arXiv subjects

Kenta Hotokezaka

Publications and source records attributed to Kenta Hotokezaka.

At least 37 records · Page 2Linked to original sources

arXiv2025

Signature of hadron-quark crossover in binary-neutron-star mergers

We study observational signatures of the hadron-quark crossover in binary-neutron-star mergers by numerical-relativity simulations with various mass configurations. We employ two equations of state (EoSs) for matter consistent with inference from the observational data. In the crossover scenario the EoS is softened in a density realized in binary-neutron-star mergers and is smoothly continued to quark matter. In the phase transition scenario without crossover, the EoS remains stiff and a first-order phase transition takes place in a density out of reach of mergers. A GW170817-like system forms a remnant massive neutron star in both scenarios, and it collapses into a black hole only in the crossover scenario due to the softening while gravitational-wave emission is strong. This difference is clearly reflected in the sudden shutdown of gravitational waves. For a given EoS, the lifetime of the merger remnant is determined primarily by the total mass of the system. Identifying these features in a variety of future events with the next generation of ground-based gravitational-wave detectors will enable us to clarify details of hadron-quark transition. The mass of the accretion disk surrounding the remnant black hole is affected not only by the lifetime of the remnant but also by the mass ratio of the system. Electromagnetic emission associated with the disk outflow will also be useful for detailed investigation of the hadron-quark transition.

astro-ph.HE↗

arXiv2025

Modeling the emission lines from r-process elements in Supernova nebulae

The origin of heavy r-process elements in the universe is still a matter of great debate, with a confirmed scenario being neutron star (NS) mergers. Additional relevant sites could be specific classes of events, such as gamma-ray burst (GRB) Supernovae (SNe), where a central engine could push neutron-rich material outwards, contributing to the ejecta of the massive exploding star. Here, we investigate our ability to infer the production of heavy elements in such scenarios, on the basis of the observed nebular emission. We solve the steady-state ionization, level population, and thermal balance, for optically thin ejecta in non-local thermodynamic equilibrium (NLTE), in order to explore the role of heavy elements in cooling the gas, and their imprint in the emergent spectrum a few hundreds days post-explosion. We find that heavy elements would be relevant in the cooling process of the nebula only if they account for at least $\sim1\%$ of the total ejected mass, at the typical kinetic temperatures of a few thousands K. However, even in the absence of such amount, a few $0.1\%$ of the total ejected mass could be instead sufficient to leave a detectable imprint around $\sim1-10~\mathrm{μm}$. This wavelength range, which would be relatively clean from features due to light elements, would be instead robustly populated by lines from heavy elements arising from forbidden transitions in their atomic fine structures. Hence, the new generation of telescopes, represented by the James Webb Space Telescope (JWST), will most likely allow for their detection.

astro-ph.HE↗

arXiv2025

Leptogenesis in the presence of density perturbations

We point out a new effect on the freeze-out process of heavy particles induced by density perturbations in the early universe, which we call ``acoustically driven freeze-out.'' This beyond-linear effect is caused by the exponential decoupling of heavy particles from the thermal bath in the presence of density perturbations, and already at moderately large values $δT / \bar{T} = O (10^{-2})$ it cannot be captured by linear perturbation theory. We illustrate this effect with leptogenesis taking the decay and inverse decay of heavy neutrinos into account, and discuss its phenomenological implications. We found that perturbations always enhance the (spatially averaged) values of the final lepton asymmetry, and as a result, constraints on the mass of heavy neutrinos are found to be relaxed in the presence of perturbations.

hep-ph↗

arXiv2025

Features and prospects for kilonova remnant detection with current and future surveys

We study the observable spectral and temporal properties of kilonova remnants (KNRs) analytically, and point out quantitative differences with respect to supernova remnants. We provide detection prospects of KNRs in the context of ongoing radio surveys. We find that there is a good chance to expect tens of these objects in future surveys with a flux threshold of $\sim 0.1$ mJy. Kilonova remnants from a postulated population of long-lived supermassive neutron star remnants of neutron star mergers are even more likely to be detected, as they are extremely bright and peak earlier. For an ongoing survey with a threshold of $\sim$ 1 mJy, we expect to find tens to hundreds of such objects if they are a significant fraction of the total kilonova (KN) population. Considering that there are no such promising KN candidates in presently ongoing surveys, we constrain the fraction of these extreme KN to be no more than 30 percent of the overall KN population. This constraint depends sensitively on the details of ejecta mass and external density distribution.

astro-ph.HE↗

arXiv2024

Revisiting Near-Infrared Features of Kilonovae: The Importance of Gadolinium

The observation of the kilonova AT2017gfo and investigations of its light curves and spectra confirmed that neutron star mergers are sites of r-process nucleosynthesis. However, the identification of elements responsible for the spectral features is still challenging, particularly at the near-infrared wavelengths. In this study, we systematically searched for all possible near-infrared transitions of heavy elements using experimentally calibrated energy levels. Our analysis reveals that most candidate elements with strong absorption lines are lanthanides (Z=57-71) and actinides (Z=89-103). This is due to their complex structures leading to many low-lying energy levels, which results in strong transitions in the near-infrared range. Domoto et al. (2022) have shown that La III and Ce III can explain the absorption features at $λ\sim$ 12,000 - 15,000 A. While our results confirm that these two elements show strong infrared features, we additionally identify Gd III as the next most promising species. Due to its unique atomic structure involving the half-filled 4f and the outer 5d orbitals, Gd III has one of the lowest-lying energy levels, between which relatively strong transitions occur. We also find absorption lines caused by Gd III in the near-infrared spectrum of a chemically peculiar star HR 465, which supports their emergence in kilonova spectra. By performing radiative transfer simulations, we confirm that Gd III lines affect the feature at $\sim$ 12,000 A previously attributed to La III. Future space-based time-series observations of kilonova spectra will allow the identification of Gd III lines.

astro-ph.HE↗

arXiv2024

An analytical joint prior for effective spins for inference on the spin distribution of binary black holes

We derive an analytical form of the joint prior of effective spin parameters, $χ_\mathrm{eff}$ and $χ_\mathrm{p}$, assuming an isotropic and uniform-in-magnitude spin distribution. This is a vital factor in performing hierarchical Bayesian inference for studying the population properties of merging compact binaries observed with gravitational waves. In previous analyses, this was evaluated numerically using kernel density estimation (KDE). However, we find that this numerical approach is inaccurate in certain parameter regions, where both $|χ_\mathrm{eff}|$ and $χ_\mathrm{p}$ are small. Our analytical approach provides accurate computations of the joint prior across the entire parameter space and enables more reliable population inference. Employing our analytic prior, we reanalyze binary black holes in the Gravitational-Wave Transient Catalog 3 (GWTC-3) by the LIGO-Virgo-KAGRA collaboration. While the results are largely unchanged, log-likelihood errors due to the use of the inaccurate prior evaluations are $\mathcal{O}(1)$. Since these errors accumulate with the increasing number of events, our analytical prior will be crucial in the future analyses.

gr-qc↗

arXiv2024

Exploring faint white dwarfs and the luminosity function with Subaru HSC and SDSS in Stripe 82

We present 5,080 white dwarf (WD) candidates selected from stars matching between the multi-band imaging datasets of the Subaru Hyper Suprime-Cam (HSC) Survey and the Sloan Digital Sky Survey (SDSS) in the Stripe82 region covering about 165~deg$^2$. We select WD candidates from the ``reduced proper motion'' diagram by combining the apparent magnitude in the range $i=19$ -- 24 and the proper motion measured from the datasets among a baseline of $\sim$ 14~years. We refine the WD candidates by fitting blackbody and template WD atmosphere models to HSC photometries for each candidate, enabling the estimation of distance and tangential velocity ($v_{\rm t}$). The deep HSC data allow us to identify low-temperature ($<4000$~K) and faint WD candidates down to absolute magnitude, $M_{\rm bol}\simeq 17$. We evaluate the selection function of our WD candidates using a mock catalogue of spatial and kinematic distributions of WDs in the (thin and thick) disc and halo regions based on a Galactic model. We construct samples of disc and halo WD candidates by selecting WDs with tangential velocity, $40<v_{\rm t}/[{\rm km}~{\rm s}^{-1}]<80$ and $200<v_{\rm t}/[{\rm km}~{\rm s}^{-1}]<500$, respectively. The total number densities of the disc and halo WDs are $(9.33 \pm 0.89) \times 10^{-3}$~pc$^{-3}$ and $(6.34 \pm 2.90) \times 10^{-4}$~pc$^{-3}$. Our luminosity functions (LF) extend down to fainter absolute magnitudes compared with previous work. The faint WDs could represent the oldest generation of building blocks over the past $\sim$10 billion years of the assembly history of our Milky Way.

astro-ph.GA↗

arXiv2024

Demystifying shock breakout spectra

The spectrum of the first supernova light (i.e., the shock breakout and early cooling emission) is an important diagnostic for the state of the progenitor star just before explosion. We consider a streamlined model describing the emergent shock breakout spectrum, which enables a straightforward classification of the possible observed spectra during the early planar phase. The overall spectral evolution is determined by a competition between three important time-scales: the diffusion time $t_{\rm{bo}}$ of the shell producing the breakout emission, the light-crossing time of this shell $t_{\rm{lc}}$, and the time $t_{\rm{eq}}$ at which the observer starts to see layers of the ejecta where the gas and radiation are in thermal equilibrium. There are five allowed orderings of these time-scales, resulting in five possible scenarios with distinct spectral behaviours. Within each scenario, the spectrum at a given time is one of five possible types, which are approximately described by broken power-laws; we provide the spectral and temporal indices of each power-law segment, and the time evolution of the break frequencies. If high-cadence multi-wavelength observations can determine the relevant breakout scenario in future events, strong constraints can be placed on the physical conditions at the site of shock breakout.

astro-ph.HE↗

arXiv2024

Revisiting GRB 060218: new insights into low-luminosity gamma-ray bursts from a revised shock breakout model

Despite two decades since the discovery of low-luminosity gamma-ray bursts, their origin remains poorly understood. In events such as GRB 060218, shock breakout from a progenitor with an extended ($10^{13}$ - $10^{14}$ cm), low-mass (0.01 - 0.1 M$_\odot$) envelope provides one possible interpretation for the smooth prompt X-ray emission lasting $\sim 1000$ s and the early optical peak at $\sim 0.5$ d. However, current shock breakout models have difficulties explaining the unexpectedly strong optical emission at $\sim 100$ s, the simultaneous presence of blackbody and power-law components in the X-ray spectrum, and the rapid evolution of the peak energy. We suggest that these peculiar features can be explained by a recently realized shock breakout scenario, in which the gas and the radiation are initially out of thermal equilibrium, but they achieve equilibrium on a time-scale faster than the light-crossing time of the envelope. In this non-standard case, due to the effects of light travel time, the observed X-ray spectrum is a multi-temperature blend of blackbody and free-free components. The free-free emission is spectrally broad, peaking in hard X-rays while also enhancing the early optical signal. As the system thermalizes, the free-free component quickly evolves toward lower energies, reproducing the observed rapid peak energy decay. To match observations, we find that more than $10^{50}$ erg must be deposited in the envelope, which may be accomplished by a choked jet. These results strengthen the case for a shock breakout origin of $ll$GRBs, and provide further evidence connecting $ll$GRBs to peculiar progenitors with extended low-mass envelopes.

astro-ph.HE↗

arXiv2024

An unexplored regime of shock breakout with a distinct spectral signature

The first light that escapes from a supernova explosion is the shock breakout emission, which produces a bright flash of UV or X-ray radiation. Standard theory predicts that the shock breakout spectrum will be a blackbody if the gas and radiation are in thermal equilibrium, or a Comptonized free-free spectrum if not. Using recent results for the post-breakout evolution which suggest that lower-temperature ejecta are probed earlier than previously thought, we show that another scenario is possible in which the gas and radiation are initially out of equilibrium, but the time when thermalized ejecta are revealed is short compared to the light-crossing time of the system. In this case, the observed spectrum differs significantly from the standard expectation, as the non-negligible light travel time acts to smear the spectrum into a complex multi-temperature blend of blackbody and free-free components. For typical parameters, a bright multi-wavelength transient is produced, with the free-free emission being spread over a wide frequency range from optical to hard X-rays, and the blackbody component peaking in soft X-rays. We explore the necessary conditions to obtain this type of unusual spectrum, finding that it may be relevant for bare blue supergiant progenitors, or for shocks with a velocity of $v_{\rm bo} \sim 0.1c$ breaking out from an extended medium of radius $R_{\rm env}$ with a sufficiently high density $ρ_{\rm bo} \gtrsim 4\times 10^{-12}\text{ g cm}^{-3} (R_{\rm env}/10^{14} \text{cm})^{-15/16}$. An application to low-luminosity gamma-ray bursts is considered in a companion paper.

astro-ph.HE↗

arXiv2024

Thorium in Kilonova Spectra: Exploring the Heaviest Detectable Element

Kilonova spectra provide us with the direct information of r-process nucleosynthesis in neutron star mergers. In this paper, we study the signatures of elements beyond the third r-process peak expected to be produced in neutron-rich ejecta in the photospheric spectra of kilonova. Ra II, Ac III, and Th III are our candidates because they have a small number of valence electrons and low-lying energy levels, which tend to result in strong absorption features. We systematically calculate the strength of bound-bound transitions of these candidates by constructing the line list based on the available atomic database. We find that Th III is the most promising species showing strong transitions at the NIR wavelengths. By performing radiative transfer simulations, we find that Th III produces broad absorption features at ~18000 A in the spectra when the mass ratio of actinides to lanthanides is larger than the solar r-process ratio and the mass fraction of lanthanides is $\lesssim 6\times10^{-4}$. Our models demonstrate that the Th feature may be detectable if the bulk of the ejecta in the line-forming region is dominated by relatively light r-process elements with the mixture of a small fraction of very neutron-rich material. Such conditions may be realized in the mergers of unequal-mass neutron stars or black hole-neutron star binaries. To detect the Th absorption features, the observations from the space (such as JWST) or high-altitude sites are important as the wavelength region of the Th features is overlapped with that affected by the strong telluric absorption.

astro-ph.HE↗

arXiv2024

Radiation hydrodynamical simulations of super-Eddington mass transfer and black hole growth in close binaries

Radiation-driven outflows play a crucial role in extracting mass and angular momentum from binary systems undergoing rapid mass transfer at super-Eddington rates. To study the mass transfer process from a massive donor star to a stellar-mass black hole (BH), we perform multi-dimensional radiation-hydrodynamical simulations that follow accretion flows from the first Lagrange point down to about a hundred times the Schwarzschild radius of the accreting BH. Our simulations reveal that rapid mass transfer occurring at over a thousand times the Eddington rate leads to significant mass loss from the accretion disk via radiation-driven outflows. Consequently, the inflow rates at the innermost radius are regulated by two orders of magnitude smaller than the transfer rates. We find that convective motions within the accretion disk drive outward energy and momentum transport, enhancing the radiation pressure in the outskirts of the disk and ultimately generating large-scale outflows with sufficient energy to leave the binary. Furthermore, we observe strong anisotropy in the outflows, which occur preferentially toward both the closest and furthest points from the donor star. However, when averaged over all directions, the specific angular momentum of the outflows is nearly comparable to the value predicted in the isotropic emission case. Based on our simulation results, we propose a formula that quantifies the mass growth rates on BHs and the mass loss rates from binaries due to radiation-driven outflows. This formula provides important implications for the binary evolution and the formation of merging binary BHs.

astro-ph.HE↗

arXiv2024

On the Formation of the W-shaped O II Lines in Spectra of Type I Superluminous Supernovae

H-poor superluminous supernovae (SLSNe-I) are characterized by O II lines around 4,000 - 4,500 A in pre-/near-maximum spectra, so-called W-shaped O II lines. As these lines are from relatively high excitation levels, they have been considered a sign of non-thermal processes, which may give a hint of power sources of SLSNe-I. However, the conditions for these lines to appear have not been understood well. In this work, we systematically calculate synthetic spectra to reproduce observed spectra of eight SLSNe-I, parameterizing departure coefficients from the nebular approximation in the SN ejecta (expressed as b_neb). We find that most of the observed spectra can be reproduced well with b_neb ~< 10, which means that no strong departure is necessary for the formation of the W-shaped O II lines. We also show that the appearance of the W-shaped O II lines is sensitive to temperature; only spectra with temperatures T ~ 14,000 - 16,000 K can produce the W-shaped O II lines without large departures. Based on this, we constrain the non-thermal ionization rate near the photosphere. Our results suggest that spectral features of SLSNe-I can give independent constraints on the power source through the non-thermal ionization rates.

astro-ph.HE↗

arXiv2024

Birefringence tests of gravity with multi-messenger binaries

Extensions to General Relativity (GR) allow the polarization of gravitational waves (GW) from astrophysical sources to suffer from amplitude and velocity birefringence, which respectively induce changes in the ellipticity and orientation of the polarization tensor. We introduce a multi-messenger approach to test this polarization behavior of GWs during their cosmological propagation using binary sources, for which the initial polarization is determined by the inclination and orientation angles of the orbital angular momentum vector with respect to the line of sight. In particular, we use spatially-resolved radio imaging of the jet from a binary neutron star (BNS) merger to constrain the orientation angle and hence the emitted polarization orientation of the GW signal at the site of the merger, and compare to that observed on Earth by GW detectors. For GW170817 we constrain the deviation from GR due to amplitude birefringence to $κ_A = -0.12^{+0.60}_{-0.61}$, while the velocity birefringence parameter $κ_V$ remains unconstrained. The inability to constrain $κ_V$ is due to the fact that Virgo did not detect GW170817, and measurements of the polarization orientation require information from a combination of multiple detectors with different alignments. For this reason, we also mock future BNS mergers with resolved afterglow proper motion and project that $κ_V$ could be constrained to a precision of $5\,$rad (corresponding to an angular shift of the GW polarization of $δϕ_V\approx 0.2\,$rad for a BNS at $100\,$Mpc) by a future network of third-generation ground-based GW detectors such as Cosmic Explorer and the radio High Sensitivity Array. Crucially, this velocity birefringence effect cannot be constrained with dark binary mergers as it requires polarization information at the emission time, which can be provided only by electromagnetic emission.

gr-qc↗

arXiv2024

Science with the Daksha High Energy Transients Mission

We present the science case for the proposed Daksha high energy transients mission. Daksha will comprise of two satellites covering the entire sky from 1~keV to $>1$~MeV. The primary objectives of the mission are to discover and characterize electromagnetic counterparts to gravitational wave source; and to study Gamma Ray Bursts (GRBs). Daksha is a versatile all-sky monitor that can address a wide variety of science cases. With its broadband spectral response, high sensitivity, and continuous all-sky coverage, it will discover fainter and rarer sources than any other existing or proposed mission. Daksha can make key strides in GRB research with polarization studies, prompt soft spectroscopy, and fine time-resolved spectral studies. Daksha will provide continuous monitoring of X-ray pulsars. It will detect magnetar outbursts and high energy counterparts to Fast Radio Bursts. Using Earth occultation to measure source fluxes, the two satellites together will obtain daily flux measurements of bright hard X-ray sources including active galactic nuclei, X-ray binaries, and slow transients like Novae. Correlation studies between the two satellites can be used to probe primordial black holes through lensing. Daksha will have a set of detectors continuously pointing towards the Sun, providing excellent hard X-ray monitoring data. Closer to home, the high sensitivity and time resolution of Daksha can be leveraged for the characterization of Terrestrial Gamma-ray Flashes.

astro-ph.HE↗

arXiv2024

Joint gravitational wave-short GRB detection of Binary Neutron Star mergers with existing and future facilities

We explore the joint detection prospects of short gamma-ray bursts (sGRBs) and their gravitational wave (GW) counterparts by the current and upcoming high-energy GRB and GW facilities from binary neutron star (BNS) mergers. We consider two GW detector networks: (1) A four-detector network comprising LIGO Hanford, Livingston, Virgo, and Kagra, (IGWN4) and (2) a future five-detector network including the same four detectors and LIGO India (IGWN5). For the sGRB detection, we consider existing satellites Fermi and Swift and the proposed all-sky satellite Daksha. Most of the events for the joint detection will be off-axis, hence, we consider a broad range of sGRB jet models predicting the off-axis emission. Also, to test the effect of the assumed sGRB luminosity function, we consider two different functions for one of the emission models. We find that for the different jet models, the joint sGRB and GW detection rates for Fermi and Swift with IGWN4 (IGWN5) lie within 0.07-0.62$\mathrm{\ yr^{-1}}$ (0.8-4.0$\mathrm{\ yr^{-1}}$) and 0.02-0.14$\mathrm{\ yr^{-1}}$ (0.15-1.0$\mathrm{\ yr^{-1}}$), respectively, when the BNS merger rate is taken to be 320$\mathrm{\ Gpc^{-3}~yr^{-1}}$. With Daksha, the rates increase to 0.2-1.3$\mathrm{\ yr^{-1}}$ (1.3-8.3$\mathrm{\ yr^{-1}}$), which is 2-9 times higher than the existing satellites. We show that such a mission with higher sensitivity will be ideal for detecting a higher number of fainter events observed off-axis or at a larger distance. Thus, Daksha will boost the joint detections of sGRB and GW, especially for the off-axis events. Finally, we find that our detection rates with optimal SNRs are conservative, and noise in GW detectors can increase the rates further.

astro-ph.HE↗

arXiv2023

The High Energy X-ray Probe (HEX-P): Sensitive broadband X-ray observations of transient phenomena in the 2030s

HEX-P will launch at a time when the sky is being routinely scanned for transient gravitational wave, electromagnetic and neutrino phenomena that will require the capabilities of a sensitive, broadband X-ray telescope for follow up studies. These include the merger of compact objects such as neutron stars and black holes, stellar explosions, and the birth of new compact objects. \hexp\ will probe the accretion and ejecta from these transient phenomena through the study of relativistic outflows and reprocessed emission, provide unique capabilities for understanding jet physics, and potentially revealing the nature of the central engine.

astro-ph.HE↗

arXiv2023

On the Testability of the Quark-Hadron Transition Using Gravitational Waves From Merging Binary Neutron Stars

Elementary particles such as quarks and gluons are expected to be fundamental degrees of freedom at ultra high temperatures or densities, while natural phenomena in our daily lives are described in terms of hadronic degrees of freedom. Massive neutron stars and remnants of binary neutron star mergers may contain quark matter, but it is not known how the transition from hadron matter to quark matter occurs. Different transition scenarios predict different gravitational waveforms emitted from binary neutron star mergers. If the difference between the equations of state occurs at sufficiently high density, it is expected that the difference between waveforms mainly appears in the merger or the post-merger phase rather than in the inspiral phase. The typical frequency of gravitational waves after the coalescence is higher than 2 kHz, which is difficult to observe using current detectors. In this study, we performed Bayesian model selection for two representative scenarios and investigated whether observations with future detectors will allow us to identify the correct model. We assume that the relatively low density equation of state around the nuclear saturation density is completely known from accumulated observations. Under this assumption, we find that it is reasonable to expect to be able to identify the correct transition scenario with third-generation detectors or specialized detectors with high sensitivity at high frequencies designed for post-merger signal observation, e.g., NEMO.

astro-ph.HE↗