The use of silicon photomultipliers (SiPMs) alongside conventional photomultiplier tubes (PMTs) is a remarkable technological development in modern ground-based very high energy gamma-ray astronomy. SiPMs exhibit comparable or even higher photon detection efficiencies (PDEs) than PMTs. The sensitivity of a PMT matches well the spectral shape of Cherenkov radiation from extended air showers. In contrast to a PMT, the sensitivity of a SiPM is shifted toward longer wavelengths, where the intensity of light of night sky (LoNS), considered as unwanted noise, increases significantly. It is obvious that a SiPM with a higher PDE will indeed measure more Cherenkov light than a PMT, but it will also detect significantly higher LoNS noise; the question is which factor will predominate in the signal-to-noise-ratio (SNR). To compare the performance of a PMT with that of a SiPM, we built SiPM-based modules and installed these and operated in parallel in the imaging camera of the 17 m diameter MAGIC telescope. Our long-term studies show that SiPM, despite their higher PDE, can deliver only a comparable to PMT performance. As already the name SiPM suggests, we use these semiconductor sensors analogously to classical PMTs: We amplify their small signals, digitize, and calibrate the converted amplitudes. Although SiPM is essentially a digital sensor, its common-anode design does not allow one to directly profit from it. Numerous arrays of single-photon avalanche diodes (SPADs) are being developed in various laboratories worldwide. Unlike SiPM, SPAD arrays digitize the incident photons from the outset and count their number. We will dwell on the potential further developments of SPADs.

Stellar intensity interferometry (SII) measures correlations in photon-arrival fluctuations recorded by telescopes observing bright celestial sources. It can resolve angular scales far smaller than those accessible to a single optical telescope and is largely insensitive to atmospheric turbulence. After the first demonstration of SII on Sirius in 1956, Hanbury Brown and Twiss used the technique to measure the diameters of 32 stars. More recently, VERITAS, MAGIC, H.E.S.S., and CTAO's LST-1 have revived the method, although observations remain restricted to bright targets because of their optical design, optimized for gamma-ray astrophysics, rather than SII. We present EON-SII, the design and performance of a two-telescope intensity interferometer intended to extend the SII technique to compact targets at magnitudes of about V=8.5 up to V=10.7. Each transportable telescope has a 4-m diameter mirror, approximately 9m2 collecting area, an actively aligned 18-panel primary mirror, and Cassegrain optics specified to concentrate at least 90

A new generation of optical intensity interferometers are emerging in recent years taking advantage of the existing infrastructure of Imaging Atmospheric Cherenkov Telescopes (IACTs). The MAGIC SII (Stellar Intensity Interferometer) in La Palma, Spain, has been operating since its first successful measurements in 2019 and its current design allows it to operate regularly. The current setup is ready to follow up on bright optical transients, as changing from regular gamma-ray observations to SII mode can be done in a matter of minutes. A paper studying the system performance, first measurements and future upgrades has been recently published. MAGIC SII's first scientific results are the measurement of the angular size of 22 stars, 13 of which with no previous measurements in the B band. More recently the Large Sized Telescope prototype from the Cherenkov Telescope Array Observatory (CTAO-LST1) has been upgraded to operate together with MAGIC as a SII, leading to its first correlation measurements at the beginning of 2024. MAGIC+CTAO-LST1 SII will be further upgraded by adding the remaining CTAO-LSTs at the north site to the system (which are foreseen to be built by the end of 2025). MAGIC+CTAO-LST1 SII shows a feasible technical solution to extend SII to the whole CTAO.

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.

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.

The high-synchrotron-peaked blazar VER J0521+211 was discovered in 2009 by VERITAS and is located at an unknown distance. In February 2020, the VERITAS collaboration alerted the multi-messenger community of an ongoing flare from the source, followed by a multi-frequency monitoring campaign led by MAGIC, covering energies from radio to very-high-energy (VHE, E>100GeV) which allowed to observe the source in VHE for 6 consecutive nights. Out of these observations, a statistical upper limit to the source redshift at z<0.243 was set using Fermi-LAT and MAGIC data, a value consistent with lower limits derived from optical spectroscopy. The modeling of the broadband spectral energy distribution (SED) and its night-wise evolution is also presented using two types of models: i) a leptonic two-zone synchrotron-self-Compton scenario; ii) a lepto-hadronic one-zone scenario. Both scenarios allow to reproduce the observed SEDs and their night-wise evolution, and have implications on optical polarization and production of neutrinos. For the first scenario, optical polarimetry measurements were compared with the predictions from the evolution of the two-zones. For the second scenario, limits on the production of neutrinos and enhanced photon emission at ultra-high-energies (E>100TeV) from $\pi^0$ decay were computed.

Very-high-energy (VHE, $E > 100\,{\rm GeV}$) gamma-ray telescopes have traditionally conducted science with proprietary data and software. After two decades of operations, they have accumulated thousands of hours of observations whose full scientific exploitation cannot be accomplished by the restricted group of scientists operating these instruments. The advent of a new generation of open gamma-ray observatories and, at the same time, the forthcoming end of their scientific operations, call for the realisation of public archives of their observations. With the objective to facilitate the exchange and dissemination of data from current- and next-generation gamma-ray instruments, the "Data formats for gamma-ray astronomy" (GADF) initiative was formed to provide an open and standardised format for gamma-ray astronomical data. In this contribution, we present the effort to produce data of the Major Atmospheric Gamma-ray Imaging Cherenkov (MAGIC) telescopes in this standardised format. A total of $150$ hours of observations of different sources were converted to the GADF format and then validated by analysing them with the open-source software Gammapy and comparing the results obtained against those produced with the MAGIC proprietary software, MARS. The effort to standardise and validate the Magic data in an open and shared format constitute the first fundamental milestone towards the realisation of its public data legacy.

Gamma-ray bursts (GRBs), the most luminous explosions in the universe, have at least two types known. One of them, short GRBs, have been thought to originate from binary neutron star (BNS) mergers. The discovery of GW170817 together with a GRB was the first and only direct proof of the hypothesis, and thus the properties of the short GRBs are poorly known yet. Aiming to clarify the underlying physical mechanisms of the short GRBs, we analyzed GRB 160821B, one of the nearest short GRBs known at z=0.162, observed with the MAGIC telescopes. A hint of a gamma-ray signal is found above 0.5 TeV at a significance of $>$3 sigma during observations from 24 seconds until 4 hours after the burst, as presented in the past. Recently, multi-wavelength data of its afterglow emission revealed a well-sampled kilonova component from a BNS merger, and the importance of GRB 160821B increased concerning GRB-GW studies. Accordingly, we investigated GRB afterglow models again, using the revised multi-wavelength data. We found that the straightforward interpretation with one-zone synchrotron self-Compton model from the external forward shock is in tension with the observed TeV flux, contradicting the suggestion reported previously. In this contribution we discuss the implication from the TeV observation, including alternative scenarios where the TeV emission can be enhanced. We also give a brief outlook of future GeV-TeV observations of short GRBs with imaging atmospheric Cherenkov telescopes, which could shed more light on the GRB-BNS merger relation.

Combined measurements of the production and decay rates of the Higgs boson, as well as its couplings to vector bosons and fermions, are presented. The analysis uses the LHC proton-proton collision data set recorded with the CMS detector in 2016 at $\sqrt{s} =$ 13 TeV, corresponding to an integrated luminosity of 35.9 fb$^{-1}$. The combination is based on analyses targeting the five main Higgs boson production mechanisms (gluon fusion, vector boson fusion, and associated production with a W or Z boson, or a top quark-antiquark pair) and the following decay modes: H $\to$ $\gamma\gamma$, ZZ, WW, $\tau\tau$, bb, and $\mu\mu$. Searches for invisible Higgs boson decays are also considered. The best-fit ratio of the signal yield to the standard model expectation is measured to be $\mu$ $=$ 1.17 $\pm$ 0.10, assuming a Higgs boson mass of 125.09 GeV. Additional results are given for parametrizations with varying assumptions on the scaling behavior of the different production and decay modes, including generic ones based on ratios of cross sections and branching fractions or coupling modifiers. The results are compatible with the standard model predictions in all parametrizations considered. In addition, constraints are placed on various two Higgs doublet models.

The MAGIC telescopes, composed of two 17-meter diameter Imaging Air Cherenkov Telescopes (IACTs) situated 2200 meters above sea level on La Palma, detect very-high-energy gamma rays from the resulting Cherenkov air showers. They utilize a digital trigger system with an energy threshold of ~55 GeV. An alternative trigger, which sums analogue signals from adjacent regions of pixel groups, reduces the trigger threshold down to ~30 GeV. This Sum-Trigger-II recently allowed MAGIC to detect the Geminga Pulsar. We present detailed performance studies and operations of the Sum-Trigger-II system following a recent re-calibration period during Fall 2018 and Spring 2019. In addition, prospects and challenges for detecting GRB events with the Sum-Trigger-II, as well as potential future upgrades to the system, are discussed.

A search for new physics using events containing an imbalance in transverse momentum and one or more energetic jets arising from initial-state radiation or the hadronic decay of W or Z bosons is presented. A data sample of proton-proton collisions at $\sqrt{s} = $ 13 TeV, collected with the CMS detector at the LHC and corresponding to an integrated luminosity of 35.9 fb$^{-1}$, is used. The observed data are found to be in agreement with the expectation from standard model processes. The results are interpreted as limits on the dark matter production cross section in simplified models with vector, axial-vector, scalar, and pseudoscalar mediators. Interpretations in the context of fermion portal and nonthermal dark matter models are also provided. In addition, the results are interpreted in terms of invisible decays of the Higgs boson and set stringent limits on the fundamental Planck scale in the Arkani-Hamed, Dimopoulos, and Dvali model with large extra spatial dimensions.

Abstract A search for a new high-mass resonance decaying to a τ lepton and a neutrino is reported. The analysis uses proton-proton collision data collected by the CMS experiment at the LHC at s = 13 TeV , corresponding to an integrated luminosity of 35.9 fb − 1 . The search utilizes hadronically decaying τ leptons. No excess in the event yield is observed at high transverse masses of the τ and missing transverse momentum. An interpretation of results within the sequential standard model excludes W ′ boson masses below 4.0 TeV at 95% confidence level. Existing limits are also improved on models in which the W ′ boson decays preferentially to fermions of the third generation. Heavy W ′ bosons with masses less than 1.7–3.9 TeV, depending on the coupling in the non-universal G(221) model, are excluded at 95% confidence level. These are the most stringent limits on this model to date.