山口 弘悦

Abstract It is generally believed that Type Ia supernovae are thermonuclear explosions of carbon–oxygen white dwarfs (WDs). However, there is currently no consensus regarding the events leading to the explosion. A binary WD (WD–WD) merger is a possible progenitor of Type Ia supernovae. Space-based gravitational wave (GW) detectors with considerable sensitivity in the decihertz range such as the DECi-hertz Interferometer Gravitational wave Observatory (DECIGO) can observe WD–WD mergers directly. Therefore, access to the decihertz band of GWs would enable multi-messenger observations of Type Ia supernovae to determine their progenitors and explosion mechanism. In this paper, we consider the event rate of WD–WD mergers and the minimum detection range to observe one WD–WD merger per year, using a nearby galaxy catalog and the relation between Ia supernovae and their host galaxies. Furthermore, we calculate DECIGO’s ability to localize WD–WD mergers and to determine the masses of binary mergers. We estimate that a decihertz GW observatory can detect GWs with amplitudes of h ∼ 10⁻²⁰ [Hz^−1/2] at 0.01–0.1 Hz, which is 1000 times higher than the detection limit of DECIGO. Assuming the progenitors of Ia supernovae are merging WD–WD (1 M_⊙ and 0.8 M_⊙), DECIGO is expected to detect 6600 WD–WD mergers within z = 0.08, and identify the host galaxies of such WD–WD mergers within z ∼ 0.065 using GW detections alone.

Abstract We report new H i observations of the Type Ia supernova remnant (SNR) SN 1006 using the Australia Telescope Compact Array with an angular resolution of $4\buildrel{\,\prime}\over{.} 5\times 1\buildrel{\,\prime}\over{.} 4$ (∼2 pc at the assumed SNR distance of 2.2 kpc). We find an expanding gas motion in position–velocity diagrams of H i with an expansion velocity of ∼4 km s⁻¹ and a mass of ∼1000 M_⊙. The spatial extent of the expanding shell is roughly the same as that of SN 1006. We here propose a hypothesis that SN 1006 exploded inside the wind-blown bubble formed by accretion winds from the progenitor system consisting of a white dwarf and a companion star, and then the forward shock has already reached the wind wall. This scenario is consistent with the single-degenerate model. We also derived the total energy of cosmic-ray protons W_p to be only ∼1.2–2.0 × 10⁴⁷ erg by adopting the averaged interstellar proton density of ∼25 cm⁻³. The small value is compatible with the relation between the age and W_p of other gamma-ray SNRs with ages below ∼6 kyr. The W_p value in SN 1006 will possibly increase up to several 10⁴⁹ erg in the next ∼5 kyr via the cosmic-ray diffusion into the H i wind shell.

Abstract We discover an unidentified strong emission feature in the X-ray spectrum of EXO 1745−248 obtained by RXTE at 40 hr after the peak of a superburst. The structure was centered at 6.6 keV and significantly broadened with a large equivalent width of 4.3 keV, corresponding to a line photon flux of 4.7 × 10−3 ph cm−2 s−1. The 3–20 keV spectrum was reproduced successfully by a power-law continuum with narrow and broad (2.7 keV in full width at half maximum) Gaussian emission components. Alternatively, the feature can be described by four narrow Gaussians, centered at 5.5 keV, 6.5 keV, 7.5 keV, and 8.6 keV. Considering the strength and shape of the feature, it is unlikely to have originated from reflection of the continuum X-rays by some optically thick material, such as an accretion disk. Moreover, the intensity of the emission structure decreased significantly with an exponential time scale of 1 hr. The feature was not detected in an INTEGRAL observation performed 10 hr before the RXTE observation with a line flux upper limit of 1.5 × 10−3 ph cm−2 s−1. The observed emission structure is consistent with gravitationally redshifted charge exchange emission from Ti, Cr, Fe, and Co. We suggest that the emission results from a charge exchange interaction between a highly metal-enriched fall-back ionized burst wind and an accretion disk, at a distance of ∼60 km from the neutron star. If this interpretation is correct, the results provide new information on nuclear burning processes during thermonuclear X-ray bursts.

Abstract The supernova remnant (SNR) 3C 397 is thought to originate from a Type Ia supernova (SN Ia) explosion of a near-Chandrasekhar-mass (M_Ch) progenitor, based on the enhanced abundances of Mn and Ni revealed by previous X-ray study with Suzaku. Here we report follow-up XMM-Newton observations of this SNR, conducted with the aim of investigating the detailed spatial distribution of the Fe-peak elements. We have discovered an ejecta clump with extremely high abundances of Ti and Cr, in addition to Mn, Fe, and Ni, in the southern part of the SNR. The Fe mass of this ejecta clump is estimated to be ∼0.06 M_⊙, under the assumption of a typical Fe yield for SNe Ia (i.e., ∼0.8 M_⊙). The observed mass ratios among the Fe-peak elements and Ti require substantial neutronization that is achieved only in the innermost regions of a near-M_Ch SN Ia with a central density of ρ_c ∼ 5 × 10⁹ g cm⁻³, significantly higher than typically assumed for standard near-M_Ch SNe Ia (ρ_c ∼ 2 × 10⁹ g cm⁻³). The overproduction of the neutron-rich isotopes (e.g., ⁵⁰Ti and ⁵⁴Cr) is significant in such high-ρ_c SNe Ia, with respect to the solar composition. Therefore, if 3C 397 is a typical high-ρ_c near-M_Ch SN Ia remnant, the solar abundances of these isotopes could be reproduced by the mixture of the high- and low-ρ_c near-M_Ch and sub-M_Ch Type Ia events, with ≲20% being high-ρ_c near-M_Ch.

Abstract The geometric structure of supernova remnants (SNR) provides a clue to unveiling the pre-explosion evolution of their progenitors. Here we present an X-ray study of N103B (0509–68.7), a Type Ia SNR in the Large Magellanic Cloud, that is known to be interacting with dense circumstellar matter (CSM). Applying our novel method for feature extraction to deep Chandra observations, we have successfully resolved the CSM, Fe-rich ejecta, and intermediate-mass element (IME) ejecta components, and revealed each of their spatial distributions. Remarkably, the IME ejecta component exhibits a double-ring structure, implying that the SNR expands into an hourglass-shape cavity and thus forms bipolar bubbles of the ejecta. This interpretation is supported by more quantitative spectroscopy that reveals a clear bimodality in the distribution of the ionization state of the IME ejecta. These observational results can be naturally explained if the progenitor binary system had formed a dense CSM torus on the orbital plane prior to the explosion, providing further evidence that the SNR N103B originates from a single-degenerate progenitor.