arXiv subjects

Fupeng Zhang

Publications and source records attributed to Fupeng Zhang.

At least 19 recordsLinked to original sources

arXiv2025

Co-evolution of Nuclear Star Clusters and Massive Black Holes: Extreme Mass-Ratio Inspirals

We explore extreme mass-ratio inspirals (EMRIs) in the co-evolution of massive black holes (MBHs) and nuclear star clusters (NSCs), which host diverse stellar populations across a wide range of masses. The dynamics are simulated self-consistently with GNC, which we have updated to incorporate gravitational wave orbital decay, the loss cone of a spinning MBH, and stellar evolution. Over $12$ Gyr, we investigate the evolution of the NSC with a mass-growing MBH, as well as the EMRIs of stellar black holes, neutron stars, white dwarfs, brown dwarfs (BDs), and low-mass main-sequence stars (MSs), along with tidal disruption events (TDEs) involving MSs, BDs, and post-MSs. The mass growth of the MBH contributed by TDEs is typically $\sim 10^7\,M_{\odot}$, $\sim 10^6\,M_{\odot}$, and $\sim 5\times10^4\,M_{\odot}$ for massive, Milky-Way-like, and smaller NSCs, respectively. Between $40\%$ and $70\%$ of the stellar mass is lost during stellar evolution, which dominates the mass growth of the MBH if a significant fraction of the lost mass is accreted. The evolution of EMRI rates is generally affected by the cluster's size expansion or contraction, stellar population evolution, MBH mass growth, and the stellar initial mass function. The EMRI rates for compact objects peak at early epochs ($\lesssim 1$ Gyr) and then gradually decline over cosmic time. LISA-band ($0.1$ mHz) EMRIs involving compact objects around Milky-Way-like MBHs tend to have high eccentricities, while those around spinning MBHs preferentially occupy low-inclination (prograde) orbits. In contrast, MS- and BD-EMRIs usually have eccentricity and inclination distributions that are distinct from those of compact objects.

astro-ph.GA↗

arXiv2025

The Secular Periodic Evolution of X-ray Quasi-periodic Eruptions Driven by Star-disc Collisions

We study the secular periodic evolution of quasi-periodic eruptions (QPEs) for GSN069 and eRO-QPE2 assuming that they are driven by star-disc collisions. We set up numerical simulations and compared them with the observed periodic decay of $\sim -3160\pm720$ s yr$^{-1}$ in GSN069 and $\sim -370\pm40$ s yr$^{-1}$ in eRO-QPE2. We find that: (1) Stellar mass black holes are unlikely the orbiters in these two sources, as their periodic decay are on the order of $<10$ s yr$^{-1}$; (2) A naked degenerate core (including white dwarf) is unlikely the orbiter in GSN069, as the decay is on the order of $<200$ s yr$^{-1}$. However, it is possible in eRO-QPE2, although the required surface density of the accretion disc is relatively high (e.g., $\Sigma\gtrsim10^7\sim 10^8$ g cm$^{-2}$); (3) Both the orbiters in GSN069 and eRO-QPE2 can be solar-like main-sequence stars (MSs). However, each collision can lead to gradual ablation of the stellar envelope in the order of $10^{-5}\sim 10^{-3}M_\odot$. To reproduce the observed decay while surviving for $\gtrsim 3$ yr, the surface density of the disc needs to be within a certain range. For example, given a $1M_\odot$ MS orbiter the surface density of the disc gas should be in the range of $3\times10^5\sim 2\times10^6$g cm$^{-2}$ for GSN069 or $5\times10^4\sim 10^6$ g cm$^{-2}$ for eRO-QPE2. In both of these two sources, the MS can not survive for more than $\sim 12$ yr. We expect that future observations of these two sources can help to distinguish whether the orbiters are degenerated compact objects or gaseous stars.

astro-ph.HE↗

arXiv2025

Self-consistent Solutions of Evolving Nuclear Star Clusters with Two-Dimensional Monte-Carlo Dynamical Simulations

We recently developed a Monte-Carlo method (GNC) that can simulate the dynamical evolution of a nuclear stellar cluster (NSC) with a massive black hole (MBH), where the two-body relaxations can be solved by the Fokker-Planck equations in energy and angular momentum space. Here we make a major update of GNC~ by integrating stellar potential and adiabatic invariant theory, so that we can study the self-consistent dynamics of NSCs with increasing mass of the MBH. We perform tests of the self-adaptation of cluster density due to MBH mass growth and Plummer core collapse, both finding consistent results with previous studies, the latter having a core collapse time of $\sim 17t_{\rm rh}$ by GNC, where $t_{\rm rh}$ is the time of half-mass relaxation. We use GNC~ to study the cosmological evolution of the properties of NSC and the mass of MBH assuming that the mass growth of the MBH is due to loss-cone accretion of stars (e.g., tidal disruption of stars) and stellar black holes, and compare the simulation results with the observations of NSCs in Milky-Way or near-by galaxies. Such scenario is possible to produce MBHs with mass $10^5\sim 10^7\,M_\odot$ for NSCs with stellar mass of $10^6\sim 10^9\,M_\odot$. In Milky-Way's NSC, to grow MBH up to $4\times 10^6\,M_\odot$, its size needs to be $\sim 1.7$ times more compact in early universe than the current value. MBHs with current masses $>6\times 10^{7}\,M_\odot$ seem difficult to explain by loss-cone accretion alone, and thus may require other additional accretion channels, such as gas accretion.

astro-ph.GA↗

arXiv2024

Improving constraints on the extended mass distribution in the Galactic Center with stellar orbits

Studying the orbital motion of stars around Sagittarius A* in the Galactic Center provides a unique opportunity to probe the gravitational potential near the supermassive black hole at the heart of our Galaxy. Interferometric data obtained with the GRAVITY instrument at the Very Large Telescope Interferometer (VLTI) since 2016 has allowed us to achieve unprecedented precision in tracking the orbits of these stars. GRAVITY data have been key to detecting the in-plane, prograde Schwarzschild precession of the orbit of the star S2, as predicted by General Relativity. By combining astrometric and spectroscopic data from multiple stars, including S2, S29, S38, and S55 - for which we have data around their time of pericenter passage with GRAVITY - we can now strengthen the significance of this detection to an approximately $10 \sigma$ confidence level. The prograde precession of S2's orbit provides valuable insights into the potential presence of an extended mass distribution surrounding Sagittarius A*, which could consist of a dynamically relaxed stellar cusp comprised of old stars and stellar remnants, along with a possible dark matter spike. Our analysis, based on two plausible density profiles - a power-law and a Plummer profile - constrains the enclosed mass within the orbit of S2 to be consistent with zero, establishing an upper limit of approximately $1200 \, M_\odot$ with a $1 \sigma$ confidence level. This significantly improves our constraints on the mass distribution in the Galactic Center. Our upper limit is very close to the expected value from numerical simulations for a stellar cusp in the Galactic Center, leaving little room for a significant enhancement of dark matter density near Sagittarius A*.

astro-ph.GA↗

arXiv2024

Relation between the local width and linear halo mass density of cosmic filaments

Large-scale cosmic filaments may have played an important role in shaping the properties of galaxies. Meanwhile, cosmic filaments are believed to harbor a substantial portion of the missing baryons at redshift z < 2. To inspect the role of filaments in these issues, many properties of filaments need to be examined, including their lengths, thicknesses, and density profiles. However, measuring some of these properties poses challenges. This study concentrates on estimating filament width/thickness, investigating potential correlations between the local width of filaments and the properties of dark matter halos within filaments. We find that the local width of filaments generally increases with the mass of dark matter halos embedded in filaments per unit length, roughly following a secondorder polynomial, although with notable scatter. We probe and discuss means that may refine our findings. After further verification and improvements, this relation could be applied to filament samples constructed from the observed galaxy distribution, aiding in understanding the roles of cosmic filaments in galaxy evolution and uncovering the missing baryons.

astro-ph.CO↗

arXiv2023

Prospects for probing small-scale dark matter models with pulsars around Sagittarius A*

Future observations with next-generation large-area radio telescopes are expected to discover radio pulsars (PSRs) closely orbiting around Sagittarius~A* (Sgr~A*), the supermassive black hole (SMBH) dwelling at our Galactic Center (GC). Such a system can provide a unique laboratory for testing General Relativity (GR), as well as the astrophysics around the GC. In this paper, we provide a numerical timing model for PSR-SMBH systems based on the post-Newtonian (PN) equation of motion, and use it to explore the prospects of measuring the black hole (BH) properties with pulsar timing. We further consider the perturbation caused by the dark matter (DM) distribution around Sgr~A*, and the possibility of constraining DM models with PSR-SMBH systems. Assuming a 5-year observation of a normal pulsar in an eccentric ($e=0.8$) orbit with an orbital period $P_b = 0.5\,$yr, we find that -- with weekly recorded times of arrival (TOAs) and a timing precision of 1 ms -- the power-law index of DM density distribution near the GC can be constrained to about 20%. Such a measurement is comparable to those measurements at the Galactic length scale but can reveal small-scale properties of the DM.

astro-ph.HE↗

arXiv2023

A centi-pc-scale compact radio core in the nearby galaxy M60

M60, an elliptical galaxy located 16.5~Mpc away, has an active nucleus with a very low luminosity and an extremely low accretion rate. Its central supermassive black hole has a mass of $M_{\rm BH}\sim4.5\times10^{9}\, M_{\odot}$ and a Schwarzschild radii corresponding to $R_{\rm S}\sim5.4\,μ\mathrm{as}$. To investigate the nature of its innermost radio nucleus, data from the Very Long Baseline Array (VLBA) at 4.4 and 7.6~GHz were reduced. The VLBA images reveal a compact component with total flux densities of $\sim$20~mJy at both frequencies, a size of $\leq$0.27~mas (99.7$\%$ confidence level), about 0.022~pc ($50\,R_{\rm S}$) at 7.6~GHz, and a brightness temperature of $\geq6\times10^{9}$~K. This suggests that the observed centi-parsec-scale compact core could be attributed to a nonthermal jet base or an advection-dominated accretion flow (ADAF) with nonthermal electrons. The extremely compact structure also supports the presence of an SMBH in the center. Our results indicate that M60 is a promising target for broad-band VLBI observations at millimeter wavelengths to probe ADAF scenarios and tightly constrain the potential photon ring (about 28\,$μ$as) around its SMBH.

astro-ph.HE↗

arXiv2023

Monte-Carlo Stellar Dynamics near Massive Black Holes: Two-dimensional Fokker-Planck solutions of multiple mass components

In this study we present a novel Monte-Carlo code, referred to as GNC, which enables the investigation of dynamical relaxation in clusters comprising multiple mass components in the vicinity of supermassive black holes at the centers of galaxies. Our method is based on two-dimensional Fokker-Planck equations in the energy and angular momentum space, and allows the evolution of multiple mass components, including stars and compact objects. The code demonstrates remarkable flexibility to incorporate additional complex dynamics, such as resonant relaxations and gravitational wave orbital decay. By employing a weighting method, we effectively enhance the statistical accuracy of rare particle results. In this initial publication, we present the fundamental version of our method, focusing on two-body relaxations and loss cone effects. Through comparisons with previous studies, we establish consistent outcomes in terms of relaxation processes, energy and angular momentum distributions, density profiles, and loss cone consumption rates. We consistently observe the development of tangential anisotropy within the cluster, while the outer regions tend to retain near-isotropic characteristics. Moving forward, GNC holds great promise for exploring a wide range of intriguing phenomena within galactic nuclei, in particular relativistic stellar dynamics, providing detailed and insightful outcomes.

astro-ph.GA↗

arXiv2022

The Eccentric and Accelerating Stellar Binary Black Hole Mergers in Galactic Nuclei: Observing in Ground and Space Gravitational Wave Observatories

We study the stellar binary black holes (BBHs) inspiralling/merging in galactic nuclei based on our numerical method GNC. We find that $3-40\%$ of all new born BBHs will finally merge due to various dynamical effects. In a five year's mission, up to $10^4$, $10^5$, $\sim100$ of BBHs inspiralling/merging in galactic nuclei can be detected with SNR$>8$ in aLIGO, Einstein/DECIGO, TianQin/LISA/TaiJi, respectively. About tens are detectable in both LISA/TaiJi/TianQin and aLIGO. These BBHs have two unique characteristics: (1) Significant eccentricities. $1-3\%$, $2-7\%$, or $30-90\%$ of them is with $e_i>0.1$ when they enter into aLIGO, Einstein, or space observatories, respectively. Such high eccentricities provide a possible explanation for that of GW 190521. Most highly-eccentric BBHs are not detectable in LISA/Tianqin/TaiJi before entering into aLIGO/Einstein as their strain become significant only at $f_{\rm GW}\gtrsim0.1$ Hz. DECIGO become an ideal observatory to detect those events as it can fully cover the rising phase. (2) Up to $2\%$ of BBHs can inspiral/merge at distances $\lesssim10^3 r_{\rm SW}$ from the massive black hole (MBH), with significant accelerations, such that the Doppler phase drift of $\sim10-10^5$ of them can be detectable with SNR$>8$ in space observatories. The energy density of the gravitational wave backgrounds (GWB) contributed by these BBHs deviate from the powerlaw slope of $2/3$ at $f_{\rm GW}\lesssim 1$mHz. The high eccentricity, significant accelerations and different profile of GWB of these sources make them distinguishable, thus interesting for future GW detections and tests of relativities.

astro-ph.HE↗

arXiv2022

Impact of cosmic filaments on the gas accretion rate of dark matter halos

We investigate the impact of cosmic filaments on the gas accretion rate, $\dot{M}_{\rm{gas}}$, of dark matter halos in filaments, based on cosmological hydrodynamic simulation. We find that for halos less massive than $10^{12.0}\ \rm{M_{\odot}}$, $\dot{M}_{\rm{gas}}$ of halos residing in prominent filaments (with width $D_{\rm{fil}}>3\ \rm{Mpc}/h$) is lower than halos residing in tenuous filaments ($D_{\rm{fil}}<3\ \rm{Mpc}/h$) by $20-30\%$ at $z=0.5$, and by a factor of 2-3 at $z=0$. However, $\dot{M}_{\rm{gas}}$ depends weakly on the physical distance between halo center and the spine of filaments from high redshift to $z=0$, only shows clear difference between the inner and outer regions in prominent filaments at $z=0$. We further probe the thermal properties of gas in prominent and tenuous filaments, which appear in relatively highly and intermediate overdense regions, respectively. The gas in prominent filaments is hotter. Around $26\%$, $38\%$ and $45\%$ of gases in prominent filaments are hotter than $10^6$ K at $z=1.0, 0.5$ and $z=0.0$ respectively. The corresponding fractions in tenuous filaments are merely $\sim 6\%, 9\%$ and $11\%$. The suppressed gas accretion rate for low-mass halos in prominent filaments at $z \lesssim 0.5$ may result from the hotter ambient gas, which could provide a physical processing mechanism to cut down the supply of gas to halos before they enter clusters. This process meets partially the need of the preheating mechanism implemented in some semi-analytical models of galaxy formation, but works only for $\sim 20\%$ of halos at $z < 1$.

astro-ph.GA↗

arXiv2021

X-ray Quasi-periodic Eruptions driven by Star-Disc Collisions : Application to GSN069 and Probing the Spin of Massive Black Holes

X-ray quasi-periodic eruptions (QPEs) are discovered recently in active galaxies with unknown driven mechanism. Under the assumption that QPEs are caused by star-disc collisions, we adopt full relativistic method and show that both the orbital parameters of the star and also the mass and spinning of the massive black hole (MBH) can be revealed by using the time of arrival (TOA) of the QPEs. By applying the model to the observed QPEs of GSN069, we find that the star is in a near-circular orbit ( $e_\bullet=0.05^{+0.02}_{-0.02}$) with semimajor axis of $\sim 365^{+54}_{-49}r_{\rm g}$ around a MBH with $M_\bullet=3.0^{+0.9}_{-0.6} \times10^5M_\odot$. The alternative short and long recurring time of the QPEs of GSN069 can be well explained by the small eccentricity and the orbital precession of the star. We find that the QPEs of GSN069 are possibly driven by a striped stellar core colliding with accretion disc after partial tidal disruption event around the MBH. For GSN069-like galaxies, if continuous X-ray monitoring of QPE events can be accumulated with uncertainties of TOA $\lesssim 100-150$s, the spin of massive black hole can be constrained by fitting to QPEs. Our results show that the timing of QPEs can provide a unique probe for measuring the spinning of MBH and tests of no-hair theorem.

astro-ph.HE↗

arXiv2021

Profiles of cosmic filaments since z=4.0 in cosmological hydrodynamical simulation

A large portion of the baryons at low redshifts are still missing from detection. Most of the missing baryons are believed to reside in large scale cosmic filaments. Understanding the distribution of baryons in filaments is crucial for the search for missing baryons. We investigate the properties of cosmic filaments since $z=4.0$ in a cosmological hydrodynamic simulation, focusing on the density and temperature profiles perpendicular to the filament spines. Our quantitative evaluation confirm the rapid growth of thick and prominent filaments after $z=2$. We find that the local linear density of filaments shows correlation with the local diameter since $z=4.0$. The averaged density profiles of both dark matter and baryonic gas in filaments of different width show self-similarity, and can be described by an isothermal single-beta model. The typical gas temperature increases as the filament width increasing, and is hotter than $10^6$ K for filaments with width $D_{fil} \gtrsim 4.0 \rm{Mpc}$, which would be the optimal targets for the search of missing baryons via thermal Sunyaev-Zel'dovich (SZ) effect. The temperature rises significantly from the boundary to the inner core regime in filaments with $D_{fil} \gtrsim 4.0 \rm{Mpc}$, probably due to heating by accretion shock, while the temperature rise gently in filaments with $D_{fil}< 4.0 \rm{Mpc}$.

astro-ph.CO↗

arXiv2020

Extracting the Possible Intrinsic Relation between Radiative Efficiency and Mass of QSOs: a Maximum Likelihood Method and its Application to the SDSS DR7 QSOs

Radiative efficiencies of QSOs and its distribution encode rich information on the evolution of both masses and spins of massive black holes (MBHs) across cosmic time. In this paper, we develop a maximum likelihood method to statistically extract the intrinsic relation between radiative efficiency ($ε$) and mass ($M_{\bullet}$) of QSOs from their distribution on the luminosity-(empirically estimated virial) mass plane. By using mock samples, we find that strong constraint can be put on the $ε-M_{\bullet}$ relation at redshift $z\lesssim 0.4$ from uniform QSO samples similar to those in Sloan Digital Sky Survey, and from QSO samples at $z \sim 0.6$ (or $\lesssim 1.0$) if the magnitude limit of the survey can be $\sim 1-2$ (or $2-3$) magnitude deeper. Applying this method to the SDSS DR7 QSOs with $z\lesssim 0.7$, we find $ε\propto M_{\bullet}^{0\sim 1.1}$ (or $ε\propto M_{\bullet}^{-1.0\sim 0}$) correlation for QSOs with the masses obtained according to the H$β$ (or Mg II) empirical mass estimator. These contradictory results may be due to the unknown systematic errors in the two mass estimators, preventing an accurate constraint on the $ε-M_{\bullet}$ relation by using current available QSO samples. We find that both the estimates of MBH mass and Eddington ratio distribution functions can be affected by the $ε-M_\bullet$ relation, suggesting that the determination of this relation is important for understanding the accretion and growth history of MBHs. In future, the intrinsic $ε-M_{\bullet}$ relation is expected to be strongly constrained by using QSO samples obtained from surveys deeper than SDSS if the host galaxy contamination and systematic errors of the mass estimator(s) can be well modeled or removed.

astro-ph.GA↗

arXiv2019

Gravitational-wave merging events from the dynamics of stellar mass binary black holes around the massive black hole in a galactic nucleus

We study the dynamical evolution of the stellar mass binary black holes (BBHs) in a galactic nucleus that contains a massive black hole (MBH). For a comprehensive study of their merging events, we consider simultaneously the non-resonant and resonant relaxations of the BBHs, the binary-single encounters of the BBHs with the field stars, the Kozai-Lidov (KL) oscillation and the close encounters between the BBHs and the central MBH, which usually lead to binaries' tidal disruptions. As the BBHs are usually heavier than the background stars, they sink to the center by mass segregation, making the KL oscillation an important effect in merging BBHs. The binary-single encounters can not only lead to softening and ionization of the BBHs, it can also make them hardening, that increases the merging rates significantly. The mergers of BBHs are mainly contributed by galaxies containing MBHs less massive than $10^8 M_\odot$ and the total event rates are likely in orders of $1$--$10$ Gpc$^{-3}$ yr$^{-1}$, depending on the detailed assumptions of the nucleus clusters. About $3-10\%$ of these BBH mergers are with eccentricity $\ge 0.01$ when their gravitational wave oscillating frequencies enter the LIGO band ($10$\,Hz). Our results show that merging the BBHs within galactic nuclei can be an important source of the merging events detected by the Advanced LIGO/Virgo detectors, and they can be distinguished from BBH mergers from the galactic fields and globular clusters when enough events are accumulated.

astro-ph.GA↗

arXiv2018

The scattering of FRBs by the intergalactic medium: variations, strength and dependence on dispersion measures

The scattering of fast radio bursts (FRBs) by the intergalactic medium (IGM) is explored using cosmological hydrodynamical simulations. We confirm that the scattering by the clumpy IGM has significant line-of-sight variations. We demonstrate that the scattering by the IGM in the voids and walls of the cosmic web is weak, but it can be significantly enhanced by the gas in clusters and filaments. The observed non-monotonic dependence of the FRB widths on the dispersion measures (DM) cannot determine whether the IGM is an important scattering matter or not. The IGM may dominate the scattering of some FRBs, and the host galaxy dominates others. For the former case, the scattering should be primarily caused by the medium in clusters. A mock sample of 500 sources shows that $τ_{\rm{IGM}} \propto \rm{DM_{IGM}}^{1.6-2.1}$ at $z<1.5$. Assuming that the turbulence follows Kolmogorov scaling, we find that an outer scale of $L_0\sim 5\,$pc is required to make $τ_{\rm{IGM}} \sim 1-10\,$ms at $ν=1\, $ GHz. The required $L_0\sim 5\, $pc can alleviate the tension in the timescales of turbulent heating and cooling but is still $\sim 4$ orders of magnitude lower than the presumed injection scale of turbulence in the IGM. The gap is expected to be effectively shortened if the simulation resolution is further increased. The mechanisms that may further reduce the gap are shortly discussed. If future observations can justify the role of the IGM in the broadening of FRBs, it can help to probe the gas in clusters and filaments.

astro-ph.CO↗

arXiv2017

Evidence for higher black hole spin in radio-loud quasars

One of the major unsolved questions on the understanding of the AGN population is the origin of the dichotomy between radio-quiet and radio-loud quasars. The most promising explanation is provided by the spin paradigm, which suggests radio-loud quasars have higher black hole spin. However, the measurement of black hole spin remains extremely challenging. We here aim at comparing the mean radiative efficiencies of carefully matched samples of radio-loud and radio-quiet SDSS quasars at 0.3<z<0.8. We use the [OIII] luminosity as an indirect average tracer of the ionizing continuum in the extreme-UV regime where differences in the SED due to black hole spin are most pronounced. We find that the radio-loud sample shows an enhancement in [OIII] line strength by a factor of at least 1.5 compared to a radio-quiet sample matched in redshift, black hole mass and optical continuum luminosity or accretion rate. We argue that this enhancement is caused by differences in the SED, suggesting higher average bolometric luminosities at fixed accretion rate in the radio-loud population. This suggests that the radio-loud quasar population has on average systematically larger radiative efficiencies and therefore higher black hole spin than the radio-quiet population, providing observational support for the black hole spin paradigm.

astro-ph.GA↗

arXiv2017

Probing the spinning of the massive black hole in the Galactic Center via pulsar timing: A Full Relativistic Treatment

Pulsars around the Massive Black Hole (MBH) in the Galactic Center (GC) are expected to be revealed by the incoming facilities (e.g., the Square Kilometre Array). Under a full relativistic framework with the pulsar approximated as a test particle, we investigate the constraints on the spinning of the MBH by monitoring the timing of surrounding pulsars. For GC pulsars orbiting closely around the MBH (e.g., $\lesssim1000$AU), we find that full relativistic treatment in modeling accurately their timing signals can be necessary, as the relativistic signals are orders of magnitude larger than the time of arrival measurement accuracies. Although usually there are near-degeneracies among MBH spin parameters, the constraints on the spinning of the MBH are still very tight. By continuously monitoring a normal pulsar in orbits with a period of $\sim2.6$yr and an eccentricity of $0.3-0.9$ under timing precision of $1-5$ms, within $\sim 8$yr the spin magnitude and the orientations of the GC MBH can be constrained with $2σ$ error of $10^{-3}-10^{-2}$ and $10^{-1}-10^\circ$, respectively. Even for pulsars in orbits similar to the detected star S2/S0-2 or S0-102, we find that the spinning of the MBH can still be constrained within $4-8$yr, with the most significant constraints provided near the pericenter passage. If the proper motion of the pulsars with astrometric accuracy of $10μ$as can also be collected along with the timing measurement, then the position, velocity, mass and the distance to the Solar System of the MBH can be constrained about $\sim10μ$as, $\sim1μ$as$/$yr, $\sim 10 M_\odot$ and $\sim1$pc, respectively.

astro-ph.GA↗

arXiv2017

X-ray Eclipses of Active Galactic Nuclei

X-ray variation is a ubiquitous feature of active galactic nuclei (AGNs), however, its origin is not well understood. In this paper, we show that the X-ray flux variations in some AGNs, and correspondingly the power spectral densities (PSDs) of the variations, may be interpreted as being caused by absorptions of eclipsing clouds or clumps in the broad line region (BLR) and the dusty torus. By performing Monte-Carlo simulations for a number of plausible cloud models, we systematically investigate the statistics of the X-ray variations resulting from the cloud eclipsing and the PSDs of the variations. For these models, we show that the number of eclipsing events can be significant and the absorption column densities due to those eclipsing clouds can be in the range from 10^{21} to 10^{24} cm^{-2}, leading to significant X-ray variations. We find that the PSDs obtained from the mock observations for the X-ray flux and the absorption column density resulting from these models can be described by a broken double power law, similar to those directly measured from observations of some AGNs. The shape of the PSDs depend strongly on the kinematic structures and the intrinsic properties of the clouds in AGNs. We demonstrate that the X-ray eclipsing model can naturally lead to a strong correlation between the break frequencies (and correspondingly the break timescales) of the PSDs and the masses of the massive black holes (MBHs) in the model AGNs, which can be well consistent with the one obtained from observations. Future studies of the PSDs of the AGN X-ray (and possibly also the optical-UV) flux and column density variations may provide a powerful tool to constrain the structure of the BLR and the torus and to estimate the MBH masses in AGNs.

astro-ph.HE↗