Diffuse Neutrino Background from Magnetorotational Stellar Core Collapses

Using state-of-the-art 3D simulations, this paper assesses how magnetorotational stellar core collapses contribute to the diffuse supernova neutrino background, finding that they enhance the high-energy spectrum and could significantly accelerate the detection of this background or allow for the measurement of their occurrence fraction in future neutrino observatories.

The Gist

Imagine the universe as a giant, noisy party where stars are constantly being born and dying. Every time a massive star dies in a spectacular explosion called a supernova, it doesn't just throw out light and debris; it also releases a massive flood of tiny, ghostly particles called neutrinos. These particles are so shy that they can pass through entire planets without stopping.

Over billions of years, the neutrinos from every single star explosion in the universe have mixed together, creating a faint, ever-present "hum" or background noise. Scientists call this the Diffuse Supernova Neutrino Background (DSNB). It's like trying to hear a single conversation in a stadium full of people shouting; the signal is there, but it's buried under the noise.

The New Suspects: The "Spinners" and "Magnetars"

For a long time, scientists thought they knew what caused most of these explosions. But this paper introduces two special types of stellar deaths that might be adding extra "volume" to the high-pitched end of the neutrino hum.

1. Protomagnetars: Imagine a star that spins incredibly fast and has a magnetic field so strong it's like a cosmic magnet the size of a city. When this star collapses, it creates a super-dense, spinning neutron star with a magnetic field trillions of times stronger than Earth's.
2. Spinars: These are similar, but they are so massive and fast-spinning that they eventually collapse further into a black hole after a few seconds.

The authors of this paper ran complex computer simulations (like a high-tech video game of physics) to see what happens when these specific "spinners" die. They found that these events are louder and hotter than normal star deaths. Specifically, they shoot out neutrinos with much higher energy (think of them as "fast" neutrinos rather than "slow" ones).

The Big Mix-Up: Why It Matters

The problem is that the "loud" neutrinos from these spinners look very similar to the neutrinos from another mysterious event: massive stars that collapse directly into black holes without a big explosion.

Think of it like this:

• Normal Star Death: A gentle pop.
• Black Hole Collapse: A loud boom.
• Magnetar/Spinner Death: A loud, high-pitched screech.

Currently, our detectors can hear the "boom" and the "screech," but they can't easily tell which one is which. If there are a lot of these "spinners" out there, they would make the high-energy part of the neutrino background much brighter than we expected.

The Detective Work: What the Paper Found

The researchers used data from the Super-Kamiokande detector in Japan (a giant tank of water buried underground that catches neutrinos) to play detective. They asked: "How many of these 'spinners' can we have before our current data says 'No, that's too many'?"

Here is what they discovered:

• The Limit: If more than about 9% of all dying massive stars were these special "spinners," the current data from Super-Kamiokande would already be screaming that something is wrong. Since the data looks okay, we know these spinners can't be the majority.
• The Future: If these spinners make up more than 10-16% of all star deaths, the next generation of detectors (like Hyper-Kamiokande or JUNO) will be able to spot them.
• Speeding Up the Search: If these spinners are common, we might detect the neutrino background 2 to 4 years earlier than we thought. It's like finding a needle in a haystack; if the needle is made of gold (high energy), it's easier to find.

The Solution: Two Senses Are Better Than One

The paper suggests a clever way to solve the mystery of "who is making the noise." We can't just listen to the neutrinos; we need to look at the stars too.

• Neutrinos tell us about the energy of the explosion.
• Telescopes (looking at light) can tell us if a star disappeared (collapsed into a black hole) or if it exploded in a specific way (like a super-bright supernova).

By combining the "hearing" (neutrino data) with the "sight" (telescope data), scientists can finally separate the "spinners" from the "black hole formers." It's like having a witness who saw the car crash and a sound engineer who recorded the crash; together, they can tell you exactly what happened.

The Bottom Line

This paper is a roadmap for the future. It tells us that while we are waiting to finally hear the "hum" of the universe's star deaths, we need to keep an eye out for these special, fast-spinning, magnetic stars. If they exist in large numbers, they will change the sound of the universe, and we will need our new, giant detectors to catch them.

Technical Summary

Technical Summary: Diffuse Neutrino Background from Magnetorotational Stellar Core Collapses

Problem Statement
The Diffuse Supernova Neutrino Background (DSNB) represents the cumulative flux of neutrinos from all core-collapse supernovae (CCSNe) throughout cosmic history. While a statistically significant detection is imminent, theoretical modeling of the DSNB is hindered by uncertainties in the CCSN rate, the initial mass function (IMF), the fraction of collapses forming black holes (BHs), neutrino flavor conversion, and galactic metallicity evolution. Furthermore, the DSNB signal in the 10–25 MeV range may receive contributions from sources other than standard neutrino-driven CCSNe, including Type Ia SNe, Population III stars, and accretion flows. A specific gap in current understanding is the potential contribution of magnetorotational core collapses—rapidly rotating massive stars where rotation and magnetic fields are dynamically significant—to the diffuse neutrino flux. These events are hypothesized progenitors of superluminous SNe and certain gamma-ray bursts, yet their impact on the DSNB spectrum remains unquantified.

Methodology
The authors assess the contribution of magnetorotational collapses to the DSNB using a suite of state-of-the-art three-dimensional neutrino-magnetohydrodynamic (MHD) simulations. The study differentiates between two remnant scenarios:

1. Protomagnetars: Rapidly rotating protoneutron stars with magnetic fields $\gtrsim 10^{15}$ G that survive the explosion.
2. Spinars: Fast-rotating magnetized relativistic objects that undergo a two-stage collapse (first to a protoneutron star, then to a BH).

The authors utilize seven 3D models with Zero Age Main Sequence (ZAMS) masses ranging from $5 M_\odot$ to $30 M_\odot$, employing the SFHo hadronic equation of state and spectral two-moment (M1) neutrino transport. These models are compared against standard 1D hydrodynamical simulations of neutrino-driven CCSNe (representing masses $11.2 M_\odot$ and $27 M_\odot$) and neutrino-driven BH-forming collapses ($40 M_\odot$).

The DSNB flux is computed by integrating the time-integrated neutrino energy spectra over the star formation history and the initial mass function (Salpeter IMF). The authors treat the fraction of magnetorotational collapses ($f_{MR}$) as a free parameter, assuming it is constant over redshift. They also account for uncertainties in the CCSN rate and the fraction of neutrino-driven BH-forming collapses ($f_{\nu BH}$). To address flavor conversion uncertainties, the study considers two extreme cases: no flavor conversion and full flavor conversion at the source.

Key Results

• Neutrino Emission Properties: Magnetorotational models exhibit neutrino luminosities $L_\nu \gtrsim 10^{52}$ erg s$^{-1}$ lasting several seconds. Crucially, the mean energies of non-electron flavor neutrinos ($\nu_x$) exceed 20 MeV for a significant portion of the emission, higher than in standard neutrino-driven CCSNe. This is attributed to neutrino decoupling occurring at higher densities in the compact protoneutron stars.
• DSNB Spectrum: The inclusion of magnetorotational collapses boosts the high-energy tail of the DSNB spectrum. The spectral shape of this high-energy tail is similar to that expected from neutrino-driven BH-forming collapses, creating a degeneracy between the two populations in the DSNB signal.
• Current Constraints: Using the latest model-independent limits on the $\bar{\nu}_e$ flux from the Super-Kamiokande Collaboration (phases VI and VII), the authors find that, under optimistic assumptions (including full flavor conversion and high star formation rates), more than 9% of all collapsing massive stars cannot undergo magnetorotational collapses.
• Future Detection Prospects:
  • Super-Kamiokande-Gadolinium (SK-Gd) and JUNO: If the fraction of magnetorotational collapses exceeds 10%, the enhanced high-energy flux could allow these detectors to reject the background-only hypothesis ($3\sigma$) 2–4 years earlier than if $f_{MR} = 0$.
  • Hyper-Kamiokande-Gadolinium (HK-Gd): With a 20-year exposure, HK-Gd could measure a magnetorotational fraction of $f_{MR} \gtrsim 11\%$ at $3\sigma$ (assuming negligible neutral-current atmospheric backgrounds). If backgrounds are present, the threshold rises to $f_{MR} \gtrsim 16\%$.
  • DUNE: While sensitive to the $\nu_e$ component, DUNE's contribution to constraining $f_{MR}$ is likely marginal due to lower statistics compared to water Cherenkov detectors, even under optimistic assumptions.

Significance and Claims
The paper claims that magnetorotational core collapses are a non-negligible source of high-energy neutrinos that could significantly alter the DSNB spectrum. The primary significance lies in the potential for the DSNB to serve as a probe for the population of rapidly rotating massive stars, which are difficult to constrain via electromagnetic observations alone due to degeneracies with BH-forming collapses.

The authors emphasize that while current data from Super-Kamiokande cannot yet tightly constrain $f_{MR}$, the upcoming generation of detectors (SK-Gd, JUNO, and HK-Gd) will have the sensitivity to either detect this population or place stringent upper limits. The paper concludes that a combination of DSNB data with electromagnetic constraints—specifically the rates of stripped-envelope SNe and searches for disappearing luminous stars—is essential to break the degeneracy between magnetorotational collapses and neutrino-driven BH-forming collapses, thereby providing crucial insights into the properties of the collapsing massive star population.