arXiv subjects

Youjun Lu

Publications and source records attributed to Youjun Lu.

At least 37 records · Page 2Linked to original sources

arXiv2025

Searching for kilonova with the SiTian prototype telescope

We simulate the optical searching of gravitational-wave electromagnetic counterpart of the binary neutron star (BNS) merger event (i.e., a kilonova) using the ground based {\it SiTian} project prototype telescope with a 5-min limiting magnitude of 22.0, triggered by LIGO and Virgo gravitational wave detectors during the ongoing O4 run. Our simulations show that an average of 0.17-0.25 kilonova events can be observed over the entire O4 period of $\sim 2$ years in the most optimistic case we set, while no kilonova can be detected in other cases. We note that it is beneficial for {\it SiTian}'s kilonova searching by extending the exposure time to gain deeper limiting magnitude despite the rapid decline of kilonova luminosity.

astro-ph.HE↗

arXiv2025

"Frog-eyes" in Astronomy: Monitoring Binary Radial Velocity Variations Through A Pair of Narrow-Band Filters

Spectroscopic observations are a crucial step in driving major discoveries in the era of time-domain surveys. However, the pace of current spectroscopic surveys is increasingly unable to meet the demands of rapidly advancing large-scale time-domain surveys. To address this issue, we propose the ``Frog-eyes" system, which employs a pair of narrow-band filters: one positioned near a strong absorption line to capture signals from Doppler shifts, and the other placed on the adjacent continuum to monitor intrinsic variations. The combination of observations from the two filters enables the extraction of radial velocity (RV) curves from a large sample of binary stars, and is particularly efficient for single-lined binaries (SB1), using photometric techniques. Comprehensive mock simulations on SB1 demonstrate that the binary orbital parameters can be precisely measured from the extracted RV curves for binary systems where the primary star has an effective temperature greater than 6000 K. With a typical ground-based photometric precision of approximately 0.3%, the uncertainties in the derived semi-amplitude K and eccentricity e are less than 10% and 0.1, respectively, for binary systems with K $\ge$ 30 km/s. These encouraging results are further validated by real observations of the hot subdwarf-white dwarf binary system HD 265435, using a non-specialized ``Frog-eyes" system installed on the Chinese 2.16m telescope. Once this system is properly installed on large-field-of-view survey telescopes, the rate of acquiring RV curves for binaries will approach their detection rate in leading time-domain photometric surveys.

astro-ph.SR↗

arXiv2025

Spin evolution and mass distribution of the Galactic Binary Neutron Stars

Binary neutron stars (BNSs) detected in the Milky Way have the total masses distributing narrowly around $\sim2.6-2.7M_\odot$, while the BNS merger GW190425 detected via gravitational wave has a significantly larger mass ($\sim3.4M_\odot$). This difference is not well understood, yet. In this paper, we investigate the BNS spin evolution via an improved binary star evolution model and its effects on the BNS observability, with implementation of various relevant astrophysical processes. We find that the first-born neutron star component in low-mass BNSs can be spun up to millisecond pulsars by the accretion of Roche-lobe overflow from its companion and its radio lifetime can be comparable to the Hubble time. However, most high-mass BNSs have substantially shorter radio lifetime than the low-mass BNSs, and thus smaller probability being detected via radio emission. Adopting the star formation and metal enrichment history of the Milky Way given by observations, we obtain the survived Galactic BNSs with pulsar components from our population synthesis model and find that their distributions on the diagrams of spin period versus spin-period-time-derivative ($P-\dot{P}$) and orbital period versus eccentricity ($P_{\rm orb}-e$) can well match those of the observed Galactic BNSs. The total mass distribution of the observed Galactic BNSs can also be matched by the model. A significant fraction ($\sim19\%-22\%$) of merging BNSs at redshift $z\sim0$ have masses $\gtrsim3M_\odot$, which seems compatible with the GW observations. Future radio observations may detect many more Galactic BNSs, which will put strong constraint on the spin evolution of BNSs during their formation processes.

astro-ph.HE↗

arXiv2024

Constraining the Binarity of Massive Black Holes in the Galactic Center and Some Nearby Galaxies via Pulsar Timing Array Observations of Gravitational Waves

Massive black holes (MBHs) exist in the Galactic center (GC) and other nearby galactic nuclei. As natural outcome of galaxy mergers, some MBHs may have a black hole (BH) companion. In this paper, assuming that the MBHs in the GC and some nearby galaxies are in binaries with orbital periods ranging from months to years (gravitational-wave frequency $\sim1-100$\,nHz), we investigate the detectability of gravitational-waves from these binary MBHs (BBHs) and constraints on the parameter space for the existence of BBHs in the GC, LMC, M31, M32, and M87, that may be obtained by current/future pulsar timing array (PTA) observations. We find that a BBH in the GC, if any, can be revealed by the Square Kilometer Array PTA (SKA-PTA) if its mass ratio $q\gtrsim10^{-4}-10^{-3}$ and semimajor axis $a\sim20-10^3$\,AU. The existence of a BH companion of the MBH can be revealed by SKA-PTA with $\sim20$-year observations in M31 if $q\gtrsim10^{-4}$ and $a\sim10^2-10^4$\,AU or in M87 if $q\gtrsim10^{-5}$ and $a\sim10^3-2\times10^4$\,AU, but not in LMC and M32 if $q\ll1$. If a number of milli-second stable pulsars with distances $\lesssim0.1-1$\,pc away from the central MBH in the GC, LMC, M32, or M31, can be detected in future and applied to PTAs, the BH companion with mass even down to $\sim100M_\odot$, close to stellar masses, can be revealed by such PTAs. Future PTAs are expected to provide an independent way to reveal BBHs and low-mass MBH companions in the GC and nearby galaxies, improving our understandings of the formation and evolution of MBHs and galaxies.

astro-ph.HE↗

arXiv2024

Constraining the Galactic Structure using Time Domain Gravitational Wave Signal from Double White Dwarfs Detected by Space Gravitational Wave Detectors

The Gravitation Wave (GW) signals from a large number of double white dwarfs (DWDs) in the Galaxy are expected to be detected by space GW detectors, e.g., the Laser Interferometer Space Antenna (LISA), Taiji, and Tianqin in the millihertz band. In this paper, we present an alternative method by directly using the time-domain GW signal detected by space GW detectors to constrain the anisotropic structure of the Galaxy. The information of anisotropic distribution of DWDs is naturally encoded in the time-domain GW signal because of the variation of the detectors' directions and consequently the pattern functions due to their annual motion around the sun. The direct use of the time-domain GW signal enables simple calculations, such as utilizing an analytical method to assess the noise arising from the superposition of random phases of DWDs and using appropriate weights to improve the constraints. We investigate the possible constraints on the scale of the Galactic thin disk and bulge that may be obtained from LISA and Taiji by using this method with mock signals obtained from population synthesis models. We further show the different constraining capabilities of the low-frequency signal (foreground) and the high-frequency signal (resolvable-sources) via the Markov Chain Monte Carlo method, and find that the scale height and length of the Galactic thin disk and the scale radius of bulge can be constrained to a fractional accuracy of ~ 30%, 30%, 40% (or 20%, 10%, 40%) by using the low-frequency (or high-frequency) signal detected by LISA or Taiji.

astro-ph.GA↗

arXiv2024

Detection Rate of Galaxy Cluster Lensed Stellar Binary Black Hole Mergers by the Third-generation Gravitational Wave Detectors

Gravitational waves (GWs) from stellar binary black hole (sBBH) mergers can be strongly gravitational lensed by intervening galaxies/galaxy clusters. Only a few works investigated the cluster-lensed sBBH mergers by adopting oversimplified models, while galaxy-lensed ones were intensively studied. In this paper, we estimate the detection rate of cluuster-lensed sBBH mergers with the third-generation GW detectors and its dependence on the lens models. We adopt detailed modeling of galaxy cluster lenses by using the mock clusters in the Synthetic Sky Catalog for Dark Energy Science with LSST (CosmoDC2) and/or approximations of the pseudo-Jaffe profile or an eccentric Navarro-Frenk-White dark matter halo plus a bright central galaxy with singular isothermal sphere profile. Considering the formation of sBBH mergers dominates by the channel of evolution of massive binary stars (EMBS), we find that the detection rate of cluster-lensed sBBHs is $\sim5-84$ yr$^{-1}$, depending on the adopted lens model and uncertainty in the merger rate density, and it is about $\sim{13_{-2.0}^{+28}}$yr$^{-1}$ if adopting relatively more realistic galaxy clusters with central main and member galaxies in the CosmoDC2 catalog, close to the estimated detection rate of sBBH mergers lensed by galaxies. In addition, we also consider the case that the production of sBBH mergers dominated by the dynamical interactions in dense stellar systems. We find that the detection rate of cluster-lensed sBBHs if from the dynamical channel is about $1.5$ times larger than that from the EMBS channel and the redshift distribution of former peaking at a higher redshift ($\sim3$) compared with that from latter ($\sim2$).

astro-ph.HE↗

arXiv2024

Constraining the origin of the nanohertz gravitational-wave background by pulsar timing array observations of both the background and individual supermassive binary black holes

The gravitational waves (GWs) from supermassive binary black holes (BBHs) are long sought by pulsar timing array experiments (PTAs), in the forms of both a stochastic GW background (GWB) and individual sources. The evidence for a GWB was reported recently by several PTAs with origins to be determined. Here we use a BBH population synthesis model to investigate the detection probability of individual BBHs by the Chinese PTA (CPTA) and the constraint on the GWB origin that may be obtained by PTA observations of both GWB and individual BBHs. If the detected GWB signal is entirely due to BBHs, a significantly positive redshift evolution ($\propto(1+z)^{2.07}$) of the mass scaling relation between supermassive black holes and their host galaxies is required. In this case, we find that the detection probability of individual BBHs is $\sim85\%$ or 64% if using a period of 3.4-year CPTA observation data, with an expectation of $\sim1.9$ or 1.0 BBHs detectable with a signal-to-noise ratio $\geq3$ or $5$, and it is expected to increase to $>95\%$ if extending the observation period to $5$ years or longer. Even if the contribution from BBHs to the GWB power signal is as small as $\sim10\%$, a positive detection of individual BBHs can still be expected within an observation period of $\sim10$ years. A non-detection of individual BBHs within several years from now jointly with the detected GWB signal can put a strong constraint on the upper limit of the BBH contribution to the GWB signal and help identify/falsify a cosmological origin.

astro-ph.HE↗

arXiv2024

A high-velocity star recently ejected by an intermediate-mass black hole in M15

The existence of intermediate-mass black holes (IMBHs) is crucial for understanding various astrophysical phenomena, yet their existence remains elusive, except for the LIGO-Virgo detection. We report the discovery of a high-velocity star J0731+3717, whose backward trajectory about 21 Myr ago intersects that of globular cluster M15 within the cluster tidal radius. Both its metallicity [Fe/H] and its alpha-to-iron abundance ratio [$α$/Fe] are consistent with those of M15. Furthermore, its location falls right on the fiducial sequence of the cluster M15 on the color-absolute magnitude diagram, suggesting similar ages. These support that J0731+3717 is originally associated with M15 at a confidence level of "seven nines". We find that such a high-velocity star ($V_{\rm ej} = 548^{+6}_{-5}$ km s$^{-1}$) was most likely tidally ejected from as close as one astronomical unit to the center of M15, confirming an IMBH ($\ge 100 M_{\odot}$ with a credibility of 98%) as the exclusive nature of the central unseen mass proposed previously.

astro-ph.GA↗

arXiv2024

Reply to Comment on "A slightly oblate dark matter halo revealed by a retrograde precessing Galactic disk warp"

In this reply, we present a comprehensive analysis addressing the concerns raised by Dehnen et al. (2024) regarding our recent measurement of the disk warp precession using the `motion-picture' method (Huang et al. 2024). We carefully examine the impact of ignoring the twist of the disk warp and the so-called $R$-$τ$ correlation on the estimation of the precession rate. The results indicate that the effect is minor and does not exceed the systematic and statistical uncertainties. Using N-body+SPH simulation data, we confirm that the `motion-picture' technique is effective in measuring retrograde precession of disk warp in stellar populations younger than 170 Myr, similar to classical Cepheids. Therefore, the overall conclusions of Huang et al. (2024) remain robust.

astro-ph.GA↗

arXiv2024

On the Cosmic Variance of the Merger Rate Density of Binary Neutron Stars

The cosmic variance on the star formation history may lead to bias to the merger rate density estimation of binary neutron star (BNS) mergers by the compact binary population synthesis. In this paper, we take the advantage of the large boxsize of the Millennium Simulation combined with the semi-analytic galaxy formation model GABE, and the parameterized population binary star evolution (BSE) model to examine how much effect will the cosmic variance introduce on the estimation of merger rate density of BNS mergers. We find that for sub-box size of $100\rm Mpc$ and $200\rm Mpc$, the variance of merger rate density $σ_{\rm R}/\rm R$ at different redshift is about $23\%-35\%$ and $13\%-20\%$ respectively. On one hand, as for the variance of the detection rate on BNS mergers with current LIGO-Virgo-KAGRA (LVK) detector network, this value is very small $\lesssim 10\%$, which indicates ignoring the cosmic variance is reasonable for estimating the merger rate density from current LVK observation. On the other hand, with next-generation gravitational wave detectors, it is possible to localize BNS mergers within sub-boxes possessing length of $\rm 40 Mpc$ for source redshift $z_{s}<0.2$. In such a small box, the cosmic variance of the merger rate density is significant, i.e., the value of $σ_{\rm R}/\rm R$ is about $\sim 55\%$. This hints that estimating the merger rate density of BNS in different sky areas may provide useful information on the cosmic variance.

astro-ph.HE↗

arXiv2024

Direct Method to Compute Doppler Beaming Factors in Binary Stars

The Doppler beaming effect, induced by the reflex motion of stars, introduces flux modulations and serves as an efficient method to photometrically determine mass functions for a large number of close binary systems, particularly those involving compact objects. In order to convert observed beaming-flux variations into a radial-velocity curve, precise determination of the beaming factor is essential. Previously, this factor was calculated as a constant, assuming a power-law profile for stellar spectra. In this study, we present a novel approach to directly compute this factor. Our new method not only simplifies the computation, especially for blue bands and cool stars, but also enables us to evaluate whether the relationship between beaming flux and radial velocity can be accurately described as linear. We develop a python code and compute a comprehensive beaming-factor table for commonly used filter systems covering main-sequence, subgiant, and giant stars, as well as hot subdwarf and white dwarf stars. Both the code and our table are archived and publicly available at http://doi.org/10.5281/zenodo.13049419.

astro-ph.SR↗

arXiv2024

A slightly oblate dark matter halo revealed by a retrograde precessing Galactic disk warp

The shape of the dark matter (DM) halo is key to understanding the hierarchical formation of the Galaxy. Despite extensive efforts in recent decades, however, its shape remains a matter of debate, with suggestions ranging from strongly oblate to prolate. Here, we present a new constraint on its present shape by directly measuring the evolution of the Galactic disk warp with time, as traced by accurate distance estimates and precise age determinations for about 2,600 classical Cepheids. We show that the Galactic warp is mildly precessing in a retrograde direction at a rate of $ω= -2.1 \pm 0.5 ({\rm statistical}) \pm 0.6 ({\rm systematic})$ km s$^{-1}$ kpc$^{-1}$ for the outer disk over the Galactocentric radius [$7.5, 25$] kpc, decreasing with radius. This constrains the shape of the DM halo to be slightly oblate with a flattening (minor axis to major axis ratio) in the range $0.84 \le q_Φ \le 0.96$. Given the young nature of the disk warp traced by Cepheids (less than 200 Myr), our approach directly measures the shape of the present-day DM halo. This measurement, combined with other measurements from older tracers, could provide vital constraints on the evolution of the DM halo and the assembly history of the Galaxy.

astro-ph.GA↗

arXiv2024

On Variation of Light Curves and Broad Emission Lines for Periodic QSOs from co-rotating Supermassive binary black holes in elliptical orbits

Context. Periodic QSOs are considered as candidates of supermassive binary black hole (BBH) systems in galactic centers. Further confirmation of these candidates may require different lines of observational evidences. Aims. Assuming the Doopler boosting scenario, in this paper we investigate the (coherent) variations of both broad emission lines (BELs) and continuum light curves for active BBH systems surrounding by a circumbinary broad line region (cBLR) and focus on their dependence on the eccentric orbital configuration. Methods. We calculate the variation of continuum light and the Doppler enhanced/weakened photoionization of each BLR cloud according to the motion of BBHs in elliptical orbits, and finally obtain the coherent variation of the continuum and BELs. Results. We find that both the amplitude and variation pattern of the continuum light curves and the evolution of the BEL profiles sensitively depend on the eccentric orbital configuration of BBH systems. If only the secondary BH is active, both the variation amplitudes of continuum light curves and BELs increase with increasing BBH inclination angles and orbital eccentricities, but decrease with increasing BBH mass ratio. If both BHs are active, the asymmetry in the ionization of BLR clouds at different areas caused by the Doppler boosting effect of the secondary BH is weakened due to that of the primary BH at the opposite direction, which leads to systematically smaller variation amplitudes of both continuum light curves and BELs than the cases with only secondary BH activated. Conclusions. The coherent variations of the BEL profiles with the continuum light for those periodic QSOs provide an important way to confirm the existence of BBHs in their center. Future joint analysis of the light curves and multi-epoch observed BEL profiles for periodic QSOs may lead to the identification of a number of BBH systems.

astro-ph.GA↗

arXiv2024

Constraining the Yukawa Gravity with Post Newtonian Approximation using S-star Orbits around the Supermassive Black Hole in our Galactic Center

A number of modified gravity theories (e.g., $f(R)$-gravity) lead to a Yukawa-like metric in the weak field limit which can be described by two Yukawa parameters, i.e., the strength $κ$ and the length scale $λ$. The S-stars, orbiting around the supermassive black hole in the Galactic Center, are unique probes to test these gravity theories in relatively strong gravitational field. The Newtonian Yukawa gravity potential or a simple approximation to the Yukawa metric was usually adopted in previous studies when using the orbital motion of S-stars to constrain such modified gravity theories, which may be not sufficiently accurate considering recent and future high resolution observations. In this paper, we first derive the Post-Newtonian (PN) Yukawa motion equation at the 2PN order, and then investigate the high order effects on the orbital motions by comparison with those from the Newtonian Yukawa gravity potential. We further obtain constraints on $κ$ by using the observations on the orbital motions of several S-stars (i.e., S2, S38, and S55). Our results show that the current observations of these stars are compatible with the General Relativity and $κ$ can be constrained to $|κ|<0.01$ with $95\%$ confidence if $λ\in(100,250)$\,AU. We also estimate the possible improvements (about an order of magnitude or more) to the constraints by future higher resolution observations and the inclusion of closer S-stars, such as S4716.

gr-qc↗

arXiv2024

Detectability of Strongly Gravitationally Lensed Tidal Disruption Events

More than 100 tidal disruption events (TDEs) have been detected at multi-bands, which can be viewed as extreme laboratories to investigate the accretion physics and gravity in the immediate vicinity of massive black holes. Future transient surveys are expected to detect several tens of thousands of TDEs, among which a small fraction may be strongly gravitationally lensed by intervening galaxies. In this paper, we statistically etsimate the detection rate of lensed TDEs, with dependence on the limiting magnitude of the transient all-sky surveys searching for them. We find that the requisite limiting magnitude for an all-sky transient survey to observe at least $1$ yr$^{-1}$ is $\gtrsim 21.3$, $21.2$, and $21.5$ mag in the $u$, $g$, and $z$ bands, respectively. If the limiting magnitude of the all-sky survey can reach $\sim 25-26$ mag in the $u$, $g$, and $z$ bands, the detection rate can be upto about several tens to hundreds per year. The discovery and identification of the first image of the lensed TDE can be taken as an early warning of the second and other subsequent images, which may enable detailed monitoring of the pre-peak photometry and spectroscopy evolution of the TDE. The additional early-stage information may help to constrain the dynamical and radiation processes involving in the TDEs.

astro-ph.HE↗

arXiv2024

On using the counting method to constrain the anisotropy of kilonova radiation

A large number of binary neutron star (BNS) mergers are expected to be detected by gravitational wave (GW) detectors and the electromagnetic (EM) counterparts (e.g., kilonovae) of a fraction of these mergers may be detected in multi-bands by large area survey telescopes. For a given number of BNS mergers detected by their GW signals, the expected numbers of their EM counterparts that can be detected by a survey with given selection criteria depend on the kilonova properties, including the anisotropy. In this paper, we investigate whether the anisotropy of kilonova radiation and the kilonova model can be constrained statistically by the counting method, i.e., using the numbers of BNS mergers detected via GW and multi-band EM signals. Adopting simple models for the BNS mergers, afterglows, and a simple two (blue and red)-component model for kilonovae, we generate mock samples for GW detected BNS mergers, their associated kilonovae and afterglows detected in multi-bands. By assuming some criteria for searching the EM counterparts, we simulate the observations of these EM counterparts and obtain the EM observed samples in different bands. With the numbers of BNS mergers detected by GW detectors and EM survey telescopes in different bands, we show that the anisotropy of kilonova radiation and the kilonova model can be well constrained by using the Bayesian analysis. Our results suggest that the anisotropy of kilonova radiation may be demographically and globally constrained by simply using the detection numbers of BNS mergers by GW detectors and EM survey telescopes in multi-bands.

astro-ph.HE↗

arXiv2023

On the detection of the electromagnetic counterparts from lensed gravitational wave events by binary neutron star mergers

Future ground-based gravitational wave (GW) detectors, i.e., Einstein telescope (ET) and Cosmic Explorer (CE), are expected to detect a significant number of lensed binary neutron star (BNS) mergers, which may provide a unique tool to probe cosmology. In this paper, we investigate the detectability of the optical/infrared electromagnetic (EM) counterparts (kilonovae/afterglows) from these lensed BNS mergers by future GW detectors and EM telescopes using simple kilonova, afterglow, and lens models. ET and CE are expected to detect $\sim5.32^{+26.1}_{-5.10}$ and $67.3^{+332}_{-64.7}$ lensed BNS mergers per year. We find that the EM counterparts associated with all these mergers will be detectable by an all sky-survey in the H-band with the limiting magnitude $m_{\textrm{lim}}\gtrsim27$, while the detectable fraction is $\lesssim0.4\%$ in the g-/z-band if with $m_{\textrm{lim}}\lesssim24$. Generally it is more efficient to search the lensed EM counterparts by adopting the infrared bands than the optical/UV bands with the same $m_{\textrm{lim}}$. Future telescopes like Vera C. Rubin Observatory, China Space Station Telescope, and Euclid can hardly detect the EM counterparts of even one lensed BNS merger. Roman Space Telescope (RST) and James Webb Space Telescope (JWST) have the capability to detect about a few or more such events per year. Moreover, the time delays and separations between the lensed image pairs are typically in the ranges from minutes to months and from $0.1$ to $1$\,arcsec, suggesting that both the GW and EM images of most lensed BNS mergers can be well resolved by not only CE/ET in the time domain but also RST/JWST spatially.

astro-ph.HE↗

arXiv2023

Diffractive lensing of nano-Hertz gravitational waves emitted from supermassive binary black holes by intervening galaxies

Pulsar timing array (PTA) experiments are expected to detect nano-Hertz gravitational waves (GWs) emitted from individual inspiralling supermassive binary black holes (SMBBHs). The GW signals from a small fraction of these SMBBHs may be diffractively lensed by intervening galaxies. In this paper, we investigate the diffractive lensing effects on the continuous GW signals from the lensed SMBBHs and estimate the detectable number of such signals by PTAs, such as the Chinese PTA (CPTA) and the Square Kilometer Array (SKA) PTA. We find that the amplitude of the lensed GW signals may be only amplified by a factor of $\sim 1.01-1.14$ ($16\%-84\%$ range) and the phase of the signals may shift somewhat due to the lensing, significantly different from those strongly lensed high frequency GW signals from compact binary mergers in the geometric optics. We estimate that $\sim 0.01\%$ of all detected nano-Hertz GW signals from individual SMBBHs by future PTA experiments are lensed by foreground galaxies (i.e., up to $\sim 106$ for CPTA and up to $\sim 289$ for SKA-PTA). However, the lensed nano-Hertz GW signals are difficult to be distinguished from those without lensing by the PTA observations only. We further discuss the possibility about the identification of the lensed nano-Hertz GW signals from SMBBHs via the electromagnetic detection of their host galaxies or active galactic nuclei.

astro-ph.CO↗