4.4.1. General Properties and Multi-Wavelength Observations

The prompt emission detected by the two instruments consists of two episodes, with the first one observed in the time interval from the trigger time to 4 s and the second brighter episode from 47 s to 61 s. The two episodes have very different spectral properties: the first one is described by a power-law with index −1.41 ± 0.08 and an exponential high-energy cutoff function with *E<sup>p</sup>* = 130 ± 20 keV, the second one can be described with a Band function with *E<sup>p</sup>* = 11 ± 1 keV, *α* = −0.92 ± 0.62 and *β<sup>T</sup>* = −2.51 ± 0.01 [189]. The (isotropic equivalent) prompt emission energy inferred from the spectral analysis of Fermi-GBM data is *<sup>E</sup>γ*,*iso* <sup>∼</sup> <sup>2</sup> <sup>×</sup> <sup>10</sup><sup>50</sup> erg.

A multi-wavelength observational campaign of the event was performed covering the entire electromagnetic spectrum. The event was not detected in the HE range by Fermi-LAT. Nevertheless, ULs have been reported in the MeV-GeV band up to <sup>3</sup> <sup>×</sup> <sup>10</sup><sup>4</sup> s [190]. The Swift-XRT started observations at 97.3 s and detected a bright X-ray afterglow, which was monitored until <sup>∼</sup>7.8 <sup>×</sup> <sup>10</sup><sup>6</sup> s [191]. The X-ray light curve in the 0.3–10 keV energy range (observer frame) shows a peculiar behavior with an initial steep decay phase followed by a plateau and a strong flare episode (Figure 23, upper panel, blue points). After the flare, the standard afterglow phase starts with a decay following a power-law with a possible steepening around 10 days. In the UV/optical/NIR band, the event was followed by several instruments. The redshift was estimated to be *z* = 0.0785 ± 0.005 [192], which makes this event one of the closest GRBs ever detected. Starting from 4.5–5.5 days after the GRB trigger, an associated supernova was reported [193]. Moreover, in the optical data, a flare is observed simultaneously with the one in the X-rays. In the radio band, the detection

was reported by several instruments, starting from ∼1 day after the trigger [194–197]. The radio flux initially slowly increases and then starts to decay after 20–30 days.

#### 4.4.2. VHE Observations and Results

The afterglow emission of GRB 190829A was followed-up by the H.E.S.S. telescopes starting at 4.3 h and continued for three consecutive nights (up to 55.9 h). The observations were performed using the four medium-size telescopes of H.E.S.S. for a total amount of 13 h divided respectively in 3.6 h (starting at 4.3 h), 4.7 h (starting at 27.2 h) and 4.7 h (starting at 51.2 h). The statistical significance at the GRB position found in the three nights are, respectively, 21.7*σ*, 5.5*σ* and 2.4*σ*.

Spectral analysis was performed for the first two nights fitting the observed photon spectrum with a power-law model. The following values are found: *αobs* = −2.59 ± 0.09 (stat.) ±0.23 (syst.) in the 0.18–3.3 TeV (first night) and *αobs* = −2.46 ± 0.22 (stat.) ±0.14 (syst.) in the 0.18–1.4 TeV energy range (second night). Fitting a power-law attenuated by EBL, the photon indices inferred for the intrinsic spectrum are: *αint* = −2.06 ± 0.10 (stat.) ±0.26 (syst.) in the 0.18–3.3 TeV energy range (first night) and *αint* = −1.86 ± 0.26 (stat.) ±0.17 (syst.) in the 0.18–1.4 TeV energy range (second night). The photon indices in each night are consistent and within the systematical uncertainties with those of the simultaneous X-ray emission. Combining all three nights, the photon index is *αint* = −2.07 ± 0.09 (stat.) ±0.23 (syst.) in the 0.18–3.3 TeV energy range.

The light-curve in the 0.2–4.0 TeV energy range derived up to 56 h is compared in Figure 24 with the XRT light-curve and the LAT upper limits. The time-evolving flux was satisfactorily modeled with a power-law decay *F*(*t*) ∝ *t <sup>α</sup>* with *<sup>α</sup>* <sup>=</sup> <sup>−</sup>1.09 <sup>±</sup> 0.05. Such a decay index is similar to the X-ray one derived in the same time interval (*α<sup>X</sup>* = −1.07 ± 0.09).

**Figure 24.** GRB 190829A: multi-wavelength light curves (**A**) (upper panel) and photon index evolution (bottom panel) in the X-ray, HE and VHE band (**B**). The BAT prompt light curve is shown in the inset (**C**). From [5].

#### 4.4.3. Interpretation

The interpretation of the VHE emission from GRB 190829A is debated and different radiation mechanisms including synchrotron, SSC or EIC emission have been proposed so far to explain the origin of the TeV emission.

The H.E.S.S. Collaboration [5] investigated both the synchrotron and the SSC emission in the external forward shock as a responsible radiation mechanism of the observed TeV component. Multi-wavelength data collected simultaneously with H.E.S.S. observations in the first two nights were modeled separately with a time-independent numerical code using the Markov-chain Monte Carlo (MCMC) approach to explore the parameter space. The results of the fitting show that the SSC mechanism fails to explain the VHE emission. The low Lorentz bulk factor predicted by the observations (Γ . 10) implies that the SSC emission occurs in KN cross-scattering regime. As a result, a steep spectrum, inconsistent with the observational VHE data, is obtained (see Figure 25, light blue shaded area). Possible improvements between the data and the model foresee a higher Γ, which is in contrast with the observations, or the presence of an additional hard component in the distribution of the accelerated electrons. However, this latter solution implies extreme assumptions on the density of the circumburst medium (*n*<sup>0</sup> = 10−<sup>5</sup> cm−<sup>3</sup> in the case of the strong magnetic field or *n*<sup>0</sup> = 10<sup>5</sup> cm−<sup>3</sup> for weak magnetic field) and a SED strongly dominated by the SSC component, which is inconsistent with the data. A better fitting of the observational data can be obtained when considering an alternative model where the maximum electron energy set by the radiative losses is ignored. In such scenario, the synchrotron emission is able to extend up to TeV energies and the observational broad-band data are described by a single synchrotron component (see Figure 25, orange shaded area). The SSC contribution is negligible while the *γ* − *γ* absorption shapes the VHE spectrum. The single synchrotron component scenario provides a better fit (>5*σ*) to the multi-wavelength data. On the other hand, this interpretation requires unknown acceleration processes or non-uniform magnetic field strength in the emission region, as described for GRB 180720B (see Section 4.2).

A complete multi-wavelength modeling of the GRB 190829A data, including contribution of the synchrotron and SSC emission for both the forward and reverse shocks, and considering a constant-density environment, is presented in [72]. The predicted broad-band light curves and the SED at the time of the H.E.S.S. detection are shown in Figure 23. A MCMC approach was adopted in order to estimate the best-fit parameters for the multi-wavelength modeling. The resulting values of the parameters related with the forward shock scenario are shown in Table 3. In contrast with the H.E.S.S. Collaboration results, the VHE emission is well reproduced with the SSC external forward shock scenario. The usual simplified assumption that *ξ<sup>e</sup>* = 1 is excluded by the fit, which provides acceptable solutions only for *<sup>ξ</sup><sup>e</sup>* . 6.5 <sup>×</sup> <sup>10</sup>−<sup>2</sup> . Moreover, an isotropic-equivalent kinetic energy at the afterglow onset *E<sup>k</sup>* = 2.5+1.9 <sup>−</sup>1.3 <sup>×</sup> <sup>10</sup><sup>53</sup> erg is estimated. Considering the observed GBM

prompt energy, such a value implies that the prompt efficiency is *η* = 1.2+1.0 <sup>−</sup>0.5 <sup>×</sup> <sup>10</sup>−<sup>3</sup> , which is much lower than the typical values derived from the previous GRB studies. The other parameters (*n*0, *e<sup>e</sup>* and *eB*) are found to be similar to the ones estimated for GRB 190114C.

**Figure 25.** GRB 190829A: modeling of X-ray, LAT and H.E.S.S. data proposed by the H.E.S.S. collaboration for the two time intervals with VHE and X-ray detections. Two scenarios are investigated for the TeV emission: synchrotron and SSC. H.E.S.S. flux contours are displayed considering the statistical uncertainty. The synchrotron and SSC component are shown in dashed and dash-dotted lines, respectively. The shaded areas represent the 68% confidence intervals determined from the posterior probability distribution of the MCMC parameter fitting for the standard SSC model (light blue) and for the model without maximum energy for synchrotron emission (orange). From [5].

A two-component off-axis jet model has also been investigated [198]. Such a model proposes that the GRB jet is observed off-axis (*θview* = 1.78◦ ) and it consists of a narrow (*θjet* = 0.86◦ ) fast (Γ = 350) jet and a slow (Γ = 20) co-axial jet. The former jet component is responsible for the emission of SSC photons in the VHE band. The calculation of the SSC flux at the time of the H.E.S.S. detection is conducted following the prescriptions of [48], considering only the Thompson scattering regime.

An EIC plus SSC scenario has also been proposed for the production of the VHE component [199]. The seed photons belong to the long-lasting X-ray flare observed for GRB 190829A, which can be up-scattered to TeV energies. A numerical calculation of the afterglow dynamics and radiative processes have been used to model the observational data. For *<sup>t</sup>* <sup>∼</sup> <sup>10</sup>3–10<sup>4</sup> s, the EIC component dominates the VHE emission, while for later times (*<sup>t</sup>* & <sup>3</sup> <sup>×</sup> <sup>10</sup><sup>4</sup> s) the EIC gradually decays and the SSC component becomes relevant. The initial afterglow kinetic energy used for the modeling (*E<sup>k</sup>* = 10<sup>52</sup> erg) suggests that GRB 190829A is not a typical low-luminosity GRB but it may have much higher kinetic energy.

**Table 3.** Parameters for modeling of GRB 190829A.

