Articles publicats en revistes (Física Quàntica i Astrofísica)

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Black holes galore in D>4
(Springer Science + Business Media, 2025-04-24) Emparan García de Salazar, Roberto A.
The black hole solutions to Einstein’s vacuum equations in four dimensions contain just one example: the Kerr black hole. Over the past two decades, we have understood that higher-dimensional black holes are far more plentiful. I give a straightforward account of the reasons for this abundance based on three key ideas: (i) Horizons in can be long. (ii) Long horizons are flexible. (iii) Long horizons are unstable. I conclude with some comments and conjectures about the classification problem.
Dynamical masses of young stellar objects with the VLBA: DYNAMO-VLBA: Trigonometric parallaxes and proper motions of YSOs in Orion
(EDP Sciences, 2026-04-24) Dzib, Sergio; Moo-Herrera, Karla; Ordóñez-Toro, Jazmín; Loinard, Laurent; Kounkel, Marina; Ortiz-León, Gisela N.; Galli, Phillip; Rodríguez, Luis F.; Mioduszewski, Amy; Masqué Saumell, Josep Maria; O'Kelly, Eoin; Forbrich, Jan
We present results from a multi-epoch Very Long Baseline Array (VLBA) survey of compact radio sources in the Orion complex, conducted within both the DYNAMO–VLBA and GOBELINS projects. We detected 216 compact radio sources, of which 58 yielded reliable multi-epoch astrometric solutions. For these sources, we derived trigonometric parallaxes and proper motions with typical precisions of about 0.05 mas and 0.10 mas yr−1, respectively. The measured parallaxes range between 2.26 and 2.65 mas, corresponding to distances of 380–440 pc, and delineate the depth of the Orion star-forming complex. We determined mean distances of 405 ± 16 pc for NGC 2068, 403 ± 5 pc for NGC 2024, 407 ± 12 pc for the σ Orionis region, 388.5 ± 1.7 pc for the Orion Nebula Cluster, and 438 ± 12 pc for L1641. A comparison with Gaia DR3 astrometry for 28 common sources revealed negligible mean parallax offsets (Δϖ= − 0.02 ± 0.01 mas) and small systematic differences in proper motions (∼0.07 mas yr−1), likely due to the residual rotation of the Gaia reference frame. Our results demonstrate the capability of high-precision radio astrometry to map embedded stellar populations and to provide an independent calibration of the Gaia reference system in obscured regions.
A Cooler Look at the Environment of Cygnus X-1: Searching for Dynamical Interactions within Cold Molecular Gas
(Institute of Physics (IOP), 2026-08-20) Bosch i Ramon, Valentí; Bosch-Cabot, Pau; Tetarenko, Alexandra J.; Miller-Jones, James C. A.; Russell, David; Motta, Sara E.; Atri, Pikky; Díaz-Trigo, María; Mariani, Isabella; Prabu, Steve
We present IRAM 30 m observations aimed at identifying potential outflow─interstellar medium interaction sites in the vicinity of the black hole X-ray binary Cygnus X-1, which displays persistent relativistic jets and a prominent stellar wind. Using this data set, we construct molecular line emission maps, identifying a never-before-seen molecular structure potentially linked to X-ray-binary-driven feedback. This structure, surrounding the source, exhibits properties consistent with an interaction powered primarily by the stellar wind of the massive stellar companion and further sculpted by the relativistic jets. Our results indicate that multiple outflow mechanisms (stellar winds and relativistic jets) may simultaneously be shaping the interstellar medium around Cygnus X-1 and that molecular line imaging can help to disentangle complex feedback processes in environments where multiple outflows take place.
Large-scale emission from γ-ray binaries: the case of LS 5039
(EDP Sciences, 2026-09) Martinez, Javier Rodrigo; Bosch i Ramon, Valentí
Context. Gamma-ray binaries hosting a non-accreting neutron star and a massive star exhibit multi-wavelength emission on different spatial scales. The interaction between the neutron star and stellar winds mixes them and produces an outflow whose interaction with the medium on large scales can inflate a bubble or form a bow shock in systems that are old enough. The well-studied but still mysterious high-energy emitting binary LS 5039 shows extended (1 pc scale) X-rays that might arise from one of these two interaction regimes. Aims. We explain and predict the large-scale emission from LS 5039. Methods. We modelled the LS 5039 large-scale emission, including the thermal and non-thermal contributions. For the latter, we assumed particle acceleration at the termination shock of the mixed-wind outflow. We considered five scenarios: a very young and an intermediate-age bubble interacting with the interstellar medium, and a bubble evolving inside a supernova remnant, studied using a one-zone model; and two older bow shocks with a moderate and a high neutron star power, respectively, for which we used a multi-zone approach. We also investigated the particles escaping from these structures and their radiation. Results. The large-scale X-rays surrounding LS 5039 are best explained as synchrotron radiation, as previously proposed. In some cases, very extended radio emission might be detectable, while particles escaping the region may be a minor but non-negligible steady contribution to the source gamma rays in the powerful bow-shock scenario. Additionally, escaping protons and nuclei with 0.1–1 PeV might inject up to ∼1036 erg s−1 into the Galactic cosmic rays for optimistic injection luminosities. Conclusions. Gamma-ray binaries can be efficient accelerators at large scales and produce non-negligible amounts of broadband emission and 0.1–1 PeV cosmic rays. Our study can help future multi-wavelength observational campaigns to better characterise the large-scale medium interaction, age, and birthplace of LS 5039.
Megahertz Gravitational Waves from Neutron Star Merger
(American Physical Society, 2026-03-10) Casalderrey Solana, Jorge; Mateos, David (Mateos Solé); Sanchez-Garitaonandia, Mikel; Blas, Diego
Neutron star mergers provide a unique laboratory for the study of strong-field gravity coupled to quantum chromodynamics in extreme conditions. The frequencies and amplitudes of the resulting gravitational waves encode invaluable information about the merger. Simulations to date have shown that these frequencies lie in the kilohertz range. They have also shown that, if quantum chromodynamics possesses a first-order phase transition at high baryon density, then this is likely to be accessed during the merger dynamics. Here, we show that this would result in the nucleation of superheated and/or supercompressed bubbles whose subsequent dynamics would produce gravitational waves in the megahertz range. We estimate the amplitude of this signal and compare it to the sensitivity of planned future detectors.
Self-sustained, out-of-equilibrium inflation
(Springer Verlag, 2026-09-04) Casalderrey Solana, Jorge; Lucía Castells-Tiestos; Gonçalves, Jéssica.; Mateos, David (Mateos Solé)
We use holography to study dS-invariant states of non-conformal, strongly coupled quantum field theories in four-dimensional de Sitter space. We show that out-of-equilibrium effects can sustain the exponential inflation within the regime of validity of semiclassical gravity, H ≪ M ≪ Msp, with H the Hubble parameter, M the characteristic scale of the quantum field theory, Msp = Mp/N the species scale, Mp the Planck scale, and N2 the number of matter fields. In the holographic description, the required fine-tuning scales only logarithmically with the ratio Msp/H. The resulting solutions exhibit apparent horizons whose increasing area indicates a continuous growth of the comoving entropy density. We suggest that this inflationary regime can arise as the late-time limit of a dynamical evolution starting from an initial Friedmann–Lemaître–Robertson–Walker universe.
Dynamical masses of young stellar objects with the VLBA: DYNAMO-VLBA: Radio binary stars in Orion
(EDP Sciences, 2026-04-24) Dzib, Sergio; Ordóñez-Toro, Jazmín; Loinard, Laurent; Kounkel, Marina; Ortiz-León, Gisela; Galli, Phillip; Rodriguez, Luis F.; Mioduszewski, Amy; Masqué Saumell, Josep Maria; O'Kelly, Eoin; Forbrich, Jan; Moo-Herrera, Karla
We present results from a multi-epoch Very Long Baseline Array (VLBA) survey conducted as part of the DYNAMO–VLBA project, aimed at measuring the dynamical masses of young stellar systems in the Orion complex. Our observations include 19 radio sources associated with 15 binary or multiple young systems. For four visual binaries in which both components were detected, the derived Keplerian orbits yield model-independent stellar masses; in particular, Brun 656 and HD 294300 show excellent agreement between VLBA-based and spectral-energy-distribution-based estimates, providing valuable benchmarks for pre-main-sequence evolutionary models. The component NU Ori C is confirmed as an intermediate-mass (∼7 M⊙) star with nonthermal radio emission, offering rare evidence of magnetic activity near the boundary with the high-mass regime. Several additional sources exhibit astrometric accelerations or periodic residuals, revealing unseen companions and extending dynamical constraints to systems with only one radio-emitting component. These results highlight the capability of very long baseline interferometry astrometry to obtain precise and model-independent masses of young binaries, providing critical empirical anchors for stellar evolution models and new insights into the origin of magnetism in intermediate-mass stars.
PENGEOM -- A general-purpose geometry package for Monte Carlosimulation of radiation transport in complex material structures (NewVersion Announcement)
(Elsevier B.V., 2025-04-18) Almansa, Julio; Salvat Pujol, Francesc; Díaz-Londoño, Gloria; Carnicer González, Arturo; Lallena, Antonio M.; Salvat Gavaldà, Francesc
A new version of the code system pengeom, which provides a complete set of tools to handle different geometries in Monte Carlo simulations of radiation transport, is presented. The distribution package consists of a set of Fortran subroutines and a Java graphical user interface that allows building and debugging the geometry-definition file, and producing images of the geometry in two- and three dimensions. A detailed description of these tools is given in the original paper [Comput. Phys. Commun. 199 (2016) 102–113] and in the code manual included in the distribution package. The present new version differs from the previous one in that 1) it implements a more systematic handling of round-off errors, 2) the set of examples has been updated, and 3) it allows including a single voxelized box as a geometry module. With the last optional feature, a Monte Carlo code can readily be used for describing irradiation processes with complex material structures, such as medical treatments.
Tracking Microhydration of the NaCl Rocksalt Molecule by Quantum Chemical Calculations and Penning Ionization Electron Spectroscopy in Helium Nanodroplets
(Wiley-VCH, 2026-05-01) Pi Pericay, Martí; Barranco Gómez, Manuel
The microhydration of rock salt (NaCl) molecules was investigated theoretically by density-functional theory and force field calculations and experimentally by high-resolution Penning ionization electron spectroscopy (PIES) in helium nanodroplets. The calculations reveal a transition from contact ion pair structures to solvent-separated ion pairs at n = 12–15. However, it takes n % 17 water molecules to form a complete solvation shell around the Cl− anion and as many as n % 34 to fully hydrate the Na+ cation, thus the entire NaCl molecule. Although NaCl molecules are predicted to be fully submerged inside the droplets, the PIES of NaCl are highly resolved, in stark contrast to other molecular species. Codoping the droplets with a controlled number of n = 1–10 water molecules leads to efficient quenching of the NaCl Penning ionization signal and to its full suppression for n ≳ 30, in line with the calculations.
Tracking Microhydration of the NaCl Rocksalt Molecule by Quantum Chemical Calculations and Penning Ionization Electron Spectroscopy in Helium Nanodroplets
(Wiley-VCH, 2026-05-01) Pi Pericay, Martí; Barranco Gómez, Manuel; Ltaief, LB; Sishodia, Keshav; Richter, Robert; Eloranta, Jussi; Krishnan, Sivarama; Calvo, Florent; Mudrich, Marcel
The microhydration of rock salt (NaCl) molecules was investigated theoretically by density-functional theory and force field calculations and experimentally by high-resolution Penning ionization electron spectroscopy (PIES) in helium nanodroplets. The calculations reveal a transition from contact ion pair structures to solvent-separated ion pairs at n = 12–15. However, it takes n % 17 water molecules to form a complete solvation shell around the Cl− anion and as many as n % 34 to fully hydrate the Na+ cation, thus the entire NaCl molecule. Although NaCl molecules are predicted to be fully submerged inside the droplets, the PIES of NaCl are highly resolved, in stark contrast to other molecular species. Codoping the droplets with a controlled number of n = 1–10 water molecules leads to efficient quenching of the NaCl Penning ionization signal and to its full suppression for n ≳ 30, in line with the calculations.
Diverse stages of star formation in the IRAS 18162-2048 region
(EDP Sciences, 2026-03-31) Fedriani, Ruben; Anglada, Guillem; Caratti o Garatti, A.; Gomez, Jose Francisco; Masqué Saumell, Josep Maria; Osorio, Mayra; Stecklum, Bringfried; Rodriguez Kamenetzky, Adriana Raquel; Galván-Madrid, Roberto; Carrasco-Gonzalez, Carlos; Blázquez Calero, Guillermo; Placinta Mitrea, Alexandru Florin; Sanna, Alberto; Cesaroni, Riccardo; Moscadelli, Luca; Ray, Tom; Coffey, Deirdre; Fuller, Gary
Context. High-mass star formation remains a major open problem in astrophysics, particularly regarding the transition between deeply embedded protostars and the onset of ionising radiation capable of producing photodissociation regions (PDRs) and compact H II regions. Aims. We aim to characterise the excitation and ionisation conditions of the high-mass star-forming region IRAS 18162−2048, which is where the parsec-scale jet HH80−81 lies. Methods. We obtained adaptive optics–assisted integral field spectroscopy in the near-IR K band (1.93–2.47 µm) with VLT/SINFONI, complemented by VLA X and C bands (3−6 cm) and ALMA band 3 (∼3.3 mm) observations. We analysed the continuum and line emission to derive visual extinction and excitation conditions and the kinematics of the gas of the region. Results. The near-IR continuum reveals two IR sources, IRS 2 and IRS 7, while the main protostellar core, IRAS 18162-2048, remains undetected up to 2.47 µm. IRS 7 shows a peculiar hydrogen recombination line (HRL) Brγ profile with a narrow emission component superimposed on a broad absorption feature, consistent with a B2/B3 zero-age main-sequence (ZAMS) star. Extended H2 emission exhibits a ‘sawtooth’ pattern in the excitation diagram, characteristic of UV radiation in a PDR rather than shock excitation. The radiative transfer model Cloudy reproduces the H2 ro-vibrational populations for Tgas = 600 K and nH = 7.9 × 103 cm−3. The VLA X and C bands observations reveal a compact radio source previously reported as a ‘stationary condensation’ (SC) and coincident with IRS 7. For the first time, we detect IRS 7/SC in millimetre wavelengths. The spectral index in the 3−6 cm and 3.3 mm regime is consistent with optically thin free–free emission. Conclusions. Our near-IR and radio observations reveal that IRS 7/SC is a B2/B3 ZAMS star that has begun to photo-ionise its environment, giving rise to an extended PDR and a compact H II region. The coexistence of this source with the deeply embedded protostar IRAS 18162-2048 and other bubble-like structures in the field suggests a multi-generational star-forming environment. Future James Webb Space Telescope observations targeting the H2 pure rotational lines (3–28 µm) and other HRLs less affected by extinction will be essential to characterising the cooler molecular and ionised gas to fully disclose the formation history of the region.
Towards a unified quantum field theory of dark energy and inflation: unstable de Sitter vacuum and running vacuum
(Institute of Physics (IOP), 2026-06-29) Solà Peracaula, Joan; González Fuentes, Alex; Moreno Pulido, Cristian
Inflation is a necessary cosmic mechanism to cure basic inconsistencies of the standard model of cosmology. In its absence, we could not understand the observed spatial flatness, the homogeneity and isotropy of the CMB, and yet the origin of structure formation, nor the large amount of entropy today. These problems are usually ‘fixed’ by postulating the existence of a scalar field (the ‘inflaton’). However, other less ad hoc options are actually possible. In the running vacuum model (RVM) framework, the vacuum energy density (VED) is a function of the Hubble rate and its time derivatives: . In this context, the VED is dynamical (there is no rigid cosmological term ). In the Friedmann–Lemaître–Robertson–Walker (FLRW) epoch, evolves very slowly with expansion, as befits the observed const. behavior. In contrast, in the very early Universe the vacuum fluctuations of the quantized matter fields induce higher powers capable of unleashing fast inflation in a short period in which const. We call this mechanism ‘RVM-inflation’. It does not require an inflaton field since inflation is brought about by pure quantum field theory (QFT) effects on the dynamical background. It is different from Starobinsky’s inflation, in which is never constant. In this work, we study a closely related scenario: the decay of the exact de Sitter vacuum into FLRW spacetime, in its radiation epoch, and its impact on the current Universe, and compare it with the RVM. The two QFT calculations are renormalized using an off-shell adiabatic prescription. We find that in both cases inflation is driven by powers accompanied by subleading contributions of order that ease a graceful-exit transition into the radiation-dominated epoch, where the FLRW regime starts and ultimately develops a mildly evolving VED in the late Universe: . The proposal presented here aims at a unified QFT approach to inflation and dark energy (conceived as dynamical vacuum energy) with potentially measurable phenomenological consequences in the present Universe, and constitutes a first step toward establishing its full theoretical and phenomenological consistency.
Long-term study of the gamma-ray emission of Cygnus X-3 with MAGIC and Fermi-LAT
(Elsevier, 2026-07) Aguasca i Cabot, Arnau; Bordas Coma, Pol; Bosch i Ramon, Valentí; Carretero-Castrillo, Mar ; Paredes i Poy, Josep Maria; Ribó Gomis, Marc; MAGIC Collaboration
Cygnus X-3 is a microquasar composed of a compact object of unknown nature closely orbiting around a Wolf-Rayet star. The particularities of this source make it a unique case among microquasars. This fact, together with its recent establishment as a PeV particle accelerator, makes Cygnus X-3 a very interesting target for the investigation of the physical processes leading to gamma-ray production. In this work, the TeV and GeV gamma-ray emission of Cygnus X-3 is studied in order to determine its origin and constrain the properties of the system. For that purpose, a point-like analysis of 130 h of data taken with the MAGIC telescopes between 2013 and 2024 was performed, which represents the largest available sample for Cygnus X-3 at ∼TeV energies. Additionally, contemporary data from Fermi-LAT were also analysed to better contextualize the MAGIC observations. For a more detailed investigation of the source physics, the data were divided into three subsets according to the flaring state of the source and orbital phase. No significant detection of Cygnus X-3 is found between 0.1 and 7 TeV for any of the datasets, and differential and integral flux upper limits are reported over the long-term monitoring of the source. The Fermi-LAT fluxes can be considered compatible with previous results, taking into account the different data samples used across studies. The MAGIC upper limits presented in this work represent the most constraining ones up to date at ∼TeV energies. An eventual detection of Cygnus X-3 at these energies would significantly constrain the source properties, and is not unreasonable to expect given that the source has already been detected in both the GeV and PeV regimes during flaring states. Further observations of Cygnus X-3 at energies above tens of GeV would be valuable for this purpose.
Efficient PINNs via multi-head unimodular regularization of the solutions space
(Springer Nature, 2025-08-15) Tarancón Álvarez, Pedro; Tejerina Pérez, Pablo; Jiménez, Raúl (Jiménez Tellado); Protopapas, Pavlos
Non-linear differential equations are a fundamental tool to describe different phenomena in nature. However, we still lack a well-established method to tackle stiff differential equations. Here we present a machine learning framework to facilitate the solution of nonlinear multiscale differential equations and, especially, inverse problems using Physics-Informed Neural Networks (PINNs). This framework is based on what is called multi-head (MH) training, which involves training the network to learn a general space of all solutions for a given set of equations with certain variability, rather than learning a specific solution of the system. This setup is used with a second novel technique that we call Unimodular Regularization (UR) of the latent space of solutions. We show that the multi-head approach, combined with Unimodular Regularization, significantly improves the efficiency of PINNs by facilitating the transfer learning process thereby enabling the finding of solutions for nonlinear, coupled, and multiscale differential equations.
Actively learning numerical relativity
(American Physical Society, 2024-07-15) Andrade Weber, Tomás; Gamba, Rossella; Trenado, J. (Juan)
Data analysis of gravitational waves detected by LIGO–Virgo–KAGRA collaboration and future observatories relies on precise modeling of the sources. In order to build, calibrate, and validate current models, we resort to expensive simulations in numerical relativity (NR), the fully-fledged simulation of Einstein’s equations. Since simulation costs and the dimensionality of parameter space are prohibitive to perform a dense coverage, approximate models interpolate among the available simulation data. We put forward the technique of Gaussian process active learning (GPAL), an adaptive, data-driven protocol for parameter space exploration and training of gravitational wave approximants. We evaluate this proposal by studying a computationally inexpensive scenario, in which we calibrate the approximant TEOBResumS using the NR-informed model as a proxy for NR. In this case study, we find that GPAL reduces the computational cost of training by a factor of 4 with respect to uniform or randomly distributed simulations. Moreover, we consider a parallel implementation which reduces computational time, and hybrid strategies which improve precalibrated models. The Gaussian process regression employed in this approach naturally endows the algorithm with notion of model uncertainty. We comment on the implications of this feature for data analysis
The tidal torque theory revisited. II. Rotational halo properties.
(Institute of Physics (IOP), 2026-01-14) Salvador Solé, Eduard; Manrique Oliva, Alberto
The peak model of structure formation was built more than 50 yr ago with the aim to address the origin of dark matter halo rotation in the tidal torque theory. Paradoxically, it has allowed one to explain and reproduce all halo properties found in cosmological simulations except their rotation, which remains to be understood. With the present two papers, we remedy this anomaly. In Paper I, we derived the angular momentum (AM) of protohalos centered on triaxial peaks of suited scale, taking into account that, to leading order, their density profile is smooth and homogeneous. Here we use that result to derive the AM of these objects, accounting for the fact that their actual density profile is slightly outward decreasing and lumpy so that they do not collapse monolithically at once, but progressively from inside out, undergoing mergers during the process. By monitoring in detail their resulting mass and AM growth, we characterize the spin distribution of the final halos and the precise mass and radial distribution of their inner mean specific AM. The results obtained explain and reproduce the rotational properties of simulated halos.
An accurate comprehensive approach to substructure: IV. Dynamical Friction
(Royal Astronomical Society, 2026-01-01) Salvador Solé, Eduard; Manrique Oliva, Alberto; Rocamora, Andreu
In three previous papers, we analysed the origin of the properties of halo substructure found in simulations. This was achieved by deriving them analytically in the peak model of structure formation, using the statistics of nested peaks (with no free parameter) plus a realistic model of subhalo stripping and shock-heating (with only one parameter). However, to simplify the treatment we neglected dynamical friction (DF). Here, we revisit that work by accounting for DF. That is also done in a fully analytic manner that avoids the numerical integration of the subhalo orbital motion. We obtain very simple expressions for the abundance and radial distribution of subhaloes of different masses that disentangle the effects of DF from those of tidal stripping and shock-heating. These analytic expressions reproduce and explain the results of simulations and allow one to extend them tohaloes of any mass, redshift, and formation times in any desired cosmology.
Very high energy observations of the Seyfert galaxy NGC 4151 with MAGIC: Indication of another gamma-ray obscured candidate neutrino source
(EDP Sciences, 2025-10-03) Aguasca Cabot, Arnau; Carretero-Castrillo, Mar; Paredes i Poy, Josep Maria; Pihet, Marine; Ribó Gomis, Marc
Seyfert galaxies are emerging as a promising source class of high-energy neutrinos. The Seyfert galaxies NGC 4151 and NGC 1068 have respectively come up as the most promising counterparts of a 3σ and of a 4.2σ neutrino excesses detected by IceCube in the TeV energy range. Constraining the very high energy (VHE) emission associated with the neutrino signal is crucial to unveiling the mechanism and site of neutrino production. In this work, we present the first results of the VHE observations (∼29 hours) of NGC 4151 with the MAGIC telescopes. We detected no gamma-ray excess in the direction of NGC 4151, and we derived constraining upper limits on the VHE gamma-ray flux. The integral flux upper limit (at the 95% confidence level) above 200 GeV is f = 2.3 × 10−12 cm−2 s−1. Comparison of the MAGIC and IceCube measurements suggests the presence of a gamma-ray obscured accelerator, and it allowed us to constrain the gamma-ray optical depth and the size of the neutrino production site.
Testing the ubiquitous presence of very high energy emission in gamma-ray bursts with the MAGIC telescopes
(EDP Sciences, 2025-08-08) Aguasca Cabot, Arnau; Carretero-Castrillo, Mar; Paredes i Poy, Josep Maria; Pihet, Marine; Ribó Gomis, Marc
Gamma-ray bursts (GRBs) are the most powerful transient objects in the Universe, and they are a primary target for the MAGIC Collaboration. Recognizing the challenges of observing these elusive objects with Imaging Atmospheric Cherenkov Telescopes (IACTs), we implemented a dedicated observational strategy that included an automated procedure for rapid re-pointing to transient sources. Since 2013, this automated procedure has enabled MAGIC to observe GRBs at a rate of approximately ten per year, which led to the successful detection of two GRBs at very high energies (VHE; E > 100 GeV). We present a comprehensive analysis of 42 non-detected GRBs (4 short GRBs) observed by MAGIC from 2013 to 2019. We derived upper limits (ULs) on the observed energy flux as well as on the intrinsic energy flux corrected for absorption by the extragalactic background light (EBL) from the MAGIC observations in selected energy and time intervals. We conducted a comprehensive study of their properties to investigate the reasons for these non-detections, including the possible peculiar properties of TeV-detected GRBs. We find that strong EBL absorption significantly hinders TeV detection for the majority of GRBs in our sample. For a subset of 6 GRBs with redshift z < 2, we compared the UL on the intrinsic flux in the VHE domain with the simultaneous X-ray flux, which is observed to be at the same level in the current population of TeV-detected GRBs. Based on these inferred MAGIC ULs, we conclude that a VHE component with a luminosity comparable to the simultaneously observed X-ray luminosity cannot be ruled out for this sample.
Detection of the Geminga pulsar at energies down to 20 GeV with the LST-1 of CTAO
(EDP Sciences, 2025-06-25) Aguasca Cabot, Arnau; Bordas Coma, Pol; Paredes i Poy, Josep Maria; Pihet, Marine; Ribó Gomis, Marc
Context. Geminga is the third gamma-ray pulsar firmly detected by imaging atmospheric Cherenkov telescopes (IACTs) after the Crab and the Vela pulsars. Most of its emission is expected at tens of giga-electronvolts, and, out of the planned telescopes of the upcoming Cherenkov Telescope Array Observatory (CTAO), the Large-Sized Telescopes (LSTs) are the only ones with optimised sensitivity at these energies. Aims. We aim to characterise the gamma-ray pulse shape and spectrum of Geminga as observed by the first LST (hereafter LST-1) of the Northern Array of CTAO. Furthermore, this study confirms the great performance and the improved energy threshold of the telescope, as low as 10 GeV for pulsar analysis, with respect to current-generation Cherenkov telescopes. Methods. We analysed 60 hours of good-quality data taken by the LST-1 between December 2022 and March 2024 at zenith angles below 50◦. Additionally, a new Fermi-LAT analysis of 16.6 years of data was carried out to extend the spectral analysis down to 100 MeV. Lastly, a detailed study of the systematic effects was performed. Results. We report the detection of Geminga in the energy range between 20 and 65 GeV. Of the two peaks of the phaseogram, the second one, P2, is detected with a significance of 12.2σ, while the first (P1) reaches a significance level of 2.6σ. The best-fit model for the spectrum of P2 was found to be a power law with a spectral index of Γ = (4.5±0.4stat)+0.2sys −0.6sys, compatible with the previous results obtained by the MAGIC Collaboration. No evidence of curvature is found in the LST-1 energy range. The joint fit with Fermi-LAT data confirms a preference for a sub-exponential cut-off over a pure exponential, even though both models fail to reproduce the data above several tens of giga-electronvolts. The overall results presented in this paper prove that the LST-1 is an excellent telescope for the observation of pulsars, and improved sensitivity is expected to be achieved with the full CTAO Northern Array.

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