Thermonuclear explosion of rotating massive stars could explain core-collapse supernovae

It is widely thought that core-collapse supernovae (CCSNe), the explosions of massive stars following the collapse of the stars' iron cores, is obtained due to energy deposition by neutrinos. So far, this scenario was not demonstrated from first principles. Kushnir and Katz (2014) have recently shown, by using one-dimensional simulations, that if the neutrinos failed to explode the star, a thermonuclear explosion of the outer shells is possible for some (tuned) initial profiles. However, the energy released was small and negligible amounts of ejected $^{56}$Ni were obtained, implying that these one-dimensional collapse induced thermonuclear explosions (CITE) are unlikely to represent typical CCSNe. Here I provide evidence supporting a scenario in which the majority of CCSNe are the result of CITE. I use two-dimensional simulations to show that collapse of stars that include slowly (few percent of breakup) rotating $\sim0.1-10\,M_{\odot}$ shells of mixed helium-oxygen, leads to an ignition of a thermonuclear detonation wave that unbinds the stars' outer layers. Simulations of massive stars with different properties show that CITE is a robust process, and results in explosions with kinetic energies in the range of $10^{49}-10^{52}\,\textrm{erg}$, and $^{56}$Ni yields of up to $\sim\,M_{\odot}$, which are correlated, in agreement with observations for the majority of CCSNe. Stronger explosions are predicted from higher mass progenitors that leave more massive remnants, in contrast to the neutrino mechanism. Neutron stars are produced in weak ($\lt10^{51}\,\textrm{erg}$) explosions, while strong ($\gt10^{51}\,\textrm{erg}$) explosions leave black hole remnants.Comment: 4 pages, 5 figures, 3 table

The progenitors of core-collapse supernovae suggest thermonuclear origin for the explosions

Core-collapse supernovae (CCSNe) are the explosions of massive stars following the collapse of the stars' iron cores. Poznanski (2013) has recently suggested an observational correlation between the ejecta velocities and the inferred masses of the red supergiant progenitors of type II-P explosions, which implies that the kinetic energy of the ejecta ($E_{\textrm{kin}}$) increases with the mass of the progenitor. I point out that the same conclusion can be reached from the model-free observed correlation between the ejected $^{56}$Ni masses ($M_{\textrm{Ni}}$) and the luminosities of the progenitors for type II supernovae, which was reported by Fraser et al. (2011). This correlation is in an agreement with the predictions of the collapse-induced thermonuclear explosions (CITE) for CCSNe and in a possible contradiction with the predictions of the neutrino mechanism. I show that a correlation between $M_{\textrm{Ni}}$ and $E_{\textrm{kin}}$ holds for all types of CCSNe (including type Ibc). This correlation suggests a common mechanism for all CCSNe, which is predicted for CITE, but is not produced by current simulations of the neutrino mechanism. Furthermore, the typical values of $E_{\textrm{kin}}$ and $M_{\textrm{Ni}}$ for type Ibc explosions are larger by an order of a magnitude than the typical values for II-P explosions, a fact which disfavors progenitors with the same initial mass range for these explosions. Instead, the progenitors of type Ibc explosions could be massive Wolf-Rayet stars, which are predicted to yield strong explosions with low ejecta masses (as observed) according to CITE. In this case, there is no deficit of high mass progenitors for CCSNe, which was suggested under the assumption of a similar mass range for the progenitors of types II-P and Ibc supernovae.Comment: 4 pages, 3 figures, 2 table

Comments on "Numerical Stability of Detonations in White Dwarf Simulations"

Katz & Zingale (2019, KZ19) recently studied a one-dimensional test problem, intended to mimic the process of detonation ignition in head-on collisions of two carbon--oxygen (CO) white dwarfs. They do not obtain ignition of a detonation in pure CO compositions unless the temperature is artificially increased or 5% He is included. In both of these cases they obtain converged ignition only for spatial resolutions better than 0.1 km, which are beyond the capability of multidimensional simulations. This is in a contradiction with the claims of Kushnir et al. (2013, K13), that a convergence to $\sim10\%$ is achieved for a resolution of a few km. Using Eulerian and Lagrangian codes we show that a converged and resolved ignition is obtained for pure CO in this test problem without the need for He or increasing the temperature. The two codes agree to within 1% and convergence is obtained at resolutions of several km. We calculate the case that includes He and obtain a similar slow convergence, but find that it is due to a boundary numerical artifact that can (and should) be avoided. Correcting the boundary conditions allows convergence with resolution of $\sim10\,\textrm{km}$ in an agreement with the claims of K13. It is likely that the slow convergence obtained by KZ19 in this case is because of a similar boundary numerical artifact, but we are unable to verify this. KZ19 further recommended to avoid the use of the burning limiter introduced by K13. We show that their recommendation is not justified.Comment: 7 pages, 6 figures. Modified following referee repor

Hard X-ray emission from accretion shocks around galaxy clusters

We show that the hard X-ray (HXR) emission observed from several galaxy clusters is naturally explained by a simple model, in which the nonthermal emission is produced by inverse Compton scattering of cosmic microwave background photons by electrons accelerated in cluster accretion shocks: The dependence of HXR surface brightness on cluster temperature is consistent with that predicted by the model, and the observed HXR luminosity is consistent with the fraction of shock thermal energy deposited in relativistic electrons being \lesssim 0.1. Alternative models, where the HXR emission is predicted to be correlated with the cluster thermal emission, are disfavored by the data. The implications of our predictions to future HXR observations (e.g. by NuStar, Simbol-X) and to (space/ground based) gamma-ray observations (e.g. by Fermi, HESS, MAGIC, VERITAS) are discussed.Comment: 7 pages, 3 figures, somewhat revised, published in JCA

Neutrino Signal of Collapse-Induced Thermonuclear Supernovae: The Case for Prompt Black Hole Formation in SN1987A

Collapse-induced thermonuclear explosion (CITE) may explain core-collapse supernovae (CCSNe). We present a preliminary analysis of the neutrino signal predicted by CITE and compare it to the neutrino burst of SN1987A. For strong CCSNe, as SN1987A, CITE predicts a proto-neutron star (PNS) accretion phase, accompanied by the corresponding neutrino luminosity, that can last a few seconds and that is cut-off abruptly by black hole (BH) formation. The neutrino luminosity can later be revived by accretion disc emission after a dead time of few to a few ten seconds. In contrast, the neutrino mechanism for CCSNe predicts a shorter PNS accretion phase, followed by a slowly declining PNS cooling luminosity. We repeat statistical analyses used in the literature to interpret the neutrino mechanism, and apply them to CITE. The first 1-2 sec of the neutrino burst are equally compatible with CITE and with the neutrino mechanism. However, the data hints to a luminosity drop at t=2-3 sec, in some tension with the neutrino mechanism while being naturally attributed to BH formation in CITE. The occurrence of neutrino events at 5 sec in SN1987A suggests that the accretion disc formed by that time. We perform 2D numerical simulations, showing that CITE may be able to accommodate this disc formation time while reproducing the ejected $^{56}$Ni mass and ejecta kinetic energy within factors 2-3 of observations. We estimate the disc neutrino luminosity and show that it can roughly match the data. This suggests that direct BH formation is compatible with the neutrino burst of SN1987A. With current neutrino detectors, the neutrino burst of the next Galactic CCSN may give us front-row seats to the formation of an event horizon in real time. Access to phenomena near the event horizon motivates the construction of a few Megaton neutrino detector that should observe extragalactic CCSNe on a yearly basis.Comment: 11 pages, 6 figure

Towards an accurate description of an accretion induced collapse and the associated ejected mass

We revisit the accretion-induced collapse (AIC) process, in which a white dwarf collapses into a neutron star. We are motivated by the persistent radio source associated with the fast radio burst FRB 121102, which was explained by Waxman as a weak stellar explosion with a small ($\sim 10^{-5}M_{\odot}$) mildly relativistic mass ejection that may be consistent with AIC. Additionally, the interaction of the relatively low ejected mass with a pre-collapse wind might be related to fast optical transients. The AIC is simulated with a one-dimensional, Lagrangian, Newtonian hydrodynamic code. We put an emphasis on accurately treating the equation of state and the nuclear burning, which is required for any study that attempts to accurately simulate AIC. We leave subjects such as neutrino physics and general relativity corrections for future work. Using an existing initial profile and our own initial profiles, we find that the ejected mass is $\sim 10^{-2}$ to $10^{-1}M_{\odot}$ over a wide range of parameters, and we construct a simple model to explain our results.Comment: Accepted for publication in ApJ, 15 pages, 9 figure

Nonthermal emission from clusters of galaxies

We show that the spectral and radial distribution of the nonthermal emission of massive, M>10^{14.5}M_sun, galaxy clusters (GCs) may be approximately described by simple analytic expressions, which depend on the GC thermal X-ray properties and on two model parameter, beta_{core} and eta_e. beta_{core} is the ratio of CR energy density (within a logarithmic CR energy interval) and the thermal energy density at the GC core, and eta_{e(p)} is the fraction of the thermal energy generated in strong collisionless shocks, which is deposited in CR electrons (protons). Using a simple analytic model for the evolution of ICM CRs, which are produced by accretion shocks (primary CRs), we find that beta_{core} ~ eta_{p}/200, nearly independent of GC mass and with a scatter Delta ln(beta_{core}) ~ 1 between GCs of given mass. We show that the HXR and gamma-ray luminosities produced by IC scattering of CMB photons by primary electrons exceed the luminosities produced by secondary particles (generated in hadronic interactions within the GC) by factors ~500(eta_e/eta_p)(T/10 keV)^{-1/2} and ~150(eta_e/eta_p)(T/10 keV)^{-1/2} respectively, where T is the GC temperature. Secondary particle emission may dominate at the radio and VHE (> 1 TeV) gamma-ray bands. Our model predicts, in contrast with some earlier work, that the HXR and gamma-ray emission from GCs are extended, since the emission is dominated at these energies by primary electrons. Our predictions are consistent with the observed nonthermal emission of the Coma cluster for eta_peta_e ~ 0.1. The implications of our predictions to future HXR observations (e.g. by NuStar, Simbol-X) and to (space/ground based) gamma-ray observations (e.g. by Fermi, HESS, MAGIC, VERITAS) are discussed. Finally, we show that our model's results agree with results of detailed numerical calculations.Comment: 22 pages, 16 figures, somewhat revised, published in JCA

Can helium envelopes change the outcome of direct white dwarf collisions?

Collisions of white dwarfs (WDs) have recently been invoked as a possible mechanism for type Ia supernovae (SNIa). A pivotal feature for the viability of WD collisions as SNIa progenitors is that a significant fraction of the mass is highly compressed to the densities required for efficient $^{56}$Ni production before the ignition of the detonation wave. Previous studies have predominantly employed model WDs composed entirely of carbon-oxygen (CO), whereas WDs are expected to have a non-negligible helium envelope. Given that helium is more susceptible to explosive burning than CO under the conditions characteristic of WD collision, a legitimate concern is whether or not early time He detonation ignition can translate to early time CO detonation, thereby drastically reducing $^{56}$Ni synthesis. We investigate the role of He in determining the fate of WD collisions by performing a series of two-dimensional hydrodynamics calculations. We find that a necessary condition for non-trivial reduction of the CO ignition time is that the He detonation birthed in the contact region successfully propagates into the unshocked shell. We determine the minimal He shell mass as a function of the total WD mass that upholds this condition. Although we utilize a simplified reaction network similar to those used in previous studies, our findings are in good agreement with detailed investigations concerning the impact of network size on He shell detonations. This allows us to extend our results to the case with more realistic burning physics. Based on the comparison of these findings against evolutionary calculations of WD compositions, we conclude that most, if not all, WD collisions will not be drastically impacted by their intrinsic He components.Comment: 5 Pages, 2 Figure, 3 Table

The structure of detonation waves in supernovae revisited

The structure of a thermonuclear detonation wave can be solved accurately and, thus, may serve as a test bed for studying different approximations that are included in multidimensional hydrodynamical simulations of supernova. We present the structure of thermonuclear detonations for the equal mass fraction of $^{12}$C and $^{16}$O (CO) and for pure $^{4}$He (He) over a wide range of upstream plasma conditions. The lists of isotopes we constructed allow us to determine the detonation speeds, as well as the final states for these detonations, with an uncertainty of the percent level (obtained here for the first time). We provide our results with a numerical accuracy of $\sim0.1\%$, which provides an efficient benchmark for future studies. We further show that CO detonations are pathological for all upstream density values, which differs from previous studies, which concluded that for low upstream densities CO detonations are of the Chapman-Jouget (CJ) type. We provide an approximate condition, independent of reaction rates, that allows to estimate whether arbitrary upstream values will support a detonation wave of the CJ type. Using this argument, we are able to show that CO detonations are pathological and to verify that He detonations are of the CJ type, as was previously claimed for He. Our analysis of the reactions that control the approach to nuclear statistical equilibrium, which determines the length-scale of this stage, reveals that at high densities, the reactions $^{11}$B$+p\leftrightarrow3^{4}$He plays a significant role, which was previously unknown.Comment: 33 pages, 27 figures, 14 tables. Revised following a referee repor

An exact integral relation between the Ni56 mass and the bolometric light curve of a type Ia supernova

An exact relation between the Ni56 mass and the bolometric light curve of a type Ia supernova can be derived as follows, using the following excellent approximations: 1. the emission is powered solely by Ni56-> Co56 ->Fe56; 2. each mass element propagates at a non-relativistic velocity which is constant in time (free coasting); and 3. the internal energy is dominated by radiation. Under these approximations, the energy E(t) carried by radiation in the ejecta satisfies: dE/dt=-E(t)/t-L(t)+Q(t), where Q(t) is the deposition of energy by the decay which is precisely known and L(t) is the bolometric luminosity. By multiplying this equation by time and integrating over time we find: E(t)*t=\int_0^t Q(t')t'dt' -\int_0^t L(t')t'dt'. At late time, t>> t_peak, the energy inside the ejecta decreases rapidly due to its escape, and thus we have \int_0^t Q(t')t'dt'=\int_0^t L(t')t'dt'. This relation is correct regardless of the opacities, density distribution or Ni56 deposition distribution in the ejecta and is very different from "Arnett's rule", L_peak ~ Q(t_peak). By comparing \int_0^t Q(t')t'dt' with \int_0^t L(t')t'dt' at t~40 day after the explosion, the mass of Ni56 can be found directly from UV, optical and infrared observations with modest corrections due to the unobserved gamma-rays and due to the small residual energy in the ejecta, E(t)*t>0.Comment: 1 paragrap