arXiv subjects

Kazumi Kashiyama

Publications and source records attributed to Kazumi Kashiyama.

At least 19 recordsLinked to original sources

arXiv2026

A Diagnostic Method for Proto-Neutron Star Magnetic Fields by Supernova Fallback Neutrinos

Young isolated neutron stars exhibit a wide range of magnetic field strengths. To understand the origin of this diversity, it is important to observationally constrain magnetic fields during the proto-neutron star (PNS) phase. We propose a novel diagnostic of PNS magnetic fields using neutrinos associated with supernova fallback. By performing one-dimensional general-relativistic magnetohydrodynamic simulations of the supernova fallback onto a magnetized PNS, we investigate how the energy spectra of the fallback neutrinos depend on the PNS magnetic field strength and the fallback accretion rate. We find that stronger magnetic fields and lower accretion rates produce softer spectra because the fallback neutrinos are emitted mainly near the magnetospheric radius, which becomes larger under these conditions. To quantify this spectral softening, we define a hardness ratio based on event rates above and below 30 MeV and introduce a hardness-event-rate diagram. Using this diagram, we identify the parameter region in which our diagnostic is applicable. Its applicability depends on the PNS magnetic field strength, fallback accretion rate, onset time of the fallback phase, and neutrino mass ordering. We also investigate the dependence on the PNS radius and find that a larger radius produces softer spectra, indicating a degeneracy between the magnetic field and PNS compactness. An independent constraint on the PNS compactness would therefore enable us to better constrain the magnetic field. These results suggest that future multimessenger observations of galactic supernovae can provide a new window into the NS diversity.

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arXiv2026

The Ultrafast Line-Driven Wind from the Double-Degenerate Merger Remnant WD J005311

We systematically construct steady-state wind solutions for double-degenerate merger remnants consisting of a degenerate oxygen-neon core and an optically thick expanding envelope powered by carbon-shell burning, with winds accelerated by radiation pressure including line driving. For a given envelope composition, each solution is characterized by two eigenvalues: the degenerate core mass, $M_{\rm WD}$, and the total envelope and wind mass, $ΔM$. We find that wind solutions exist for $M_{\rm WD} \gtrsim 1.0\,M_\odot$. For a given $M_{\rm WD}$, the solutions transition with decreasing $ΔM$ from slow, continuum-driven winds to ultrafast, line-driven winds, forming a sequence that can be interpreted as the temporal evolution of a merger remnant. We apply these solutions to WD J005311, a Galactic double-degenerate merger-remnant candidate with an ultrafast wind of $v_{\rm w} \sim 0.05\,c$. Its luminosity, effective temperature, and mass-loss rate are consistently reproduced with $M_{\rm WD} \sim 1.15-1.25\,M_\odot$ and $ΔM \sim (1.8-5.7)\times 10^{-3}\,M_\odot$. Combining our evolutionary analysis with observations of the surrounding nebula Pa 30 and historical records of SN 1181, we argue that WD J005311 began launching a continuum-driven wind $400-650$ yr after the putative merger and transitioned to the ultrafast-wind phase approximately $100$ yr ago. This phase is expected to continue for another $\sim 1000$ yr, during which the WD will remain below the Chandrasekhar mass and avoid collapse into a neutron star. Before the continuum-driven wind phase, the remnant may have passed through a more bloated, hydrostatic giant phase with a slower but more massive wind. Future observations of the wind nebula could test this scenario by revealing signatures of interactions between the slow, massive wind and the ultrafast wind.

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arXiv2026

A Type Ia Supernova Candidate at $z\sim4.3$: A Transient Interloper in the Search for $z\sim14$ Galaxies

The James Webb Space Telescope (JWST) is opening a new window into the distant Universe by discovering galaxies and transients in the early Universe. We investigate a high-redshift transient candidate, beacon_1420-5253_4770. This object was initially identified as a high-redshift galaxy candidate at $z\sim14$. However, the source was not detected in epochs before and after the detection epoch, suggesting that the object is a transient source rather than a persistent galaxy. We classify the source by comparing the colors, magnitudes, light curves, and spectral energy distribution with various spectral templates of transients. Our analysis shows that the observed properties are consistent with a Type Ia supernova at $z\sim4.3$. Strong absorption by Fe-group elements seen in Type Ia supernova spectra can mimic the Lyman break used to detect high-redshift galaxies. At $z \sim 4.3$, corresponding to a cosmic age of only $\sim 1.5$ Gyr, our detection provides a direct probe of the delay time between star formation and supernova explosion. Our estimate of the event rate implies a minimum delay time shorter than 1 Gyr. We also discuss the implications of transient contamination for searches of galaxies in the early Universe.

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arXiv2026

Successive Partial Disruptions with Orbital Precession in a White Dwarf-Black Hole System for Repeating GRB 250702B

The peculiar gamma-ray burst GRB 250702B is the longest event ever observed, lasting about one day and exhibiting four prompt-emission flares of $\sim100$ s with irregular recurrence intervals of at least one hour. To explain this hierarchy of timescales, we consider a scenario in which a stellar object undergoes repeated partial tidal disruptions by a black hole (BH). We find that if a white dwarf (WD) is on a highly eccentric orbit ($e\approx0.97$) around an intermediate-mass black hole (BH) with $M_{\rm BH}\lesssim10^{6}\,M_\odot$ and $a = 50\,R_\odot\left(M_{\rm BH}/10^{6}\,M_\odot\right)^{1/3}$, the observed properties of GRB 250702B can be naturally reproduced. In this framework, the duration of each flare is determined by the viscous accretion timescale of material stripped near pericenter, with a typical mass $ΔM \approx 2\times10^{-2}\,M_\odot$. The minimum recurrence time corresponds to the orbital period, while the total activity period is set by the secular orbital evolution timescale leading to the complete disruption of the WD. Furthermore, if $M_{\rm BH}\gtrsim10^{5}\,M_\odot$ and the orbit has a minimum polar angle relative to the BH equatorial plane of $θ_{\rm min}\gtrsim0.12 {\rm rad}$, relativistic frame dragging induces $\gtrsim0.1$ rad precession of the orbital angular momentum between successive pericenter passages, comparable to a typical GRB jet half-opening angle, resulting in intermittent alignment with the observer and irregular flare spacing. The WD experiences $\approx40$ jet-launch episodes before complete disruption, but only four are expected to be observed on-axis. The remaining off-axis jets become visible at late times, enhancing the radio afterglow by about an order of magnitude, providing a testable prediction of this scenario.

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arXiv2026

Spectral Uniformity of Little Red Dots: A Natural Outcome of Coevolving Seed Black Holes and Nascent Starbursts

The birth of seeds of massive black holes (BHs) and nascent galaxies at cosmic dawn takes place in dense gaseous environments, which play a crucial role in shaping their coevolution and radiation spectra. We investigate gas accretion during the assembly of massive halos with $M_{\rm h}\gtrsim 10^{10-11}~M_\odot$ at redshifts $z\simeq 4-10$, driving both rapid BH feeding and concurrent nuclear starbursts. As the BH grows to $\sim 10^{6-7}~M_\odot$ via super-Eddington accretion, the accretion power inflates a dense envelope whose effective temperature approaches the Hayashi limit at $T_{\rm eff}\simeq 5000~{\rm K}$, producing red optical emission, while a coeval young stellar population of $\sim 10^7~M_\odot$ provides blue UV emission. This early coevolving system naturally reproduces the characteristic spectral features of the so-called little red dots (LRDs), a population of broad-line active galactic nuclei (AGNs), including the V-shaped UV-to-optical spectra and weakness of X-ray, infrared, and radio emission. Massive stars in the nuclear starburst soon explode as supernovae, injecting energy and momentum that expel gas from the nucleus, quench gas supply to the BH envelope, and ultimately drive a transition into normal AGN phases. For individual LRDs, the optical-to-UV luminosity ratio remains nearly constant at $L_{\rm opt}/L_{\rm UV}\simeq 2-10$ from the onset of accretion bursts for $\simeq 15~{\rm Myr}$, one-third of the Salpeter time, until quenching by stellar feedback. While this ratio is sustained for the LRD population at $z\simeq 4-8$, it declines toward lower redshifts as BHs can no longer maintain red envelopes, thereby losing the LRD characteristics.

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arXiv2026

Quasi-steady emission from repeating fast radio bursts can be explained by magnetar wind nebulae

Among more than 1000 known fast radio bursts (FRBs), only five sources - FRBs 20121102A, 20190520B, 20201124A, 20240114A and 20190417A - have confirmed associations with persistent radio sources (PRS). The observed quasi-steady emission is consistent with synchrotron radiation from a composite of magnetar wind nebula (MWN) and supernova (SN) ejecta. Using a phenomenological model that incorporates simplified treatments of the nebular dynamics and particle acceleration, we compute the synchrotron flux by solving kinetic equations for energized electrons, accounting for electromagnetic cascades of electron-positron pairs interacting with nebular photons. Within the framework of our model, the rotation-powered scenario requires a young neutron star (NS) with age $t_{\rm age}\approx 20\,{\rm yr}$, dipolar magnetic field $B_{\rm dip}\approx (3-5)\times10^{12}\,{\rm G}$ and initial spin period $P_i\approx 1.5-3\,{\rm ms}$ in an ultra-stripped SN progenitor to account for emissions from FRBs 20121102A and 20190520B. In contrast, FRB 20201124A requires $t_{\rm age}\approx 10\,{\rm yr}$, $B_{\rm dip}\approx 5.5\times10^{13}\,{\rm G}$ and $P_i\approx 10\,{\rm ms}$ in a conventional core-collapse SN progenitor. For the magnetar-flare-powered model, NS aged $t_{\rm age} \approx 25\,/40\,{\rm yr}$ in a USSN progenitor and $t_{\rm age} \approx 12.5\,{\rm yr}$ in a CCSN progenitor explains the observed flux for FRB 20121102A/20190520B and FRB 20201124A, respectively. Finally, we estimate a minimum NS age $t_{\rm age,min} \sim 1-3\,{\rm yr}$ based on the near-source plasma contribution to observed DM, and $t_{\rm age,min} \sim 6.5-10\,{\rm yr}$ from the absence of radio signal attenuation.

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arXiv2026

Blowouts of Nascent Wind Bubbles in Pulsar-Driven Supernovae

Formation of a rapidly spinning, strongly magnetized neutron star (NS) may occur in various classes of core-collapse events. If the NS injects an amount of energy comparable to the explosion energy of the accompanying supernova (SN) before the SN ejecta becomes transparent, the nascent NS wind bubble can overtake the outer ejecta and undergo a blowout driven by hydrodynamic instabilities. Based on multidimensional numerical studies, we construct a minimal semi-analytic framework to follow the post-blowout dynamics and radiative evolution, map the blowout conditions by scanning the ejecta and NS parameters, and compute survey-ready multi-band light curves. For stripped-envelope SNe with an ejecta mass of $M_\mathrm{ej} \sim 10\,M_\odot$ and an explosion energy of $E_\mathrm{sn} \sim 10^{51}\,\mathrm{erg}$, blowout occurs for NSs with magnetic field strengths of $B_{\mathrm{dip}} \gtrsim 10^{13}\,\mathrm{G}$ and spin periods of $P_\mathrm{NS} \lesssim \mathrm{a\ few}\,\mathrm{ms}$. Relatively weak-field cases with $B_\mathrm{dip} \lesssim 10^{14}\,\mathrm{G}$ produce luminous double-peaked UV/optical light curves, as observed in the superluminous SN LSQ14bdq, while stronger-field cases with $B_\mathrm{dip} \gtrsim 10^{14}\,\mathrm{G}$ result in hypernovae preceded by X-ray blowout precursors. We also examine weaker and lower-mass SN explosions representing ultra-stripped SNe and accretion- or merger-induced collapse events, in which blowout is more readily achieved over a broader range of NS parameters, producing fast X-ray transients with durations of $ 10^{2\mbox{--}4}\,\mathrm{s}$ and peak luminosities of $10^{42\mbox{--}48}\,\mathrm{erg\,s^{-1}}$. Our results encourage coordinated UV, optical, and X-ray observations which constrain the formation of the most energetic NSs in the universe.

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arXiv2026

Super-knee cosmic rays from interacting supernovae

There is increasing evidence that, in the very late phase of stellar evolution before core collapse, massive stars have winds with large mass loss rates that give rise to a dense circumstellar medium (CSM) surrounding the progenitor star. After core collapse, a shock wave forms when the supernova ejecta interacts with this CSM. In such an interaction, the nuclei in the CSM can undergo diffusive shock acceleration and reach very high energies. We consider such a model, which includes magnetic field amplification from the non-resonant streaming instability, enhancement to the abundance of heavy-ions, and composition-dependent acceleration. Applying this to several supernova subclasses, we find that IIn supernovae can supply a dominant fraction of the observed super-knee cosmic-ray (CR) flux from $\sim{\rm few}\times10^{15}\,{\rm eV}$ to $\sim{\rm few}\times10^{17}\,{\rm eV}$ and is consistent with recent LHAASO measurements above the CR knee. This systematic model also explains the increasingly heavy nuclear composition in this energy range.

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arXiv2025

A Necessary Condition for the Submergence of Proto-Neutron Star Magnetic Fields by Supernova Fallback

Central compact objects (CCOs) are a subclass of neutron stars with a dipole magnetic field strength considerably weaker than those of radio pulsars and magnetars. One possible explanation for such weak magnetic fields in the CCOs is the hidden magnetic field scenario, in which supernova fallback submerges the magnetosphere of a proto-neutron star beneath a newly formed crust. However, the fallback mass and timescale required for this submergence process remain uncertain. We perform one-dimensional general relativistic magnetohydrodynamic simulations of the supernova fallback onto a magnetized proto-neutron star, while considering neutrino cooling. In our simulations, the infalling material compresses the magnetic field and drives a strong shock. The shock initially expands outward, but eventually stalls and recedes as neutrino cooling becomes significant. After the shock stalls, the gas density above the magnetosphere increases rapidly, potentially leading to the formation of a new crust. To understand the shock dynamics, we develop semi-analytic models that describe the resulting magnetospheric and shock radii when the shock stalls. By comparing the fallback time scale with the shock stalling time scale, corresponding to the waiting time for the new crust formation, we derive a necessary condition for the submergence of the PNS magnetic field. Our results will provide guidance for investigating the diversity of young isolated neutron stars through multidimensional simulations.

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arXiv2025

Relaxation of time-variable neutron-loaded relativistic jets across the photosphere and their GeV-TeV neutrino counterparts

Both observational and theoretical studies indicate that the central engine of a gamma-ray burst (GRB) is intrinsically time-variable, implying jet inhomogeneity. A jet with an inhomogeneous Lorentz factor distribution develops internal shocks both below and above the photosphere, relaxing toward homologous expansion. Below the photosphere, neutrons, whose mean free paths are much longer than those of charged particles, play an essential role in the dissipation process. Using neutron-inclusive shell simulations with initial conditions based on the collapsar scenario, we link the statistical inhomogeneity of the jet at the breakout of the progenitor to the dissipation that occurs inside and outside the photosphere, and calculate the GeV-TeV neutrino counterpart originated from inelastic neutron-proton interactions consistently with the prompt gamma-ray emission. We find that the peak energy of the GeV-TeV neutrinos is in 10-30 GeV irrespective to the baryon loading factor of the jet, with the high-energy tail extending into the TeV range as the amplitude of the time variability becomes stronger. When gamma-ray emission is efficient as in typical GRBs (i.e., the gamma-ray radiation efficiency with respect to the total jet power is approximately 100%, the radiative efficiency of GeV-TeV neutrinos remains 0.1-10%. By contrast, when the gamma-ray radiation efficiency is relatively low (< 10%) for jets where a large fraction of the energy is dissipated below the photosphere, the neutrino efficiency can increase up to 20%. This suggests that GRBs with relatively low gamma-ray luminosities, as well as X-ray-rich transients, can be promising targets for ongoing and future GeV-TeV neutrino transient searches.

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arXiv2025

Gamma-Ray Burst Jets in Circumstellar Material: Dynamics, Breakout, and Diversity of Transients

Recent observations indicate that stripped-envelope core-collapse supernovae are often surrounded by dense circumstellar material (CSM). Motivated by this, we develop an analytic model to systematically study the dynamics of long gamma-ray burst (LGRB) jet propagation in various CSM environments. We derive a general expression for the jet head velocity ($β_{\rm h}$) and breakout time ($t_{\rm b}$) valid across Newtonian, relativistic, and intermediate regimes, accounting for a previously unrecognized dependence on $1 - β_{\rm h}$. Our results highlight a fundamental distinction between jet propagation in massive stars, where $β_{\rm h}\ll 1$, and in extended CSM, where $1-β_{\rm h}\ll 1$. We establish an analytic success/failure criterion for jets and express it in terms of jet and CSM parameters, revealing a strong dependence on CSM radius. To quantify the relativistic nature of the jet-cocoon system, we introduce the energy-weighted proper velocity $\overline{Γβ}$. We identify three possible jet outcomes-(a) successful jets ($\overline{Γβ} \sim 10-100$), (b) barely failed jets ($\overline{Γβ} \sim 1$), and (c) completely failed jets ($\overline{Γβ} \sim 0.1$)-and constrain their respective jet/CSM parameter spaces. We show that in (b) and (c), large CSM radii can result in luminous fast blue optical transients via cocoon cooling emission. This theoretical framework provides a basis for future observational and theoretical studies to understand the link between LGRBs, intermediate GRBs, low-luminosity LGRBs, and their environments.

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arXiv2025

EP240414a: A Gamma-Ray Burst Jet Weakened by an Extended Circumstellar Material

The recent Einstein Probe (EP) event EP240414a exhibits several unusual observational features. Its prompt and afterglow emissions place it between long gamma-ray bursts (LGRBs) and low-luminosity GRBs (LLGRBs). The event is followed by a fast optical transient (AT 2024gsa), initially exhibiting a thermal-like spectrum but later evolving into an unusually red peak at $\sim 3-5$ days, which is difficult to explain with thermal emission. Using our generalized analytic framework for jet propagation in a circumstellar material (CSM; Hamidani et al. 2025), we explore a scenario in which a conventional LGRB jet is launched in a progenitor surrounded by a dense CSM. For a CSM of $\sim 0.03 M_\odot$ extending to $\sim 3\times 10^{13}$ cm, we find that the jet is significantly weakened before breaking out, becoming "barely failed", an intermediate state between successful (LGRB) and completely failed (LLGRB) jets. This scenario naturally explains EP240414a's multi-wavelength observations, with the early thermal component produced by cocoon cooling emission, and the red peak explained by non-thermal afterglow emission from the mildly relativistic barely failed jet (and its inner-cocoon). Our work demonstrates the important role of extended CSM in shaping GRB jets and illustrates how early multi-wavelength follow-up observations can reveal the physically diverse nature of jet-driven transients.

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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.

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arXiv2024

Exploring Low-Mass Black Holes through Tidal Disruption Events in the Early Universe: Perspectives in the Era of JWST, RST, and LSST Surveys

The James Webb Space Telescope (JWST) has recently uncovered the presence of low-luminosity active galactic nuclei (AGNs) at $z=4-11$. Spectroscopic observations have provided estimates of the nuclear black hole (BH) masses for these sources, extending the low-mass boundary down to $M_{\rm BH} \sim 10^{6-7}~M_\odot$. Despite this breakthrough, the observed lowest mass of BHs is still $\gtrsim 1-2$ orders of magnitude heavier than the predicted mass range of their seed population, thereby leaving the initial mass distribution of massive BHs poorly constrained. In this paper, we focus on UV-to-optical (in rest frame) flares of stellar tidal disruption events (TDEs) embedded in low-luminosity AGNs as a tool to explore low-mass BH populations with $\lesssim 10^{4-6}~M_\odot$. We provide an estimate of the TDE rate over $z=4-11$ associated wth the properties of JWST-detected AGN host galaxies, and find that deep and wide survey programs with JWST and Roman Space Telescope (RST) can detect and identify TDEs up to $z\simeq 4-7$. The predicted detection numbers of TDEs at $z>4$ in one year are $N_{\rm TDE} \sim 2-10~(0.2-2)$ for the JADES-Medium (and COSMOS-Web) survey with JWST, and $N_{\rm TDE} \sim 2-10~(8-50)$ for the Deep (and Wide) tier of the High-latitude Time Domain Survey with RST. We further discuss the survey strategies to hunt for the transient high-redshift TDEs in wide-field surveys with RST, as well as a joint observation campaign with the Vera C. Rubin Observatory for enhancing the detection number. The high-redshift TDE search will give us a unique opportunity to probe the mass distribution of early BH populations.

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arXiv2024

A dynamical model for IRAS 00500+6713: the remnant of a type Iax supernova SN 1181 hosting a double degenerate merger product WD J005311

IRAS 00500+6713 is a hypothesized remnant of a type Iax supernova SN 1181. Multi-wavelength observations have revealed its complicated morphology; a dusty infrared ring is sandwiched by the inner and outer X-ray nebulae. We analyze the archival X-ray data taken by XMM-Newton and Chandra to constrain the {angular radius}, mass, and metal abundance of the X-ray nebulae, and construct a theoretical model describing the dynamical evolution of IRAS 00500+6713, including the effects of the interaction between the SN ejecta and the intense wind enriched with carbon burning ashes from the central white dwarf (WD) J005311. We show that the inner X-ray nebula corresponds to the wind termination shock while the outer X-ray nebula to the shocked interface between the SN ejecta and the interstellar matter. The observed X-ray properties can be explained by our model with an {ejecta kinetic} energy of $E_\mathrm{ej} = (0.77 \mbox{--} 1.1)\times 10^{48}$~erg, an ejecta mass of $M_\mathrm{ej} = 0.18\mbox{--}0.53~M_\odot$, if the currently observed wind from WD J005311 started to blow $t_\mathrm{w} \gtrsim 810$ yr after the explosion, i.e., approximately after A.D. 1990. The inferred SN properties are compatible with those of Type Iax SNe and the timing of the wind launch may correspond to the Kelvin-Helmholtz contraction of the oxygen-neon core of WD J005311 that triggered a surface carbon burning. Our analysis supports that IRAS 00500+6713 is the remnant of SN Iax 1181 produced by a double degenerate merger of oxygen-neon and carbon-oxygen WDs, and WD J005311 is the surviving merger product.

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arXiv2024

On the X-ray efficiency of the white dwarf pulsar candidate ZTF J190132.9+145808.7

Strongly magnetized, rapidly rotating massive white dwarfs (WDs) emerge as potential outcomes of double degenerate mergers. These WDs can act as sources of non-thermal emission and cosmic rays, gethering attention as WD pulsars. In this context, we studied the X-ray emissions from ZTF J190132.9+145808.7 (hereafter ZTF J1901+14), a notable massive isolated WD in the Galaxy, using the Chandra X-ray observatory. Our results showed 3.5sigma level evidence of X-ray signals, although it is marginal. Under the assumption of a photon index of 2, we derived its intrinsic flux to be 2.3 (0.9--4.7) $\times 10^{-15}$~erg~cm$^{-2}$s$^{-1}$ and luminosity 4.6 (2.0--9.5) $\times 10^{26}$~erg~s$^{-1}$ for a 0.5--7 keV band in the 90% confidence range, given its distance of 41 pc. We derived an X-ray efficiency (eta) concerning the spin-down luminosity to be 0.012 (0.0022--0.074), a value comparable to that of ordnary neutron star pulsars. The inferred X-ray luminosity may be compatible with curvature radiation from sub-TeV electrons accelerated within open magnetic fields in the magnetosphere of ZTF J1901+14. Conducting more extensive X-ray observations is crucial to confirm whether ZTF J1901+14-like isolated WDs are also significant sources of X-rays and sub-TeV electron cosmic rays, similar to other WD pulsars in accreting systems.

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arXiv2024

Gravitational Collapse of White Dwarfs to Neutron Stars. I. From Initial Conditions to Explosions with Neutrino-radiation Hydrodynamics Simulations

This paper provides collapses of massive, fully convective, and non-rotating white dwarfs (WDs) formed by accretion-induced collapse or merger-induced collapse and the subsequent explosions with the general relativistic neutrino-radiation hydrodynamics simulations. We produce initial WDs in hydrostatic equilibrium, which have super-Chandrasekhar mass and are about to collapse. The WDs have masses of 1.6$M_\odot$ with different initial central densities specifically at $10^{10}$, $10^{9.6}$, $10^{9.3}$ and $10^{9.0}\,{\rm g\,cm^{-3}}$. First, we check whether initial WDs are stable without weak interactions. Second, we calculate the collapse of WDs with weak interactions. We employ hydrodynamics simulations with Newtonian gravity in the first and second steps. Third, we calculate the formation of neutron stars and accompanying explosions with general relativistic simulations. As a result, WDs with the highest density of $10^{10}\,{\rm g\,cm^{-3}}$ collapse not by weak interactions but by the photodissociation of the iron, and three WDs with low central densities collapse by the electron capture as expected at the second step and succeed in the explosion with a small explosion energy of $\sim 10^{48}$ erg at the third step. By changing the surrounding environment of WDs, we find that there is a minimum value of ejecta masses being $\sim 10^{-5}M_{\odot}$. With the most elaborate simulations of this kind so far, the value is one to two orders of magnitude smaller than previously reported values and is compatible with the estimated ejecta mass from FRB~121102.

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arXiv2023

The optically thick rotating magnetic wind from a massive white dwarf merger product -- II. axisymmetric magnetohydrodynamic simulations

We numerically construct a series of axisymmetric rotating magnetic wind solutions, aiming at exploring the observation properties of massive white dwarf (WD) merger remnants with a strong magnetic field, a fast spin, and an intense mass loss, as inferred for WD J005311. We investigate the magnetospheric structure and the resultant spin-down torque exerted to the merger remnant with respect to the surface magnetic flux $Φ_*$, spin angular frequency $Ω_*$ and the mass loss rate $\dot M$. We confirm that the wind properties for $σ\equiv Φ^2_* Ω_*^2/\dot M v_\mathrm{esc}^3 \gtrsim 1$ significantly deviate from those of the spherical Parker wind, where $v_\mathrm{esc}$ is the escape velocity at stellar surface. For such a rotating magnetic wind sequence, we find: (i) quasi-periodic mass eruption triggered by magnetic reconnection along with the equatorial plane (ii) a scaling relation for the spin-down torque $T \approx (1/2) \times \dot{M} Ω_* R^2_* σ^{1/4}$. We apply our results to discuss the spin-down evolution and wind anisotropy of massive WD merger remnants, the latter of which could be probed by a successive observation of WD J005311 using Chandra.

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