Searcharxiv⌕ Search

arXiv subjects

Kenta Hotokezaka

Publications and source records attributed to Kenta Hotokezaka.

At least 109 records · Page 6Linked to original sources
arXiv2016

Are Ultra-Long Gamma-Ray Bursts Caused by Blue Supergiant Collapsars, Newborn Magnetars, or White Dwarf Tidal Disruption Events?

Ultra-long gamma-ray bursts (ulGRBs) are a new population of GRBs with extreme durations of $\sim 10^{4}$ s. Leading candidates for their origin are blue supergiant collapsars, magnetars, and white dwarf tidal disruption events (WD-TDEs) caused by massive black holes (BHs). Recent observations of supernova-like (SN-like) bumps associated with ulGRBs challenged both the WD-TDE and the blue supergiant models because of the detection of SNe and the absence of hydrogen lines, respectively. We propose that WD-TDEs can accommodate the observed SN-like bumps if the fallback WD matter releases energy into the unbound WD ejecta. The observed ejecta energy, luminosity, and velocity are explained by the gravitational energy, Eddington luminosity, and escape velocity of the formed accretion disk, respectively. We also show that the observed X-rays can ionize the ejecta, eliminating lines. The SN-like light curves (SN 2011kl) for the ulGRB 111209A are consistent with all three models, although a magnetar model is unnatural because the spin-down time required to power the SN-like bump is a hundred times longer than the GRB. Our results imply that TDEs are a possible energy source for SN-like events in general and for ulGRBs in particular.

astro-ph.HE↗
arXiv2016

r-process Production Sites as inferred from Eu Abundances in Dwarf Galaxies

Recent observations of $r$-process material in ultra-faint dwarf galaxies (UFDs) shed light on the sources of these elements. Strong upper limits on the Eu mass in some UFDs combined with detections of much larger masses in a UFD, Reticulum II, and other dwarf galaxies imply that Eu production is dominated by rare events, and that the minimal Eu mass observed in any UFD is approximately the amount of Eu mass produced per event. This is consistent with other independent observations in the Galaxy. We estimate, using a model independent likelihood analysis, the rate and Eu (Fe) mass produced per $r$-process (Fe production) event in dwarf galaxies including classical dwarfs and UFDs. The mass and rate of the Fe production events are consistent with the normal core-collapse supernova~(ccSN) scenario. The Eu mass per event is $3\times 10^{-5}M_{\odot}<\tilde{m}_{\rm Eu}<2\times 10^{-4}M_{\odot}$, corresponding to a total $r$-process mass per event of $6\times 10^{-3}M_{\odot}<\tilde{m}_{r-process}<4\times 10^{-2}M_{\odot}$. The rate of $r$-process events is $2.5\times 10^{-4}<R_{rp/SN}<1.4\times 10^{-3}$ as compared with the ccSNe rate. These values are consistent with the total Eu mass observed in our own Galaxy, suggesting that the same mechanism is behind the production of $r$-process events in both dwarf galaxies and the Milky Way, and that it may be the dominant mechanism for production of $r$-process elements in the Universe. The results are consistent with neutron star mergers estimates, but cannot rule out other rare core collapse scenarios, provided that they produce a significant amounts of $r$-process material per event.

astro-ph.HE↗
arXiv2016

Radio Counterparts of Compact Binary Mergers detectable in Gravitational Waves: A Simulation for an Optimized Survey

Mergers of binary neutron stars and black hole-neutron star binaries produce gravitational-wave (GW) emission and outflows with significant kinetic energies. These outflows result in radio emissions through synchrotron radiation. We explore the detectability of these synchrotron generated radio signals by follow-up observations of GW merger events lacking a detection of electromagnetic counterparts in other wavelengths. We model radio light curves arising from (i) sub-relativistic merger ejecta and (ii) ultra-relativistic jets. The former produces radio remnants on timescales of a few years and the latter produces $γ$-ray bursts in the direction of the jet and orphan-radio afterglows extending over wider angles on timescales of weeks. Based on the derived light curves, we suggest an optimized survey at $1.4$ GHz with five epochs separated by a logarithmic time interval. We estimate the detectability of the radio counterparts of simulated GW-merger events to be detected by advanced LIGO and Virgo by current and future radio facilities. The detectable distances for these GW merger events could be as high as 1 Gpc. $20$--$60\%$ of the long-lasting radio remnants will be detectable in the case of the moderate kinetic energy of $3\cdot 10^{50}$ erg and a circum-merger density of $0.1 {\rm cm^{-3}}$ or larger, while $5$--$20\%$ of the orphan radio afterglows with kinetic energy of $10^{48}$ erg will be detectable. The detection likelihood increases if one focuses on the well-localizable GW events. We discuss the background noise due to radio fluxes of host galaxies and false positives arising from extragalactic radio transients and variable Active Galactic Nuclei and we show that the quiet radio transient sky is of great advantage when searching for the radio counterparts.

astro-ph.HE↗
arXiv2016

Natal Kicks and Time Delays in Merging Neutron Star Binaries - Implications for r-process nucleosynthesis in Ultra Faint Dwarfs and in the Milky Way

Merging neutron star binaries are prime candidate sources for heavy r-process nucleosynthesis. The amount of heavy r-process material is consistent with the mass ejection and rates of mergers, and abundances of relic radioactive materials suggest that heavy r-process material is produced in rare events. Observations of possible macronovae provide further support for this model. Still, some concerns remain. One is the observation of heavy r-process elements in Ultra Faint Dwarf (UFD) galaxies. The escape velocities from UFDs are so small that the natal kicks, taking place at neutron stars birth, might eject such binaries from UFDs. Furthermore the old stellar populations of UFDs requires that r-process nucleosynthesis must have taken place very early on, while it may take several Gyr for compact binaries to merge. This last problem arises also within the Milky Way where heavy r-process materials has been observed in some low metallicity stars. We show here that since a significant fraction of neutron star binaries form with a very small proper motion those won't be ejected even from a UFD. Furthermore, a significant fraction of these binaries merge within a few hundred Myr and in some cases the merger time is even shorter than a hundred Myr. This population of "rapid mergers" explains the appearance of heavy r-process material in both UFDs and in the Milky Way.

astro-ph.HE↗
arXiv2016

The Macronova in GRB 050709 and the GRB/macronova connection

GRB 050709 was the first short Gamma-ray Burst (sGRB) with an identified optical afterglow. In this work we report a re-analysis of the publicly available data of this event and the discovery of a Li-Paczynski macronova/kilonova that dominates the optical/IR signal at t $>$ 2.5 days. Such a signal would arise from ~0.05 M$_\odot$ r-process material launched by a compact binary merger. The implied mass ejection supports the suggestion that compact binary mergers are significant and possibly main sites of heavy r-process nucleosynthesis. We have re-analyzed all afterglow data from nearby short and hybrid GRBs. A statistical study of sGRB/macronova connection reveals that macronova may have taken place in all these GRBs though the fraction as low as 0.18 cannot be ruled out. The identification of two of the three macronova candidates in the I-band implies a more promising detection prospect for ground-based surveys.

astro-ph.HE↗
arXiv2016

Effects of neutron-star dynamic tides on gravitational waveforms within the effective-one-body approach

Extracting the unique information on ultradense nuclear matter from the gravitational waves emitted by merging, neutron-star binaries requires robust theoretical models of the signal. We develop a novel effective-one-body waveform model that includes, for the first time, dynamic (instead of only adiabatic) tides of the neutron star, as well as the merger signal for neutron-star--black-hole binaries. We demonstrate the importance of the dynamic tides by comparing our model against new numerical-relativity simulations of nonspinning neutron-star--black-hole binaries spanning more than 24 gravitational-wave cycles, and to other existing numerical simulations for double neutron-star systems. Furthermore, we derive an effective description that makes explicit the dependence of matter effects on two key parameters: tidal deformability and fundamental oscillation frequency.

gr-qc↗
arXiv2016

Measurability of the tidal deformability by gravitational waves from coalescing binary neutron stars

Combining new gravitational waveforms derived by long-term (14--16 orbits) numerical-relativity simulations with waveforms by an effective-one-body (EOB) formalism for coalescing binary neutron stars, we construct hybrid waveforms and estimate the measurability for the dimensionless tidal deformability of the neutron stars, $Λ$, by advanced gravitational-wave detectors. We focus on the equal-mass case with the total mass $2.7M_\odot$. We find that for an event at a hypothetical effective distance of $D_{\rm eff}=200$ Mpc, the distinguishable difference in the dimensionless tidal deformability will be $\approx 100$, 400, and 800 at 1-$σ$, 2-$σ$, and 3-$σ$ levels, respectively, for advanced LIGO. If the true equation of state is stiff and the typical neutron-star radius is $R \gtrsim 13 $ km, our analysis suggests that the radius will be constrained within $\approx 1$ km at 2-$σ$ level for an event at $D_{\rm eff}=200$ Mpc. On the other hand, if the true equation of state is soft and the typical neutron-star radius is $R\lesssim 12$ km , it will be difficult to narrow down the equation of state among many soft ones, although it is still possible to discriminate the true one from stiff equations of state with $R\gtrsim 13$ km. We also find that gravitational waves from binary neutron stars will be distinguished from those from spinless binary black holes at more than 2-$σ$ level for an event at $D_{\rm eff}=200$ Mpc. The validity of the EOB formalism, Taylor-T4, and Taylor-F2 approximants as the inspiral waveform model is also examined.

gr-qc↗
arXiv2016

Testing the Magnetar Model via Late Time Radio Observations of Two Macronova Candidates

Compact binary mergers may have already been observed as they are the leading model for short gamma-ray bursts (sGRBs). Radioactive decay within the ejecta from these mergers is expected to produce an infra-red flare, dubbed macronova (or kilonova), on a time scale of a week. Recently two such macronova candidates were identified in followup observations of sGRBs, strengthening the possibility that those indeed arise from mergers. The same ejecta will also produce a long term (months to years) radio emission due to its interaction with the surrounding ISM. In search for this emission, we observed the two macronova candidates, GRB 130603B and GRB 060614 with the Jansky very large array (VLA) and the Australia Telescope Compact Array (ATCA). Our observations resulted in null-detections, putting strong upper limits on the kinetic energy and mass of the ejecta. A possible outcome of a merger is a highly magnetized neutron star (a magnetar), which has been suggested as the central engine for GRBs. Such a magnetar will deposit a significant fraction of its energy into the ejecta leading to a brighter radio flare. Our results, therefore, rule out magnetars in these two events.

astro-ph.HE↗
arXiv2015

Radioactive decay products in neutron star merger ejecta: heating efficiency and $γ$-ray emission

The radioactive decay of the freshly synthesized $r$-process nuclei ejected in compact binary mergers power optical/infrared macronovae (kilonovae) that follow these events. The light curves depend critically on the energy partition among the different products of the radioactive decay and this plays an important role in estimates of the amount of ejected $r$-process elements from a given observed signal. We study the energy partition and $γ$-ray emission of the radioactive decay. We show that $20$-$50\%$ of the total radioactive energy is released in $γ$-rays on timescales from hours to a month. The number of emitted $γ$-rays per unit energy interval has roughly a flat spectrum between a few dozen keV and $1$ MeV so that most of this energy is carried by $\sim 1$ MeV $γ$-rays. However at the peak of macronova emission the optical depth of the $γ$-rays is $\sim 0.02$ and most of the $γ$-rays escape. The loss of these $γ$-rays reduces the heat deposition into the ejecta and hence reduces the expected macronova signals if those are lanthanides dominated. This implies that the ejected mass is larger by a factor of $2$-$3$ than what was previously estimated. Spontaneous fission heats up the ejecta and the heating rate can increase if a sufficient amount of transuranic nuclei are synthesized. Direct measurements of these escaping $γ$-rays may provide the ultimate proof for the macronova mechanisms and an identification of the $r$-process nucleosynthesis sites. However, the chances to detect these signals are slim with current X-ray and $γ$-ray missions. New detectors, more sensitive by at least a factor of ten, are needed for a realistic detection rate.

astro-ph.HE↗
arXiv2015

Short-lived $^{244}$Pu Points to Compact Binary Mergers as Sites for Heavy r-process Nucleosynthesis

Measurements of the radioactive $^{244}$Pu abundances can break the degeneracy between high-rate/low-yield and low-rate/high-yield scenarios for the production of heavy $r$-process elements. The first corresponds to production by core collapse supernovae (cc-SNe) while the latter corresponds to production by e.g. compact binary mergers. The estimated $^{244}$Pu abundance in the current interstellar medium inferred from deep-sea measurements (Wallner et al. 2015) is significantly lower than that corresponding Early Solar System abundances (Turner et al 2007). We estimate the expected median value of the $^{244}$Pu abundances and fluctuations around this value in both models. We show that while the current and Early Solar System abundances are naturally explained within the low-rate/high-yield (e.g. merger) scenario, they are incompatible with the high-rate/low-yield (cc-SNe) model. The inferred event rate remarkably agrees with compact binary merger rates estimated from Galactic neutron star binaries and from short gamma-ray bursts. Furthermore, the ejected mass of $r$-process elements per event agrees with both theoretical and observational macronova/kilonova estimates.

astro-ph.HE↗
arXiv2015

A possible Macronova in the late afterglow of the `long-short' burst GRB 060614

Long-duration ($>2$ s) $γ$-ray bursts that are believed to originate from the death of massive stars are expected to be accompanied by supernovae. GRB 060614, that lasted 102 s, lacks a supernova-like emission down to very stringent limits and its physical origin is still debated. Here we report the discovery of near-infrared bump that is significantly above the regular decaying afterglow. This red bump is inconsistent with even the weakest known supernova. However, it can arise from a Li-Paczyński macronova $-$ the radioactive decay of debris following a compact binary merger. If this interpretation is correct GRB 060614 arose from a compact binary merger rather than from the death of a massive star and it was a site of a significant production of heavy r-process elements. The significant ejected mass favors a black hole-neutron star merger but a double neutron star merger cannot be ruled out.

astro-ph.HE↗
arXiv2015

Exploring tidal effects of coalescing binary neutron stars in numerical relativity II: Longterm simulations

We perform new longterm (15-16 orbits) simulations of coalescing binary neutron stars in numerical relativity using an updated Einstein's equation solver, employing low-eccentricity initial data, and modeling the neutron stars by a piecewise polytropic equation of state. A convergence study shows that our new results converge more rapidly than the third order and using the determined convergence order, we construct an extrapolated waveform for which the estimated total phase error should be less than 1 radian. We then compare the extrapolated waveforms with those calculated by the latest effective-one-body (EOB) formalism in which the so-called tidal deformability, higher post-Newtonian corrections, and gravitational self-force effects are taken into account. We show that for a binary of compact neutron stars with their radius 11.1 km, the waveform by the EOB formalism agrees quite well with the numerical waveform so that the total phase error is smaller than 1 radian for the total phase of $\sim 200$ radian up to the merger. By contrast, for a binary of less compact neutron stars with their radius 13.6 km, the EOB and numerical waveforms disagree with each other in the last few wave cycles, resulting in the total phase error of $\sim 3$ radian.

gr-qc↗
arXiv2015

Mass ejection from neutron star mergers: different components and expected radio signals

In addition to producing a strong gravitational signal, a short gamma-ray burst (GRB), and a compact remnant, neutron star mergers eject significant masses at significant kinetic energies. This mass ejection takes place via dynamical mass ejection and a GRB jet but other processes have also been suggested: a shock-breakout material, a cocoon resulting from the interaction of the jet with other ejecta, and viscous and neutrino driven winds from the central remnant or the accretion disk. The different components of the ejected masses include up to a few percent of a solar mass, some of which is ejected at relativistic velocities. The interaction of these ejecta with the surrounding interstellar medium will produce a long lasting radio flare, in a similar way to GRB afterglows or to radio supernovae. The relative strength of the different signals depends strongly on the viewing angle. An observer along the jet axis or close to it will detect a strong signal at a few dozen days from the radio afterglow (or the orphan radio afterglow) produced by the highly relativistic GRB jet. For a generic observer at larger viewing angles, the dynamical ejecta, whose contribution peaks a year or so after the event, will generally dominate. Depending on the observed frequency and the external density, other components may also give rise to a significant contribution. We also compare these estimates with the radio signature of the short GRB 130603B. The radio flare from the dynamical ejecta might be detectable with the EVLA and the LOFAR for the higher range of external densities $n\gtrsim 0.5{\rm cm^{-3}}$.

astro-ph.HE↗
arXiv2014

Possible Indirect Confirmation of the Existence of Pop III Massive Stars by Gravitational Wave

We perform population synthesis simulations for Population III (Pop III) coalescing binary neutron stars (NS-NSs), neutron star - black hole binaries (NS-BHs), and binary black holes (BH-BHs) which merge within the age of the universe. We found that the typical mass of Pop III BH-BHs is $\sim 30\ \rm{M}_{\odot}$ so that the inspiral chirp signal of gravitational waves can be detected up to z=0.28 by KAGRA, Adv. LIGO, Adv. Virgo and GEO network. Our simulations suggest that the detection rate of the coalescing Pop III BH-BHs is $140 (68) \cdot ({\rm SFR}_{\rm p}/10^{-2.5} \rm{M}_{\odot} {\rm yr}^{-1} {\rm Mpc}^{-3}) \cdot {\rm Err}_{\rm sys} ~{\rm events} ~{\rm yr}^{-1}$ for the flat (Salpeter) initial mass function (IMF), respectively, where $\rm SFR_p$ and $\rm Err_{sys}$ are the peak value of the Pop III star formation rate and the possible systematic errors due to the assumptions in Pop III population synthesis, respectively. $\rm Err_{sys}=1$ correspond to conventional parameters for Pop I stars. From the observation of the chirp signal of the coalescing Pop III BH-BHs, we can determine both the mass and the redshift of the binary for the cosmological parameters determined by Planck satellite. Our simulations suggest that the cumulative redshift distribution of the coalescing Pop III BH-BHs depends almost only on the cosmological parameters. We might be able to confirm the existence of Pop III massive stars of mass $\sim 30~\rm M_{\odot}$ by the detections of gravitational waves if the merger rate of the Pop III massive BH-BHs dominates that of Pop I BH-BHs.

astro-ph.HE↗
arXiv2014

Jet collimation in the ejecta of double neutron star merger: new canonical picture of short gamma-ray bursts

The observations of jet breaks in the afterglows of short gamma-ray bursts (SGRBs) indicate that the jet has a small opening angle of < 10°. The collimation mechanism of the jet is a longstanding theoretical problem. We numerically analyze the jet propagation in the material ejected by double neutron star merger, and demonstrate that if the ejecta mass is > 10^{-2} M_{sun}, the jet is well confined by the cocoon and emerges from the ejecta with the required collimation angle. Our results also suggest that there are some populations of choked (failed) SGRBs or low-luminous new types of event. By constructing a model for SGRB 130603B, which is associated with the first kilonova/macronova can- didate, we infer that the equation-of-state of neutron stars would be soft enough to provide sufficient ejecta to collimate the jet, if this event was associated with a double neutron star merger.

astro-ph.HE↗
arXiv2013

Radioactively Powered Emission from Black Hole-Neutron Star Mergers

Detection of electromagnetic counterparts of gravitational wave (GW) sources is important to unveil the nature of compact binary coalescences. We perform three-dimensional, time-dependent, multi-frequency radiative transfer simulations for radioactively powered emission from the ejecta of black hole (BH) - neutron star (NS) mergers. Depending on the BH to NS mass ratio, spin of the BH, and equations of state of dense matter, BH-NS mergers can eject more material than NS-NS mergers. In such cases, radioactively powered emission from the BH-NS merger ejecta can be more luminous than that from NS-NS mergers. We show that, in spite of the expected larger distances to BH-NS merger events, observed brightness of BH-NS mergers can be comparable to or even higher than that of NS-NS mergers. We find that, when the tidally disrupted BH-NS merger ejecta are confined to a small solid angle, the emission from BH-NS merger ejecta tends to be bluer than that from NS-NS merger ejecta for a given total luminosity. Thanks to this property, we might be able to distinguish BH-NS merger events from NS-NS merger events by multi-band observations of the radioactively powered emission. In addition to the GW observations, such electromagnetic observations can potentially provide independent information on the nature of compact binary coalescences.

astro-ph.HE↗
arXiv2013

Progenitor models of the electromagnetic transient associated with the short GRB 130603B

An electromagnetic transient powered by the radioactive decay of r-process elements, a so-called kilonova/macronova, is one of the possible observable consequences of compact binary mergers including at east one neutron star. Recent observations strongly suggest the first discovery of the electromagnetic transient, which is associated with the short GRB 130603B. We explore a possible progenitor of this event combining the numerical-relativity simulations and radiative transfer simulations of the dynamical ejecta of binary neutron star and black hole - neutron star mergers. We show that the ejecta models within a realistic parameter range consistently reproduce the observed near-infrared excess. We also show that the soft equation of state models for binary neutron star mergers and the stiff equation of state models for black hole - neutron star mergers are favored to reproduce the observed luminosity.

astro-ph.HE↗
arXiv2013

Remnant massive neutron stars of binary neutron star mergers: Evolution process and gravitational waveform

Massive (hypermassive and supramassive) neutron stars are likely to be often formed after the merger of binary neutron stars. We explore the evolution process of the remnant massive neutron stars and gravitational waves emitted by them, based on numerical-relativity simulations for binary neutron star mergers employing a variety of equations of state and choosing a plausible range of the neutron-star mass of binaries. We show that the lifetime of remnant hypermassive neutron stars depends strongly on the total binary mass and also on the equations of state. Gravitational waves emitted by the remnant massive neutron stars universally have a quasiperiodic nature of an approximately constant frequency although the frequency varies with time. We also show that the frequency and time-variation feature of gravitational waves depend strongly on the equations of state. We derive a fitting formula for the quasiperiodic gravitational waveforms, which may be used for the data analysis of a gravitational-wave signal.

astro-ph.HE↗