arXiv subjects

Yi-Zhong Fan

Publications and source records attributed to Yi-Zhong Fan.

At least 19 recordsLinked to original sources

arXiv2026

GW230729\_082317 and GW231118\_005626: A Candidate of Hierarchical Triple Mergers

Hierarchical triple mergers denote a sequential merger process in which the black hole remnant from an initial binary merger subsequently merges with a third black hole. In this study, we identify a related pair of binary black hole mergers, GW230729\_082317 and GW231118\_005626, which exhibit consistent sky localization and luminosity distances; the latter is identified as a candidate hierarchical merger binary back holes. The Bayes factor in favor of a hierarchical triple merger origin over the non-association hypothesis is $110$. However, after accounting for the priors odds, the corresponding odds ratio decreases to $0.1$, indicating that the current evidence is insufficient to favor the hierarchical triple merger scenario. GW230720\_0823 show no detectable signatures of orbit eccentricity or line-of-sight acceleration. The inferred properties of these two gravitational wave events suggest a dynamical formation channel, such as few-body scattering or gas-assisted mergers, while a primordial triple origin is disfavored. Among the proposed dynamical environments, the accretion disk of an active galactic nucleus provides the most plausible site. We further derive constraints on the local rate of hierarchical triple mergers rate under both the null and candidate scenarios. Under the assumption of no detected hierarchical triple mergers, the most stringent $90\%$ confidence upper limit is $4.2\,{\rm Gpc^{-3}yr^{-1}}$ for $t_{\rm max}=10$ years using the GWTC-4 catalog.

astro-ph.HE↗

arXiv2026

Dark Matter Particle Explorer: Ten-Year On-Orbit Performance and Scientific Outcomes

The Dark Matter Particle Explorer (DAMPE, also known as ``Wukong'') is the first scientific space mission within the framework of the Strategic Pioneer Program on Space Science of the Chinese Academy of Sciences. The main scientific objectives of DAMPE include indirect detection of dark matter, cosmic-ray physics, and high energy gamma-ray astronomy. Thanks to its large geometrical factor and thick imaging calorimeter, DAMPE could effectively detect cosmic-ray electrons (including positrons) and gamma-rays up to 10~TeV, and nuclei up to PeV. Since launched on 17 December 2015 into a Sun-synchronous orbit, DAMPE has been smoothly operating on orbit for more than ten years. Here we present an overview of instrument, on-orbit status, and highlight scientific results of DAMPE, including the measurements of the all-electron energy spectrum and the spectra of protons and other nuclei, and high energy gamma-ray line search.

astro-ph.HE↗

arXiv2026

Models of spectral bumps of primary cosmic rays and multi-messenger tests

The energy spectra of individual species of cosmic ray nuclei by recent direct detection experiments show complicated structures with spectral hardenings and softenings rather than featureless power laws, which challenge the traditional understanding of the production, acceleration, and propagation of cosmic rays. Particularly, the DAMPE measurements reveal charge-dependent softenings and reject mass-dependent softenings with a high significance. Taking into account the correlated structures at similar energies in the large-scale anisotropies of cosmic rays, one of the most natural interpretations of the spectral structures is the presence of a nearby cosmic ray source. In this case, the softening energies correspond to the acceleration upper limits of such a source, forming the so-called Peters cycle of the spectra. Alternative models such as the injection/acceleration and propagation models may also reproduce the spectral bumps, but show difficulty to explain the anisotropies. Fluxes of secondary nuclei and $γ$ rays offer additional tests of different models. Future multi-messenger observations are expected to be able to pin down the origin of cosmic rays.

astro-ph.HE↗

arXiv2026

Black Holes in the Red-sequence Elliptical Galaxies at Redshifts $\sim 0.7-2.5$: Not Dark Energy Source but Remnants of Little Red Dots

The nature of dark energy remains one of the most profound mysteries in modern cosmology. One intriguing proposal is that black holes (BHs) could be the astrophysical source of dark energy through a cosmological coupling mechanism, and strong evidence has been claimed via analyzing the growth of the black hole masses in the red-sequence elliptical galaxies at redshifts $\leq 2.5$. In this work, with a group of very high redshift AGNs detected by the James Webb Space Telescope (JWST) in the red-sequence elliptical galaxies, we show that the possibility of BHs being the astrophysical source of dark energy has been rejected at a confidence level exceeding 10$σ$. Moreover, it turns out that the Little Red Dots recently discovered by JWST, characterized by the low accretion rates, can naturally evolve into the red-sequence elliptical galaxies hosting the relatively low mass black holes at the redshifts of $\sim 0.7-2.5$, without the need of black hole cosmological coupling.

astro-ph.CO↗

arXiv2026

A dense dark matter core of the subhalo in the strong lensing system JVAS B1938+666

The nature of dark matter remains unknown, motivating the study of fuzzy/wave dark matter (FDM/$ψ$DM) and self-interacting dark matter (SIDM) as alternative frameworks to address small-scale discrepancies in halo profiles inferred from observations. This study presents a non-parametric reconstruction of the mass distribution of the previously-found, dark subhalo in the strong-lensing system JVAS B1938+666. Compared with the standard Navarro-Frenk-White (NFW) profile, both SIDM and $ψ$DM ($m_ψ=1.32^{+0.22}_{-0.31}\times 10^{-22} \, \rm eV$) provide significantly better fits to the resulting density profile. Moreover, the SIDM model is favored over $ψ$DM with a Bayes factor of 14.44. The reconstructed density profile features a characteristic kiloparsec-scale core ($r_c \approx 0.5 \, \rm kpc$) with central density $ρ_c \approx 2.5\times 10^{7}\, \rm M_{\odot} \, kpc^{-3} $, exhibiting remarkable consistency with the core-halo mass scaling relations observed in Local Group dwarf spheroidals. These findings offer insights that may help address the core-cusp discrepancy in $Λ$CDM substructure predictions.

astro-ph.CO↗

arXiv2026

A Common $\sim$55 GeV Inelastic Dark Matter Origin of the LZ, Fermi-LAT, and AMS-02 Excesses and Paleo-Detector Prospects

We investigate the 248~keV nuclear recoil excess reported by LUX-ZEPLIN (LZ) in a leptophobic inelastic dark matter framework, in which the mass splitting naturally suppresses low-energy recoils. We find two viable branches, including the endothermic solution with a high dark matter mass $m_χ\gtrsim400$~GeV and the exothermic one with a mass of tens GeV. Remarkably, the exothermic dark matter scenario with $m_χ\sim55$~GeV can simultaneously account for the LZ excess, the Galactic Center GeV gamma-ray excess, and the AMS-02 antiproton excess. The favored annihilation cross-section is slightly below the one required for the observed relic abundance, resulting in an overabundant relic in the minimal setup. An additional neutrino-portal mediator can provide the required depletion while leaving the gamma-ray and antiproton signals essentially unchanged. We further show that paleo-detectors provide a powerful probe of the inelastic interpretation, requiring exposures of $\sim10$ and $\sim100$~mg Gyr for the endothermic and exothermic scenarios, respectively.

hep-ph↗

arXiv2026

The impact of primordial magnetic fields on the formation of galaxies

Primordial magnetic fields (PMFs), potentially generated during cosmic inflation or early-universe phase transitions, can modify cosmic structure formation by enhancing the post-recombination matter power spectrum. In this work, we investigate the impact of PMFs on galaxy formation using a suite of high-resolution cosmological hydrodynamical simulations evolved to $z=0$. Our simulations are initialized with PMF-induced enhancements to the matter power spectrum and include more comprehensive baryonic subgrid models than previous work, incorporating both stellar and AGN feedback. We find that PMFs systematically accelerate early structure formation, producing more abundant halos and galaxies, higher baryon fractions, enhanced star formation rates, and rapid growth of supermassive black holes at high redshifts ($z \sim 10$). However, the resulting enhanced stellar and AGN feedback efficiently processes and expels gas, reducing the late-time differences between PMF and standard $Λ$CDM models. The PMF signatures are therefore strongest at high redshift and gradually weaken toward low redshift as nonlinear evolution and baryonic feedback dominate over the initial PMF-induced enhancements. The impact of PMFs depends strongly on the characteristic peak scale, $k_{\rm peak}$, of the initial power enhancement, with models yielding similar non-linear evolution if they share a similar $k_{\rm peak}$, even when other PMF parameters differ. Our results highlight the critical role of comprehensive baryonic physics in accurately quantifying PMF signatures.

astro-ph.GA↗

arXiv2026

Mass-Dependent Dark Matter Deficit from Inelastic Scattering

Recent stellar-kinematic and neutral-hydrogen observations indicate a dark matter deficit within the central tens of kiloparsecs of nearby galaxies that grows systematically with stellar mass relative to hydrodynamical simulations. We show that this mass dependence can arise from exothermic inelastic dark matter with strongly velocity-dependent scattering. Two nearly degenerate dark matter states interact through vector and scalar mediators with opposite-sign contributions and unequal ranges. The resulting coupled-channel dynamics suppresses $s$-wave conversion at low velocity while retaining a $p$-wave enhancement at several hundred $\rm km/s$, where down-scattering injects kinetic energy and lowers central dark matter densities. At dwarf velocities, conversion remains suppressed while elastic scattering can still drive core formation consistent with the observed dwarf-clustering pattern. A late dark-sector phase transition, along with the low-velocity suppression, preserves a large excited-state fraction until halo formation. Using representative halos spanning the four observed stellar-mass bins, we find that the model accounts for the inferred dark matter deficit in all four bins, including its systematic growth with stellar mass, while its impact weakens toward cluster velocities. The growing discrepancy with hydrodynamical simulations may therefore offer a glimpse of previously hidden dark matter microphysics.

astro-ph.GA↗

arXiv2026

Binary-pulsar Timing: A 3D Solar-System Accelerometer and Unknown-Source Probe

The acceleration of the Solar System barycenter (SSB) offers a unique precision probe of the local gravitational environment, enabling searches for otherwise invisible gravitating sources such as Planet Nine and nearby (primordial) black holes. Using the binary-pulsar timing measurements, we develop an analysis framework that combines the line-of-sight differential accelerations of 26 binary pulsars to jointly reconstruct the three-dimensional acceleration of the SSB and place directional upper limits. Across three smooth Galactic-potential baselines, the residual acceleration is consistent with zero. We obtain directional 95\% upper limits of $0.198$, $0.262$, and $0.693~μ\mathrm{as}\,\mathrm{yr}^{-1}$ over 50\%, 75\%, and all sampled directions, respectively. These limits are tighter by factors of 3.5, 3.6, and 1.6 than a matched Gaia EDR3 benchmark and improve previous pulsar constraints by more than an order of magnitude. We further extend the framework to the full SSB--pulsar two-endpoint response, enabling direct position-dependent mass constraints on unknown gravitational sources. A $10\,M_\odot$ object is excluded within 0.39, 0.34, and 0.21 pc over the same sky fractions. Our unknown-mass limits surpass the tidal-equivalent INPOP19a reference beyond 0.2 pc by about an order of magnitude over most of the sky at 1 pc.

astro-ph.GA↗

arXiv2026

Updated estimates of post-merger gravitational wave energy released in GW170817 and the maximum neutron star mass

In this work, benefiting from the increased sample of neutron stars with measured masses and radii as well as the incorporation of the chiral effective field theory and perturbative QCD constraints, the tidal parameter $κ_2^T$ is constrained to be $78^{+17}_{-11}$ (68.3% credible interval, mainly adopted in this work unless mentioned specifically) for the binary neutron stars involved in GW170817. Such a $κ_2^T$ is in favor of strong gravitational wave radiation in the post-merger phase and the corresponding energy is estimated to be $E_{\rm GW,p} \simeq 0.051^{+0.022}_{-0.017}\,M_\odot c^2$. Assuming the remnant from binary neutron star merger is a supramassive neutron star, as suggested by the modeling of the electromagnetic counterparts of GW170817, we examine the maximum mass of nonrotating neutron stars ($M_{\rm TOV}$) while accounting for the uncertainty in the remnant's lifetime ($t_{\rm c}$). Our results show that $M_{\rm TOV}$ varies from $2.09^{+0.11}_{-0.09}\,M_\odot$ for $t_{\rm c}=0.1$ s to $2.18^{+0.10}_{-0.09}\,M_\odot$ for $t_{\rm c}=1$ s. This result is consistent with that independently inferred from the re-construction of the equation of state of neutron star matter, i.e., $M_{\rm TOV,exc}=2.16^{+0.10}_{-0.07}M_\odot$, particularly if the very massive neutron stars with masses measured indirectly have been removed in constructing the prior distribution of $M_{\rm TOV}$. The consistency of the maximum mass of nonrotating neutron stars found in different approaches suggests a reasonable understanding of this key parameter for dense-matter physics.

astro-ph.HE↗

arXiv2026

Population-Level Verification of the Black-Hole Area Law with First- and Second-Generation Black Holes

Hawking's area theorem states that the total event-horizon area of classical black holes can never decrease. Gravitational-wave tests of this law have so far focused on individual loud mergers, thus probing only a sparse subset of binary parameter space. Here we perform the \emph{first} population-level test. We exploit the decomposition of the coalescing black holes in the latest gravitational-wave catalogue into a low-spin subpopulation of stellar-collapse origin and a high-spin subpopulation assembled through hierarchical mergers of the former. If the high-spin black holes are merger remnants, the area theorem requires their horizon areas to exceed the total pre-merger areas of the low-spin binaries. Using parameter estimation restricted to the inspiral of 241 events, so that no merger-ringdown information enters the inference, we find that both peaks of the horizon-area distribution of second-generation black holes lie above their first-generation counterparts. The displacement is significant at each peak ($2.1$--$3.7σ$), and when we tie the two subpopulations with a single common shift, an area decrease is excluded decisively ($\gtrsim5.0σ$). The second law of black-hole mechanics thus holds statistically across the quasicircular, moderately spinning mergers that dominate current catalogues, which in turn underpins the robustness of our classification of stellar-collapse and hierarchical-merger black holes.

astro-ph.HE↗

arXiv2026

Bekenstein--Hod Bound: A $3.3σ$ Confirmation from GW250114

Black holes link gravity, thermodynamics and information via limits on the relaxation rate of a perturbed system. The Bekenstein-Hod bound imposes a minimum relaxation time at fixed temperature, but its observational test demands both black hole thermodynamic characterization and decay time measurement. Here we test this bound with GW250114, the loudest gravitational-wave signal yet observed from a binary black-hole merger. We infer the remnant temperature from pre-merger data truncated at least $10\,M$ before the peak and its longest-lived decay time from post-merger data, thereby avoiding direct reuse of the same strain samples. The ringdown frequencies and damping times are allowed to vary independently rather than being fixed to the Kerr spectrum. For the primary $t_{<}=-10\,M$ analysis, the bound is verified at $3.3-3.6σ$ across the focal ringdown start times, representing a substantial improvement over the $91\%$ confidence level set by GW150914. The conclusion remains robust under varied pre-merger cutoffs and explicit inclusion of the short-lived first overtone in waveform modelling. This separated-data measurement converts an information-theoretic relaxation bound into a precision test of a single astrophysical black hole.

gr-qc↗

arXiv2026

Evidence for a $\sim 43$ GeV $γ$-ray line signal in a stacking analysis of the Virgo, Fornax, and Ophiuchus Galaxy clusters

As the largest gravitationally bound objects in the Universe, galaxy clusters have provided the first piece of evidence for the presence of dark matter and may be suitable targets for indirect dark matter searches. Among various signals, the GeV-TeV $γ$-ray line has been taken as the smoking-gun signal of the dark matter annihilation and decay since no known astrophysical or physical process(es) could generate such a peculiar spectrum. With 15.5 years of Fermi-LAT P8R3 publicly available data, we search for the $γ$-ray line emission in the directions of 13 massive galaxy clusters at redshifts $z \leq 0.028$ with an unbinned likelihood analysis. A $γ$-ray line signal at $\sim 43.2$ GeV has a net test statistic (TS) value of $\approx 30$ if we take into account only the data in the directions of the Virgo, Fornax, and Ophiuchus clusters, three massive clusters with the highest J-factors expected to generate the dark matter annihilation signal. The signal still presents when the data of 10 other nearby massive clusters have also been included, though the TS value decreases to $\approx 21$, likely because of their lower signal-to-noise ratios. The absence of this signal in the inner Galaxy disfavors both the instrumental effect and the canonical dark matter annihilation interpretation, and a more sophisticated dark matter model or very peculiar astrophysical scenario might be needed. This $γ$-ray line signal, if intrinsic, could be unambiguously verified by the Very Large Area $γ$-ray Space Telescope in its first two years of performance.

astro-ph.HE↗

arXiv2026

Revealing the $χ_{\rm eff}$-$q$ Correlation among Coalescing Binary Black Holes and Tentative Evidence for AGN-driven Hierarchical Mergers

The origin of the correlation between the effective spins ($χ_{\rm eff}$) and mass ratios ($q$) of LIGO-Virgo-KAGRA's binary black holes (BBHs) is still an open question. Motivated by the recent identification of two subpopulations of the BBHs, in this work we investigate the potential $χ_{\rm eff}-q$ correlation for each subpopulation. Surprisingly, the $χ_{\rm eff}$-$q$ correlation {either significantly weakens or disappears} for the low-mass subpopulation if we introduce a second $χ_{\rm eff}$ distribution for the high-mass subpopulation, which likely originates from hierarchical mergers. {This suggests that the $χ_{\rm eff}$-$q$ correlation in the overall population can be explained by the superposition of two distinct subpopulations.} {We find Bayesian evidence strongly favoring two separate $χ_{\rm eff}$ distributions over a single mass-ratio-dependent distribution, with Bayes factors $\ln\mathcal{B}>4.2$.} The first subpopulation has a narrow $χ_{\rm eff}$ distribution peaking at $\sim0.05$, whose primary-mass function {showing a rapid decline beyond} $\sim 40M_{\odot}$, in agreement with first-generation BBHs. The second $χ_{\rm eff}$ distribution is broad and peaks at $μ_{χ,2} \sim 0.4$, aligning with predictions for hierarchical mergers in active galactic nucleus (AGN) disks. {However, we cannot exclude negative $χ_{\rm eff}$values in the second subpopulation, suggesting hierarchical mergers might occur both in AGN disks and stellar clusters. Furthermore, the inferred second $χ_{\rm eff}$ distribution might alternatively arise from other formation channels, such as stable mass transfer or chemically homogeneous evolution, if not interpreted as hierarchical mergers.}

astro-ph.HE↗

arXiv2026

Aligned Hierarchical Black Hole Mergers in Active-Galactic-Nuclei Disks Revealed by GWTC-4

The active galactic nucleus (AGN) accretion disks are ideal sites for hierarchical black hole (BH) mergers. To robustly probe such a possibility, we analyze binary black hole mergers in the GWTC-4 with a flexible mixture population model for component masses, spin magnitudes, and spin tilt angles, and identify two distinct subpopulations. In the second subpopulation characterized by high spin magnitudes $χ\sim 0.8$ as well as the broad mass distribution up to $\gtrsim 150M_\odot$, we find a pronounced preference for spins aligned with the orbital angular momentum: an isotropic tilt distribution is strongly disfavored (logarithmic Bayes factor = 4.5). The aligned events account for $\sim 0.57^{+0.23}_{-0.31}$ of the second subpopulation, corresponding to a local rate of $\sim 0.25^{+0.38}_{-0.16} ~ {\rm Gpc}^{-3} {\rm yr}^{-1}$ (all values reflect central 90\% credible intervals). These notable features naturally arise from hierarchical mergers embedded in AGN disks, where gas torques may effectively align spins. Our results suggest that AGN-disk hierarchical assembly may be one important channel for the present gravitational-wave sample, and provide concrete, testable predictions for future detection.

astro-ph.HE↗

arXiv2026

Fast Radio Bursts Trace Cosmic Star Formation with Little Delay

The progenitor channels of fast radio bursts (FRBs) remain debated, with a central question being whether their cosmic rate traces star formation promptly or instead follows it with the long time-delay characteristic of compact-binary mergers. We perform a forward-modeling, hierarchical Bayesian analysis of the CHIME/FRB population, jointly fitting the catalog sample, baseband fluences, and localized host redshifts, while self-consistently incorporating the survey selection function through the injection framework. Across a range of delay-time models, the reconstructed FRB rate robustly peaks at the same redshift as the cosmic star-formation history, with a mean delay of only $0.1-0.3$ Gyr that remains consistent with a prompt, zero-delay origin at the $2σ$ level. For dominant FRB population, this finding rules out the multi-Gyr delays reported previously and interpreted as the evidence for compact binary merger origin, and instead points toward progenitor systems linked to young stellar remnants, most notably magnetars formed in core-collapse supernovae.

astro-ph.HE↗

arXiv2026

Smoking-gun evidence for hierarchical black-hole mergers

How stellar-mass black holes grow after their birth is a central open question in astrophysics. Gravitational-wave observations have revealed a subpopulation of coalescing black holes with both high masses and high spins, but whether these properties arise from hierarchical mergers in dense stellar environments or from accretion onto isolated black holes has remained unresolved. Here, using a flexible mixture population model applied to the 259 binary black hole mergers in GWTC-5, we show that the mass function of the high-spin subpopulation traces, peak by peak, the predicted remnant-mass distribution of the low-spin, stellar-collapse-origin subpopulation up to $\sim80\,M_\odot$. This morphological match, quantified by a Bhattacharyya coefficient as high as $\sim0.95$, is naturally expected if the high-spin black holes are themselves the products of earlier mergers, whereas any alternative scenario would require fine-tuning, thereby providing smoking-gun evidence for hierarchical mergers. In addition, the sharp upper-mass cutoff of the low-spin subpopulation at $m_{\rm max,1}=54.2^{+7.7}_{-7.2}\,M_\odot$ yields an astrophysical $S$-factor of $S_{300}=151^{+30}_{-26}$~keV~b (68\% credible interval) for the $^{12}{\rm C}(α,γ)^{16}{\rm O}$ reaction, in agreement with the benchmark theoretical value. These results establish that the entire observed black-hole population can be accounted for by stellar collapse followed by dynamical hierarchical assembly, without invoking primordial black holes.

astro-ph.HE↗

arXiv2026

Challenges in Binary Pulsar Timing Detection of Dark Matter Subhalos

Recently, binary pulsar timing has been proposed as a viable probe of dark matter subhalos with masses of $\sim 10^7\,M_{\odot}$ in the solar neighborhood. We present a comprehensive analytical framework that incorporates the subhalo mass function, projection effects of line-of-sight acceleration, and the spatiotemporal geometric requirements for joint detection by binary systems, enabling a quantitative evaluation of the detectability of nearby subhalos. Applying this framework to the current binary pulsar sample, we find a probability $\leq 1.7 \times 10^{-4}$ of detecting at least one subhalo within the effective volume. An independent timing residual analysis shows no statistically significant excess in line-of-sight accelerations beyond predictions from data-driven Galactic gravitational potential models. These results place stringent constraints on detecting $<10^8~M_{\odot}$ dark matter subhalos with existing pulsar timing data, aligning with the theoretical expectation that such subhalos have a low survival probability in the solar neighborhood. A low detection prospect still holds even for future Square Kilometre Array observations.

astro-ph.HE↗