arXiv subjects

Yefei Yuan

Publications and source records attributed to Yefei Yuan.

9 recordsLinked to original sources

arXiv2025

Probing the Strong Gravity Region of Black Holes with eXTP

We present the novel capabilities of the enhanced X-ray Timing and Polarimetry (eXTP) mission to study the strong gravity region around stellar-mass black holes in X-ray binary systems and supermassive black holes in active galactic nuclei. eXTP can combine X-ray spectral, timing, and polarimetric techniques to study the accretion process near black holes, measure black hole masses and spins, and test Einstein's theory of General Relativity in the strong field regime. We show how eXTP can improve the current measurements of black holes of existing X-ray missions and we discuss the scientific questions that can be addressed.

astro-ph.HE↗

arXiv2023

Sciences with the 2.5-meter Wide Field Survey Telescope (WFST)

The Wide Field Survey Telescope (WFST) is a dedicated photometric surveying facility being built jointly by the University of Science and Technology of China and the Purple Mountain Observatory. It is equipped with a 2.5-meter diameter primary mirror, an active optics system, and a mosaic CCD camera with 0.73 gigapixels on the primary focal plane for high-quality image capture over an FOV of 6.5-square-degree. It is anticipated that WFST will be set up at the Lenghu site in the summer of 2023 and begin to observe the northern sky in four optical bands (u, g, r, and i) with a range of cadences, from hourly/daily in the Deep High-Cadence Survey (DHS) program to semiweekly in the Wide-Field Survey (WFS) program, three months later. During a photometric night, a nominal 30 s exposure in the WFS program will reach a depth of 22.27, 23.32, 22.84, and 22.31 (AB magnitudes) in these four bands, respectively, allowing for the detection of a tremendous amount of transients in the low-z universe and a systematic investigation of the variability of Galactic and extragalactic objects. In the DHS program, intranight 90 s exposures as deep as 23 (u) and 24 mag (g), in combination with target of opportunity follow-ups, will provide a unique opportunity to explore energetic transients in demand for high sensitivities, including the electromagnetic counterparts of gravitational wave events, supernovae within a few hours of their explosions, tidal disruption events and fast, luminous optical transients even beyond a redshift of unity. In addition, the final 6-year co-added images, anticipated to reach g=25.8 mag in WFS or 1.5 mags deeper in DHS, will be of fundamental importance to general Galactic and extragalactic science. The highly uniform legacy surveys of WFST will serve as an indispensable complement to those of LSST that monitor the southern sky.

astro-ph.IM↗

arXiv2021

The Powers of Relativistic Jets depend on the Spin of Accreting Supermassive Black Hole

Theoretical models show that the power of relativistic jets of active galactic nuclei depends on the spin and mass of the central supermassive black holes, as well as the accretion. Here we report an analysis of archival observations of a sample of blazars. We find a significant correlation between jet kinetic power and the spin of supermassive black holes. At the same time, we use multiple linear regression to analyze the relationship between jet kinetic power and accretion, spin and black hole mass. We find that the spin of supermassive black holes and accretion are the most important contribution to the jet kinetic power. The contribution rates of both the spin of supermassive black holes and accretion are more than 95\%. These results suggest that the spin energy of supermassive black holes powers the relativistic jets. The jet production efficiency of almost all Fermi blazars can be explained by moderately thin magnetically arrested accretion disks around rapidly spinning black holes.

astro-ph.HE↗

arXiv2018

Accretion in Strong Field Gravity with eXTP

In this paper we describe the potential of the enhanced X-ray Timing and Polarimetry (eXTP) mission for studies related to accretion flows in the strong field gravity regime around both stellar-mass and supermassive black-holes. eXTP has the unique capability of using advanced 'spectral-timing-polarimetry' techniques to analyze the rapid variations with three orthogonal diagnostics of the flow and its geometry, yielding unprecedented insight into the inner accreting regions, the effects of strong field gravity on the material within them and the powerful outflows which are driven by the accretion process.

astro-ph.HE↗

arXiv2014

Descope of the ALIA mission

The present work reports on a feasibility study commissioned by the Chinese Academy of Sciences of China to explore various possible mission options to detect gravitational waves in space alternative to that of the eLISA/LISA mission concept. Based on the relative merits assigned to science and technological viability, a few representative mission options descoped from the ALIA mission are considered. A semi-analytic Monte Carlo simulation is carried out to understand the cosmic black hole merger histories starting from intermediate mass black holes at high redshift as well as the possible scientific merits of the mission options considered in probing the light seed black holes and their coevolution with galaxies in early Universe. The study indicates that, by choosing the armlength of the interferometer to be three million kilometers and shifting the sensitivity floor to around one-hundredth Hz, together with a very moderate improvement on the position noise budget, there are certain mission options capable of exploring light seed, intermediate mass black hole binaries at high redshift that are not readily accessible to eLISA/LISA, and yet the technological requirements seem to within reach in the next few decades for China.

gr-qc↗

arXiv2010

Numeric Spectrum of Relic Gravitational Waves in Accelerating Universe

The accelerating expansion of the Universe at the present stage is a process that will change the spectrum of relic gravitational waves. Here we present a numerical calculation for the power spectrum of relic gravitational waves in the accelerating Universe. The results show that although the overall features of the power spectrum is similar to that in the non-accelerating models, the amplitude is smaller by an order of $10^{-1}$. We also find that the spectrum is very sensitive to the index $β$ of the inflationary expansion with the scale factor $a(τ) \propto |τ|^{1+β}$. With increase of $β$, the resulting spectrum is tilted to be flatter with more power on high frequencies, and the sensitivity of the second science run of the LIGO detectors puts a restriction on the parameter $β< -1.8$. The influence of reheating following the inflation has been examined.

astro-ph↗

arXiv2005

Relic Gravitational Waves in the Accelerating Universe

Observations have recently indicated that the Universe at the present stage is in an accelerating expansion, a process that has great implications. We evaluate the spectrum of relic gravitational waves in the current accelerating Universe and find that there are new features appearing in the resulting spectrum as compared to the decelerating models. In the low frequency range the peak of spectrum is now located at a frequency $ν_E \simeq (\frac{Ω_m}{Ω_Λ})^{1/3} ν_H$, where $ν_H$ is the Hubble frequency, and there appears a new segment of spectrum between $ν_E$ and $ν_H$. In all other intervals of frequencies $\geq ν_H$, the spectral amplitude acquires an extra factor $\frac{Ω_m}{Ω_Λ}$, due to the current acceleration, otherwise the shape of spectrum is similar to that in the decelerating models. The recent WMAP result of CMB anisotropies is used to normalize the amplitude for gravitational waves. The slope of the power spectrum depends sensitively on the scale factor $a(τ) \propto |τ|^{1+β}$ during the inflationary stage with $β=-2$ for the exact de Sitter space. With the increasing of $β$, the resulting spectrum is tilted to be flatter with more power on high frequencies, and the sensitivity of the second science run of the LIGO detectors puts a restriction on the parameter $β\leq -1.8$. We also give a numerical solution which confirms these features.

astro-ph↗

arXiv2003

Electron Positron Capture Rates and Beta-Equilibrium Condition for Electron-Positron Plasma with Nucleons

The exact reaction rates of beta processes for all particles at arbitrary degeneracy are derived, and an analytic beta-equilibrium condition $μ_n=μ_p+2μ_e$ for the hot electron-positron plasma with nucleons is found, if the matter is transparent to neutrinos. This simple analytic formula is valid only if electrons are nondegenerate, which is generally satisfied in the hot electron-positron plasma. Therefore, it can be used to efficiently determine the steady state of the hot matter with positrons.

astro-ph↗

arXiv2003

Rotational Evolution of Protoneutron Stars

We study the rotational evolution of a protoneutron star with hyperons and nucleons or solely nucleons in its core due to the escape of the trapped neutrinos. It is found that at the early stage of its evolution, the stellar crust contracts significantly, consequently the star spins up. At the late stage, a the protoneutron star with hyperons, it keeps shrinking and spinning up till all the trapped neutrinos escape. Consequently, the distribution of the stellar initial spin periods is skewed toward shorter periods. For a protoneutron star with only nucleons, the expansion of its core dominates, and the stellar rotation slows down. After the neutrinos escape, the range of the spin periods is narrower than the initial one, but the distribution is still nearly uniform. If the hyperonic star is metastable, its rotational frequency accelerates distinguishedly before it collapses to a black hole.

astro-ph↗