arXiv subjects

Nicolás Viaux

Publications and source records attributed to Nicolás Viaux.

6 recordsLinked to original sources

arXiv2026

An Analytic Threshold for LESA-Driven Negative ELN Flux Directions in Core-Collapse Supernovae: Derivation and Population Census

In core-collapse supernovae (CCSNe), deleptonization normally favors $ν_e$ over $\barν_e$ emission. However, lepton-number emission self-sustained asymmetry (LESA) can make the energy-integrated emitted lepton-number flux negative along some directions. We derive a simple diagnostic for this transition and test it in 33 independent 3D CCSN simulations: 25 Princeton/Fornax models ($8.1$--$100\,M_\odot$) and 8 Garching models, including non-, slow-, and fast-rotating $15\,M_\odot$ cases. Of 23 non-black-hole-forming Princeton models, 22 cross the threshold, with median onset $t_c=225\,\mathrm{ms}$, IQR $162$--$264\,\mathrm{ms}$, and cross-model scatter $\mathrm{CV}=18.6\%$. Full-sky flux-sign searches show that the threshold identifies the anti-LESA-pole transition, distinguishing the global LESA-driven crossing from early localized turbulent crossings. The fast-rotating Garching $15\,M_\odot$ model, where rapid rotation suppresses the LESA dipole, is correctly classified as a non-crosser without using any rotation parameter. Both black-hole-forming Princeton models cross near $250\,\mathrm{ms}$ post-bounce and remain above threshold for $1807$ and $2463\,\mathrm{ms}$ before collapse. Thus, in the next nearby CCSN, the emitted $\barν_e$ energy flux may exceed the $ν_e$ flux along some lines of sight. Such directions may also correlate with sustained fast flavor instability, although testing this requires local phase-space distributions or dedicated linear stability analysis. The relevant quantity here is the energy-integrated emitted flux field, i.e. a luminosity difference per steradian, not a neutrino number flux.

astro-ph.HE↗

arXiv2026

Which Neutron Stars Reach the Stiffening Regime? Multimessenger Constraints on Core Sound Speed and Stellar-Mass Thresholds

We present a concise multimessenger inference of the neutron-star core sound-speed profile using GW170817 and three \textit{NICER} mass--radius posteriors (PSR J0030$+$0451, PSR J0740$+$6620, and PSR J0437$-$4715). The main result is not only a preference for intermediate-density stiffening within smooth equation-of-state families, but a translation of that inference into the stellar masses that access the relevant density regime. In the baseline smooth peaked family, the posterior probability that $c_s^2 > 1/3$ at $3.5\,n_{\rm sat}$ is $85.4\,\%$, while equal-prior averaging over peaked, monotonic, piecewise, and transition-capable families gives a more conservative $79.0\,\%$. Posterior-resampled exact Tolman--Oppenheimer--Volkoff solutions show that the onset density of the inferred stiffening is typically reached near $1.6\,M_\odot$, whereas the peak region is accessed only near $2.1\,M_\odot$. A J0740-like $2.07\,M_\odot$ pulsar reaches the onset in $91\,\%$ of posterior draws but the peak in only $46\,\%$, showing that current data mainly constrain whether massive stars have entered the stiffening regime rather than traversed its full peak.

astro-ph.HE↗

arXiv2020

Probing the anomalous positron fraction origin with fully leptonic decaying gravitino dark matter candidates

The flux of electron and positron cosmic rays measured by the space-based experiments PAMELA, AMS-02, CALET, DAMPE and Fermi-LAT shows an unexpected behaviour at high energies, in comparison with expectations from standard astrophysical sources. In particular, AMS-02 observations provide compelling evidence for a new source of positrons and electrons whose origin is still unknown. Plausible scenarios include either the contribution of dark matter or unresolved astrophysical sources, such as nearby pulsars. It has been shown that explanations based mostly on dark matter, tend to overproduce gamma-rays, entering in conflict with measurements of the extra-galactic gamma-ray background (EGB). Although this situation seems to be quite generic, it ultimately depends on the properties of the dark matter candidate. In this work we revisit a model in which the gravitino is a dark matter candidate decaying to SM particles through R-parity violating couplings. We show that decay channels allowed by trilinear couplings produce more electrons and positrons and fewer photons in comparison to channels allowed by bilinear couplings. Indeed, when considering AMS-02 data alone we find that the trilinear model is compatible with EGB constraints. However, this result may change when DAMPE or CALET measurements of the sum of electrons and positrons fluxes are considered instead of AMS-02 data. Therefore, in order to evaluate this model further, discrepancies between the data collected by these experiments need to be clarified

hep-ph↗

arXiv2013

Neutrino and axion bounds from the globular cluster M5 (NGC 5904)

The red-giant branch (RGB) in globular clusters is extended to larger brightness if the degenerate helium core loses too much energy in "dark channels." Based on a large set of archival observations, we provide high-precision photometry for the Galactic globular cluster M5 (NGC 5904), allowing for a detailed comparison between the observed tip of the RGB with predictions based on contemporary stellar evolution theory. In particular, we derive 95% confidence limits of $g_{ae}<4.3\times10^{-13}$ on the axion-electron coupling and $μ_ν<4.5\times10^{-12}\,μ_{\rm B}$ (Bohr magneton $μ_{\rm B}=e/2m_e$) on a neutrino dipole moment, based on a detailed analysis of statistical and systematic uncertainties. The cluster distance is the single largest source of uncertainty and can be improved in the future.

astro-ph.SR↗

arXiv2013

Particle-physics constraints from the globular cluster M5: Neutrino Dipole Moments

Stellar evolution is modified if energy is lost in a "dark channel" similar to neutrino emission. Comparing modified stellar evolution sequences with observations provides some of the most restrictive limits on axions and other hypothetical low-mass particles and on non-standard neutrino properties. In particular, a putative neutrino magnetic dipole moment mu_nu enhances the plasmon decay process, postpones helium ignition in low-mass stars, and therefore extends the red-giant branch (RGB) in globular clusters (GCs). The brightness of the tip of the RGB (TRGB) remains the most sensitive probe for mu_nu and we revisit this argument from a modern perspective. Based on a large set of archival observations, we provide high-precision photometry for the Galactic GC M5 (NGC5904) and carefully determine its TRGB position. On the theoretical side, we add the extra plasmon decay rate brought about by mu_nu to the Princeton-Goddard-PUC stellar evolution code. Different sources of uncertainty are critically examined. The main source of systematic uncertainty is the bolometric correction and the main statistical uncertainty derives from the distance modulus based on main-sequence fitting. (Other measures of distance, e.g., the brightness of RR Lyrae stars, are influenced by the energy loss that we wish to constrain.) The statistical uncertainty of the TRGB position relative to the brightest RGB star is less important because the RGB is well populated. We infer an absolute I-band brightness of M_I=-4.17+/-0.13 mag for the TRGB compared with the theoretical prediction of -3.99+/-0.07 mag, in reasonable agreement with each other. A significant brightness increase caused by neutrino dipole moments is constrained such that mu_nu<2.6x10^-12mu_B(68% CL), where mu_B is the Bohr magneton, and mu_nu<4.5x10^-12 mu_B(95% CL). In these results, statistical and systematic errors have been combined in quadrature.

astro-ph.SR↗

arXiv2013

The meV mass frontier of axion physics

We explore consequences of the idea that the cooling speed of white dwarfs can be interpreted in terms of axion emission. In this case the Yukawa coupling to electrons has to be g_{ae} 10^-13, corresponding to an axion mass of a few meV. Axions then provide only a small fraction of the cosmic cold dark matter, whereas core-collapse supernovae release a large fraction of their energy in the form of axions. We estimate the diffuse supernova axion background (DSAB) in the universe, consisting of 30 MeV-range axions with a radiation density comparable to the extra-galactic background light. The DSAB would be challenging to detect. However, axions with white-dwarf inspired parameters can be accessible in a next generation axion helioscope.

hep-ph↗