DarkSide-20k (DS-20k) is a dark matter detection experiment under construction at the Laboratori Nazionali del Gran Sasso (LNGS) in Italy. It utilises similar to 100 t of low radioactivity argon from an underground source (UAr) in its inner detector, with half serving as target in a dual-phase time projection chamber (TPC). The UAr cryogenics system must maintain stable thermodynamic conditions throughout the experiment's lifetime of over 10 years. Continuous removal of impurities and radon from the UAr is essential for maximising signal yield and mitigating background. We are developing an efficient and powerful cryogenics system with a gas purification loop with a target circulation rate of 1000 slpm. Central to its design is a condenser operated with liquid nitrogen which is paired with a gas heat exchanger cascade, delivering a combined cooling power of more than 8 kW. Here we present the design choices in view of the DS-20k requirements, in particular the condenser's working principle and the cooling control, and we show test results obtained with a dedicated benchmarking platform at CERN and LNGS. We find that the thermal efficiency of the recirculation loop, defined in terms of nitrogen consumption per argon flow rate, is 95% and the pressure in the test cryostat can be maintained within +/-(0.1-0.2) mbar. We further detail a 5-day cool-down procedure of the test cryostat, maintaining a cooling rate typically within -2K/h, as required for the DS-20k inner detector. Additionally, we assess the circuit's flow resistance, and the heat transfer capabilities of two heat exchanger geometries for argon phase change, used to provide gas for recirculation. We conclude by discussing how our findings influence the finalisation of the system design, including necessary modifications to meet requirements and ongoing testing activities.

The dual-phase liquid argon time projection chamber is presently one of the leading technologies to search for dark matter particles with masses below 10 GeV c(-2). This was demonstrated by the DarkSide-50 experiment with approximately 50 kg of low-radioactivity liquid argon as target material. The next generation experiment DarkSide-20k, currently under construction, will use 1,000 times more argon and is expected to start operation in 2027. Based on the DarkSide-50 experience, here we assess the DarkSide-20k sensitivity to models predicting light dark matter particles, including Weakly Interacting Massive Particles (WIMPs) and sub-GeV c(-2) particles interacting with electrons in argon atoms. With one year of data, a sensitivity improvement to dark matter interaction cross-sections by at least one order of magnitude with respect to DarkSide-50 is expected for all these models. A sensitivity to WIMP-nucleon interaction cross-sections below 1x10(-42) cm(2) is achievable for WIMP masses above 800 MeV c(-2). With 10 years exposure, the neutrino fog can be reached for WIMP masses around 5 GeV c(-2).

Experiments aimed at direct searches for WIMP dark matter require highly effective reduction of backgrounds and control of any residual radioactive contamination. In particular, neutrons interacting with atomic nuclei represent an important class of backgrounds due to the expected similarity of a WIMP-nucleon interaction, so that such experiments often feature a dedicated neutron detector surrounding the active target volume. In the context of the development of DarkSide-20k detector at INFN Gran Sasso National Laboratory (LNGS), several R D projects were conceived and developed for the creation of a new hybrid material rich in both hydrogen and gadolinium nuclei to be employed as an essential element of the neutron detector. Thanks to its very high cross-section for neutron capture, gadolinium is one of the most widely used elements in neutron detectors, while the hydrogen-rich material is instrumental in efficiently moderating the neutrons. In this paper results from one of the R Ds are presented. In this effort the new hybrid material was obtained as a poly(methyl methacrylate) (PMMA) matrix, loaded with gadolinium oxide in the form of nanoparticles. We describe its realization, including all phases of design, purification, construction, characterization, and determination of mechanical properties of the new material.

The need to push the timing performances to the limit finds breeding ground in several fields from high energy physics to biomedical applications such as Time of Flight Positron Emission Tomography (ToF-PET). In the last years, excellent results have been achieved thanks to the improvement of the scintillator crystal materials, the electronics readout and the detector development. In this context, SiPM Coincidence Time Resolution (CTR) is a key parameter in order to assess the device timing performance. In this work we will present the CTR of the recently introduced FBK NUV-HD-MT Silicon Photomultiplier (SiPM) technology. Thanks to the addition of the optically insulating material inside the trenches, FBK NUV-HDMT devices show an extremely low CrossTalk (CT) of about ≃ 5% at 47.5V (≃ 15V excess bias). The Photon Detection Efficiency (PDE) reaches the ≃ 65% at the same excess bias at 420nm. The CTR was measured using a 4mm × 4mm SiPM to match the 3mm × 3mm × 5mm LYSO:Ce:Ca crystal and comparing different microcell sizes. By using a standard readout electronics we achieved a CTR of ~ 95ps FWHM for all the devices thanks to the extremely low CT of the technology that allows to push the voltage bias to high values. We also compare the CTR between the SiPM version with a metal mask outside the active area (capacitive coupling) and the SiPM without it in order to asses the role of the masking in the timing performance, to discuss about limitations and further improvements.

Extended and delayed emission around distant TeV sources induced by the effects of propagation of gamma rays through the intergalactic medium can be used for the measurement of the intergalactic magnetic field (IGMF). We search for delayed GeV emission from the hard-spectrum TeV blazar 1ES 0229+200 with the goal to detect or constrain the IGMF-dependent secondary flux generated during the propagation of TeV gamma rays through the intergalactic medium. We analyze the most recent MAGIC observations over a 5 year time span and complement them with historic data of the H.E.S.S. and VERITAS telescopes along with a 12-year long exposure of the Fermi/LAT telescope. We use them to trace source evolution in the GeV-TeV band over one-and-a-half decade in time. We use Monte Carlo simulations to predict the delayed secondary gamma-ray flux, modulated by the source variability, as revealed by TeV-band observations. We then compare these predictions for various assumed IGMF strengths to all available measurements of the gamma-ray flux evolution. We find that the source flux in the energy range above 200 GeV experiences variations around its average on the 14 years time span of observations. No evidence for the flux variability is found in 1-100 GeV energy range accessible to Fermi/LAT. Non-detection of variability due to delayed emission from electromagnetic cascade developing in the intergalactic medium imposes a lower bound of B>1.8e-17 G for long correlation length IGMF and B>1e-14 G for an IGMF of the cosmological origin. Though weaker than the one previously derived from the analysis of Fermi/LAT data, this bound is more robust, being based on a conservative intrinsic source spectrum estimate and accounting for the details of source variability in the TeV energy band. We discuss implications of this bound for cosmological magnetic fields which might explain the baryon asymmetry of the Universe.

Context. Diffusive shock acceleration (DSA) is the most promising mechanism to accelerate Galactic cosmic rays (CRs) in the shocks of supernova remnants (SNRs). The turbulence upstream is supposedly generated by the CRs, but this process is not well understood. The dominant mechanism may depend on the evolutionary state of the shock and can be studied via the CRs escaping upstream into the interstellar medium (ISM). Aims. Previous observations of the $\gamma$-Cygni SNR showed a difference in morphology between GeV and TeV energies. Since this SNR has the right age and is at the evolutionary stage for a significant fraction of CRs to escape, we aim to understand $\gamma$-ray emission in the vicinity of the $\gamma$-Cygni SNR. Methods. We observed the region of the $\gamma$-Cygni SNR with the MAGIC Imaging Atmospheric Cherenkov telescopes between May 2015 and September 2017 recording 87 h of good-quality data. Additionally we analysed Fermi-LAT data to study the energy dependence of the morphology as well as the energy spectrum in the GeV to TeV range. The energy spectra and morphology were compared against theoretical predictions, which include a detailed derivation of the CR escape process and their $\gamma$-ray generation. Results. The MAGIC and Fermi-LAT data allowed us to identify three emission regions, which can be associated with the SNR and dominate at different energies. Our hadronic emission model accounts well for the morphology and energy spectrum of all source components. It constrains the time-dependence of the maximum energy of the CRs at the shock, the time-dependence of the level of turbulence, and the diffusion coefficient immediately outside the SNR shock. While in agreement with the standard picture of DSA, the time-dependence of the maximum energy was found to be steeper than predicted and the level of turbulence was found to change over the lifetime of the SNR.

The activation of materials due to exposure to cosmic rays may become an important background source for experiments investigating rare event phenomena. DarkSide-20k, currently under construction at the Laboratori Nazionali del Gran Sasso, is a direct detection experiment for galactic dark matter particles, using a two-phase liquid-argon Time Projection Chamber (TPC) filled with 49.7 tonnes (active mass) of Underground Argon (UAr) depleted in 39Ar. Despite the outstanding capability of discriminating gamma/beta background in argon TPCs, this background must be considered because of induced dead time or accidental coincidences mimicking dark-matter signals and it is relevant for low-threshold electron-counting measurements. Here, the cosmogenic activity of relevant long-lived radioisotopes induced in the experiment has been estimated to set requirements and procedures during preparation of the experiment and to check that it is not dominant over primordial radioactivity; particular attention has been paid to the activation of the 120 t of UAr used in DarkSide-20k. Expected exposures above ground and production rates, either measured or calculated, have been considered in detail. From the simulated counting rates in the detector due to cosmogenic isotopes, it is concluded that activation in copper and stainless steel is not problematic. The activity of 39Ar induced during extraction, purification and transport on surface is evaluated to be 2.8% of the activity measured in UAr by DarkSide-50 experiment, which used the same underground source, and thus considered acceptable. Other isotopes in the UAr such as 37Ar and 3H are shown not to be relevant due to short half-life and assumed purification methods.

The emerging field of ultra-fast timing has enormous potential in high-energy physics, nuclear medicine, and imaging, and is driving advances in detector and readout electronics. To achieve improved time resolution, significant efforts have been devoted to this field, resulting in impressive coincidence time resolution (CTR) values for silicon photomultiplier (SiPM) and scintillator crystals using state-of-the-art high-frequency (HF) readout. However, scaling this readout method for system-level applications is not yet feasible, hence requiring dedicated application-specific integrated circuits (ASICs). In this study, we evaluate the performance of the FastIC, an 8-channel readout ASIC suitable for TOP-PET application, in terms of CTR. The study employs Cherenkov photons produced in bismuth germanium oxide (BGO) scintillator crystals and novel SiPMs with metal-trenching. We obtain CTR values of 257±5 ps and 152±4 ps, respectively, for 2x2x20mm³ and 2x2x3mm³ BGO crystals using HF readout as a reference, with minimal electronic interference, by exploiting the full potential of SiPMs with metal trenching. In comparison, the CTR values obtained using the FastIC for the same crystals were 490±3 ps and 330±4 ps, respectively. We also assess the limits of the FastIC and suggest areas for further improvements.

The Aria cryogenic distillation plant, located in Sardinia, Italy, is a key component of the DarkSide-20k experimental program for WIMP dark matter searches at the INFN Laboratori Nazionali del Gran Sasso, Italy. Aria is designed to purify the argon, extracted from underground wells in Colorado, USA, and used as the DarkSide-20k target material, to detector-grade quality. In this paper, we report the first measurement of argon isotopic separation by distillation with the 26 m tall Aria prototype. We discuss the measurement of the operating parameters of the column and the observation of the simultaneous separation of the three stable argon isotopes: ^36Ar , ^38Ar , and ^40Ar . We also provide a detailed comparison of the experimental results with commercial process simulation software. This measurement of isotopic separation of argon is a significant achievement for the project, building on the success of the initial demonstration of isotopic separation of nitrogen using the same equipment in 2019.

Since their discovery in the late 1960s Gamma-Ray Burst (GRB) emission has been deeply investigated with the help of the huge amount of data collected covering the entire electromagnetic spectrum. This large and broadband dataset was essential to constitute a general picture describing the GRB physics, revealing the most credible underlying physical processes and environmental conditions ongoing at the GRB site. A key leap in the comprehension of the GRB physics have been achieved recently, thanks to the detection of the newly energetic component in the Very High Energy (VHE, E > 100 GeV) domain. The possible presence of a TeV spectral window in GRBs was predicted and theorized for several decades, but the first observational proofs of its existence were reached only in 2019 thanks to the discoveries claimed by the MAGIC and H.E.S.S. telescopes. GRB190114C was successfully detected in the TeV band by the MAGIC telescopes starting from around one minute after its trigger time and lasting for nearly 40 minutes. A successful follow-up campaign was performed and the multi-wavelength afterglow emission of the event was collected from 1 to about 2 x 1017 GHz. Such very broad dataset allows to perform unique studies on the radiation mechanisms and on the physical properties of such event. In this contribution I will describe the main results and the theoretical interpretations that have been derived from the multi-wavelength dataset of GRB190114C. In particular, the description of the TeV component detected by the MAGIC telescopes as produced via the Synchrotron Self-Compton (SSC) mechanism and its connection with the emission at lower energy bands will be presented. Such studies are a fundamental starting point for the interpretation of the current and upcoming events that will be observed in the VHE domain.

Classical novae are cataclysmic binary star systems in which the matter of a companion star is accreted on a white dwarf1,2. Accumulation of hydrogen in a layer eventually causes a thermonuclear explosion on the surface of the white dwarf3, brightening the white dwarf to ~105 solar luminosities and triggering ejection of the accumulated matter. Novae provide the extreme conditions required to accelerate particles, electrons or protons, to high energies. Here we present the detection of gamma rays by the MAGIC telescopes from the 2021 outburst of RS Ophiuchi, a recurrent nova with a red giant companion, which allowed us to accurately characterize the emission from a nova in the 60 GeV to 250 GeV energy range. The theoretical interpretation of the combined Fermi LAT and MAGIC data suggests that protons are accelerated to hundreds of gigaelectronvolts in the nova shock. Such protons should create bubbles of enhanced cosmic ray density, of the order of 10 pc, from the recurrent novae. Detection of the 2021 outburst of the nova RS Oph in very-high-energy gamma rays by the MAGIC telescopes is reported. Investigation of the gamma-ray emission provides evidence for acceleration of protons within the nova shock, which then propagate outwards to create bubbles of enhanced cosmic ray density.

The blazar TXS 0506+056 got into the spotlight of the astrophysical community in 2017 September, when a high-energy neutrino detected by IceCube (IceCube-170922A) was associated at the 3σ level with a γ-ray flare from this source. This multi-messenger photon-neutrino association remains, as per today, the most significant association ever observed. TXS 0506+056 was a poorly studied object before the IceCube-170922A event. To better characterize its broadband emission, we organized a multiwavelength campaign lasting 16 months (2017 November to 2019 February), covering the radio band (Metsähovi, OVRO), the optical/UV (ASAS-SN, KVA, REM, Swift/UVOT), the X-rays (Swift/XRT, NuSTAR), the high-energy γ rays (Fermi/LAT), and the very high-energy (VHE) γ rays (MAGIC). In γ rays, the behavior of the source was significantly different from the behavior in 2017: MAGIC observations show the presence of flaring activity during 2018 December, while the source only shows an excess at the 4σ level during the rest of the campaign (74 hr of accumulated exposure); Fermi/LAT observations show several short (on a timescale of days to a week) flares, different from the long-term brightening of 2017. No significant flares are detected at lower energies. The radio light curve shows an increasing flux trend that is not seen in other wavelengths. We model the multiwavelength spectral energy distributions in a lepto-hadronic scenario, in which the hadronic emission emerges as Bethe-Heitler and pion-decay cascade in the X-rays and VHE γ rays. According to the model presented here, the 2018 December γ-ray flare was connected to a neutrino emission that was too brief and not bright enough to be detected by current neutrino instruments.

Milky Way dwarf spheroidal galaxies (dSphs) are among the best candidates to search for signals of dark matter annihilation with Imaging Atmospheric Cherenkov Telescopes, given their high mass-to-light ratios and the fact that they are free of astrophysical gamma-ray emitting sources. Since 2011, MAGIC has performed a multi-year observation program in search for Weakly Interacting Massive Particles (WIMPs) in dSphs. Results on the observations of Segue 1 and Ursa Major II dSphs have already been published and include some of the most stringent upper limits (ULs) on the velocity-averaged cross-section 〈σannv〉 of WIMP annihilation from observations of dSphs. In this work, we report on the analyses of 52.1h of data of Draco dSph and 49.5h of Coma Berenices dSph observed with the MAGIC telescopes in 2018 and in 2019 respectively. No hint of a signal has been detected from either of these targets and new constraints on the 〈σannv〉 of WIMP candidates have been derived. In order to improve the sensitivity of the search and reduce the effect of the systematic uncertainties due to the J-factor estimates, we have combined the data of all dSphs observed with the MAGIC telescopes. Using 354.3h of dSphs good quality data, 95% CL ULs on 〈σannv〉 have been obtained for 9 annihilation channels. For most of the channels, these results reach values of the order of 10−24cm3/s at ∼1 TeV and are the most stringent limits obtained with the MAGIC telescopes so far.

The first Large Size Telescope (LST-1) of the Cherenkov Telescope Array has been operational since October 2018 at La Palma, Spain. We report on the results obtained during the camera commissioning. The noise level of the readout is determined as a 0.2 p.e. level. The gain of PMTs are well equalized within 2% variation, using the calibration flash system. The effect of the night sky background on the signal readout noise as well as the PMT gain estimation are also well evaluated. Trigger thresholds are optimized for the lowest possible gamma-ray energy threshold and the trigger distribution synchronization has been achieved within 1 ns precision. Automatic rate control realizes the stable observation with 1.5% rate variation over 3 hours. The performance of the novel DAQ system demonstrates a less than 10% dead time for 15 kHz trigger rate even with sophisticated online data correction.

The flat spectrum radio quasar (FSRQ) PKS 1510−089 is known for its complex multiwavelength behaviour and it is one of only a few FSRQs detected in very-high-energy (VHE,E > 100 GeV)γrays. The VHEγ-ray observations with H.E.S.S. and MAGIC in late May and early June 2016 resulted in the detection of an unprecedented flare, which revealed, for the first time, VHEγ-ray intranight variability for this source. While a common variability timescale of 1.5 h has been found, there is a significant deviation near the end of the flare, with a timescale of ∼20 min marking the cessation of the event. The peak flux is nearly two orders of magnitude above the low-level emission. For the first time, a curvature was detected in the VHEγ-ray spectrum of PKS 1510–089, which can be fully explained by the absorption on the part of the extragalactic background light. OpticalR-band observations with ATOM revealed a counterpart of theγ-ray flare, even though the detailed flux evolution differs from the VHEγ-ray light curve. Interestingly, a steep flux decrease was observed at the same time as the cessation of the VHEγ-ray flare. In the high-energy (HE,E > 100 MeV)γ-ray band, only a moderate flux increase was observed withFermi-LAT, while the HEγ-ray spectrum significantly hardens up to a photon index of 1.6. A search for broad-line region (BLR) absorption features in theγ-ray spectrum indicates that the emission region is located outside of the BLR. Radio very-long-baseline interferometry observations reveal a fast-moving knot interacting with a standing jet feature around the time of the flare. As the standing feature is located ∼50 pc from the black hole, the emission region of the flare may have been located at a significant distance from the black hole. If this is indeed a true correlation, the VHEγrays must have been produced far down in the jet, where turbulent plasma crosses a standing shock.

University of Trieste, Department of Physics, via Valerio 2, Trieste, Italy INFN, Sezione di Trieste,via Valerio 2, Trieste, Italy Institute for Fundamental Physics of the Universe (IFPU), via Beirut 12, Trieste, Italy Max Planck Institute for Physics, Föhringer Ring 6, Munich, Germany University of Zagreb, Faculty of Electrical Engineering and Computing (FER), Zagreb, Croatia 5 University of Udine, Dipartimento di Scienze Matematiche, Informatiche e Fisiche via delle Scienze 206, Udine, Italy Institut de Física d’Altes Energies (IFAE), The Barcelona Institute of Science and Technology (BIST), Bellaterra (Barcelona), Spain Institute for Cosmic Ray Research, The University of Tokyo, Kashiwanoha 5-1-5, Kashiwa, Japan Osservatorio Astronomico di Roma, INAF, Via Frascati, 33, Monte Porzio Catone (RM), Italy Laboratoire d’Annecy de Physique des Particules (LAPP), CNRS-IN2P3, 9 Chemin de Bellevue BP 110, Annecy Cedex, France Osservatorio Astronomico di Brera, INAF, via E. Bianchi 46, Merate (LC), Italy E-mail: francesco.longo@ts.infn.it

Aims. We present a detailed characterisation and theoretical interpretation of the broadband emission of the paradigmatic TeV blazar Mrk 421, with a special focus on the multi-band flux correlations. Methods. The dataset has been collected through an extensive multi-wavelength campaign organised between 2016 December and 2017 June. The instruments involved are MAGIC, FACT, Fermi -LAT, Swift , GASP-WEBT, OVRO, Medicina, and Metsähovi. Additionally, four deep exposures (several hours long) with simultaneous MAGIC and NuSTAR observations allowed a precise measurement of the falling segments of the two spectral components. Results. The very-high-energy (VHE; E > 100 GeV) gamma rays and X-rays are positively correlated at zero time lag, but the strength and characteristics of the correlation change substantially across the various energy bands probed. The VHE versus X-ray fluxes follow different patterns, partly due to substantial changes in the Compton dominance for a few days without a simultaneous increase in the X-ray flux (i.e., orphan gamma-ray activity). Studying the broadband spectral energy distribution (SED) during the days including NuSTAR observations, we show that these changes can be explained within a one-zone leptonic model with a blob that increases its size over time. The peak frequency of the synchrotron bump varies by two orders of magnitude throughout the campaign. Our multi-band correlation study also hints at an anti-correlation between UV-optical and X-ray at a significance higher than 3 σ . A VHE flare observed on MJD 57788 (2017 February 4) shows gamma-ray variability on multi-hour timescales, with a factor ten increase in the TeV flux but only a moderate increase in the keV flux. The related broadband SED is better described by a two-zone leptonic scenario rather than by a one-zone scenario. We find that the flare can be produced by the appearance of a compact second blob populated by high energetic electrons spanning a narrow range of Lorentz factors, from γ ′ min =2×10 4 to γ ′ max =6×10 5 .

PSR J0218+4232 is one of the most energetic millisecond pulsars known and has long been considered as one of the best candidates for very high-energy (VHE; >100 GeV) γ-ray emission. Using 11.5 yr of Fermi Large Area Telescope (LAT) data between 100 MeV and 870 GeV, and ∼90 hr of Major Atmospheric Gamma Imaging Cherenkov (MAGIC) observations in the 20 GeV to 20 TeV range, we searched for the highest energy γ-ray emission from PSR J0218+4232. Based on the analysis of the LAT data, we find evidence for pulsed emission above 25 GeV, but see no evidence for emission above 100 GeV (VHE) with MAGIC. We present the results of searches for γ-ray emission, along with theoretical modeling, to interpret the lack of VHE emission. We conclude that, based on the experimental observations and theoretical modeling, it will remain extremely challenging to detect VHE emission from PSR J0218+4232 with the current generation of Imaging Atmospheric Cherenkov Telescopes, and maybe even with future ones, such as the Cherenkov Telescope Array.

Context. QSO B1420+326 is a blazar classified as a flat-spectrum radio quasar (FSRQ). At the beginning of the year 2020, it was found to be in an enhanced flux state and an extensive multiwavelength campaign allowed us to trace the evolution of the flare.Aims. We search for very high-energy (VHE) gamma-ray emission from QSO B1420+326 during this flaring state. We aim to characterize and model the broadband emission of the source over different phases of the flare.Methods. The source was observed with a number of instruments in radio, near-infrared, optical (including polarimetry and spectroscopy), ultraviolet, X-ray, and gamma-ray bands. We use dedicated optical spectroscopy results to estimate the accretion disk and the dust torus luminosity. We performed spectral energy distribution modeling in the framework of combined synchrotron-self-Compton and external Compton scenario in which the electron energy distribution is partially determined from acceleration and cooling processes.Results. During the enhanced state, the flux of both SED components of QSO B1420+326 drastically increased and the peaks were shifted to higher energies. Follow-up observations with the MAGIC telescopes led to the detection of VHE gamma-ray emission from this source, making it one of only a handful of FSRQs known in this energy range. Modeling allows us to constrain the evolution of the magnetic field and electron energy distribution in the emission region. The gamma-ray flare was accompanied by a rotation of the optical polarization vector during a low -polarization state. Also, a new superluminal radio knot contemporaneously appeared in the radio image of the jet. The optical spectroscopy shows a prominent FeII bump with flux evolving together with the continuum emission and a MgII line with varying equivalent width.