Session G02: Multimessenger Astrophysics and Cataclysmic Events I

ASASSN-15hy: An Underluminous, red 03fg-like Type Ia Supernova

We present observations of the 03fg-like (super-Chandrasekhar) SN Ia ASASN-15hy. It is bright in the UV and NIR, lacks a clear $i$-band secondary maximum, shows a strong and persistent C II feature, and has a low Si II $\lambda$6355 velocity. However, some properties are also extreme among the subgroup. ASASN-15hy is underluminous, red, yet slowly declining. It has the most delayed onset of the $i$-band maximum of any 03fg-like SN. ASASN-15hy lacks the $H$-band break that is typically present during the first month in normal SNe Ia. Such events may be a potential problem for high-redshift SN Ia cosmology. ASASN-15hy may be an explosion of a degenerate core inside a nondegenerate envelope. The explosion impacting the nondegenerate envelope with a large mass provides additional luminosity and low ejecta velocities. An initial deflagration burning phase is critical in reproducing the low $^{56}$Ni mass and luminosity, while the large core mass provides the diffusion time scales to produce the broad light curves. The model consists of a rapidly rotating 1.47 $M_{\odot}$ degenerate core and a 0.8 $M_{\odot}$ nondegenerate envelope. This deflagration core-degenerate scenario may result from the merger between a white dwarf and an asymptotic giant branch star’s core.   

Thermonuclear Supernovae as a Source of Galactic Positrons

We present a study on the emission of positrons from Type Ia Supernovae (SNe~Ia). We evaluate their escape fractions and energy spectra and then address their role in the Galactic positron puzzle. Our simulations encompass a wide variety of explosion scenarios, including the explosion of Chandrasekhar mass limit ($M_{\mathrm{Ch}}$) white dwarfs (WD), He-triggered explosions of sub-$M_{\mathrm{Ch}}$ WDs, and dynamical mergers of two WDs. For each model, we study the influence of the size and morphology of the progenitor magnetic field between 1 and $10^{13}$ G. Based on the observed brightness distribution of SNe~Ia, we find that the resulting positron injection is dominated by normal bright SNe~Ia, which may reduce the dependence on the explosion mechanism. The morphology of the progenitor B-field dominates the positron escape at about $(1.96 \pm 0.75)$ and $(0.94 \pm 0.56) \times 10^{52}$ e$^{+}$ per SN~Ia for large scale dipole and turbulent fields, respectively. Assuming a Galactic SN~Ia rate between $0.13$ and $ 0.76$ per century, we find positron injection rate ranges from (0.48 to 6.42) and (0.20 to 3.91) $\times 10^{42}$ e$^{+}$~s$^{-1}$ for dipole and turbulent fields, respectively. SNe~Ia may be contributed to $\approx 1-13\%$ of the Galactic positron annihilation rate.   

Nebular Phase NIR Spectroscopy of Sibling Supernovae

We present multi-wavelength nebular-phase time-series spectroscopy of SN 2013aa and SN 2017cbv, two SNe Ia on the outskirts of the same host galaxy NGC 5643, by the Carnegie Supernova Project-II. These sibling SNe were well observed at both early and nebular phases, providing an excellent opportunity to study the SN Ia intrinsic diversity. The new nebular-phase near-infrared (NIR) spectra are supplemented with previously published optical and NIR spectra. The explosion kinematics were assessed by measuring multiple nebular-phase [Fe II] lines in both the optical and NIR. The NIR [Fe II] 1.644 micron line provides the most robust velocity measurements against the choice of the fit method and line blending compared to its optical counterparts. Compared to the optical, the NIR velocities have the same radial shift direction, but the sizes of the shifts are consistently and substantially lower, pointing to a potential issue in optical studies. Methods for detecting asymmetries and viewing angle effects were also tested and provided corroborating evidence in both the optical and NIR.   

Effects of dynamical tides on gravitational wave signals from eccentric double white dwarf systems

The tidal response of compact stars in a binary system is a useful probe of the stellar interior. While the equilibrium part of the tide dominates the matter response for systems with a large orbital separation, the dynamical tide starts to become important for small separation as the orbital period gets closer to the matter response time. In this presentation, I will discuss the effect of dynamical tides on the gravitational wave signals from eccentric binary white dwarf systems. These systems are one of the target sources of the proposed space-based gravitational wave detector (LISA). We show numerically that for orbits with high eccentricities and small separations, the dynamical tide can cause a chaotic growth of the normal mode amplitudes and cause the orbit to evolve in a random manner, leading to a chaotic waveform. Meanwhile, for systems with lower eccentricities and larger separations, the dynamical tide affects the orbit slightly by introducing extra dependence on the normal mode frequencies. This causes slight amplitude modulations of the gravitational wave emitted, which implies that normal mode oscillations can potentially be detected with the inspiral signal.   

Recent Progress in Multimessenger Nuclear Astrophysics

Neutron stars offer us an excellent testbed to explore dense-matter physics that is difficult to access with terrestrial experiments. Recent multimessenger observations through radio, x-ray, and gravitational waves have advanced our understanding of the nature of supranuclear matter. In this talk, I will briefly review recent progress on probing nuclear physics with multimessenger astronomy. For example, I will discuss how one can measure certain nuclear parameter (the curvature of symmetry energy at nuclear saturation density) by adopting correlations between this parameter and neutron star observables, such as radius and tidal deformability, and using observations by LIGO/Virgo and NICER.