Alberto Manfreda

Lorentz invariance is a fundamental symmetry underlying both the Standard Model of particle physics and General Relativity. Testing its validity provides a direct means of searching for new physics emerging near the Planck scale. A search for isotropic Lorentz invariance violation using 5828 selected atmospheric-neutrino candidate events is presented. The data were collected over 1.4 years with a partial configuration of the KM3NeT/ORCA detector comprising six detection units. No evidence for such violation is found; thus, competitive constraints are set on off-diagonal isotropic Lorentz-invariance-violating coefficients of mass dimension 3. In addition, the first direct constraints on isotropic flavour-diagonal coefficient differences between the muon and tau sectors are obtained for mass dimensions 3 to 8.

We report on the average and orbital phase-resolved polarization of Cyg X-3 in the hard state (HS) during the 2023 Imaging X-ray Polarimetry Explorer (IXPE) observational campaign. We find the polarization degree of 21.4% ± 0.4% and polarization angle of 92 . ° 2 ± 0 . ° 5 , compatible within 3 σ with the first HS IXPE observation in 2022. As the observed polarization depends on both the accretion geometry and the X-ray emission mechanism, and assuming it arises from reflection from the optically thick envelope surrounding the central source, our result indicates that both are similar in the two HS epochs on year-long timescales despite the source transitioning through an intermediate state in between. We discuss time- and energy-dependent polarization properties and their implications for the geometry and stability of the accretion funnel, finding significant orbital modulation of the polarization properties.

Context. The detection of the highest energy neutrino observed to date by KM3NeT, with an estimated energy of 220 PeV, opens up new possibilities for the study and identification of the astrophysical sources responsible for a diffuse flux of such ultra-high-energy neutrinos, among which gamma-ray bursts are longstanding candidates. Aims. Based on the event KM3-230213A, we derived constraints on the baryon loading and density of the surrounding environment in models of blast waves in long-duration gamma-ray bursts. Methods. We computed the diffuse flux from gamma-ray burst blast waves, either expanding in a constant density interstellar medium or developing in a radially decreasing density of a wind-like environment surrounding the gamma-ray burst progenitor star, by taking into account the expected neutrino spectra and luminosity function. We used a Poisson likelihood method to constrain the blast wave model parameters by calculating the expected number of neutrino events within the 90% confidence level energy range of KM3-230213A and by using the joint exposure of KM3NeT/ARCA, IceCube, and Pierre Auger. Results. We constrain the baryon loading to be f _b ≤ 51 at 90% confidence, with the best-fit and 68% confidence interval being + = 26.9 ^+11.4 _−17.2 for a constant interstellar medium particle density of n ₀ = 1 cm ⁻³ . In the wind-like environment case, the baryon loading is f _b ≤ 1095 at 90% confidence, with the corresponding 68% confidence interval being f _b ∈ [8, 231], which is proportional to the sixth power of a variable density parameter of A _* = 0.1.

Core-collapse supernovae mark the end of life of massive stars. However, despite their importance in astrophysics, their underlying mechanisms remain unclear. Neutrinos that emerge from the dense core of the star offer a promising way to study supernova dynamics. A strategy is presented to improve the potential of the KM3NeT neutrino telescope to detect core-collapse supernovae in our Galaxy or the Large Magellanic Cloud by further exploiting the properties of its optical modules equipped with multiple photomultipliers. A supernova burst is expected to produce a sudden hit rate increase in the KM3NeT detectors, which could be used to detect a supernova even in the absence of triggers from other experiments. New observables have been defined for individual optical modules that exploit the geometry and time distribution of the detected hits, enabling a better discrimination between signal and background signatures. In addition, a thorough investigation of the related systematic uncertainties is presented for the first time. When implemented, this new methodology allowed KM3NeT to probe 46% more Galactic core-collapse supernova candidates than with the previous trigger strategy, reaching the dense Galactic bulge. It is now expected that, once completed, KM3NeT will achieve full Galactic sensitivity to core-collapse supernovae independently from other experiments.

The KM3NeT collaboration has reported the detection of the highest energy neutrino event observed to date. The energy of the event is of the order of 220 PeV hinting towards a neutrino flux at the highest energies. In this article, the potential blazar origin for this event is explored. The publicly available Astro-Multimessenger Modeling software is used to model the blazar gamma-ray and neutrino fluxes. It is concluded that a population of blazars could produce the diffuse flux compatible with the observation of the ultra-high energy event detected by the KM3NeT/ARCA detector. At the same time, the gamma-ray flux produced by such a population of blazars is consistent with the diffuse gamma-ray flux measured by the Fermi Large Area Telescope.

RCW 86 is a supernova remnant whose origin has recently been linked to an off-center explosion within a cavity created by its progenitor star. In the southwestern region, the forward shock is thought to have reached the cavity wall, encountering diverse environmental conditions. We report on the spatially resolved X-ray polarimetric observation of RCW 86 with the Imaging X-ray Polarimetry Explorer (IXPE). In the 2–4.5 keV energy band we find no significant detection of polarization. Employing a dedicated background subtraction procedure and Bayesian spectropolarimetric fitting, we derive 99% upper limits on the polarization degree of the synchrotron component: 15% in higher-statistics regions and 30%–40% in lower-statistics regions. These upper limits on the polarization degree in several regions exclude the possibility of a strongly coherent magnetic field down to the subparsec scale, and that of a moderately coherent one on the scale of the synchrotron features as resolved by IXPE. The results indicate that the shocks in the southwestern rim of RCW 86 propagate more slowly than the unshocked ejecta at their locations, yet exceed the measured proper motion speeds. This behavior is consistent with reflected shocks occurring in tenuous regions of the shocked ejecta, distinct from regions that are radio-bright.

We present a comprehensive characterization of the on-orbit imaging performance of the three telescopes on board the Imaging X-ray Polarimetry Explorer (IXPE). Each telescope comprises a Wolter-I mirror module assembly and a Gas Pixel Detector focal-plane detector unit. We analyze data from point-like X-ray sources and fit a composite point-spread function (PSF) model that we compare with ground calibrations. We study the dependence of the PSF parameters and of the angular resolution, in terms of half-power diameter (HPD), on the time and source counting rate. We find no significant secular evolution of PSF parameters or HPD over 30 months on orbit, with average HPDs of 26 . ″ 1 ± 0 . ″ 5 (Telescope 1), 32 . ″ 1 ± 0 . ″ 5 (Telescope 2), and 30 . ″ 9 ± 0 . ″ 6 (Telescope 3) and rate trends consistent with zero up to source counting rates of ∼60 cts s ⁻¹ in the 2–3 keV energy band for all three telescopes. We set a 99% C.L. upper limit of 4.4% on the optics-induced polarization in the PSF halo and find no measurable degradation of the polarization modulation factor in the wings versus the core due to misreconstructed photoelectron tracks. IXPE’s imaging performance thus is consistent with the ≤30″ observatory requirement with high stability, ensuring robust spatially resolved polarization measurements for the mission’s projected lifetime through 2030.