Young massive stellar clusters (YMSCs) have emerged as potential γ-ray sources after the recent association of a dozen YMSCs with extended γ-ray emission. The large size of the detected halos, comparable to that of the wind-blown bubble expected around YMSCs, makes the γ-ray detection of individual YMSCs rather challenging. As a result, the emission from most of the Galactic YMSCs could be unresolved, thus contributing to the diffuse γ-ray radiation observed along the Galactic Plane. In this study, we estimate the possible contribution to the Galactic diffuse γ-ray emission from a synthetic population of YMSCs, and we compare it with observations obtained with different experiments, from 1 GeV to hundreds of teraelectronvolt in three regions of the Galactic Plane. As the population of galactic YMSCs is only known locally, we evaluated the contribution of γ-ray emission relying on the simulation of synthetic populations of YMSCs based on the observed properties of local clusters. We computed the γ-ray emission from each cluster assuming that the radiation is purely hadronic in nature and produced by cosmic rays that are accelerated at the cluster's collective wind termination shock. We find that the γ-ray emission from unresolved YMSCs can significantly contribute to the observed Galactic diffuse flux, especially in the inner part of the Galaxy, and that an important role is played by kinetic power injected by the Wolf-Rayet stellar winds. The predicted γ-ray flux should be considered as a lower limit, given that our calculation does not include the contribution of supernovae exploding in YMSCs.

Recently, the Large High-Altitude Air Shower Observatory (LHAASO) collaboration has obtained a measurement of the gamma-ray diffuse emission in the ultra-high energy range, 10-103 TeV after masking the contribution of known sources. The measurement is provided in two Galactic regions and appears to be 2-3 times higher than the gamma-ray signal expected from the hadronic interactions of diffuse cosmic rays with the interstellar medium, potentially implying that either additional emission sources exist or cosmic ray intensities have spatial variations. In this work, we calculate the hadronic gamma-ray diffuse emission outside the masks, considering a realistic gas distribution. We present a comprehensive calculation of the emission, which includes systematic uncertainties in the gas content of the Galactic disk, in the energy and spatial distribution of cosmic rays, as well as in the hadronic interaction cross-sections. Our results show that these factors mitigate the tension between data and predictions. The LHAASO data appear compatible with our baseline model in the outer Galactic region. In the inner region, the data show an excess with respect to the predictions below similar to 50 TeV, while at higher energies they are well described by our model. We argue that two plausible explanations for enhanced gamma-ray emission unresolved sources and CR spectral hardening in the inner Galaxy are likely suppressed by the LHAASO masking strategy, which excludes regions where both effects are expected to be most prominent.

Massive star clusters (SCs) have been proposed as additional contributors to Galactic Cosmic rays (CRs), to overcome the limitations of supernova remnants (SNR) to reach the highest energy end of the Galactic CR spectrum. Thanks to fast mass losses through collective stellar winds, the environment around SCs is potentially suitable for particle acceleration up to PeV energies. A handful of star clusters has been detected in gamma-rays confirming the idea that particle acceleration is taking place in these environments. Here we present a new analysis of Fermi-LAT data collected towards a few massive young star clusters and estimate the contribution of these types of sources to the bulk of CRs. We then briefly discuss the observational prospects for ASTRI and CTAO.

The Large High-Altitude Air Shower Observatory (LHAASO) collaboration has recently measured the ultra-high energy gamma-ray diffuse emission (10 − 103 TeV) after masking the contribution of known sources. The observed signal appears to be 2 − 3 times higher than expected from the hadronic interactions of diffuse cosmic rays with the interstellar medium, potentially suggesting a contribution from unresolved sources. However, estimates of the diffuse emission are affected by large uncertainties. In this work, we calculate the hadronic gamma-ray diffuse emission, accounting for uncertainties in the gas content of the Galactic disk, the energy and spatial distribution of cosmic rays, and the hadronic interaction cross-section. We show that the LHAASO data above ∼ 30 TeV are consistent with this model, not requiring (nor probing) the existence of any further contribution due to unresolved sources or cosmic ray spectral variations in the inner Galaxy.

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.

Young & massive stellar clusters (SCs) are a potential source of galactic cosmic rays up to very high energies as a result of two possible acceleration scenarios. Collective stellar winds from massive member stars form a wind-blown bubble with a termination shock (TS) at which particle acceleration to PeV energies may occur. Furthermore, shock acceleration may occur at SNRs expanding inside the bubble. By applying a model of CR acceleration at both the wind TS and SNR shocks to catalogues of known SCs derived from Gaia DR2, we identify the most promising targets to search for evidence of PeVatron activity. Predictions for the secondary fluxes of gamma-ray and neutrino emission are derived based on particle acceleration at the collective wind TS and the subsequent hadronic interactions with the surrounding medium. Predictions from our modelling under baseline and optimistic scenarios are compared to data, finding consistent results. We estimate the detection prospects for future gamma-ray and neutrino experiments. We find that degree-scale angular sizes of the wind-blown bubbles are typical, that may pose a challenge for experimental detection. A shortlist of the most promising candidates is provided, with an anticipated flux range. Of order 10 SCs may be detectable with future facilities, and 1-5 could be currently operating as PeVatrons. Of these, three gamma-ray detected SCs have data within our predicted range. Our model can consistently describe gamma-ray measurements of SC emission. Several further as-yet-undetected SCs offer promising targets for future observations, although the flux range allowed by our model can be large (> factor 10). The large angular size of the wind-blown bubble may lead to low surface brightness emission, worsening the problem of source confusion. Nevertheless, we discuss how further work will help to constrain SCs as PeVatron candidates. (abridged)

In recent years, the MAGIC telescopes have been equipped with a setup that allows its Imaging Atmospheric Cherenkov Telescopes (IACTs) to function as an Intensity Interferometer. The deadtime-free setup includes a 4-channel GPU-based real-time correlator together with optical filters in the 350-450 nm wavelength range and specialized Active Mirror Control (AMC) configurations. This implementation allows MAGIC to perform measurements of the spatial coherence (visibility) of the intensity fluctuations of an object’s starlight over several separations (baselines) and construct a model of said object. The accessible baseline range for MAGIC is ~40-90 m which translates into an angular resolution of 0.5-1 mas. Additionally, thanks to the AMC it can access even smaller baselines, of less than 17 m (which is the diameter of each of both dishes) to measure objects of greater angular size (>1 mas) and even measure the zero-baseline correlation, which is key to calibrate the system. We present the latest measurements that allow us to understand the performance and systematics of our setup and validate our analysis.

Recently, several gamma-ray bursts (GRBs) have been detected in the very-high-energy (VHE) gamma-ray energy range by ground-based gamma-ray experiments such as MAGIC, H.E.S.S., and LHASSO. For some GRBs, the VHE emission is consistent with synchrotron self-Compton (SSC) emission from high-energy electrons accelerated in the forward shock of the relativistic jet. However, more statistics are needed to further constrain the emission models. GRB 201216C is a long bright GRB detected in a broad energy range from radio to VHE. The redshift is estimated to be 1.1, making the GRB the most distant source detected in the VHE energy range. MAGIC started the observation 56 seconds after the GRB was triggered by the Swift-BAT telescope. We performed a detailed analysis and detected the signal with about 6 sigma in the first 20 minutes. MAGIC continued the observation for 2.2 hours on the same night and 4 hours on the next night. No signal was detected later than 40 minutes after the GRB trigger. We have performed modelling of the multi-wavelength emission using the MAGIC data. We analysed simultaneous optical data from Liverpool Telescope with MAGIC and included the results in the modelling. The sub-TeV emission is consistent with the single-zone SSC model in the forward shock. In this presentation, we show the final results of the MAGIC data analysis of GRB 201216C and discuss the emission mechanism of the multi-wavelength data.

The instrumentation for gamma-ray astronomy has advanced tremendously during the last two decades. The study of the most violent environments in the Universe has opened a new window to understand the frontier of physics, exploring processes that are beyond the capabilities of Earth- based laboratories to replicate. One of the instruments at the forefront of gamma-ray astronomy is the MAGIC stereoscopic system, which consists of two 17-m diameter mirror dish telescopes located at 2200m a.s.l. on the Canary Island of La Palma, in Spain. The year 2023 marks the 20th anniversary of MAGIC, reaching the milestone of 200 publications in peer-reviewed journals over a wide range of research areas, covering astrophysics with Galactic and extragalactic objects, dark matter searches, and cosmology. MAGIC has established itself as a world-wide leading instrument for gamma-ray astronomy in the energy range from 20 GeV to beyond 100 TeV. MAGIC is an active participant in multiple multiwavelength and multimessenger observational campaigns, contributing to our understanding of the universe. In the conference, I will provide a status report of MAGIC, including the discussion of a few outstanding results during the last two decades and the prospects for the near future.

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 2020, the MAGIC collaboration started an initiative to improve diversity, equity and inclusion (DEI) in the collaboration. One of the major actions undertaken by this initiative was a survey which was distributed among all MAGIC members with the goal of getting a clear picture of the status quo and to identify potential problems or opportunities concerning DEI related topics. Two surveys have since been performed - one in December 2020 and one in June 2022 - covering questions related to demographics, working conditions, recognition, harassment, bullying and discrimination. In this contribution, we present the questionnaire and strategy used to run these surveys. Additionally, we describe the impact the survey had within the collaboration and which actions have been taken as a direct consequence of the survey results.

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.

In recent years, several young massive star clusters (YMSCs) have been associated with extended $\gamma$-ray sources, suggesting that some acceleration process, able to accelerate particles at least up to hundreds of TeV, is at work. The number of YMSCs with associated $\gamma$-ray emission is of order ten, however, the number of potential sources is much larger, probably up to several hundreds. It is plausible that many of such objects have not been observed yet due to their low surface brightness. However, such unresolved sources may contribute to the diffuse Galactic $\gamma$-ray background. In this work, we aim at estimating the total contribution of unresolved YMSCs to the diffuse $\gamma$-ray flux, considering a synthetic population built from observed properties of local (within ~2 kpc from the Sun) clusters. We simulate the Galactic population of YMSCs using a Montecarlo approach. For every cluster, we build the stellar population, in order to estimate the collective wind luminosity and mass loss rate. The $\gamma$-ray emission of each cluster is then computed assuming a pure hadronic scenario, where protons are accelerated at the collective wind termination shock of the stellar cluster and subsequently interact with the material embedded inside the wind-blown bubble. We also account for different scenarios for the particle diffusion inside the bubble, which determines both the maximum energy and the escape time from the bubble, thus affecting the final $\gamma$-ray spectrum. The results are then compared with measurements of the diffuse $\gamma$-ray flux provided by several experiments, from a few GeV to hundreds of TeV.

In the last decade, the detection by diverse experiments of diffuse $\gamma$-ray emissions toward several galactic young massive star clusters has renewed attention to these objects as potential galactic cosmic ray accelerators. Indeed, the conversion of a few percent of the power supplied by the strong winds from the massive stars into accelerated particles is enough to explain the observed $\gamma$-ray luminosities in a purely hadronic scenario. Cygnus OB2 is one of the massive star clusters found in coincidence with diffuse $\gamma$-ray emission detected in a broad range of energies, from a few GeV up to 1.4 PeV. In this work, we aim to compare the morphology and spectrum of the observed gamma emission with those predicted from a theoretical model where particles are accelerated at the termination shock of the cluster wind. The expected properties of the $\gamma$-ray emission depend on the distribution of accelerated cosmic rays, which is determined by the physics of acceleration at the termination shock and propagation in the hot expanding bubble created by the cluster wind. Propagation and acceleration are in turn strongly linked to the type of turbulence spectrum in the hot plasma filling the bubble. After testing different turbulence spectra, we found our model predictions to reproduce the observed spectral energy distribution well in the case of Kraichnan turbulence. The predicted radial profile agrees well with HAWC observations but not with Fermi results. According to our best fit model, Cygnus OB2 should be able to accelerate cosmic rays up to 1 PeV, and would hence be a PeVatron.

The next generation facility for gamma-ray ground-based observations is the Cherenkov Telescope Array (CTA) observatory, which comprises three types of imaging atmospheric Cherenkov telescopes (IACTs). The Large-Sized Telescopes (LSTs) of CTA are the largest telescope type with a mirror dish of 23 m diameter. They cover the low energy end of the accessible gamma-ray energies for IACTs, starting from about 20 GeV up to a few TeV. The first LST prototype, known as LST-1, was officially inaugurated at the Observatorio del Roque de Los Muchachos in La Palma (Canary Islands, Spain) in 2018 and has since performed calibration observations of various known gamma-ray sources. Additionally, the site houses the MAGIC telescopes, two 17 m IACTs situated approximately 100 m away from LST-1, which have been performing stereo observations since 2009. Currently, joint observations between LST-1 and MAGIC are being carried on, and the data taken independently by the two IACT systems is analyzed by combining events via software. However, this method increases the energy threshold, as it discards all but the relatively high-energy events triggered by all three telescopes. To address this issue, we have developed a novel hardware stereo trigger system between LST-1 and MAGIC, which is capable of handling events triggered by any-two out of the three telescopes. In this contribution, we will report on the performance estimation of joint LST-1 and MAGIC observations using the hardware trigger

The blazars B2 1811+31 and GB6 J1058+2817 were found to be in flaring state during 2020 and 2021, respectively. The high states of the sources were registered by the Fermi-LAT at energies below 100 GeV, triggering observations at higher energies with the MAGIC telescopes, in the UV/X rays with the Swift satellite and with ground-based radio and optical telescopes. The observations in the very-high-energy (VHE, 100 GeV < E < 100 TeV) gamma-ray band led to the first detection of both sources in this energy range. A long-term gamma-ray lightcurve was derived using Fermi-LAT data, identifying the time intervals in which the two sources persisted in a quiet state. Archival data collected in the radio to X-ray wavelengths showed that the two sources exhibited intermediate-synchrotron-peaked BL Lac behaviors in their low states, rather rare sources in the TeV sky. The high state of the two sources was deeply investigated thanks to the coverage provided by multi-wavelength (MWL) observational campaigns. In this contribution, we present the results of the gamma-ray observations which are included in a MWL observational campaign organized on these sources during their high-states. We discuss the flare spectral properties and temporal variability. In the high-energy gamma band, sub-daily-scale variability and strong spectral hardening give evidence for compact emission regions responsible for the radiative output at high energies during the flare.

Young and massive stellar clusters are a promising source class for the acceleration of galactic cosmic rays to PeV energies. Supernova explosions of massive stars lead to semi-continuous injection of cosmic rays into the cluster environment. Collective stellar winds form a wind-blown bubble with a termination shock at which further particle acceleration may occur. Interactions of such energetic hadronic particles with target material - such as molecular clouds - will generate signature gamma-ray and neutrino emission. We apply a model of cosmic ray acceleration in stellar clusters to catalogues of known stellar clusters, identifying the most promising targets to search for evidence of PeVatron activity. Predictions for the secondary fluxes of gamma-ray and neutrino emission are derived based on particle acceleration at the collective wind termination shock. Our results are compared to data available from the H.E.S.S. Galactic Plane Survey, and we estimate the detection prospects for current and future generation neutrino experiments.

The blazar S5 0716+714 is an intermediate BL Lacertae object remarkable for its variability in many energy bands. It was discovered by MAGIC in the very-high-energy (VHE) gamma-ray range in 2008. Later in 2015 an impressive electric vector polarization angle (EVPA) swing was detected in connection with a multiwavelength flaring event including the VHE gamma-ray band. This generated interest in further studies of the jet of this source and its electromagnetic emission at all wavelengths. Since then, MAGIC has monitored the source in coordination with other observatories and here we present the long-term study using data from 2015 to 2022 in a MWL context. The data set also includes the extraordinary flaring activity of 2017, so far the historical maximum detected for this source in the optical and VHE gamma-ray band.

MAGIC is a system of two Imaging Atmospheric Cherenkov Telescopes (IACTs), designed to detect very-high-energy gamma rays, and is operating in stereoscopic mode since 2009 at the Observatorio del Roque de Los Muchachos in La Palma, Spain. In 2018, the prototype IACT of the Large-Sized Telescope (LST-1) for the Cherenkov Telescope Array, a next-generation ground-based gamma-ray observatory, was inaugurated at the same site, at a distance of approximately 100 meters from the MAGIC telescopes. Using joint observations between MAGIC and LST-1, we developed a dedicated analysis pipeline and established the threefold telescope system via software, achieving the highest sensitivity in the northern hemisphere. Based on this enhanced performance, MAGIC and LST-1 have been jointly and regularly observing the Galactic Center, a region of paramount importance and complexity for IACTs. In particular, the gamma-ray emission from the dynamical center of the Milky Way is under debate. Although previous measurements suggested that a supermassive black hole Sagittarius A* plays a primary role, its radiation mechanism remains unclear, mainly due to limited angular resolution and sensitivity. The enhanced sensitivity in our novel approach is thus expected to provide new insights into the question. We here present the current status of the data analysis for the Galactic Center joint MAGIC and LST-1 observations.

RS Ophiuchi (RS Oph) is a symbiotic recurrent nova that shows eruptive events roughly every 15 years. On August 8th, 2021, RS Oph erupted with its latest outburst. This event was detected by a wide range of multi-wavelength (MWL) instruments from radio up to very-high-energy (VHE) gamma rays. The MAGIC telescopes followed up on optical and high-energy triggers and initiated an observation campaign from August 9th till September 1st. RS Oph is the first nova detected in the VHE gamma-ray energy range. We report on the detection of VHE gamma rays at a significant level of 13.2σ during the first 4 days of RS Oph with the MAGIC telescopes. We combine the VHE emission detected by MAGIC with optical and high energy observations and conclude RS Oph accelerated hadrons during its eruption. We will present the MWL modeling revealing this hadronic emission, and its further implications for Galactic cosmic-rays.