F</sup>*0,*int* = (7.52 <sup>±</sup> 2.03 (statistical) <sup>+</sup>4.53 <sup>−</sup>3.84) (systematic) <sup>×</sup>10−<sup>10</sup> TeV−<sup>1</sup> cm−<sup>2</sup> s −1 , evaluated at an energy *E*0,*int* = 0.154 TeV.



**Figure 14.** GRB 180720B: broad-band light curves and photon index evolution including optical, X-ray and VHE data. For the late time H.E.S.S. observation (18 days after the trigger) an UL has been derived. A flux temporal decay with index 1.2 (similar to what observed in optical and X-ray) is also shown in black dashed line. From [4].

**Figure 15.** GRB 180720B: spectral fit to the observed emission in the 0.1–0.44 TeV energy range. In the left panel (**a**), the observed spectrum is assumed to be described by a power-law model, while in the panel on the right (**b**), the spectrum is an power-law attenuated by EBL. Statistical and systematic uncertainties at a 1*σ* confidence level are shown with shaded areas. From [4].
