Neutrino signal from the hadron-quark phase transition in the conversion of Neutron Stars into Quark Stars

TL;DR

This work investigates neutrino-time-domain signatures of a hadron–quark phase transition during the conversion of a neutron star into a quark star, using a 1D general-relativistic hydrodynamics framework with DD2 hadronic matter and an MIT bag quark EOS. By tracking latent-heat release and neutrino microphysics across key emission channels, the authors show a short (10–50 ms) spectrally hard feature in the neutrino luminosity emerging at deconfinement, accompanied by a characteristic lag and a hardening of the spectrum that are relatively robust to hadronic uncertainties. After MSW flavor conversion, the signature remains detectable, with predictions calibrated for a Galactic event at ~10 kpc showing potential observability by IceCube and Hyper-K, and offering a multimessenger pathway alongside GW diagnostics. These results provide concrete, model-driven observables for identifying HQPT in compact objects, highlighting the practical impact for next-generation neutrino detectors and informing the interpretation of future Galactic transients.

Abstract

We calculate in a 1D General Relativistic (GR) hydrodynamic simulation the neutrino luminosity in an astrophysical scenario where a neutron star (NS) displays a hadron-quark phase transition (HQPT) into a Quark Star (QS). Deconfinement is triggered once the central density exceeds a critical threshold above $\sim 3n_0$ being $n_0$, saturation density. We use descriptions based on DD2 and the MIT Bag model equations of state (EOSs). We account for neutrinos using a microphysics forward emission model including $e^-e^+$ annihilation, plasmon decay, nucleon (N) modified (or direct) Urca processes, and $NN$ bremsstrahlung, and, for the post transition, the quark direct Urca and an opacity-based leakage scheme with GR redshift. We find that the neutrino light curve generically develops a short $\simeq$10-50 ms, spectrally harder feature near deconfinement, appearing as either a prompt shoulder or a distinct secondary peak. Heavy lepton neutrinos result in a delayed peak with respect to the previous. We identify three diagnostics that are only mildly degenerate with hadronic uncertainties: (i) an enhanced peak-to-plateau ratio $R_{\rm pp}$ sourced by latent-heat release, (ii) a characteristic lag $Δt$ between the collapse rise and the HQPT feature that tracks the central density trajectory, and (iii) a flavor hardening $Δ\!\langle E_ν\rangle$ driven by quark-matter phase space. After MSW flavor conversion, these signatures remain detectable with current experiments. For a Galactic event ($d\sim 10$ kpc), IceCube and Hyper-K should resolve the HQPT feature and distinguish it from both no transition NS collapse and canonical core-collapse supernova (CCSN) templates.

Figures (3)

• Figure 1: Density (upper panel) and temperature (lower panel) time evolution in a NS collapsing under the DD2 EOS with quark transition described by MIT bag model (green dashed line), the same DD2 EOS from initial, $t=0$$\rm T_{0} = 10$ MeV for a warm NS (orange curve) , and as a comparison we plot the case $\rm T_{0} = 0.5$ MeV with only SLy4 EOS (blue dash line). For the latter no transition $t=0$ to QS arises culminating in prompt black hole formation instead.
• Figure 2: Time evolution of individual neutrino luminosity components $L_i$ during the collapse of a NS to a QS. Pair annihilation, URCA, bremsstrahlung, and plasmon decay channels are shown along with exponential suppression due to neutrino trapping. The olive dots represents the deconfinement burst from hadronic to quark matter. The purple dash line is the $\nu_x$ emission, building more gradually to peak slightly later, due to thermal pair processes dominating.
• Figure 3: Upper panel: Electron anti-neutrino differential luminosity after MSW flavor conversion for the normal hierarchy (NH, solid red) and inverted hierarchy (IH, blue dashed). Bottom panel: Predicted IceCube detection rates from various scenarios at a distance of $d = 10$ kpc. The solid orange curve shows a NS-QS collapse with HQPT at ($t \approx 45\,\mathrm{ms}$), which induces a sharp neutrino burst, in deep contrast to the dashed blue line in absence of it. Green dash line represents a canonical CCSN; SN1987A-like, while the purple curve represents a short, high-luminosity signal from a $2.8M_\odot$ BNS.