Searcharxiv⌕ Search

arXiv subjects

Liying Zhu

Publications and source records attributed to Liying Zhu.

14 recordsLinked to original sources
arXiv2026

IY Lyr: A Thick-Disk first-overtone RR Lyrae Star with a Possible Neutron Star Companion

IY Lyr, historically misclassified as an eclipsing binary, has been previously identified as a first-overtone RR Lyrae star (RRc star). Using multiband photometry (All-Sky Automated Survey for Supernovae, Zwicky Transient Facility, TESS, and our BVRI data), Large Sky Area Multi-Object Fiber Spectroscopic Telescope spectroscopy, and Gaia astrometry, we investigate its pulsation, binarity, and Galactic population. From O-C analysis, we detect a long-term period decrease and a light-travel time effect with an orbital period of 3.94 $\pm$ 0.09 years, eccentricity of 0.46 $\pm$ 0.15, and a mass function of 0.65 $\pm$ 0.14 M$_{\odot}$. The companion is independently supported by radial velocity residuals and Gaia proper motions. Combined constraints yield an orbital inclination of 94.2$^{\circ}$ $\pm$ 1.1$^{\circ}$ and a companion mass of 1.37 $\pm$ 0.19 M$_{\odot}$. Chemical abundances ([Fe/H] $\simeq$ -1.0 $\pm$ 0.1, [$α$/Fe] $\simeq$ +0.27 $\pm$ 0.03, Xiang et al. 2019) and dynamics ($L_{\rm z}$ $\simeq$ 1287 $\pm$ 35 kpc km s$^{-1}$, $Z_{\rm max}$ $\simeq$ 1.17 $\pm$ 0.10 kpc) identify IY Lyr as likely an old, high-$α$, thick-disk star. The companion mass lies at the peak of the neutron star mass distribution, and the system's age excludes a main-sequence star; we conclude the companion is most likely a typical neutron star, although a massive white dwarf near the Chandrasekhar limit cannot be ruled out. IY Lyr is among the few RRc binaries with a compact companion supported by multiple methods, and it has important implications for thick-disk binary evolution and neutron star formation.

astro-ph.SR↗
arXiv2026

CW Cas: A solar-type contact binary system with an unseen third companion in a hierarchical quadruple system

We present a comprehensive multiband photometric and spectroscopic study of the G-type binary CW Cas whose parameters have not been well determined. Our double-lined spectroscopic radial velocity curve of this system yields a reliable mass ratio of $q = 1.88(9)$. By combining $BVR_{c}I_{c}$ bands, TESS light curves and radial velocity curves, we found that CW Cas is a W-subtype shallow contact binary with a fill-out factor of 15\%. The components have masses of $0.98(6)M_{\odot}$ and $0.52(4)M_{\odot}$, separated by $2.25(5)R_{\odot}$. A notable asymmetry in the maxima of the light curves was detected and explained by a dark spot located on the surface of at least one component. Comparison of light curves from different years revealed that these dark spot activities exhibit cyclic variations with an approximate period of 1250 days. Orbital period analysis via O-C diagram spanning 125 years shows a long-term decrease superimposed with periodic oscillation caused by the light-travel-time effect (LTTE) due to a third body. This tertiary component has an orbital period of $P_{3} = 99.4(6)$ years and a minimal mass of $M_{3}=0.91(1)M_{\odot}$. The absence of detectable signatures for this massive object in either spectroscopic or photometric datasets implies it must be a compact object such as a white dwarf or neutron star. Furthermore, a visual companion was identified based on Gaia DR3 astrometric data, suggesting that CW Cas is part of a hierarchical quadruple system. As such, CW Cas represents a valuable laboratory for probing 2+1+1 hierarchical multiple system hosting compact object.

astro-ph.SR↗
arXiv2026

Long Photometric Cycles in Double Periodic Variables from Nodal Precession of a Tilted Accretion Disk

We investigate whether the long photometric cycles observed in double-periodic variables (DPVs) can arise from nodal precession of a tilted accretion disk driven by the tidal torque of the companion. Within a simple analytical framework, we derive testable relations linking the long-to-orbital period ratio to the binary mass ratio, the normalized disk size, and the disk tilt angle $β$, which itself can be inferred from the long-cycle amplitude, orbital inclination $i$, and disk luminosity fraction. The model naturally reproduces the two observed long-cycle light-curve morphologies -- sinusoidal and double-hump -- distinguished by the geometric criterion $i+β\le 90^\circ$ versus $i+β>90^\circ$. Applying these relations to a sample of DPVs, we find that the inferred disk sizes are physically reasonable and consistent with independent light-curve modeling for a non-negligible subset of systems. Our results show that tidal nodal precession represents a viable and potentially important contributor to the long-period variability of DPVs and provide a quantitative framework for future observational and theoretical studies.

astro-ph.SR↗
arXiv2026

A unified framework for hot accretion flows with finite angular momentum: from Bondi-like to disc-like regimes

Observations of X-ray luminous elliptical galaxies suggest that the accretion rate onto the central supermassive black hole can reach a substantial fraction of the Bondi rate. However, classical accretion theory applicable to such hot accretion flows treats spherically symmetric Bondi accretion and disc-like advection-dominated accretion flows (ADAFs) as two distinct limiting cases, lacking a unified framework for flows with finite angular momentum. In this work, we develop such a framework that continuously connects these two regimes. Our model naturally recovers the Bondi solution in the limit of vanishing angular momentum and approaches the properties of classical ADAFs at high angular momentum, while providing a physically well-defined description of the intermediate regime where neither limiting case is strictly applicable. We further demonstrate that the accretion rate is jointly regulated by the angular momentum of the ambient gas and the gas viscosity. For sufficiently large but physically reasonable viscosity, the accretion rate can remain at a significant fraction of the Bondi rate even in the presence of substantial gas rotation. These results offer a natural explanation for how such accretion rates can be sustained despite finite angular momentum in realistic galactic environments.

astro-ph.HE↗
arXiv2025

Accretion rates of stellar-mass compact objects embedded in AGN discs

Stellar-mass compact objects (COs) embedded in active galactic nucleus (AGN) discs are commonly assumed to accrete via Bondi or Bondi-Hoyle-Lyttleton (BHL) prescriptions, neglecting gas angular momentum. We show that differential rotation in AGN discs can impart non-negligible angular momentum, in which case accretion proceeds through a viscous disc rather than Bondi/BHL flow. Our model provides a new framework estimating the CO accretion rate as $\dot{M}_\mathrm{CO} = \min\{\dot{M}_\mathrm{vis}, \dot{M}_\mathrm{BHL}\}$, where the viscous rate $\dot{M}_\mathrm{vis}$ accounts for gas--CO relative motion decomposed into a local gradient term (due to differential rotation) and bulk motion (from differing orbital parameters). This rate can be expressed as $\dot{M}_\mathrm{vis} = αξ(r_\mathrm{H}/r_\mathrm{BHL})^3\dot{M}_\mathrm{BHL}$, where $ξ$ is a coefficient of order unity. It can also be approximately scaled to the global AGN accretion rate as $\dot{M}_\mathrm{vis} \propto \dot{M}_1$, with the scaling coefficients in both forms determined by the specific dynamical configuration. The accretion is viscosity-limited when $q > [αξ(1+\mathcal{M}^2)^{3}/3]^{1/2} h^3$, where $q$ is the mass ratio between the CO and the supermassive black hole, $α$ the viscosity parameter, $\mathcal{M}$ the Mach number of the bulk relative motion, and $h$ the aspect ratio of the AGN disc. In thin AGN discs this condition is satisfied for most stellar-mass or more massive COs. Our framework also naturally allows for the inclusion of established outflow corrections, thereby enabling a more realistic treatment of super-Eddington flows. Our formulation thus improves upon Bondi/BHL prescriptions and offers a more physically motivated basis for studying CO evolution in AGN environments.

astro-ph.HE↗
arXiv2025

Spherically Symmetric Accretion with Self-Gravity: Analytical Formulae and Numerical Validation

Spherically symmetric accretion incorporating self-gravity constitutes a three-point boundary value problem (TPBVP) governed by constraints at the outer boundary, sonic point, and accretor surface. Previous studies have two limitations: either employing an incorrect formula for self-gravity potential in analytical treatments, or introducing additional input parameters in numerical implementations to circumvent solving the full TPBVP. To address these issues, we present a self-consistent TPBVP formulation, solved using the relaxation method. We also derive approximate analytical formulae that enable rapid estimates of self-gravity effects. Our analysis identifies a dimensionless parameter $β\equiv 2G \barρ r_\mathrm{out}^2/a_\mathrm{out}^2$ that characterizes the strength of self-gravity, where $\barρ$ and $r_\mathrm{out}$ are the mean density and outer radius of the flow, respectively, and $a_\mathrm{out}$ is the adiabatic sound speed of the external medium. For practical estimation, $\barρ$ may be approximated by the external medium density $ρ_\mathrm{out}$. We identify an upper limit for $β$, beyond which steady accretion becomes unsustainable -- a behavior consistent with classical gravitational instability that previous studies failed to capture. The accretion rate enhancement decreases monotonically as the adiabatic index $γ$ increases. For $γ=5/3$, self-gravity ceases to augment the accretion rate. These theoretical predictions are validated by our numerical solutions. We further apply our results to two astrophysical scenarios: hyper-Eddington accretion onto supermassive black hole seeds in the early Universe, where self-gravity is significant; and accretion onto stellar-mass objects embedded in active galactic nuclei (AGN) disks, where self-gravity is non-negligible under certain conditions and should be evaluated using $β$.

astro-ph.HE↗
arXiv2024

KM UMa: An active short-period detached eclipsing binary in a hierarchical quadruple system

The first detailed photometric and spectroscopic analysis of the G-type eclipsing binary KM UMa is presented, which indicates that the system is a short-period detached eclipsing binary. The radial velocity curves were calculated using the cross-correlation function method based on Large Sky Area Multi-Object Fiber Spectroscopic Telescope, Sloan Digital Sky Survey, and our observations, which determined the mass ratio as $q=0.45\ (\pm0.04)$. Based on the light curves from the Transiting Exoplanet Survey Satellite, other survey data, and our multiband observations, the positive and negative O'Connell effects have been detected evolving gradually and alternately over the last 20 yr, which can be explained by the presence of spots on the primary component. A superflare event was detected in the SuperWASP data on 2007 February 28, further indicating that KM UMa is a very active system. We calculated its energy to be $5\times10^{34}$ erg by assuming it occurred on the primary star. Utilizing hundreds of medium-resolution spectra and one low-resolution spectrum, the equivalent width variations of the $H_α$ line were calculated, indicating the presence of a 5.21 ($\pm0.67$) yr magnetic activity cycle. The orbital period variations were analyzed using the O-C method, detecting a long-term decrease superimposed with a periodic variation. The amplitude of the cyclic variation is $0.01124\ (\pm0.00004)$ day, with a period of $33.66\ (\pm 0.0012)$ yr, which exceeds the 5.21 yr activity cycle, suggesting that this is more likely attributable to the light travel time effect of a third body. Simultaneously, a visual companion has been detected based on the Gaia astrometric data, indicating that KM UMa is actually in a 2+1+1 hierarchical quadruple system.

astro-ph.SR↗
arXiv2024

Deep and low mass-ratio contact binaries and their third bodies

Deep and low mass-ratio contact binaries (DLMCBs) are believed to be in the final stage of their contact phase, potentially leading to the formation of fast-rotating single stars such as FK Com-type stars and blue stragglers, as well as luminous red novae. These systems serve as an excellent laboratory for studying stellar coalescence and merging processes. Our search for DLMCBs began in 2004 and has since identified a group of such systems. Together with that collected from the literature, more than 100 DLMCBs have been detected so far. Half of them have had their periods investigated based on O-C curves. Some have shown period increases, while others have exhibited period decreases. Among them, more than half DLMCBs have cyclic variations, suggesting the possibility of the existence of a third body orbiting around the DLMCBs. Furthermore, with more data obtained extending the span of the O-C curve, more cyclic variations could be detected. The high proportion of signs of the presence of third bodies makes them an essential factor to consider when studying the merger of contact binaries.

astro-ph.SR↗
arXiv2024

Electrical Impedance Tomography Based Closed-loop Tumor Treating Fields in Dynamic Lung Tumors

Tumor Treating Fields (TTFields) is a non-invasive anticancer modality that utilizes alternating electric fields to disrupt cancer cell division and growth. While generally well-tolerated with minimal side effects, traditional TTFields therapy for lung tumors faces challenges due to the influence of respiratory motion. We design a novel closed-loop TTFields strategy for lung tumors by incorporating electrical impedance tomography (EIT) for real-time respiratory phase monitoring and dynamic parameter adjustments. Furthermore, we conduct theoretical analysis to evaluate the performance of the proposed method using the lung motion model. Compared to conventional TTFields settings, we observed that variations in the electrical conductivity of lung during different respiratory phases led to a decrease in the average electric field intensity within lung tumors, transitioning from end-expiratory (1.08 V/cm) to end-inspiratory (0.87 V/cm) phases. Utilizing our proposed closed-Loop TTFields approach at the same dose setting (2400 mA, consistent with the traditional TTFields setting), we can achieve a higher and consistent average electric field strength at the tumor site (1.30 V/cm) across different respiratory stages. Our proposed closed-loop TTFields method has the potential to improved lung tumor therapy by mitigating the impact of respiratory motion.

physics.med-ph↗
arXiv2024

The first low-mass eclipsing binary within the fully convective zone from TMTS

We present a comprehensive photometric and spectroscopic analysis of the short-period ($\sim$5.32 hours) and low-mass eclipsing binary TMTSJ0803 discovered by Tsinghua-Ma Huateng Telescope for Survey (TMTS). By fitting the light curves and radial velocity data with the Wilson--Devinney code, we find that the binary is composed of two late spotted active M dwarfs below the fully convective boundary. This is supported by the discovery of a significant Balmer emission lines in the LAMOST spectrum and prominent coronal X-ray emission. In comparison with the typical luminosity of rapidly rotating fully convective stars, the much brighter X-ray luminosity ($L_{X}/L_{\rm{bol}} = 0.0159 \pm 0.0059$) suggests the stellar magnetic activity of fully convective stars could be enhanced in such a close binary system. Given the metallicity of [M/H] = $-$ 0.35 dex as inferred from the LAMOST spectrum, we measure the masses and radii of both stars to be $M_{1} = 0.169 \pm 0.010~M_{\odot}$, $M_{2} = 0.162 \pm 0.016~M_{\odot}$, $R_{1} = 0.170 \pm 0.006~R_{\odot}$, and $R_{2} = 0.156 \pm 0.006~R_{\odot}$, respectively. Based on the luminosity ratio from the light curve modeling, the effective temperatures of two components are also estimated. In comparison with the stellar evolution models, the radii and effective temperatures of two components are all below the isochrones. The radius deflation might be mainly biased by a small radial velocity (RV) data or (and) a simple correction on RVs, while the discrepancy in effective temperature might be due to the enhanced magnetic activity in this binary.

astro-ph.SR↗
arXiv2021

Revisiting the X-ray Emission of the Asynchronous Polar V1432 Aql

As the only eclipsing asynchronous polar, V1432 Aql provides an excellent laboratory to study the interaction between the accreted matter and the magnetic field. Here, we report an analysis of the X-ray data from the contemporaneous \nustar\ and \xrt\ observations. The X-ray data present a profile with a low-intensity state for almost half an orbital period, a dip at 0.6 phase, and a peak at 0.75 phase, which suggests that there was only one accretion region during the observation and the claim is supported by the spectral analysis. The comparison with the previous data indicates that the X-ray data have an orbital modulation, as the case in \sax, rather than a spin one observed in \rosat. We attribute the orbit and spin modulations to the different accretion geometries at work. The spectral analysis of the wide-band data presents a significant reflection effect, a commonly observed soft X-ray temperature, and the energy balance in V1432 Aql . Additionally, we obtained a low total accretion rate of 1.3 $\times$ 10$^{-10}$ M$_{\odot}$ yr$^{-1}$ and a high specific accretion rate of 3.8 g cm$^{-2}$ s$^{-1}$ which explains the strong reflection from the surface of the white dwarf. However, due to its complex emission, a more physical understanding of its accretion geometry is still outstanding.

astro-ph.HE↗
arXiv2021

The Accretion Geometry of the Asynchronous Polar V1432 Aql

As the only eclipsing asynchronous polar (AP), V1432 Aql provides an excellent laboratory to study the interaction between the accreted matter and the magnetic field. However, due to its complex emission, a more physical understanding of its accretion geometry is still outstanding. Here, we report an X-ray spectral study using contemporaneous observations from \nustar\ and \swift. We detect significant Compton reflection and confirm earlier reports of a high soft X-ray temperature $\sim52$ keV. We suggest that the multi-temperature emission is due to a distribution of the specific accretion rate over the accretion region, which leads to a continuous temperature distribution over the heated area and explains the high temperature of the soft X-rays. We interpret these characteristics as the results of the inefficient accretion in APs. Thus the accretion stream can punch deeply into the magnetosphere and feed the white dwarf (WD) over a much narrower accretion region near its equator. Additionally, the broad-band X-rays provide details of the accretion; the low total accretion rate of $\sim 1 \times 10^{-10} ~M_{\odot} ~yr^{-1}$ contradicts the speculation that V1432 Aql was a recent nova, while the high specific accretion rate of $\sim 5.6 ~g ~cm^{-2} ~s^{-1}$ explains the significant reflection from the surface of the WD.

astro-ph.HE↗
arXiv2012

Visual minima timings of eclipsing binaries: To use or not to use?

Plenty of mid-eclipse timings of short-periodic eclipsing binaries derived from series of visual observations appear to be an acceptable source of information for orbital period analyses, namely if they were done in time-intervals not covered by other types of observations. However, our thorough period analysis of the nearly contact eclipsing binary BS Vulpeculae proves that visually determined timings done in 1979--2003 were undoubtedly biased to accommodate the existing linear ephemeris. The heavily subjective character of visual observations disqualifies them as a source of true phase information apt for fine eclipsing binary period analyses. Consequently we warn against the use of visual timings without a preceding careful verification.

astro-ph.SR↗