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Renyi Ma

Publications and source records attributed to Renyi Ma.

At least 19 recordsLinked to original sources
arXiv2024

A New Model for the Extragalactic Radio Sky at Low Frequency Calibrated Using the LOFAR Two-metre Survey

Building the radio sky template are crucial for detecting the 21 cm emission line signal from the Epoch of Reionization (EoR), as well as for other cosmological research endeavors. Utilizing data from the LOFAR Two-meter Sky Survey (LoTSS) at 150 MHz, we recalibrated the luminosity function for various types of radio sources, including High Excitation Radio Galaxies (HERGs), Low Excitation Radio Galaxies (LERGs), Radio-Quiet Active Galactic Nuclei (RQ-AGNs), and Star-Forming Galaxies (SFGs). We subsequently updated the Tiered Radio Extragalactic Continuum Simulation (T-RECS) code to generate refined mock radio source catalogues. The simulated source counts from this work align more closely with observed data at redshifts greater than $z>4$. Additionally, the differential source counts in total intensity within the flux density range of $0.1-1~\mathrm{mJy}$ closely mirror actual observations. Due to our model incorporating a lower number of faint sources compared to T-RECS, it predicts a reduced power spectrum for point sources, suggesting a potential advantage in studies in low frequency band.

astro-ph.GA↗
arXiv2018

XMM-Newton Survey of Local OVII Absorption Lines in the Spectra of Galactic X-ray Sources

The detection of highly ionized metal absorption lines in the X-ray spectra of the Galactic X-ray binaries (XRBs) implies the distribution of hot gas along the sightline toward the background sources. However, the origin of this hot gas is still unclear: it can arise in the hot interstellar medium (ISM), or is intrinsic to the XRBs. In this paper, we present an XMM-Newton survey of the OVII absorption lines in the spectra of Galactic XRBs. A total of 33 XRBs were selected, with 29 low mass XRBs and 4 high mass XRBs. At more than 3$σ$ threshold, OVII absorption line was detected in 16 targets, among which 4 were newly discovered in this work. The average line equivalent width (EW) is centered around $\sim$ 20 mÅ. Additionally we do not find strong correlations between the OVII EWs and the Galactic neutral absorption N$_{\rm H}$, the Galactic coordinates or the distance of background targets. Such non-correlation may suggest the contamination of the circumstellar material, or the lack of constrains on the line Doppler-b parameter. We also find regardless the direction of the XRBs, the OVII absorption lines were always detected when the flux of the background XRBs reach a certain level, suggesting a uniform distribution of this hot gas. We estimate a ratio of 0.004 --- 0.4 between hot and neutral phase of the interstellar medium. This is the second paper in the series following Fang et al. 2015, in which we focused on the local OVII absorption lines detected in the background AGN spectra. Detailed modeling of the hot ISM distribution will be investigated in a future paper.

astro-ph.GA↗
arXiv2016

A luminous hot accretion flow in the low-luminosity active galactic nucleus NGC 7213

The active galactic nucleus (AGN) NGC 7213 shows a complex correlation between the monochromatic radio luminosity $L_R$ and the 2--10 keV X-ray luminosity $L_X$, i.e. the correlation is unusually weak with $p\sim 0$ (in the form $L_R\propto L_X^p$) when $L_X$ is below a critical luminosity, and steep with $p>1$ when $L_X$ is above that luminosity. Such a hybrid correlation in individual AGNs is unexpected as it deviates from the Fundamental Plane of AGN activity. Interestingly, a similar correlation pattern is observed in the black hole X-ray binary H1743--322, where it has been modelled by switching between different modes of accretion. We propose that the flat $L_R$--$L_X$ correlation of NGC 7213 is due to the presence of a luminous hot accretion flow, an accretion model whose radiative efficiency is sensitive to the accretion rate. Given the low luminosity of the source, $L_X\sim 10^{-4}$ of the Eddington luminosity, the viscosity parameter is determined to be small, $α\approx 0.01$. We also modelled the broad-band spectrum from radio to $γ$-rays, the time lag between the radio and X-ray light curves, and the implied size and the Lorentz factor of the radio jet. We predict that NGC 7213 will enter into a two-phase accretion regime when $L_X > 1.5 \times 10^{42}\, {\rm erg\,s^{-1}}$. When this happens, we predict a softening of the X-ray spectrum with the increasing flux and a steep radio/X-ray correlation.

astro-ph.HE↗
arXiv2016

A Neutron Star-White Dwarf Binary Model for Repeating Fast Radio Burst 121102

We propose a compact binary model for the fast radio burst (FRB) repeaters, where the system consists of a magnetic white dwarf (WD) and a neutron star (NS) with strong bipolar magnetic fields. When the WD fills its Roche lobe, mass transfer will occur from the WD to the NS through the inner Lagrange point. The accreted magnetized materials may trigger magnetic reconnection when they approach the NS surface, and therefore the electrons can be accelerated to an ultra-relativistic speed. In this scenario, the curvature radiation of the electrons moving along the NS magnetic field lines can account for the characteristic frequency and the timescale of an FRB. Owing to the conservation of angular momentum, the WD may be kicked away after a burst, and the next burst may appear when the system becomes semi-detached again through the gravitational radiation. By comparing our analyses with the observations, we show that such an intermittent Roche lobe overflow mechanism can be responsible for the observed repeating behavior of FRB 121102.

astro-ph.HE↗
arXiv2015

XMM-Newton Survey of Local O VII Absorption Lines in the Spectra of Active Galactic Nuclei

Highly ionized, z=0 metal absorption lines detected in the X-ray spectra of background active galactic nuclei (AGNs) provide an effective method to probe the hot ($T\sim10^6$ K) gas and its metal content in and around the Milky Way. We present an all-sky survey of the $K_α$ transition of the local O VII absorption lines obtained by Voigt-profile fitting archival XMM-Newton observations. A total of 43 AGNs were selected, among which 12 are BL Lac-type AGNs, and the rest are Seyfert 1 galaxies. At above the $3σ$ level the local O VII absorption lines were detected in 21 AGNs, among which 7 were newly discovered in this work. The sky covering fraction, defined as the ratio between the number of detections and the sample size, increases from at about 40% for all targets to 100% for the brightest targets, suggesting a uniform distribution of the O VII absorbers. We correlate the line equivalent width with the Galactic coordinates and do not find any strong correlations between these quantities. Some AGNs have warm absorbers that may complicate the analysis of the local X-ray absorber since the recession velocity can be compensated by the outflow velocity, especially for the nearby targets. We discuss the potential impact of the warm absorbers on our analysis. A comprehensive theoretical modelling of the X-ray absorbers will be presented in a later paper.

astro-ph.CO↗
arXiv2014

Jet-dominated quiescent states in black hole X-ray binaries: the case of V404 Cyg

The dynamical and radiative properties of the quiescent state (X-ray luminosity $<10^{34}\ {\rm erg s^{-1} cm^{-2}}$) of black hole X-ray transients (BHXTs) remains unclear, mainly because of low-luminosity and poor data quantity. We demonstrate that, the simultaneous multi-wavelength (including radio, optical, ultraviolet and X-ray bands) spectrum of V404 Cyg in its bright quiescent state can be well described by the radiation from the companion star and more importantly, the compact jet. Neither the outer thin disc nor the inner hot accretion flow is important in the total spectrum. Together with several additional recent findings, i.e. the power-law X-ray spectrum and the constant X-ray spectral shape (or constant photon index) in contrast to the dramatic change in the X-ray luminosity, we argue the quiescent state spectrum of BHXTs is actually jet-dominated. Observational features consistent with this jet model are also discussed as supporting evidences.

astro-ph.HE↗
arXiv2013

Relationship Between the Kinetic Power and Bolometric Luminosity of Jets: limitation from black hole X-ray binaries, active galactic nuclei, and gamma-ray bursts

The correlation between the kinetic power $P_{\rm jet}$ and intrinsic bolometric luminosity $L_{\rm jet}$ of jets may reveal the underlying jet physics in various black hole systems. Based on the recent work by Nemmen et al. (2012), we re-investigate this correlation with additional sources of black-hole X-ray binaries (BXBs) in hard/quiescent states and low-luminosity active galactic nuclei (LLAGNs). The new sample includes 29 sets of data from 7 BXBs and 20 LLAGNs, with $P_{\rm jet}$ and $L_{\rm jet}$ being derived from spectral modeling of the quasi-simultaneous multi-band spectra under the accretion-jet scenario. Compared to previous works, the range of luminosity is now enlarged to more than $20$ decades, i.e. from $10^{31}{\rm erg/s}$ to $10^{52}{\rm erg/s}$, which allows for better constraining of the correlation. One notable result is that the jets in BXBs and LLAGNs almost follow the same $P_{\rm jet} - L_{\rm jet}$ correlation that was obtained from blazars and gamma-ray bursts (GRBs). The slope indices we derived are $1.03\pm0.01$ for the whole sample, $0.85\pm0.06$ for BXB subsample, $0.71\pm0.11$ for LLAGN subsample, and $1.01\pm0.05$ for the LLAGN-blazar subsample, respectively. The correlation index around unit implies the independence of jet efficiency on the luminosity or kinetic power. Our results may further support the hypothesis that similar physical processes exist in the jets of various black hole systems.

astro-ph.HE↗
arXiv2013

A Simple Proof On Poincaré Conjecture

We give a simple proof on the Poincaré's conjecture which states that every compact smooth $3-$manifold which is homotopically equivalent to $S^3$ is diffeomorphic to $S^3$.

math.GM↗
arXiv2013

Existence Of Contact Structure

Let $(Σ,ξ',ω)$ be a close almost contact $(2n-1)$-manifold. Then, by McDuff's theorem, we prove that $ξ'$ is homotopic to a contact structure $ξ$. This answers a question proposed by Chern.

math.GM↗
arXiv2013

Hamilton-Arnold Chord And Periodic Orbits

In this article, we first prove that every Hamilton flow has at least as many Hamilton- Arnold chords as a smooth function on the Legendre submanifold of zero first cohomology has critical points. Second, we prove that every Hamilton flow has at least as many close Hamilton orbits as a smooth round function on the close Hamilton manifold of zero first cohomology has critical circles which implies that the so called Arnold-Ginzburg or Seifert conjecture holds.

math.SG↗
arXiv2013

Proofs On Arnold Conjectures

In this article, we give proofs on the Arnold Lagrangian intersection conjecture on the cotangent bundles, Arnold-Givental Lagrangian intersection conjecture and the Arnold fixed point conjecture.

math.SG↗
arXiv2013

A Proof On Arnold Chord Conjecture

In this article, we first give a proof on the Arnold chord conjecture which states that every Reeb flow has at least as many Reeb chords as a smooth function on the Legendre submanifold has critical points on contact manifold. Second, we prove that every Reeb flow has at least as many close Reeb orbits as a smooth round function on the close contact manifold has critical circles on contact manifold. This also implies a proof on the fact that there exists at least number $n$ close Reeb orbits on close $(2n-1)$-dimensional convex hypersurface in $R^{2n}$ conjectured by Ekeland.

math.GM↗
arXiv2012

A Disc-Corona Model for a Rotating Black Hole

We propose a disc-corona model in which a geometrically thin, optically thick disc surrounds a Kerr black hole, and magnetic fields exert a time-steady torque on the inner edge of the accretion disc. The analytical expression of the total gravitational power is derived from the thin-disc dynamics equations by using this new boundary condition. It is shown that the magnetic torque can considerably enhance the amount of energy released in the disc-corona system. Furthermore, the global solutions of this disc-corona system are obtained numerically. We find that the fraction of the power dissipated into the corona in the total for such disc-corona system increases with the increasing dimensionless black hole spin parameter $a_\ast $, but is insensitive on the $Δ\varepsilon $ which is the additional radiative efficiency parameter relevant to magnetic torque, for $Δ\varepsilon > 1$. In addition, the emerged spectra from this disc-corona system are simulated by using Monte-Carlo method, and the effect of the different parameters on the output spectra is discussed.

astro-ph.HE↗
arXiv2012

J-holomorphic Curves And Periodic Reeb Orbits

We study the $J-$holomorphic curves in the symplectization of the contact manifolds and prove that there exists at least one periodic Reeb orbits in any closed contact manifold with any contact form by using the well-known Gromov's nonlinear Fredholm alternative for $J-$holomorphic curves. As a corollary, we give a complete solution on the well-known Weinstein conjecture.

math.DG↗