Gamma-Ray Bursts (GRBs) afterglows are rapidly decaying signals that pose significant detection challenges, requiring improved methods to track their temporal evolution. In this study, we systematically compare various techniques for detecting GRB-like transient emissions at very high energies (VHE, >100 GeV). Our analysis includes time-dependent extension of the standard method (i.e., Li Ma 1983) and other previously developed methods in the literature, alongside a novel likelihood-based approach, which directly fits spectral and temporal decay features to IACT data. Through dedicated observation simulations, we evaluate the performance of these methods for the Large-Sized Telescope prototype (LST-1) of the Cherenkov Telescope Array Observatory (CTAO). We characterised the enhanced sensitivity found in some of these methods compared to the standard significance estimation. For GRBs simulated with very fast afterglow decay, an improvement of more than a factor of two is seen in detections for some of the methods presented here, compared to the standard Li Ma approach.

OT 081 is a well-known, luminous blazar that is remarkably variable in many energy bands. We present the first broadband study of the source, which includes very high energy (VHE, E > 100 GeV) gamma-ray data taken by the MAGIC (Major Atmospheric Gamma-ray Imaging Cherenkov telescopes) and H.E.S.S. (High Energy Stereoscopic System) imaging Cherenkov telescopes. The discovery of VHE gamma-ray emission happened during a high state of gamma-ray activity in July 2016, observed by many instruments from radio to VHE gamma-rays. We identify four states of activity of the source, one of which includes VHE gamma-ray emission. Variability in the VHE domain is found on daily time-scales. The intrinsic VHE spectrum can be described by a power law with index 3 . 27 +/- 0.44(stat) +/- 0.15(sys) (MAGIC) and 3 . 39 +/- 0.58(stat) +/- 0.64(sys) (H.E.S.S.) in the energy range of 55-300 and 120-500 GeV, respectively. The broadband emission cannot be successfully reproduced by a simple one-zone synchrotron self-Compton model. Instead, an additional external Compton component is required. We test a lepto-hadronic model that reproduces the data set well and a proton-synchrotron-dominated model that requires an extreme proton luminosity. Emission models that are able to successfully represent the data place the emitting region well outside of the broad-line region to a location at which the radiative environment is dominated by the infrared thermal radiation field of the dusty torus. In the scenario described by this flaring activity, the source appears to be a flat spectrum radio quasar (FSRQ), in contrast with past categorizations. This suggests that the source can be considered to be a transitional blazar, intermediate between BL Lac and FSRQ objects.

During its first phase, from 2004 up to the end of 2012, the H.E.S.S. (High Energy Stereoscopic System) experiment observed the extragalactic skies for more than 2700 hours. These data have been re-analysed in a single consistent framework, leading to the derivation of a catalog of 23 sources. In total, about 5.7

On October 9th, 2022, the brightest gamma-ray burst (GRB) since the first GRB observation in the late sixties was detected by the Fermi-GBM and Swift-BAT telescopes (GRB 221009A). The outstanding characteristics of this GRB triggered extensive follow-up observations of the source across all wavebands, including at very-high-energy (VHE) gamma rays with the Large-Sized Telescope prototype (LST-1) of the upcoming Cherenkov Telescope Array Observatory (CTAO). In this contribution, we present the analysis and results of the LST-1 observation campaign in October 2022, focusing on the data taken under nominal observing conditions and above 200 GeV.

Observations with imaging atmospheric Cherenkov telescopes (IACTs) have enhanced our knowledge of nearby supernova (SN) remnants with ages younger than 500 years by establishing Cassiopeia A and the remnant of Tycho's SN as very-high-energy (VHE) gamma-ray sources. The remnant of Kepler's SN, which is the product of the most recent naked-eye supernova in our Galaxy, is comparable in age to the other two, but is significantly more distant. If the gamma-ray luminosities of the remnants of Tycho's and Kepler's SNe are similar, then the latter is expected to be one of the faintest gamma-ray sources within reach of the current generation IACT arrays. Here we report evidence at a statistical level of 4.6 sigma for a VHE signal from the remnant of Kepler's SN based on deep observations by the High Energy Stereoscopic System (H.E.S.S.) with an exposure of 152 hours. The measured integral flux above an energy of 226 GeV is ~0.3% of the flux of the Crab Nebula. The spectral energy distribution (SED) reveals a gamma-ray emitting component connecting the VHE emission observed with H.E.S.S. to the emission observed at GeV energies with Fermi-LAT. The overall SED is similar to that of the remnant of Tycho's SN, possibly indicating the same non-thermal emission processes acting in both these young remnants of thermonuclear SNe.

Most gamma-ray detected active galactic nuclei are blazars with one of their relativistic jets pointing towards the Earth. Only a few objects belong to the class of radio galaxies or misaligned blazars. Here, we investigate the nature of the object PKS 0625-354, its gamma-ray flux and spectral variability and its broad-band spectral emission with observations from H.E.S.S., Fermi-LAT, Swift-XRT, and UVOT taken in November 2018. The H.E.S.S. light curve above 200 GeV shows an outburst in the first night of observations followed by a declining flux with a halving time scale of 5.9 h. The gamma gamma-opacity constrains the upper limit of the angle between the jet and the line of sight to similar to 10 degrees. The broad-band spectral energy distribution shows two humps and can be well fitted with a single-zone synchrotron self Compton emission model. We conclude that PKS 0625-354, as an object showing clear features of both blazars and radio galaxies, can be classified as an intermediate active galactic nuclei. Multi-wavelength studies of such intermediate objects exhibiting features of both blazars and radio galaxies are sparse but crucial for the understanding of the broad-band emission of gamma-ray detected active galactic nuclei in general.

The detection of gravitational waves from a binary neutron star merger by Advanced LIGO and Advanced Virgo (GW170817), along with the discovery of the electromagnetic counterparts of this gravitational wave event, ushered in a new era of multimessenger astronomy, providing the first direct evidence that BNS mergers are progenitors of short gamma-ray bursts (GRBs). Such events may also produce very-high-energy (VHE, > 100GeV) photons which have yet to be detected in coincidence with a gravitational wave signal. The Cherenkov Telescope Array (CTA) is a next-generation VHE observatory which aims to be indispensable in this search, with an unparalleled sensitivity and ability to slew anywhere on the sky within a few tens of seconds. New observing modes and follow-up strategies are being developed for CTA to rapidly cover localization areas of gravitational wave events that are typically larger than the CTA field of view. This work will evaluate and provide estimations on the expected number of of gravitational wave events that will be observable with CTA, considering both on- and off-axis emission. In addition, we will present and discuss the prospects of potential follow-up strategies with CTA.

Galaxy clusters are expected to be dark matter (DM) reservoirs and storage rooms for the cosmic-ray protons (CRp) that accumulate along the cluster's formation history. Accordingly, they are excellent targets to search for signals of DM annihilation and decay at gamma-ray energies and are predicted to be sources of large-scale gamma-ray emission due to hadronic interactions in the intracluster medium. We estimate the sensitivity of the Cherenkov Telescope Array (CTA) to detect diffuse gamma-ray emission from the Perseus galaxy cluster. We perform a detailed spatial and spectral modelling of the expected signal for the DM and the CRp components. For each, we compute the expected CTA sensitivity. The observing strategy of Perseus is also discussed. In the absence of a diffuse signal (non-detection), CTA should constrain the CRp to thermal energy ratio within the radius $R_{500}$ down to about $X_{500}<3\times 10^{-3}$, for a spatial CRp distribution that follows the thermal gas and a CRp spectral index $\alpha_{\rm CRp}=2.3$. Under the optimistic assumption of a pure hadronic origin of the Perseus radio mini-halo and depending on the assumed magnetic field profile, CTA should measure $\alpha_{\rm CRp}$ down to about $\Delta\alpha_{\rm CRp}\simeq 0.1$ and the CRp spatial distribution with 10% precision. Regarding DM, CTA should improve the current ground-based gamma-ray DM limits from clusters observations on the velocity-averaged annihilation cross-section by a factor of up to $\sim 5$, depending on the modelling of DM halo substructure. In the case of decay of DM particles, CTA will explore a new region of the parameter space, reaching models with $\tau_{\chi}>10^{27}$s for DM masses above 1 TeV. These constraints will provide unprecedented sensitivity to the physics of both CRp acceleration and transport at cluster scale and to TeV DM particle models, especially in the decay scenario.

Context. To date, three pulsars have been firmly detected by imaging atmospheric Cherenkov telescopes (IACTs). Two of them reached the TeV energy range, challenging models of very high-energy (VHE) emission in pulsars. More precise observations are needed to better characterize pulsar emission at these energies. The LST-1 is the prototype of the large-sized telescopes, which will be part of the Cherenkov Telescope Array Observatory (CTAO). Its improved performance over previous IACTs makes it well suited for studying pulsars. Aims. In this work we study the Crab pulsar emission with the LST-1, improving upon and complementing the results from other telescopes. Crab pulsar observations can also be used to characterize the potential of the LST-1 to study other pulsars and detect new ones. Methods. We analyzed a total of similar to 103 hours of gamma-ray observations of the Crab pulsar conducted with the LST-1 in the period from September 2020 to January 2023. The observations were carried out at zenith angles of less than 50 degrees. To characterize the Crab pulsar emission over a broader energy range, a new analysis of the Fermi/LAT data, including similar to 14 years of observations, was also performed. Results. The Crab pulsar phaseogram, long-term light curve, and phase-resolved spectra are reconstructed with the LST-1 from 20 GeV to 450 GeV for the first peak and up to 700 GeV for the second peak The pulsed emission is detected with a significance level of 15.2 sigma. The two characteristic emission peaks of the Crab pulsar are clearly detected (> 10 sigma), as is the so-called bridge emission between them (5.7 sigma). We find that both peaks are described well by power laws, with spectral indices of similar to 3.44 and similar to 3.03, respectively. The joint analysis of Fermi/LAT and LST-1 data shows a good agreement between the two instruments in their overlapping energy range. The detailed results obtained from the first observations of the Crab pulsar with the LST-1 show the potential that CTAO will have to study this type of source.

Context. Imaging atmospheric Cherenkov telescopes (IACTs) are used to observe very high-energy photons from the ground. Gamma rays are indirectly detected through the Cherenkov light emitted by the air showers they induce. The new generation of experiments, in particular the Cherenkov Telescope Array Observatory (CTAO), sets ambitious goals for discoveries of new gamma-ray sources and precise measurements of the already discovered ones. To achieve these goals, both hardware and data analysis must employ cuttingedge techniques. This also applies to the LST-1, the first IACT built for the CTAO, which is currently taking data on the Canary island of La Palma. Aims. This paper introduces a new event reconstruction technique for IACT data, aiming to improve the image reconstruction quality and the discrimination between the signal and the background from misidentified hadrons and electrons. Methods. The technique models the development of the extensive air shower signal, recorded as a waveform per pixel, seen by CTAO telescopes’ cameras. Model parameters are subsequently passed to random forest regressors and classifiers to extract information on the primary particle. Results. The new reconstruction was applied to simulated data and to data from observations of the Crab Nebula performed by the LST-1. The event reconstruction method presented here shows promising performance improvements. The angular and energy resolution, and the sensitivity, are improved by 10 to 20% over most of the energy range. At low energy, improvements reach up to 22%, 47%, and 50%, respectively. A future extension of the method to stereoscopic analysis for telescope arrays will be the next important step.

A decade has passed since high-energy astrophysical neutrinos have been discovered by IceCube, however the corresponding sources have not been fully identified yet. The reported coincidence of the high-energy IceCube-170922A with the gamma-ray blazar TXS 0506+056 is not enough to claim that blazars are the dominant high-energy neutrino emitters in the Universe. In fact, recently IceCube announced a second correlation with NGC 1068, a nearby Seyfert galaxy, which is significantly different from a gamma-emitting blazars. The hunt for counterparts of the IceCube neutrinos using gamma-ray telescopes started in 2012. Nonetheless, these efforts will continue with the next-generation gamma-ray telescopes, such as the CTA Large Size Telescopes (LSTs) and other telescopes, by means of an improved and revised observation strategy. These new observations will allow us to detect enough sources in order to elucidate the mystery of the neutrino emitters. In this contribution, we introduces the efforts made thus far in the search for gamma-ray counterpart of high-energy IceCube events using the current generation IACTs, focusing on alerts made of multiple neutrinos events, and present an idea to improve in the observational strategies with the CTA LSTs that will become operational in the coming decade. We will discuss how to reduce the bias to gamma-ray emitters in order to search for possible neutrino counterparts.

Despite the advancement in background rejection techniques, observation of the very-high-energy gamma-ray sky by imaging atmospheric Cherenkov telescopes (IACTs) are subject to an irreducible background from gamma-like hadron- or electron-induced air showers. The determination of this residual background is crucial for accurate spectral and spatial measurements. The Cherenkov Telescope Array (CTA) will become the next generation of IACTs. To unveil its full potential, the improved reconstruction performance of CTA needs to be coupled with a reliable background estimate across the entire field of view. This may become especially important in the case of the planned surveys of large areas of the sky. In this contribution we will present pybkgmodel, an open-source python software package developed for CTA. It aims at providing in a consistent way the various background modelling methods, based on the experience from current IACTs such as H.E.S.S, MAGIC, and VERITAS. It is designed as a toolbox allowing a user to easily choose the optimal reconstruction approach for various target regions or a combination of several algorithms. We will introduce the design of the package as well as demonstrate its performance using simulations for the CTA Large-Sized Telescope prototype (LST-1).

In 2021 July, PKS 1510−089 exhibited a significant flux drop in the high-energy γ-ray (by a factor 10) and optical (by a factor 5) bands and remained in this low state throughout 2022. Similarly, the optical polarization in the source vanished, resulting in the optical spectrum being fully explained through the steady flux of the accretion disk and the broad-line region. Unlike the aforementioned bands, the very-high-energy γ-ray and X-ray fluxes did not exhibit a significant flux drop from year to year. This suggests that the steady-state very-high-energy γ-ray and X-ray fluxes originate from a different emission region than the vanished parts of the high-energy γ-ray and optical jet fluxes. The latter component has disappeared through either a swing of the jet away from the line of sight or a significant drop in the photon production efficiency of the jet close to the black hole. Either change could become visible in high-resolution radio images.

LMC P3 is a gamma-ray binary comprising of an unconfirmed compact object and an O-star and is located in the Large Magellanic Cloud. Initially discovered in Fermi-LAT data, it shows an orbital period of 10.3 days. H.E.S.S. has reported the detected VHE gamma-ray emission during only 20% of the orbit, between orbital phases 0.2 and 0.4, which roughly corresponds to the inferior conjunction of the compact object. H.E.S.S. has continued the observations of this object since then. Here we will present new results obtained with a much deeper data set. The new data allow a more precise measurement of the location of the VHE gamma-ray peak along the orbit of the system to be made. We will interpret these results with respect to emission and absorption mechanisms in gamma-ray binary systems.

The prototype Large-Sized Telescope (LST-1) of the Cherenkov Telescope Array Observatory (CTAO) is in commissioning phase at the Observatorio del Roque de Los Muchachos at 2200 m a.s.l. in La Palma (Canary Islands, Spain). LST-1 is a 23-m diameter telescope and is the first of four that will compose the LST part of the CTAO Northern array. The LST subarray is optimized to provide the best sensitivity for gamma rays in the 20 GeV - 200 GeV energy range. The MAGIC telescopes, which are located approximately 100 m from the LST-1, is operating as a two 17-m telescope stereoscopic system for more than 14 years. LST-1 and MAGIC routinely perform joint observations of gamma-ray sources to exploit the potential of the three-telescope system. This contribution describes the analysis pipeline and evaluates the performance of the system using Monte Carlo simulations and data on the Crab Nebula. The sensitivity achieved during joint observations with MAGIC and LST-1 is about 30% higher than that of MAGIC alone.

The prototype Large-Sized Telescope (LST-1) of the Cherenkov Telescope Array (CTA) was inaugurated on La Palma, Canary Islands, in 2018. Since then, the telescope is in the commissioning phase and takes regular gamma-ray data on astrophysical sources while waiting for other CTA telescopes in La Palma to be constructed. Here we present the status of the commissioning, lessons learned, the telescope performance, and scientific highlights achieved in the last couple of years. The science results include the detection of active galactic nuclei flares, studies of pulsar wind nebulae and pulsars, the detection of a Nova, and searches for gamma-ray emission from gamma-ray bursts. We also present the status of the construction of the three further LST telescopes (LST-2-4) in La Palma and plans for their commissioning.

Clusters of galaxies are the largest gravitationally-bound structures in the Universe. They are composed of galaxies and gas (approximately 15% of the total mass) mostly dark matter (DM, accounts up to 85% of the total mass). If the DM is composed of Weakly Interacting Massive Particles (WIMPs), galaxy clusters represent one of the best targets to search for gamma-ray signals induced by the decay of WIMPs, with masses around the TeV scale. Due to its sensitivity and energy range of operation (from 20 GeV to 300 TeV), the Cherenkov Telescope Array (CTA) Observatory has a unique opportunity to test WIMPs with masses close to the unitarity limit. This will complement the searches for DM from other gamma-ray observatories as well as direct and collider experiments. The CTA Observatory is planning to search for gamma-ray emission, either its origin may be cosmic-ray (CR) or DM related, in the Perseus galaxy cluster during the first years of operation. In this poster, we will present the software created to perform the analysis using the \texttt{ctools} software and the corresponding results.

The Galactic Centre (GC) region is a highly interesting region for very high energy gamma-ray studies due to its proximity and diverse sources. It is also a unique place for Dark Matter (DM) searches, since we expect a large amount of DM in this region and it is nearby. Currently a new detector is under development to observe the GC region, called the Southern Wide field-of-view Gamma-ray Observatory (SWGO). This instrument will be the first water Cherenkov detector located in the Southern Hemisphere sensitive to >100 GeV gamma rays. In our work, we are going to present a simulation of the GC gamma-ray mission as seen by a SWGO-like observatory. To do so we are using the models published by the Cherenkov Telescope Array (CTA) Collaboration together with simulated Instrument Response Functions (IRFs) for testing purposes. This also allows us to predict the expected sensitivity of SWGO to WIMP DM annihilations using a template based method. A sensitivity study during the design phase of the observatory is important in order to choose the best detector design to get outstanding physics results.

The origin of the gamma-ray emission from M87 is currently a matter of debate. This work aims to localize the VHE (100 GeV-100 TeV) gamma-ray emission from M87 and probe a potential extended hadronic emission component in the inner Virgo Cluster. The search for a steady and extended gamma-ray signal around M87 can constrain the cosmic-ray energy density and the pressure exerted by the cosmic rays onto the intra-cluster medium, and allow us to investigate the role of the cosmic rays in the active galactic nucleus feedback as a heating mechanism in the Virgo Cluster. H.E.S.S. telescopes are sensitive to VHE gamma rays and have been utilized to observe M87 since 2004. We utilized a Bayesian block analysis to identify M87 emission states with H.E.S.S. observations from 2004 until 2021, dividing them into low, intermediate, and high states. Because of the causality argument, an extended ($\gtrsim$kpc) signal is allowed only in steady emission states. Hence, we fitted the morphology of the 120h low state data and found no significant gamma-ray extension. Therefore, we derived for the low state an upper limit of 58"(corresponding to $\approx$4.6kpc) in the extension of a single-component morphological model described by a rotationally symmetric 2D Gaussian model at 99.7% confidence level. Our results exclude the radio lobes ($\approx$30 kpc) as the principal component of the VHE gamma-ray emission from the low state of M87. The gamma-ray emission is compatible with a single emission region at the radio core of M87. These results, with the help of two multiple-component models, constrain the maximum cosmic-ray to thermal pressure ratio $X_{{CR,max.}}$$\lesssim$$0.32$ and the total energy in cosmic-ray protons (CRp) to $U_{CR}$$\lesssim$5$\times10^{58}$ erg in the inner 20kpc of the Virgo Cluster for an assumed CRp power-law distribution in momentum with spectral index $\alpha_{p}$=2.1.