Cosmic rays are at the foundation of astroparticle physics and the extensive air showers (EAS) is one indirect way to detect them. Air showers, however, have been used to infer information not just of cosmic rays particles, but also to localize gamma rays sources. The shower maximum of an EAS, defined as the position at the atmosphere where the maximum quantity of charged particles is reached, is an observable of air showers that can permit to infer the mass composition of cosmic rays. For this reason, it is important to propose methods to measure it. Several methods to determine the shower maximum have been implemented in the last decades with the development of different kinds of telescopes. This work discusses the possibility of determining the maximum of air showers using imaging atmospheric Cherenkov telescopes (IACT). The Cherenkov telescopes can detect the Cherenkov radiation produced by the interaction of charged particles with the atmosphere. Those Cherenkov photons are projected back into the plane containing the longitudinal development of the air shower. Each plane is saved as a 2D histogram with the longitudinal and lateral development in the vertical and horizontal axis, respectively. A detailed analysis of each 2D histogram is presented and used to obtain the depth of the maximum of the Cherenkov profile. The main effect seen is a decrease in the shower maximum of Cherenkov photons as a function of the telescope position from the shower axis to 150 m. After 150m from the shower axis, there is a constant behavior that is correlated to the real depth of the maximum of an EAS. Based on this constant behavior after 150 m, the shower maximum is reconstructed and it is shown the resolution of the method as a function of the energy, which is around 55 g/cm2 considering just one telescope, and 15 g/cm2 for the best case considering zenith angle of 20 degrees. Moreover, the method is tested with some simulations took from Very Energetic Radiation Imaging Telescope Array System (VERITAS) experiment to compare with the results of our simulations. The resolution of the reconstruction of the shower maximum for proton and iron showers was also done which ranges around 80 g/cm2 for proton and around 30 g/cm2 for iron in the case of 20° of zenith angle. / Os raios cósmicos estão na base da física das astropartículas e os chuveiros atmosféricos extensos (EAS pela sigla em inglês) são uma maneira indireta de detectá-los. Os chuveiros atmosféricos, no entanto, têm sido usados para inferir informações não apenas sobre partículas de raios cósmicos, mas também para localizar fontes de raios gama. A profundidade de máximo num EAS, definido como a profundidade atmosférica onde a quantidade máxima de partículas carregadas é atingida, é um observável de EAS que permite inferir a composição de massa dos raios cósmicos. Por esse motivo, é importante propor métodos para medí-lo. Vários métodos para determinar a profundidade de máximo foram implementados nas últimas décadas com o desenvolvimento de diferentes tipos de telescópios. Este trabalho discute a possibilidade de determinar a profundidade de máximo de chuveiros atmosféricos utilizando os telescópios atmosféricos Cherenkov (IACT). Os telescópios Cherenkov podem detectar a radiação Cherenkov produzida pela interação de partículas carregadas com a atmosfera. Esses fótons Cherenkov são projetados de volta ao plano que contém o desenvolvimento longitudinal do chuveiro. Cada plano é salvo num histograma 2D com o desenvolvimento longitudinal e lateral no eixo vertical e horizontal, respectivamente. Uma análise detalhada de cada histograma 2D é apresentada e usada para obter a profundidade máxima do perfil de emissão de luz Cherenkov. O principal efeito visto é uma diminuição na profundidade de máximo dos fótons Cherenkov do chuveiro como função da posição do telescópio a partir do eixo do chuveiro até 150 m. A partir de 150 m do eixo do chuveiro, há um comportamento constante que está correlacionado com a profundidade real de máximo do EAS. Com base nesse comportamento constante após 150 m, o máximo do chuveiro é reconstruído e é mostrada a resolução do método em função da energia, que é cerca de 55 g/cm2 considerando apenas um telescópio, e 15 g/cm2 para o melhor caso, considerando o ângulo zenital de 20 graus. Além disso, o método é testado com algumas simulações cedidas pelo experimento VERITAS (Very Energetic Radiation Imaging Telescope Array System, pela sigla em inglês) para comparar com os resultados de nossas simulações. Também foi feita a resolução da reconstrução da profundidade de máximo para chuveiros atmosféricos de prótons e ferro, que varia em torno de 80 g/cm2 para prótons e em torno de 30 g/cm2 para ferro no caso chuveiros inclinados a um ângulo de 20°.

Zeitabhängige Multimessenger-Astronomie ist die Studie von vorübergehenden astrophysikalischen Phänomenen mithilfe verschiedener Botschafterteilchen und -wellen. Das High Energy Stereoscopic System (H.E.S.S.) ist ein bodengestütztes Teleskop-Array, das Gammastrahlen im Bereich von 30 GeV bis 100 TeV misst. In dieser Arbeit werden drei H.E.S.S.-Programme zur Nachverfolgung von vorübergehenden Phänomenen diskutiert. Zuerst wird das Nova-Programm vorgestellt, zusammen mit der Entdeckung des Ausbruchs von RS Ophiuchi (RS Oph) im Jahr 2021. RS Oph ist eine Nova, welche schon mehrmals ausgebrochen ist. Die Analyse der Nova, zeigt eine klare Detektion über die ersten fünf Nächte sowie eine marginale Detektion bis zu vier Wochen nach dem Ausbruch. RS Oph ist damit das erste galaktische vorübergehende Phänomen, das bei Energien von ~1 TeV detektiert wurde. Eine klare Variabilität des Spektrums wird gezeigt. Eine Diskussion zeigt, dass die beobachteten Gammastrahlen höchstwahrscheinlich durch beschleunigte Protonen innerhalb eines astrophysikalischen Schocks stammt. Die Ergebnisse zeigen eine zeitaufgelöster Teilchenbeschleunigung. Das zweite Programm ist das Gravitationswellen (GW)-Programm. Hier wird die Analyse von vier beobachteten Verschmelzungsereignissen von binären schwarzen Löchern vorgestellt. Es wird keine Detektion gemeldet, stattdessen werden Himmelskarten mit Höchstgrenzen produziert. Die Effektivität der GW-Nachverfolgung mit H.E.S.S. wird diskutiert. Das dritte Programm ist das Pionierprogramm für Gezeiten-Sternzerrissereignisse (TDEs), das in den letzten Jahren etabliert wurde. Herausforderungen des Programms werden diskutiert, und die H.E.S.S.-Nachverfolgung des Ereignisses AT2019uqv wird vorgestellt. Es werden keine Detektionen gemeldet, stattdessen werden obere Grenzen für AT2019uqv angegeben. Abschließend erfolgt ein Vergleich der Programme sowie ein Ausblick auf das nächste Jahrzehnt bodengestützter Gammastrahlenbeobachtungen. / Time-domain multi-messenger astronomy is the study of transient astrophysical phenomena using several messenger particles and waves. The High Energy Stereoscopic System (H.E.S.S.) is a ground-based telescope array, which measures very-high-energy gamma rays between 30 GeV and 100 TeV. Within this work, three H.E.S.S. transients follow-up programs and their results are shown. At first, the nova program is shown together with the detection of the 2021 RS Ophiuchi (RS Oph) outburst. RS Oph is a known nova with past eruptions. The analysis of the nova presented in this work shows a clear detection over the first five nights of observations, and a marginal detection two to four weeks after the eruption. RS Oph is thus the first Galactic transient phenomenon detected at ~1 TeV energies. A clear variability of the spectrum is shown. A discussion of the underlying physics concludes, that the observed gamma-ray emission most likely stems from cooled protons accelerated within an astrophysical shock. The results show time-resolved particle acceleration. The second program is the gravitational-waves (GWs) program. Here, the analysis of four observed binary black hole merger events is shown. No detection is reported, and upper limit sky maps are derived instead. The viability of GW follow-up with H.E.S.S. is discussed, and a case is made for a potential counterpart detection. The third program is the tidal disruption event (TDE) program, a pioneered program established at the emergence of the source class within the last few years. Unique challenges of the follow-up program are discussed, and the H.E.S.S. follow-up of the event AT2019uqv is shown. No detection is reported, and upper limits for AT2019uqv are given instead. The interest in follow-up observations of TDEs with gamma-ray instruments is stressed. In conclusion, a comparison of the different programs and their requirements is given, together with an outlook for the next decade of ground-based gamma-ray observations.