The detection of neutrinos from massive stellar collapses can teach us a lot not only about source objects but also about microphysics working deep inside them. In this study we discuss quantitatively the possibility to extract information on the properties of dense and hot hadronic matter from neutrino signals coming out of black-hole-forming collapses of non-rotational massive stars. Based on our detailed numerical simulations we evaluate the event numbers for SuperKamiokande with neutrino oscillations being fully taken into account. We demonstrate that the event numbers from a Galactic event are large enough not only to detect it but also to distinguish one hadronic equation of state from another by our statistical method assuming the same progenitor model and non-rotation. This means that the massive stellar collapse can be a unique probe into hadron physics and will be a promising target of the nascent neutrino astronomy.
One of the important roles of astrophysics is to explore physics under extreme conditions that are difficult to realize in terrestrial experiments. In this sense, hadron physics at supra-nuclear densities and non-zero temperatures (e.g., hyperon appearance, quark deconfinement and so on) is a natural target of astrophysics and the gravitational collapse of massive stars at the end of their lives will set the stage [1,2]. In particular, black-hole-forming collapses expected for very massive stars with a mass larger than ∼30 solar masses (M ⊙ ) will be the most promising site. Although such an event has not been observed yet, a black hole candidate with an estimated mass of 24-33M ⊙ was discovered [3] and this might be a remnant of the collapse of such a massive star. Recently, a regular monitoring of ∼10 6 supergiants within a distance of 10 Mpc is proposed to detect their silent disappearances [4] and the black-hole-forming collapse, which would be invisible optically, might be observed that way.
In our previous studies [5][6][7], we showed that the event, which we refer to as the “failed supernova” hereafter, is as bright in neutrino emissions as ordinary core-collapse supernovae. We showed also that its time evolutions of luminosities and spectra are qualitatively different from those of the ordinary supernova explosion and the ensuing proto-neutron star cooling [8], which may lead to the delayed black hole formation for some reasons [9,10]. Our numerical data were adopted as a reliable basis to predict the relic neutrino background from stellar collapses [11]. More importantly, however, we also demonstrated by employing different hadronic equations of state (EOS) that the duration of neutrino emissions from the failed supernova is sensitive to the stiffness of EOS at supra-nuclear densities and, therefore, that the observation of neutrinos from such an event will provide us with valuable information on the properties of dense and hot hadronic matter as well as on the maximum mass of proto-neutron stars.
Although this approach is simple and robust, valid irrespective of neutrino oscillations, it can not distinguish EOS’s with a similar duration of neutrino emissions: a soft nucleonic EOS and a hyperonic EOS, for example. In this study, we attempt to break this degeneracy by analyzing more in detail the time variation of neutrino numbers observed at a terrestrial detector, which we refer to as the “light curve” hereafter. While we have studied the detection of failed supernova neutrinos taking fully into account the neutrino oscillation and its parameter dependence so far [12], we innovate a new method here by employing the Kolmogolov-Smirnov (KS) test, which is free from the ambiguity of the distance to the progenitor. We adopt the results of our detailed numerical simulations and evaluate the neutrino event number for SuperKamiokande (SK) as a representative of currently operating neutrino detectors. This is the FIG. 1: Luminosities and average energies of νe for four different EOS’s, LS180-EOS (dash-dotted), LS220-EOS (dashed), Shen-EOS (dotted) and Hyperon-EOS (solid). Time is measured from the bounce.
first ever serious self-contained attempt to demonstrate that for Galactic events it is indeed possible to break the degeneracy for hadronic EOS’s by the statistical analysis.
We arrange this paper as follows. A brief review of the neutrino detection and a description of the newly proposed statistical method are given in Sec. II. The main results of our study are reported in Sec. III. In Sec. IV, we mention the possible uncertainties and observational issues. Sec. V is devoted to a summary.
The evaluation of the light curve of neutrinos from the failed supernova can be roughly divided into three parts. The first step is a computation of the neutrino luminosity and spectrum at the source. The general relativistic ν-radiation-hydrodynamics code, which solves the Boltzmann equations for neutrinos together with the Lagrangian hydrodynamics under spherical symmetry, is utilized to quantitatively compute the dynamics as well as the neutrino luminosities and spectra up to the black hole formation. This code passed a couple of well known standard tests and a detailed comparison with Monte Carlo simulations [13][14][15]. The numerical errors are estimated to be ∼10% from the computations with lower resolutions in Ref. [16]. The progenitor model with 40M ⊙ [17] is adopted as the initial condition for the dynamical simulations.
A hadronic EOS is needed at this stage. It should be emphasized that it is not the intention of the paper to endorse a particular EOS but that EOS’s which are available for astrophysical numerical simulations, that is, subroutines or tables that provide thermodynamic variables in wide ranges of density, temperature and proton fraction are very limited at present. For example, an EOS table including hyperons has been provided only by Ishizuka et al. [18] so far, based on the relativisti
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