arXiv subjects

Shigehiro Nagataki

Publications and source records attributed to Shigehiro Nagataki.

At least 37 records · Page 2Linked to original sources

arXiv2023

Constraints on the Neutron-Star Structure from the Clocked X-Ray Burster 1RXS J180408.9$-$342058

Type-I X-ray bursts are rapid-brightening transient phenomena on the surfaces of accreting neutron stars (NSs). Some X-ray bursts, called {\it clocked bursters}, exhibit regular behavior with similar light curve profiles in their burst sequences. The periodic nature of clocked bursters has the advantage of constraining X-ray binary parameters and physics inside the NS. In the present study, we compute numerical models, based on different equations of state and NS masses, which are compared with the observation of a recently identified clocked burster, 1RXS J180408.9$-$342058. We find that the relation between accretion rate and recurrence time is highly sensitive to the NS mass and radius. We determine, in particular, that 1RXS J180408.9$-$342058 appears to possess a mass less than $1.7M_{\odot}$ and favors a stiffer nuclear equation of state (with an NS radius $\gtrsim12.7{\rm km}$). Consequently, the observations of this new clocked burster may provide additional constraints for probing the structure of NSs.

astro-ph.HE↗

arXiv2023

Potential of the Cherenkov Telescope Array for studying the young nearby supernova remnants

Modern imaging atmospheric Cherenkov telescopes have extensively observed young nearby supernova remnants (SNRs), with ages of about 1000 years or less, in the very-high-energy (VHE) gamma-ray band. These efforts resulted in the detection of VHE emission from three young SNRs - Cassiopeia A, Tycho, and SN 1006 - and provided significant evidence for emission from the more distant Kepler's SNR. However, many questions on the production of VHE gamma rays in these remnants remain unanswered. Using detailed physical models for Tycho's SNR based on the CR-hydro-NEI code and physically motivated models for the other young nearby remnants, we simulated observations with the Cherenkov Telescope Array (CTA) of these gamma-ray sources. We highlight properties of these remnants accessible for investigation with future CTA observations and discuss which questions are expected to be answered.

astro-ph.HE↗

arXiv2023

Numerical simulation of photospheric emission in long gamma-ray bursts: prompt correlations, spectral shapes, and polarizations

We explore the properties of photospheric emission in the context of long gamma-ray bursts (LGRBs) using three numerical models that combine relativistic hydrodynamical simulations and Monte Carlo radiation transfer calculations in three dimensions. Our simulations confirm that the photospheric emission gives rise to correlations between the spectral peak energy and luminosity that agree with the observed Yonetoku, Amati, and Golenetskii correlations. It is also shown that the spectral peak energy and luminosity correlate with the bulk Lorentz factor, as indicated in the literature. On the other hand, synthetic spectral shapes tend to be narrower than those of the observations. The result indicates that an additional physical process that can provide non-thermal broadening is needed to reproduce the spectral features. Furthermore, the polarization analysis finds that, while the degree of polarization is low for the emission from the jet core ($Π< 4~\%$), it tends to increase with the viewing angle outside the core and can be as high as $Π\sim 20-40~\%$ in an extreme case. This suggests that the typical GRBs show systematically low polarization compared to softer, dimmer counterparts (X-ray-rich GRBs and X-ray flashes). Interestingly, our simulations indicate that photospheric emission exhibits large temporal variation in the polarization position angle ($Δψ\sim 90^{\circ}$), which may be compatible with those inferred in observations. A notable energy dependence of the polarization property is another characteristic feature found in the current study. Particularly, the difference in the position angle among different energy bands can be as large as $\sim 90^{\circ}$.

astro-ph.HE↗

arXiv2023

Examining Neutrino-Matter Interactions in the Cassiopeia A Supernova

Neutrino interactions with stellar material are widely believed to be fundamental to the explosion of massive stars. However, this important process has remained difficult to confirm observationally. We propose a new method to verify it using X-ray observations of the supernova remnant Cassiopeia A. The elemental composition in its Fe-rich ejecta that could have been produced at the innermost region of the supernova, where neutrinos are expected to interact, allows us to examine the presence of neutrino interactions. Here we demonstrate that the amount of Mn produced without neutrino nucleosynthesis processes (i.e., the $ν$- and $ν$p-process) is too small to explain the Mn/Fe mass ratio we measure (0.14--0.67\%). This result supports the operation of significant neutrino interactions in the Cassiopeia A supernova. If the observed Mn/Fe mass ratio purely reflects the production at the innermost region of the supernova, this would be the first robust confirmation of neutrino-matter interactions in an individual supernova. We further show that the Mn/Fe mass ratio has the potential to constrain supernova neutrino parameters (i.e., total neutrino luminosity, neutrino temperature). Future spatially-resolved, high-resolution X-ray spectroscopy will allow us to investigate the details of neutrino-supernova astrophysics through its signatures in elemental composition not only in Cassiopeia A but also in other remnants.

astro-ph.HE↗

arXiv2023

Quasars: standard candles up to z=7.5 with the precision of Supernovae Ia

Currently, the $Λ$ Cold Dark Matter model, which relies on the existence of cold dark matter and a cosmological constant $Λ$, best describes the Universe. However, we lack information in the high-redshift ($z$) region between Type Ia Supernovae (SNe Ia) (up to $z=2.26$) and the Cosmic Microwave Background ($z=1100$), an interval crucial to test cosmological models and their possible evolution. We have defined a sample of 983 Quasars up to $z=7.54$ with reduced intrinsic dispersion $δ=0.007$ which determines the matter density parameter $Ω_M$ with the same precision of SNe Ia. Although previous analysis have been used Quasars as cosmological tools (e.g. Risaliti and Lusso 2019), this is the first time that high-redshift sources, in this case Quasars, as standalone cosmological probes yield such tight constraints on $Ω_M$. Our results show the importance of correcting cosmological relationships for selection biases and redshift evolution and how the choice of a golden sample reduces considerably the intrinsic scatter. This proves the reliability of Quasars as standard cosmological candles.

astro-ph.CO↗

arXiv2023

Investigating Time Evolution of Thermal Emission from the Putative Neutron Star in SN 1987A for 50+ Years

Observations collected with the Atacama Large Millimeter/submillimeter Array (ALMA) and analysis of broadband X-ray spectra have recently suggested the presence of a central compact object (CCO) in SN 1987A. However, no direct evidence of the CCO has been found yet. Here we analyze Chandra X-ray observations of SN 1987A collected in 2007 and 2018, and synthesize the 2027 Chandra and 2037 Lynx spectra of the faint inner region of SN 1987A. We estimate the temporal evolution of the upper limits of the intrinsic luminosity of the putative CCO in three epochs (2018, 2027 and 2037). We find that these upper limits are higher for higher neutron star (NS) kick velocities due to the increased absorption from the surrounding cold ejecta. We compare NS cooling models with both the intrinsic luminosity limits obtained from the X-ray spectra, and the ALMA constraints with the assumption that the observed blob of SN 1987A is primarily heated by thermal emission. We find that the synthetic Lynx spectra are crucial to constrain physical properties of the CCO, which will be confirmed by future observations in the 2040s. We draw our conclusions based on two scenarios, namely the non-detection and detection of NS by Lynx. If the NS is not detected, its kick velocity should be ~700 km/s. Furthermore, the non-detection of the NS would suggest rapid cooling processes around the age of 40 years, implying strong crust superfluidity. Conversely, in the case of NS detection, the mass of the NS envelope must be high.

astro-ph.HE↗

arXiv2023

Impacts of the direct URCA and Superfluidity inside a Neutron Star on Type-I X-Ray Bursts and X-Ray Superbursts

We investigate the impacts of neutrino cooling mechanism inside the neutron star (NS) core on the light curves of type-I X-ray bursts and X-ray superbursts. From several observations of NS thermal evolution, physical processes of fast neutrino cooling, such as the direct Urca (DU) process, are indicated. They significantly decrease the surface temperature of NSs, though the cooling effect could be suppressed by nucleon superfluidity. In the present study, focusing on the DU process and nucleon superfluidity, we investigate the effects of NS cooling on the X-ray bursts using a general-relativistic stellar-evolution code. We find that the DU process leads find the longer recurrence time and the higher peak luminosity, which could be obstructed by the neutrons superfluidity. We also apply our burst models to the comparison with {\it Clocked burster} GS 1826$-$24, and to the recurrence time of superburst triggered by carbon ignition. These effects are significant within a certain range of binary parameters and uncertainty of the NS equation of state.

astro-ph.HE↗

arXiv2023

A bias-free cosmological analysis with quasars alleviating $H_0$ tension

Cosmological models and their parameters are widely debated because of theoretical and observational mismatches of the standard cosmological model, especially the current discrepancy between the value of the Hubble constant, $H_{0}$, obtained by Type Ia supernovae (SNe Ia), and the Cosmic Microwave Background Radiation (CMB). Thus, considering high-redshift probes like quasars (QSOs), having intermediate redshifts between SNe Ia and CMB, is a necessary step. In this work, we use SNe Ia and the most updated QSO sample, reaching redshifts up to $z\sim7.5$, applying the Risaliti-Lusso QSO relation based on a non-linear relation between ultraviolet and X-ray luminosities. We consider this relation both in its original form and corrected for selection biases and evolution in redshift through a reliable statistical method also accounting for the circularity problem. We also explore two approaches: with and without calibration on SNe Ia. We then investigate flat and non-flat standard cosmological models and a flat $w$CDM model, with a constant dark energy equation of state parameter $w$. Remarkably, when correcting for the evolution as a function of cosmology, we obtain closed constraints on $Ω_M$ using only non-calibrated QSOs. We find that considering non-calibrated QSOs combined with SNe Ia and accounting for the same correction, our results are compatible with a flat $Λ$CDM model with $Ω_M = 0.3$ and $H_0 = 70 \, \mathrm{km\,s^{-1}\,Mpc^{-1}}$. Intriguingly, the $H_0$ values obtained place halfway between the one from SNe Ia and CMB, paving the way for new insights into the $H_0$ tension.

astro-ph.CO↗

arXiv2022

Non-Hermitian topological Fermi superfluid near the $p$-wave unitary limit

We theoretically discuss the non-Hermitian superfluid phase transition in one-dimensional two-component Fermi gases near the $p$-wave Feshbach resonance accompanied by the two-body loss associated with the dipolar relaxation. For the first time we point out that this system gives us an opportunity to explore the interplay among various non-trivial properties such as universal thermodynamics at divergent $p$-wave scattering length, topological phase transition at vanishing chemical potential, and non-Hermitian Bardeen-Cooper-Schrieffer(BCS) to Bose-Einstein condensate (BEC) transition, in a unified manner. In the BCS phase, the loss-induced superfluid-normal transition occurs when the exceptional point appears in the effective non-Hermitian Hamiltonian. In the BEC phase, the diffusive gapless mode can be regarded as a precursor of the instability of the superfluid state. Moreover, we show that the superfluid state is fragile against the two-body loss near the topological phase transition point.

cond-mat.quant-gas↗

arXiv2022

Merger and post-merger of binary neutron stars with a quark-hadron crossover equation of state

Fully general-relativistic binary-neutron-star (BNS) merger simulations with quark-hadron crossover (QHC) equations of state (EOSs) are studied for the first time. In contrast to EOSs with purely hadronic matter or with a first-order quark-hadron phase transition (1PT), in the transition region QHC EOSs show a peak in sound speed, and thus a stiffening. We study the effects of such stiffening in the merger and post-merger gravitational (GW) signals. Through simulations in the binary-mass range $2.5 < M/M_{\odot} < 2.75$, characteristic differences due to different EOSs appear in the frequency of the main peak of the post-merger GW spectrum ($f_2$), extracted through Bayesian inference. In particular, we found that (i) for lower-mass binaries, since the maximum baryon number density ($n_{\rm max}$) after the merger stays below $3\text{--}4$ times the nuclear-matter density ($n_0$), the characteristic stiffening of the QHC models in that density range results in a lower $f_2$ than that computed for the underlying hadronic EOS and thus also than that for EOSs with a 1PT, (ii) for higher-mass binaries, where $n_{\rm max}$ may exceed $4\text{--}5 n_0$ depending on the EOS model, whether $f_2$ in QHC models is higher or lower than that in the underlying hadronic model depends on the height of the sound-speed peak. Comparing the values of $f_2$ for different EOSs and BNS masses gives important clues on how to discriminate different types of quark dynamics in the high-density end of EOSs and is relevant to future kilohertz GW observations with third-generation GW detectors.

astro-ph.HE↗

arXiv2022

R-process viable outflows are suppressed in global alpha-viscosity models of collapsar disks

Collapsar disks have been proposed to be rich factories of heavy elements, but the major question of whether their outflows are neutron-rich, and could therefore represent significant sites of the rapid neutron-capture (r-) process, or dominated by iron-group elements remains unresolved. We present the first global models of collapsars that start from a stellar progenitor and self-consistently describe the evolution of the disk, its composition, and its outflows in response to the imploding stellar mantle, using energy-dependent M1 neutrino transport and an alpha-viscosity to approximate turbulent angular-momentum transport. We find that a neutron-rich, neutrino-dominated accretion flow (NDAF) is established only marginally--either for short times or relatively low viscosities--because the disk tends to disintegrate into an advective disk (ADAF) already at relatively high mass-accretion rates, launching powerful outflows but preventing it from developing a hot, dense, and therefore neutron-rich core. Viscous outflows disrupt the star within ~100s with explosion energies close to that of hypernovae. If viscosity is neglected, a stable NDAF with disk mass of about 1Msun is formed but is unable to release neutron-rich ejecta, while it produces a relatively mild explosion powered by a neutrino-driven wind blown off its surface. With ejecta electron fractions close to 0.5, all models presumably produce large amounts of Ni56. Our results suggest that collapsar models based on the alpha-viscosity are inefficient r-process sites and that genuinely magnetohydrodynamic effects may be required to generate neutron-rich outflows. A relatively weak effective viscosity generated by magnetohydrodynamic turbulence would improve the prospects for obtaining neutron-rich ejecta.

astro-ph.HE↗

arXiv2022

Optical and X-ray GRB Fundamental Planes as Cosmological Distance Indicators

Gamma-Ray Bursts (GRBs), can be employed as standardized candles, extending the distance ladder beyond Supernovae Type Ia (SNe Ia, $z=2.26$). We standardize GRBs using the 3D fundamental plane relation (the Dainotti relation) among the rest-frame end time of the X-ray plateau emission, its corresponding luminosity, and the peak prompt luminosity. Combining SNe Ia and GRBs, we constrain $Ω_{\text{M}}= 0.299 \pm 0.009$ assuming a flat $Λ$CDM cosmology with and without correcting GRBs for selection biases and redshift evolution. Using a 3D optical Dainotti correlation, we find this sample is as efficacious in the determination of $Ω_{\text{M}}$ as the X-ray sample. We trimmed our GRB samples to achieve tighter planes to simulate additional GRBs. We determined how many GRBs are needed as standalone probes to achieve a comparable precision on $Ω_{\text{M}}$ to the one obtained by SNe Ia only. We reach the same error measurements derived using SNe Ia in 2011 and 2014 with 142 and 284 simulated optical GRBs, respectively, considering the errorbars on the variables halved. These error limits will be reached in 2038 and in 2047, respectively. Using a doubled sample (obtained by future machine learning approaches allowing a lightcurve reconstruction and the estimates of GRB redhifts when z is unknown) compared to the current sample, with errorbars halved we will reach the same precision as SNe Ia in 2011 and 2014, now and in 2026, respectively. If we consider the current SNe precision, this will be reached with 390 optical GRBs by 2054.

astro-ph.CO↗

arXiv2022

Additional evidence for a pulsar wind nebula in the heart of SN 1987A from multi-epoch X-ray data and MHD modeling

Since the day of its explosion, supernova (SN) 1987A has been closely monitored to study its evolution and to detect its central compact relic. In fact, the formation of a neutron star is strongly supported by the detection of neutrinos from the SN. However, besides the detection in the Atacama Large Millimeter/submillimeter Array (ALMA) data of a feature that is compatible with the emission arising from a proto-pulsar wind nebula (PWN), the only hint for the existence of such elusive compact object is provided by the detection of hard emission in NuSTAR data up to ~ 20 keV. We report on the simultaneous analysis of multi-epoch observations of SN 1987A performed with Chandra, XMM-Newton and NuSTAR. We also compare the observations with a state-of-the-art 3D magnetohydrodynamic (MHD) simulation of SN 1987A. A heavily absorbed power-law, consistent with the emission from a PWN embedded in the heart of SN 1987A, is needed to properly describe the high-energy part of the observed spectra. The spectral parameters of the best-fit power-law are in agreement with the previous estimate, and exclude diffusive shock acceleration as a possible mechanism responsible for the observed non-thermal emission. The information extracted from our analysis are used to infer the physical characteristics of the pulsar and the broad-band emission of its nebula, in agreement with the ALMA data. Analysis of the synthetic spectra also show that, in the near future, the main contribution to Fe K emission line will originate in the outermost shocked ejecta of SN 1987A.

astro-ph.HE↗

arXiv2022

The double detonation of a double degenerate system, from Type Ia supernova explosion to its supernova remnant

Type Ia supernovae (SNe) are believed to be caused by the thermonuclear explosion of a white dwarf (WD), but the nature of the progenitor system(s) is still unclear. Recent theoretical and observational developments have led to renewed interest in double degenerate models, in particular the "helium-ignited violent merger" or "dynamically-driven double-degenerate double-detonation" (D$^6$). In this paper we take the output of an existing D$^6$ SN model and carry it into the supernova remnant (SNR) phase up to 4000 years after the explosion, past the time when all the ejecta have been shocked. Assuming a uniform ambient medium, we reveal specific signatures of the explosion mechanism and spatial variations intrinsic to the ejecta. The first detonation produces an ejecta tail visible at early times, while the second detonation leaves a central density peak in the ejecta that is visible at late times. The SNR shell is off-centre at all times, because of an initial velocity shift due to binary motion. The companion WD produces a large conical shadow in the ejecta, visible in projection as a dark patch surrounded by a bright ring. This is a clear and long-lasting feature that is localized, and its impact on the observed morphology is dependent on the viewing angle of the SNR. These results offer a new way to diagnose the explosion mechanism and progenitor system using observations of a Type Ia SNR.

astro-ph.HE↗

arXiv2022

Can one-zone hadronuclear model explain the hard-TeV spectrum of BL Lac objects?

Context. The intrinsic TeV emission of some BL Lacs are characterized by a hard spectrum (the hard-TeV spectrum) after correcting for the extragalactic background light. The hard-TeV spectra pose a challenge to conventional one-zone models, including the leptonic model, the photohadronic model, the proton synchrotron model, etc. Aims. In this work, we study if the one-zone hadronuclear (pp) model can be used to interpret the hard-TeV spectra of BL Lacs without introducing extreme parameters. Methods. We give analytical calculations to study if there is a parameter space and the charge neutrality condition of jet can be satisfied when interpreting the hard-TeV spectra of BL Lacs without introducing a super-Eddington jet power. Results. We find that in a sample of hard-TeV BL Lacs collected by Xue et al. (2019a), only the hard-TeV spectrum of 1ES 0229+200 could be explained by gamma-ray from pi-0 decay produced in the pp interactions, but at the cost of setting a small radius of the radiation region that comparable to the Schwarzschild radius of the central black hole. Combining with previous studies of other one-zone models, we suggest that the hard-TeV spectra of BL Lacs cannot be explained by any one-zone models without introducing extreme parameters, and should originate from the multiple radiation regions.

astro-ph.HE↗

arXiv2021

Neutrino absorption and other physics dependencies in neutrino-cooled black-hole accretion disks

Black-hole (BH) accretion disks formed in compact-object mergers or collapsars may be major sites of the rapid-neutron-capture (r-)process, but the conditions determining the electron fraction (Y_e) remain uncertain given the complexity of neutrino transfer and angular-momentum transport. After discussing relevant weak-interaction regimes, we study the role of neutrino absorption for shaping Y_e using an extensive set of simulations performed with two-moment neutrino transport and again without neutrino absorption. We vary the torus mass, BH mass and spin, and examine the impact of rest-mass and weak-magnetism corrections in the neutrino rates. We also test the dependence on the angular-momentum transport treatment by comparing axisymmetric models using the standard alpha-viscosity with viscous models assuming constant viscous length scales (l_t) and three-dimensional magnetohydrodynamic (MHD) simulations. Finally, we discuss the nucleosynthesis yields and basic kilonova properties. We find that absorption pushes Y_e towards ~0.5 outside the torus, while inside increasing the equilibrium value Y_e^eq by ~0.05-0.2. Correspondingly, a substantial ejecta fraction is pushed above Y_e=0.25, leading to a reduced lanthanide fraction and a brighter, earlier, and bluer kilonova than without absorption. More compact tori with higher neutrino optical depth, tau, tend to have lower Y_e^eq up to tau~1-10, above which absorption becomes strong enough to reverse this trend. Disk ejecta are less (more) neutron-rich when employing an l_t=const. viscosity (MHD treatment). The solar-like abundance pattern found for our MHD model marginally supports collapsar disks as major r-process sites, although a strong r-process may be limited to phases of high mass-infall rates, Mdot>~ 2 x 10^(-2) Msun/s.

astro-ph.HE↗

arXiv2021

High-entropy ejecta plumes in Cassiopeia A from neutrino-driven convection

Recent multi-dimensional simulations suggest that high-entropy buoyant plumes help massive stars to explode. Outwardly protruding iron-rich fingers in the galactic supernova remnant Cassiopeia A are uniquely suggestive of this picture. Detecting signatures of specific elements synthesized in the high-entropy nuclear burning regime (i.e., $α$-rich freeze out) would be among the strongest substantiating evidence. Here we report the discovery of such elements, stable Ti and Cr, at a confidence level greater than 5$σ$ in the shocked high-velocity iron-rich ejecta of Cassiopeia A. We found the observed Ti/Fe and Cr/Fe mass ratios require $α$-rich freeze out, providing the first observational demonstration for the existence of high-entropy ejecta plumes that boosted the shock wave at explosion. The metal composition of the plumes agrees well with predictions for strongly neutrino-processed proton-rich ejecta. These results support the operation of the convective supernova engine via neutrino heating in the supernova that produced Cassiopeia A.

astro-ph.HE↗

arXiv2021

A semi-analytic afterglow with thermal electrons and synchrotron self-Compton emission

We extend previous work on gamma-ray burst (GRB) afterglows involving hot thermal electrons at the base of a shock-accelerated tail. Using a physically-motivated electron distribution based on first-principles simulations, we compute broadband emission from radio to TeV gamma-rays. For the first time, we present the effects of a thermal distribution of electrons on synchrotron self-Compton (SSC) emission. The presence of thermal electrons causes temporal and spectral structure across the entire observable afterglow, which is substantively different from models that assume a pure power-law distribution for the electrons. We show that early-time TeV emission is enhanced by more than an order of magnitude for our fiducial parameters, with a time-varying spectral index that does not occur for a pure power law of electrons. We further show that the X-ray "closure relations" take a very different, also time-dependent, form when thermal electrons are present; the shape traced out by the X-ray afterglows is a qualitative match to observations of the traditional decay phase.

astro-ph.HE↗