Irene Tamborra

The flavor evolution of neutrinos in dense astrophysical sources, such as core-collapse supernovae or compact binary mergers, is non-linear due to the coherent forward scattering of neutrinos among themselves. Recent work in this context has been addressed to figure out whether flavor equipartition could be a generic flavor outcome of fast flavor conversion. We investigate the flavor conversion physics injecting random perturbations in the neutrino field in two simulation setups: 1. a spherically symmetric simulation shell without periodic boundaries, with angular distributions evolving dynamically thanks to non-forward scatterings of neutrinos with the background medium, and neutrino advection; 2. a periodic simulation shell, with angular distributions of neutrinos defined a priori and neutrino advection. We find that, independent of the exact initial flavor configuration and type of perturbations, flavor equipartition is …

The large distances travelled by neutrinos emitted from the Sun and core-collapse supernovae together with the characteristic energy of such neutrinos provide ideal conditions to probe their lifetime, when the decay products evade detection. We investigate the prospects of probing invisible neutrino decay capitalising on the detection of solar and supernova neutrinos as well as the diffuse supernova neutrino background (DSNB) in the next-generation neutrino observatories Hyper-Kamiokande, DUNE, JUNO, DARWIN, and RES-NOVA. We find that future solar neutrino data will be sensitive to values of the lifetime-to-mass ratio and of s/eV. From a core-collapse supernova explosion at kpc, lifetime-to-mass ratios of the three mass eigenstates of s/eV could be tested. After years of data taking, the DSNB would extend the sensitivity reach of to s/eV. These results promise an improvement of about orders of magnitude on the values of the decay parameters with respect to existing limits.

The origin of the observed Band-like photon spectrum in short gamma-ray bursts (sGRBs) is a long-standing mystery. We carry out the first general relativistic magnetohydrodynamic simulation of an sGRB jet with initial magnetization σ 0= 150 in dynamical ejecta from a binary merger. From this simulation, we identify regions along the jet of efficient energy dissipation due to magnetic reconnection and collisionless subshocks. Taking into account electron and proton acceleration processes, we solve for the first time the coupled transport equations for photons, electrons, protons, neutrinos, and intermediate particle species up to close to the photosphere (ie, up to 1× 10 12 cm), accounting for all relevant radiative and cooling processes. We find that the subphotospheric multimessenger signals carry strong signatures of the hadronic interactions and their resulting particle cascades. Importantly, the spectral energy …

Neutron star merger remnants are unique sites for exploring neutrino flavor conversion in dense media. Because of the natural excess of over , the neutrino-neutrino potential can cancel the matter potential, giving rise to matter-neutrino resonant flavor conversion. Under the assumption of two (anti)neutrino flavors and spatial homogeneity, we solve the neutrino quantum kinetic equations to investigate the occurrence of the matter-neutrino resonance within a multi-angle framework. We find that isotropy is broken spontaneously, regardless of the mass ordering. Relying on a hydrodynamical simulation of a binary neutron star merger remnant with a black hole of and an accretion torus of , we find that complete flavor conversion caused by the matter-neutrino resonance is unlikely, although the matter and neutrino potentials cancel at various locations above the disk. Importantly, the matter-neutrino resonant flavor conversion crucially depends on the shape of the neutrino angular distributions. Our findings suggest that an accurate modeling of the neutrino angular distributions is necessary to understand flavor conversion physics in merger remnants, its implications on the disk physics and synthesis of the elements heavier than iron.

We explore the impact of small-scale flavor conversions of neutrinos, the so-called fast flavor conversions (FFCs), on the dynamical evolution and neutrino emission of core-collapse supernovae (CCSNe). In order to do that, we implement FFCs in the spherically symmetric (1D) CCSN simulations of a 20 M⊙ progenitor model parametrically, assuming that FFCs happen at densities lower than a systematically varied threshold value and lead to an immediate flavor equilibrium consistent with lepton number conservation. We find that besides hardening the ν e and ν e spectra, which helps the expansion of the shock by enhanced postshock heating, FFCs can cause significant, nontrivial modifications of the energy transport in the SN environment via increasing the ν μ, τ luminosities. In our nonexploding models this results in extra cooling of the layers around the neutrinospheres, which triggers a faster contraction of the …

Observations of some supernovae (SNe), such as SN 2014C, in the x-ray and radio wavebands revealed a rebrightening over a timescale of about a year since their detection. Such a discovery hints towards the evolution of a hydrogen-poor SN of type Ib/Ic into a hydrogen-rich SN of type IIn, the late time activity being attributed to the interaction of the SN ejecta with a dense hydrogen-rich circumstellar medium (CSM) far away from the stellar core. We compute the neutrino and γ-ray emission from these SNe, considering interactions between the shock accelerated protons and the nonrelativistic CSM protons. Assuming three CSM models inspired by recent electromagnetic observations, we explore the dependence of the expected multimessenger signals on the CSM characteristics. The detection prospects of existing and upcoming γ-ray (Fermi-Large Area Telescope (LAT) and Cherenkov Telescope Array) and …

The large neutrino density in the deep interior of core-collapse supernovae and compact binary merger remnants makes neutrino flavor evolution nonlinear because of the coherent forward scattering of neutrinos among themselves. Under the assumption of spherical symmetry, we model neutrino decoupling from matter in an idealized setup and present the first nonlinear simulation of flavor evolution in the presence of charged current and neutral current collisions, as well as neutrino advection. Within our framework, we find that flavor transformation occurs before neutrinos fully decouple from matter, dynamically affecting the flavor distributions of all neutrino species and shifting the location of the neutrino decoupling surfaces. Our results call for further work as they may have implications on the explosion mechanism of supernovae, the nucleosynthesis of the heavy elements, as well as the observable neutrino …

The metastable hypermassive neutron star produced in the coalescence of two neutron stars can copiously produce axions that radiatively decay into ~MeV photons. These photons can form a fireball with characteristic temperature smaller than . By relying on X-ray observations of GW170817/GRB 170817A with CALET CGBM, Konus-Wind, and Insight-HXMT/HE, we present new bounds on the axion-photon coupling for axion masses in the range -. We exclude couplings down to , complementing and surpassing existing constraints. Our approach can be extended to any feebly-interacting particle decaying into photons.

We present the first simulations of core-collapse supernovae in axial symmetry with feedback from fast neutrino flavor conversion (FFC). Our schematic treatment of FFCs assumes instantaneous flavor equilibration under the constraint of lepton-number conservation individually for each flavor. Systematically varying the spatial domain where FFCs are assumed to occur, we find that they facilitate SN explosions in low-mass (9–12 M⊙) progenitors that otherwise explode with longer time delays, whereas FFCs weaken the tendency to explode of higher-mass (around 20 M⊙) progenitors.

Collapsing massive stars lead to a broad range of astrophysical transients, whose multiwavelength emission is powered by a variety of processes including radioactive decay, activity of the central engine, and interaction of the outflows with a dense circumstellar medium. These transients are also candidate factories of neutrinos with energy up to hundreds of PeV. We review the energy released by such astrophysical objects across the electromagnetic wavebands as well as neutrinos, in order to outline a strategy to optimize multimessenger follow-up programs. We find that, while a significant fraction of the explosion energy can be emitted in the infrared-optical-ultraviolet (UVOIR) band, the optical signal alone is not optimal for neutrino searches. Rather, the neutrino emission is strongly correlated with the one in the radio band, if a dense circumstellar medium surrounds the transient, and with x-rays tracking the …

At high densities in compact astrophysical sources, the coherent forward scattering of neutrinos onto each other is responsible for making the flavor evolution nonlinear. Under the assumption of spherical symmetry, we present the first simulations tracking flavor transformation in the presence of neutrino-neutrino forward scattering, neutral and charged current collisions with the matter background, as well as neutrino advection. We find that, although flavor equipartition could be one of the solutions, it is not a generic outcome, as often postulated in the literature. Intriguingly, the strong interplay between flavor conversion, collisions, and advection leads to a spread of flavor conversion across the neutrino angular distributions and neighboring spatial regions. Our simulations show that slow and fast flavor transformation can occur simultaneously. In the light of this, looking for crossings in the electron neutrino lepton …

In core-collapse supernovae, the neutrino density is so large that neutrino flavor instabilities, leading to flavor conversion, can be triggered by the forward scattering of neutrinos among each other, if a crossing between the angular distributions of electron neutrino and antineutrinos exists (fast instability) or in the presence of perturbations induced by the neutrino vacuum frequency (slow instability). Recently, it has been advanced the conjecture that neutrino collisions with the medium could be another mean to kickstart flavor change (collisional instability). We rely on a spherically symmetric core-collapse supernova model with mass , compute the neutrino angular distributions solving the kinetic equations and investigate the occurrence of flavor instabilities at different post-bounce times, ranging from the accretion phase to the early cooling phase. We find that fast and slow flavor instabilities largely dominate over the collisional ones in the decoupling region for all post-bounce times. While more work is needed to assess the relevance of collisional instabilities in neutrino-dense environments, our findings suggest that neutrino collisions with matter affect the flavor evolution in the decoupling region, but are not responsible for triggering flavor conversion.

The recently reported observation of VFTS 243 is the first example of a massive black-hole binary system with negligible binary interaction following black-hole formation. The black-hole mass () and near-circular orbit () of VFTS 243 suggest that the progenitor star experienced complete collapse, with energy-momentum being lost predominantly through neutrinos. VFTS 243 enables us to constrain the natal kick and neutrino-emission asymmetry during black-hole formation. At 68% C.L., the natal kick velocity (mass decrement) is km/s (). Most likely were ejected, presumably in neutrinos, and the black hole experienced a natal kick of km/s. The neutrino-emission asymmetry is %, with best fit values of 0-0.2%. Such a small neutrino natal kick accompanying black-hole formation is in agreement with theoretical predictions.

Hydrodynamical instabilities, such as the standing accretion shock instability (SASI), play an essential role in the dynamics of core-collapse supernovae, with observable imprints in the neutrino and gravitational wave signals. Yet, the impact of stellar rotation on the development of SASI is poorly explored. We investigate the conditions favoring the growth of SASI in the presence of rotation through a perturbative analysis. The properties of SASI are compared in two stationary configurations, cylindrical and spherical equatorial, which mainly differ by their advection timescales from the shock to the proto-neutron star surface. Without rotation, the mode m= 1, corresponding to a one-armed spiral SASI deformation, can be significantly more unstable in the spherical equatorial configuration. In fact, the shorter advection time in the spherical equatorial geometry allows for a larger contribution of the entropic-acoustic coupling …

A growing number of gamma-ray burst (GRB) afterglows is observed at very-high energies (VHE, ≳ 100 GeV). Yet, our understanding of the mechanism powering the VHE emission remains baffling. We make use of multiwavelength observations of the afterglow of GRB 180720B, GRB 190114C, and GRB 221009A to investigate whether the bursts exhibiting VHE emission share common features. We assume the standard afterglow model and microphysical parameters consistent with a synchrotron self-Compton (SSC) scenario for the VHE radiation. By requiring that the blastwave should be transparent to γ–γ pair production at the time of observation of the VHE photons and relying on typical prompt emission efficiencies and data in the radio, optical, and X-ray bands, we infer for those bursts that the initial energy of the blastwave is  erg and the circumburst density is  cm−3 for a …

In core-collapse supernovae and neutron star mergers, the neutrino density is so large that neutrino-neutrino refraction can lead to flavor conversion, classified as ``fast'' since the neutrino self-interaction strength represents the characteristic time-scale of the system. However, it has been empirically realized that the vacuum frequency affects the development of flavor conversion even if , as is the case in the core of compact astrophysical sources. Focusing on a homogeneous and axially symmetric neutrino gas, we explore the role of in the onset of flavor instabilities. Relying on a perturbative approach, we find that the odd powers of are linked to the angular distribution of the neutrino flavor particle number (FPN). Hence, when , the flavor conversion dynamics does not depend on the neutrino flavor lepton number only (FLN), like for fast flavor conversion, but also on the FPN. A non-zero vacuum frequency is also responsible for inducing flavor instabilities with a non-negligible growth rate in a neutrino gas that would be otherwise stable for . Such a neutrino ensemble with can be formally mapped into an effective system with , whose angular distributions have non-zero imaginary components. Our work highlights the overlooked role of vacuum mixing in the development of flavor instabilities in neutrino systems with FLN zero-crossings in the angular distributions.

Neutrinos are fascinating elementary particles heralding the dawn of the multi-messenger astronomy era. Neutrinos affect the stellar dynamics, drive the formation of the heavy elements, and carry signatures of supernova physics. Recent developments on the modeling of neutrino emission from stripped-envelope supernovae will be reviewed within a multi-messenger framework together with the most exciting detection prospects.

The recent body of work points out that the mean-field approximation, widely employed to mimic the neutrino field within a neutrino-dense source, might give different results in terms of flavor evolution with respect to the correspondent many-body treatment. In this paper, we investigate whether such conclusions derived within a constrained framework should hold in an astrophysical context. We show that the plane waves, commonly adopted in the many-body literature to model the neutrino field, provide results that are crucially different with respect to the ones obtained using wave packets of finite size streaming with a nonzero velocity. The many-body approach intrinsically includes coherent and incoherent scatterings. The mean-field approximation, on the other hand, only takes into account the coherent scattering in the absence of the collision term. Even if incoherent scatterings are included in the mean-field …

Mounting evidence suggests that the launching of collapsar jets is magnetically driven. Recent general relativistic magnetohydrodynamic simulations of collapsars reveal that the jet is continuously loaded with baryons, owing to strong mixing with the cocoon. This results in a high photosphere at≳ 10 12 cm. Consequently, collisionless internal shocks below the photosphere are disfavored, and neutrino production in the deepest jet regions is prevented, in contrast to what has been assumed in the literature. We find that subphotospheric neutrino production could take place in the presence of collisionless subshocks or magnetic reconnection. Efficient particle acceleration is not possible in the cocoon, at the cocoon-countercocoon shock interface, or at the shock driven by the cocoon in the event of a jet halted in an extended envelope. These subphotospheric neutrinos have energy E ν≲ 10 5 GeV for initial jet …

The gravitational wave (GW) and neutrino signals from core-collapse supernovae (CCSNe) are expected to carry pronounced imprints of the standing accretion shock instability (SASI). We investigate whether the correlation between the SASI signatures in the GW and neutrino signals could be exploited to enhance the detection efficiency of GWs. We rely on a benchmark full-scale three-dimensional CCSN simulation with zero-age main sequence mass of 27 M⊙. Two search strategies are explored: 1. the inference of the SASI frequency range and/or time window from the neutrino event rate detectable at the IceCube Neutrino Observatory; 2. the use of the neutrino event rate to build a matched filter template. We find that incorporating information from the SASI modulations of the IceCube neutrino event rate can increase the detection efficiency compared to standard GW excess energy searches up to 30% for …