arXiv subjects

S. Casanova

Publications and source records attributed to S. Casanova.

At least 19 recordsLinked to original sources

arXiv2026

Evidence for a spectral steepening of the gamma-ray emission from the Galactic Center ridge

Very-high-energy (VHE) $\gamma$-ray emission detected from the central molecular zone (CMZ) hints at diffusive propagation of cosmic rays (CRs) injected continuously near the Galactic center (GC). Using H.E.S.S. VHE $\gamma$-ray observations, we aim to construct a multi-component description of the region in order to derive the spatial and spectral distributions of CRs and discuss their potential origin. We rely on a spectro-morphological analysis, in which spectral and spatial parameters of different components are fit simultaneously, and on a three-dimensional description of the CMZ and CR distributions. The GC ridge emission is well reproduced with the assumed gas distribution model weighted by a $1/r^{\alpha}$ CR density profile with $\alpha = 1.10 \pm 0.05_{\rm{stat}} \pm 0.10_{\rm{syst}}$. We detect no significant spectral variations across the CMZ, which supports a continuous injection from the GC. We report a significant (> 3 $\sigma$) curvature in the GC ridge $\gamma$-ray spectrum, implying a parent proton break (or cutoff energy) of $E_b \sim 15-50$ TeV ($E_{\rm{cut}} \sim 60-160$ TeV). We also derive the best-fit spectral parameters of the other sources in the region and report a significant cutoff at $E \sim 5$ TeV in the spectrum of the pulsar wind nebula G0.9+0.1. Finally we constrain the position of the injection site to be close to the GC (ruling out the Arches and Quintuplet clusters as major contributors to the emission). We discuss possible origins for the GC ridge emission, potential mechanisms for causing the observed curvature, a possible contribution of the gas content to the emission of HESS J1745-290 and derive qualitative constraints on the line-of-sight positions of the clouds shaping the CMZ.

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arXiv2026

Search for gamma-ray spectral lines from dark matter annihilation with the H.E.S.S. Inner Galaxy Survey

Spectral gamma-ray line features are expected as key signatures from dark matter (DM) annihilations of TeV-scale particle DM. Observations of the Galactic Centre with atmospheric Cherenkov telescopes are unique to probe thermal-relic TeV particle DM, well beyond the reach of direct detection and collider searches. We report here on the search for line signals in very-high-energy gamma rays using data from the Inner Galaxy Survey, consisting of 546 hours of H.E.S.S. observations of the inner few degrees of the Galactic Centre. No significant signal is detected. We then compute the exclusion limits on the annihilation line cross section $\langle \sigma v \rangle_{\rm line}$, with a two-dimensional log-likelihood ratio test statistics, exploiting spectral and spatial features of the DM signal. Assuming an Einasto DM density profile for the Milky Way, our results provide the most constraining limits so far, reaching $\langle \sigma v \rangle_{\rm line} = 2.3$ $\times$ $10^{-28}$ and $2.4 \times$ $10^{-27}$ cm$^3$s$^{-1}$ for DM masses of 1 and 10 TeV, respectively. The present limits are used to constrain the widely searched Wino, Higgsino and Quintuplet models. For the first time, thermal Higgsino DM is probed for DM Milky Way models.

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arXiv2026

Prospects for the detection of gamma rays using Cherenkov telescopes enhanced by a ground array observatory

We study through detailed simulated data and their optimized analysis the expected performance of the Single-Mirror Small-Size imaging atmospheric Cherenkov Telescopes (SST-1M) potentially located inside a high-altitude array of Water-Cherenkov Detectors (WCDs) inspired by the current foreseen design of the Southern Wide-field Gamma-ray Observatory (SWGO). For such a hybrid setup, we show an improvement in the flux sensitivity above 10 TeV by about 60% for monocular and 30% for stereoscopic SST-1M observation, due to the improved gamma/hadron separation when additional parameters from the WCD array are used. We also discuss further benefits of the hybrid gamma observatory concept and its technical challenges.

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arXiv2026

Gamma-Ray Constraints on Heavy Axion-Like-Particle Decays from Fermi-LAT and H.E.S.S. Blazar Spectra

The propagation of very-high-energy (VHE; $E_γ \geq 100$ GeV) gamma rays from extragalactic sources is affected by interactions with photons of the extragalactic background light (EBL), resulting in pair production that attenuates the intrinsic gamma-ray flux. This interaction renders the Universe increasingly opaque to VHE photons at high energies and redshifts. New physics scenarios involving axion-like particles (ALPs) could modify this expected optical depth. In particular, ALPs with masses $m_a \sim 10$ eV can decay into two photons over cosmological timescales, thereby contributing to the diffuse EBL. If such ALPs constitute a significant fraction of the dark matter density, their decay would enhance the EBL intensity and consequently increase the gamma-ray optical depth. In this study, we investigate this scenario using a large sample of gamma-ray spectra observed with the High Energy Stereoscopic System (H.E.S.S.) and the Fermi Large Area Telescope. We model the contribution of decaying ALPs to the EBL and assess their impact on the spectra of blazars across redshifts. By comparing these observations with standard EBL models, we place constraints on the properties of heavy ALPs, specifically their mass and photon coupling, and evaluate their viability as a dark matter candidate capable of modifying the gamma-ray transparency of the Universe. From the combined analysis, and under the assumption that ALPs constitute the entire dark matter density, we derive 95% confidence exclusion limits on the photon-ALP coupling down to $g_{aγ} \sim 7 \times 10^{-12}$ GeV$^{-1}$ for masses $m_a\sim 15$ eV. These constraints are competitive with existing astrophysical bounds and provide complementary sensitivity to other techniques, closing a previously unconstrained region of parameter space in the $m_a \sim 2.5$-$20$ eV range.

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arXiv2026

Constraints on axion-like particles from ultra-high-energy observations of M87 with the HAWC observatory

In this work, we perform an indirect search for axion-like particles (ALPs) through their hypothesized mixing with photons in the presence of magnetic fields. ALPs are a well-motivated dark-matter candidate class, and the photon-ALP conversion mechanism provides a unique channel to constrain their mass and coupling constant using very-high-energy gamma-ray observations. The photon-ALP mixing could alter the observed gamma-ray spectrum from extragalactic sources by effectively reducing the apparent attenuation due to extragalactic-background-light absorption. We analyze 7.5 years of data from the High Altitude Water Cherenkov (HAWC) Observatory, targeting the nearby radio galaxy M87. This source is located within the Virgo cluster and is an ideal environment for photon-ALP conversion due to its low redshift and the large-scale, strongly magnetized medium of the cluster. We find no evidence for a photon-ALP conversion signal and, consequently, set constraints on the ALP mass and photon-ALP coupling constant with emission from M87 which are consistent with previous results. Our analysis places competitive constraints on the ALP parameter space, defining an exclusion region in the mass range of approximately $10^{-8}$ to $10^{-6}$ eV for coupling constants above $5\times10^{-12}$ GeV$^{-1}$, complementing previous constraints from other gamma-ray observatories.

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arXiv2026

Chasing Gamma-Ray Signals from Binary Neutron Star Coalescences with the Cherenkov Telescope Array: Prospects and Observing Strategies

The detection of gravitational waves (GWs) from a binary neutron star (BNS) merger by Advanced LIGO and Advanced Virgo (GW170817), together with its electromagnetic counterpart, the short gamma-ray burst GRB~170817A, heralded the birth of multi-messenger astronomy. The detection of TeV emission from GRBs motivates follow-up observations with the Cherenkov Telescope Array Observatory (CTAO), ideal for detecting such signals due to its unprecedented sensitivity, rapid response, and wide-field survey capabilities. The aim of this work is to evaluate GeV--TeV GW follow-up strategies for CTAO using a multi-step simulation pipeline and to estimate the expected rate of joint GW-GRB detections during observing run O5. Using a simulated sample of BNS systems with corresponding GW detections, gamma-ray emission is simulated through phenomenological prescriptions based on the observed population of short GRBs, including off-axis jet scenarios. CTAO observations are simulated to account for instrument response, sky tiling strategies, integration times, and varying observing conditions. Strategies with variable and constant integration times are investigated. We find that, via an optimized follow-up strategy, about 5% of simulated GW-associated short GRBs produce GeV--TeV radiation detectable by CTAO. Detectability is strongly influenced by the jet opening angle and viewing angle, suggesting that even rough estimates of the viewing angle in GW alerts could enhance targeting. This framework motivates future follow-ups of GW-detectable events, including neutron star-black hole mergers, and further supports the development of advanced strategies incorporating galaxy distributions and synergies with future detectors such as the Einstein Telescope.

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arXiv2026

Search for TeV emission from spider millisecond pulsars with HAWC

Millisecond pulsars (MSPs) are observed to emit multi-wavelength radiation, from radio to GeV. Spider MSPs, which interact with their low-mass companion in close orbit (orbital periods $< 1$ day), may lead to strong intrabinary shocks that can further accelerate electron and positron pairs produced in the magnetosphere, possibly emitting very-high-energy (0.1--100 TeV; VHE) photons through inverse Compton scattering. Using 2565 days of HAWC Pass 5 data, we search for VHE emission from spider MSPs and present upper limits on individual sources. We also perform a stacking analysis to examine whether the two sets of spider systems, classified as redbacks and black widows depending on the companion mass, exhibit different spectral properties. Our study places constraints on TeV emission from MSPs and suggests that they are unlikely to contribute significantly to the Galactic diffuse emission at TeV and higher energies.

astro-ph.HE↗

arXiv2026

H.E.S.S. observations of composite Seyfert-starburst galaxies

Context: Composite galaxies that contain both Seyfert and starburst components may produce very high-energy (VHE; >100 GeV) gamma-ray emission at a wide range of spatial scales, from a few Schwarzschild radii of a supermassive black hole to dimensions of kiloparsec-size jet-driven outflows. In addition to supernova remnants, various sources have been suggested to explain data collected on composite galaxies, including multi-messenger neutrino and ultra-high-energy cosmic-ray data. Aims: The closest composite Seyfert-starburst galaxies (NGC 1068, the Circinus galaxy, and NGC 4945) are observed with the High Energy Stereoscopic System (H.E.S.S.) to provide constraints on cosmic-ray populations in these systems. Methods: Data obtained in H.E.S.S. observations have been analyzed to search for VHE gamma-ray counterparts to the GeV gamma-ray signals detected with Fermi-LAT and for potential spectral components in the VHE range. Results: No significant signals have been found in these H.E.S.S. data. Upper limits on the VHE gamma-ray fluxes were applied to constrain theoretical models involving different spectral components.

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arXiv2026

A Multi-messenger Search for Ultra-high-energy Gamma Rays in Coincidence with Neutrinos

The last five years have shown us that ultra-high-energy (UHE; $>$100 TeV) gamma-ray sources are ubiquitous, but the nature of these sources remain highly uncertain. UHE gamma rays can be produced via either leptonic (Inverse compton) or hadronic (pion decay) emission mechanisms. To decisively determine the emission mechanisms, multimessenger searches are essential. Neutrinos are of particular interest as they are only created via hadronic channels. In this work, we describe a metric to select high-quality UHE events from the High Altitude Water Cherenkov (HAWC) Observatory. We use this metric to search for correlations between HAWC archival data and IceCube public neutrino alerts. 24 spatial coincidences are found, which is higher than the number of events expected by random chance. Therefore, we conclude that there are likely associations between HAWC gamma rays and IceCube neutrinos, but the angular resolutions of the two instruments prevent us from conclusively making any definitive associations between the coincidences and specific astrophysical sources. More sensitive detectors are needed.

astro-ph.HE↗

arXiv2026

The second H.E.S.S. gamma-ray burst catalogue: 15 years of observations with the H.E.S.S. telescopes

Recent observational efforts using imaging atmospheric Cherenkov telescopes (IACTs) have led to firm detections of very-high-energy (VHE) signals from bright gamma-ray bursts (GRBs), often at moderate redshifts. This work presents 15 years of H.E.S.S. GRB observations and examines their implications through population comparisons and selected modelling cases. GRBs are a key science target of the High Energy Stereoscopic System (H.E.S.S.). With a low-energy threshold ($\lesssim$100 GeV) and rapid repointing capabilities, H.E.S.S. can begin follow-up observations within tens of seconds after a GRB trigger, covering the late prompt or early afterglow phases. We report GRB follow-up observations with H.E.S.S. from 2004 to 2019, which resulted in no significant VHE signals (aside from the detections of GRB~180720B and GRB~190829A). The resulting upper limits comprise the largest set available for GRBs at VHE. A subset of bursts with favourable conditions were selected for X-ray analysis and emission modelling. Population studies were performed to compare detected and non-detected GRBs. The results indicate that VHE-detected GRBs are not a distinct population, but tend to feature luminous X-ray emission and favourable redshift and observing conditions. This highlights the potential of next-generation IACTs such as the Cherenkov Telescope Array Observatory (CTAO), whose lower energy threshold will enhance the detection of fainter and more distant GRBs.

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arXiv2026

HAWC Study on the Ultra-High-Energy Gamma-Ray Emissions from the Pulsar Wind Nebula G32.64+0.53

Multi-TeV gamma-ray emission around eHWC J1850+001 (a source from the first HAWC catalog of gamma-ray sources emitting above 56 TeV) is spatially coincident with the pulsar wind nebula (PWN) G32.64+0.53, powered by PSR J1849-0001. The absence of counterparts in radio, optical, and GeV energy ranges, contrasted with clear detections in X-rays and very-high-energy (VHE) gamma-rays, is indicative of a non-thermal leptonic origin for the nebula. We apply a systematic analysis pipeline, including a sophisticated model for the Galactic diffuse emission, to 2860 days of data from the HAWC Observatory. Our detailed analysis confirms that the ultra-high-energy (UHE) emission originates from G32.64+0.53, and we measure its spectrum up to 270 TeV with significant emission well beyond 100 TeV. We fit the multi-wavelength observations with a time-dependent leptonic model powered by the pulsar's rotational energy, and the results establish the nebula as a leptonic PeV accelerator, capable of accelerating electrons to a maximum energy of $E_{\mathrm{cut}}\geq2.9~\mathrm{PeV}$ (95\% one-sided lower bound). The model also constrains the nebular magnetic field to $4.28 ~\mathrm{\mu G}$, supporting a leptonic PWN origin for the observed UHE emission.

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arXiv2026

Scrutinizing the 2020 multiwavelength outburst of PKS 0903-57 through observations with H.E.S.S

The blazar PKS 0903-57 has recently been classified as a flat spectrum radio quasar at a redshift of $z=0.2621$. In March and April 2020, Fermi-LAT and AGILE reported tremendous activity in high-energy $γ$ rays with the flux increasing by $\sim$2 orders of magnitude compared to quiescence. The flare was observed with H.E.S.S. in very-high-energy $γ$ rays for six nights with a total observation time of 13.1 h, resulting in the discovery of PKS 0903-57 in this energy band with an average flux of $1.5\times 10^{-10}$ ph cm$^{-2}$s$^{-1}$ above an energy threshold of $\sim 180$ GeV corresponding to $60\%$ of the Crab Nebula flux above the same threshold. The very-high-energy $γ$-ray flux was strongly variable. X-ray and optical data were collected with Swift and ATOM, and also indicate significant variability. The observed multiwavelength flux and spectral variability during the H.E.S.S. observation window suggest variability time scales on the order of a few hours and reveal complex correlation patterns. The lack of absorption beyond that of the extragalactic background light in the $γ$-ray domain suggests that the emission region was located outside of the broad-line region. A leptonic one-zone modeling of the six H.E.S.S. observation nights using the dusty torus as seed photons for the inverse-Compton scattering, results in a low magnetization of the emission region. This implies that shock acceleration is likely the main driver during the event.

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arXiv2026

Search for Signatures of Dark Matter Annihilation in the Galactic Center with HAWC

We conduct an indirect dark matter (DM) search in the Galactic Center, focusing on a square region within $\pm 9^{\circ}$ in Galactic longitude and latutide, using 2,865 days of data ($\sim$8 years) from the High-Altitude Water Cherenkov (HAWC) Observatory. We explore DM particles within the Weakly Interacting Massive Particles framework with masses from 1 TeV to 10 PeV. Analyzing three annihilation channels ($b\bar{b}$, $τ^{+}τ^{-}$, $W^{+}W^{-}$) and three density profiles (Navarro-Frenk-White, Einasto, Burkert), we find no significant excess and set 95\% confidence-level upper limits on the velocity-weighted annihilation cross section. Our results provide the first constraints on DM particles well above 100 TeV using gamma-ray data from the Galactic Center, with the strongest limits $\mathcal{O}(10^{-24})$~cm$^{3}$/s, from the $τ^{+}τ^{-}$ channel and the Einasto profile.

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arXiv2026

Investigating IceCube Neutrino Alerts with the HAWC $\gamma$-Ray Observatory

Neutrino emission from astrophysical sources has long been considered a signature of cosmic-ray acceleration. The IceCube neutrino observatory has observed a diffuse flux of TeV-PeV neutrinos, but very few confirmed sources have emerged. With the recent publication of IceCube Event Catalog (IceCat-1), IceCube has released a list of the most promising astrophysical neutrino events since May 2011. Using the archival data from the High Altitude Water Cherenkov (HAWC) Gammma-ray observatory, we perform a coincidence search for gamma rays and neutrinos using a Bayesian Block algorithm with the public IceCube alerts from IceCat-1, along with additional alerts issued later. In this work, we consider 368 alerts, up to July 8, 2025, that are within HAWC's field of view. We observe approximately a 5\% coincident detection rate, which is consistent with expectations from background. Two of these detections contain the Active Galactic Nuclei (AGN) Markarian 421 and Markarian 501. We discuss the likelihood that the neutrino/$\gamma$-ray coincidences are false positives and a brief overview of the results.

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arXiv2026

H.E.S.S. detection of the PSR J0855-4644 nebula

HESS J0852-463 is a TeV γ-ray source located in the Galactic plane. The region consists of a supernova remnant (SNR, RX J0852.0-4622) with a shell-like morphology, commonly referred to as Vela Junior, and a pulsar denoted PSR J0855-4644. Pulsars are among the most efficient leptonic accelerators in our Galaxy, making this region particularly interesting to study. We utilise the most recent data taken by the High Energy Stereoscopic System (H.E.S.S.), to investigate any γ-ray emission associated with the pulsar in this region, PSR J0855-4644. We applied a full forward folding method on the H.E.S.S. data. Utilising 3D modelling techniques, we evaluated the TeV γ-ray emission towards the various components of this complex system. The distinct energy-dependent morphology observed in our data motivates further investigation of this source. We resolved the emission in the Vela Junior region into various components, several of which correspond to the SNR itself. In particular, we find a new extended component which is coincident with the position of PSR J0855-4644. The spectrum follows a power-law distribution with a best-fit index of ΓE = 1.81 \pm 0.07stat which differs from the properties of the surrounding γ-ray emission of the Vela Junior SNR. A one-zone leptonic joint fit between the X-rays (from XMM-Newton) and γ-rays (from H.E.S.S.) leads to a lower limit on the magnetic field of 1.6μG and a spectral index of α = 1.88 \pm 0.01, in line with expectations of pulsar wind nebulae (PWNe). In this paper, we report the first detection of the PWN of PSR J0855-4644 at TeV energies with the H.E.S.S. experiment at a significance of 12.2σ. This is attributed to the advanced techniques of the 3D analysis. Based on the pulsar's characteristics, its PWN is consistent with the known TeV PWNe population in the Galaxy.

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arXiv2026

Improved Heavy Dark Matter Annihilation Search from Dwarf Galaxies with HAWC

Understanding dark matter's elusive nature is crucial for the framework of particle physics and expanding the Standard Model. This analysis utilizes the High Altitude Water Cherenkov (HAWC) gamma ray Observatory to indirectly search for dark matter (DM) by studying gamma ray emission from dwarf spheroidal galaxies (dSphs). Selected for their high ratio of dark matter to baryonic matter, dSphs are useful for this type of search owing to the low background emission. In comparison to previous HAWC studies, we significantly improve our sensitivity to DM from dSphs due to improvements to our event reconstruction and reduced hadronic contamination. We expanded the number of dSphs studied, DM annihilation channels into the Standard Model (SM), and the amount of data collected on each previously studied dSph. We searched for DM signals in each dSph using the latest version of the algorithms used to reconstruct data from the primary detector of the HAWC instrument. We report that we do not detect evidence of DM from dSphs, so we place upper limits for the velocity-weighted DM annihilation cross-section ($\langle\sigma v \rangle$) on the order of $10^{-23}~\text{cm}^3\text{s}^{-1}$ for a DM mass range of $1-10^4$ TeV.

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arXiv2026

The Fourth HAWC Catalog of Very-High-Energy Gamma-Ray Sources

We present an updated catalog of TeV gamma-ray sources based on the fifth pass of data from the High-Altitude Water Cherenkov (HAWC) Observatory. This release benefits from improved event reconstruction and nearly three additional years of observations. It also incorporates a systematic multi-source fitting framework, enabling more flexible and accurate modeling of the gamma-ray sky. This fitting procedure was modeled after the manual approach used in HAWC analyses of individual sources and regions, as well as other gamma-ray catalogs, like the 4FGL. In addition to more varied modeling of source morphology and spectral parameters compared to previous HAWC catalogs, this catalog uses a robust modeling of Galactic diffuse TeV emission. The fitting procedure uses both point-like and symmetric Gaussian spatial templates to model the source morphology. The spectral shape of the emission is modeled with either a simple power-law or log-parabola to explore curvature in the spectral energy distribution. We report 85 sources at the 4σ level, including 11 sources not associated with any TeVCat source using a distance-based association criterion. Distance-based association with the 1LHAASO catalog results in 22 4HWC sources without a counterpart. Additionally, there are 12 sources not associated with any physical counterpart in the Low- or High-Mass X-Ray Binary, the ATNF, or Fermi Pulsar, or SNR catalogs of sources. Five of the aforementioned sources have no counterpart in any of the catalogs searched and represent an opportunity for follow-up observations.

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arXiv2026

Constraints on Axion-Like Particles from Ultra-High-Energy Observations of 3HWC J1908+063 with HAWC

Axion-like particles (ALPs) are hypothetical particles and compelling candidates for cold dark matter. Their existence could be probed through their conversions into photons in the presence of magnetic fields. In this work, we explore the effect of these photon-ALP conversions by searching for an attenuation in the observed gamma ray spectra of galactic sources that emit at energies of hundreds of TeV. We analyze data from the High-Altitude Water Cherenkov (HAWC) Observatory for the source 3HWC J1908+063. No evidence of photon-ALP conversions was found, and we set constraints on the ALP parameter space. Specifically, we derive exclusion limits for ALPs with masses in the range $10^{-8}~\mathrm{eV} \leq m_a \leq 10^{-6}~\mathrm{eV}$ and photon-ALP couplings in the range $10^{-12}~\mathrm{GeV}^{-1} \leq g_{aγ} \leq 10^{-10}~\mathrm{GeV}^{-1}$, based on HAWC observations.

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