Collapsar Disk Outflows III: Detectable Neutrino and Gravitational Wave Signatures

TL;DR

This work quantifies MeV neutrino and gravitational-wave signatures from collapsar accretion disks that form after BH formation, focusing on the NDAF and later ADAF phases and their time variability. Using post-processed, axisymmetric viscous hydrodynamic simulations with neutrino leakage and a 19-isotope network, the authors extract neutrino spectra and matter-driven GW signals, exploring multiple flavor-transformation scenarios. They find a prominent NDAF neutrino plateau with typical energies of order 10–20 MeV, easily detectable by IceCube within the Galaxy for favorable flavor mixing, along with SMB-like GW signals in the 10–100 Hz band detectable by LIGO A+ and future detectors; a memory GW signature could be accessible to space-based DECIGO. Time variability during the NDAF phase, tied to shock oscillations and disk advection timescales, imprints characteristic bands in neutrino and GW spectrograms, encoding inner-disk dynamics and shock evolution. These results demonstrate that joint neutrino–GW observations can probe angular-momentum transport and NDAF physics, with detectability strongly dependent on progenitor structure and flavor transformation during propagation.

Abstract

We investigate the neutrino and gravitational wave (GW) signals from accretion disks formed during the failed collapse of a rotating massive star (a collapsar). Following black hole formation, a neutrino-cooled, shocked accretion disk forms, which displays non-spherical oscillations for a period of seconds before becoming advective and exploding the star. We compute the neutrino and GW signals (matter quadrupole, frequencies $\lesssim 100$ Hz) from collapsar disks using global axisymmetric, viscous hydrodynamic simulations. The neutrino signal with typical energies of O$(10)$ MeV is maximal during the neutrino-cooled (NDAF) phase that follows shock formation. This phase lasts for a few seconds and is easily detectable within O$(10-100)$ kpc by the IceCube Neutrino Telescope. Additional neutrino signatures from a precursor equatorial shock and by stochastic accretion plumes during the advective phase are detectable within the galaxy. The GW signal during the NDAF phase is detectable in the galaxy by current and next-generation ground-based observatories. The explosion (memory) GW signal is similar to that of standard core-collapse supernovae and can be probed with a deci-Hertz space-based detector. Shock oscillations during the NDAF phase impart time variations with frequency O$(10-100)$ Hz to the neutrino and GW signals, encoding information about the shock dynamics and inner disk. These time variations can be detectable in neutrinos by IceCube within O$(1-10)$ kpc depending on progenitor model, flavor transformation scenario, and detailed properties of the angular momentum transport mechanism.

Figures (9)

• Figure 1: Key physical quantities and relevant phases in the evolution of the baseline model 16TI-SFHo. Shown are the shock radius coefficients (top row, Eq. \ref{['eq:legendre_coeff']}), product of number luminosity times mean energy for electron antineutrinos $\bar{\nu}_e$ and one species of heavy lepton neutrinos $\nu_x$ (middle row), and GW displacement for an equatorial observer (bottom row, Eq. \ref{['eq:Dhplus']} with $\sin\theta=1$). Panels on the right are zoom-ins of panels to their left, as shown by dashed lines. The pink, gray, and yellow shaded regions encompass shock formation, NDAF phase, and ADAF phase, respectively. The average shock radius (monopole) $a_0$ illustrates the timing of formation, oscillations, and expansion of the shock, while the normalized dipole ($a_1/a_0$) and quadrupole ($a_2/a_0$) coefficients show the amplitude of large-scale oscillations during the NDAF phase. The product $N_{\nu_i}\langle \epsilon_{\nu_i}\rangle^2$ is proportional to the neutrino absorption rate via inverse beta decay, and thus assesses detectability in IceCube. The large difference between $\bar{\nu}_e$ and $\nu_x$ illustrates the sensitivity of neutrino detection to flavor transformation.
• Figure 2: IceCube event rate over the period of shock formation, NDAF phase, and onset of explosion, for models 16TI-SFHo-dt (upper left) and 35OC-SFHo-dt (upper right), 16TI-lo$\alpha$-dt (lower left), and 16TI-hi$\alpha$-dt (lower right), assuming a fiducial distance of $D = 10$ kpc. Blue, red, and black curves correspond to unoscillated, adiabatic, and full swap flavor transformation scenarios, respectively (§\ref{['s:neutrinos']}). The semi-transparent region below each curve indicates the uncertainty range obtained by using $\langle \epsilon_\nu\rangle = 4.1kT_\nu$ instead of Eq. (\ref{['eq:mean_energy_temp']}) when computing the neutrino spectrum for input in SNOwGLoBES (§\ref{['s:neutrinos']}). This uncertainty is larger than the shot noise in the signal. The light gray horizontal band at the bottom indicates the $1\sigma$ detection threshold, and the dashed line the $5\sigma$ threshold. The dotted lines in the upper left panel indicate the time ranges and event rate levels (adiabatic case with default mean energy coefficient) used to compute the detection horizons in Table \ref{['tab:neutrino']}, see text for details. At this distance, the NDAF phase is detectable at high significance for likely flavor transformation scenarios. The high viscosity model 16TI-hi$\alpha$-dt does not experience an NDAF phase.
• Figure 3: Left: Characteristic strain $h_{\rm char}$ (Eq. \ref{['eq:hchar_definition']}) at $D=10$ kpc for an equatorial observer, for models 16TI-SFHo and 35OC-SFHo, calculated from the waveform over the entire simulation (cf. Table \ref{['tab:models']}), and applying a Hann window (i.e., a "short Fourier transform"). Also shown with dashed lines are the strain noise sensitivities for LIGO A+ ligo_lrr_2020, Cosmic Explorer for 20 km post-merger (CE-pm) and 40 km low-frequency (CE-lf) configurations srivastava_2022, Einstein Telescope (ET-D hild_2011), and Deci-Herz Interferometer Gravitational Wave Observatory (DECIGO, yagi_2017), as labeled on each curve. Right: Same as the left panel, but now using the -dt models, and restricting the time period to the NDAF phase (cf. Fig. \ref{['fig:neutrino_rate']}), over which the Hann window is applied.
• Figure 4: Gravitational wave displacement (Eq. \ref{['eq:Dhplus']}) as a function of time, for the models originally presented in Paper I, as labeled.
• Figure 5: Left: Discrete Fourier power spectrum (periodogram estimate, Hann window) of the IceCube event rate at $D = 10$ kpc assuming adiabatic MSW flavor transformation and default mean energy coefficient for model 16TI-SFHo-dt over the time period shown, which covers the NDAF phase (cf. Fig. \ref{['fig:neutrino_rate']}). The spectrum is normalized to the power of the IceCube noise as described in Appendix \ref{['app:noise_power']}. The dashed horizontal line shows equality between signal power and noise power. The green curve shows a smoothing fit used to assess the detection prospects at a fiducial frequency of $10$ Hz (cf. Table \ref{['tab:variability']}). Right: Discrete Fourier power spectrum (periodogram estimate, Hann window) of each normalized shock coefficient $a_\ell/a_0$ (multiplied by the sampling time $\Delta t = 1$ ms), as labeled. For both neutrino and shock coefficients, the power spectrum is nearly flat up to $\sim 20-30$ Hz, decreasing as a power-law for higher frequencies.