arXiv subjects

C. Hu

Publications and source records attributed to C. Hu.

143 records · Page 8Linked to original sources

arXiv2013

Super-Eddington accreting massive black holes as long-lived cosmological standards

Super-Eddington accreting massive black holes (SEAMBHs) reach saturated luminosities above a certain accretion rate due to photon trapping and advection in slim accretion disks. We show that these SEAMBHs could provide a new tool for estimating cosmological distances if they are properly identified by hard X-ray observations, in particular by the slope of their 2-10 keV continuum. To verify this idea we obtained black hole mass estimates and X-ray data for a sample of 60 narrow line Seyfert 1 galaxies that we consider to be the most promising SEAMBH candidates. We demonstrate that the distances derived by the new method for the objects in the sample get closer to the standard luminosity distances as the hard X-ray continuum gets steeper. The results allow us to analyze the requirements for using the method in future samples of active black holes and to demonstrate that the expected uncertainty, given large enough samples, can make them into a useful, new cosmological ruler.

astro-ph.CO↗

arXiv2013

Partial wave analysis of $J/ψ\to γηη$

Based on a sample of $2.25\times 10^{8}$ $J/ψ$ events collected with the BESIII detector at BEPCII, a full partial wave analysis on $J/ψ\toγηη$ was performed using the relativistic covariant tensor amplitude method. The results show that the dominant $0^{++}$ and $2^{++}$ components are from the $f_0(1710)$, $f_0(2100)$, $f_0(1500)$, $f_2'(1525)$, $f_2(1810)$ and $f_2(2340)$. The resonance parameters and branching fractions are also presented.

hep-ex↗

arXiv2012

Study of the near-threshold $ωϕ$ mass enhancement in doubly OZI suppressed $J/ψ\to γωϕ$ decays

A 2.25$\times10^8$ $\jpsi$ event sample accumulated with the BESIII detector is used to study the doubly OZI suppressed decay modes $\jpsi\toγ\of$, $ω\to\ppp$, $ϕ\to\kk$. A strong deviation ($>$ 30$σ$) from three-body $\jpsi\toγωϕ$ phase space is observed near the $ωϕ$ mass threshold that is consistent with a previous observation reported by the BESII experiment. A partial wave analysis with a tensor covariant amplitude that assumes that the enhancement is due to the presence of a resonance, the X(1810), is performed, and confirms that the spin-parity of the X(1810) is $0^{++}$. The mass and width of the X(1810) are determined to be $M=1795\pm7$(stat)$^{+13}_{-5}$(syst)$\pm$19(mod) MeV/$c^2$ and $Γ=95\pm10$(stat)$^{+21}_{-34}$(syst)$\pm$75(mod) MeV/$c^2$, respectively, and the product branching fraction is measured to be ${\cal B}(\jpsi\toγX(1810))\times{\cal B}(X(1810)\to\of)=(2.00\pm0.08$(stat)$^{+0.45}_{-1.00}$(syst)$\pm$1.30(mod))$\times10^{-4}$. %where the first error is the statistical error and the second the systematical error. These results are consistent within errors with those of the BESII experiment.

hep-ex↗

arXiv2012

Star formation in self-gravitating disks in active galactic nuclei. II. Episodic formation of broad line regions

(abridged) We study the consequence of star formation (SF) in an self-gravity dominated accretion disk in quasars. The warm skins of the SF disk are governed by the radiation from the inner part of the accretion disk to form Compton atmosphere (CAS). The CAS are undergoing four phases to form broad line regions. Phase I is the duration of pure accumulation supplied by the SF disk. During phase II clouds begin to form due to line cooling and sink to the SF disk. Phase III is a period of preventing clouds from sinking to the SF disk through dynamic interaction between clouds and the CAS. Finally, phase IV is an inevitable collapse of the entire CAS through line cooling. This CAS evolution drives the episodic appearance of BLRs. Geometry and dynamics of BLRs can be self-consistently derived from the thermal instability of the CAS during phases II and III by linear analysis. The metallicity gradient of SF disk gives rise to different properties of clouds from outer to inner part of BLRs. We find that clouds have column density N_H < 10^22cm^{-2} in the metal-rich regions whereas they have N_H > 10^22 cm^{-2} in the metal-poor regions. The metal-rich clouds compose the high ionization line (HIL) regions whereas the metal-poor clouds are in low ionization line (LIL) regions. Metal-rich clouds in HIL regions will be blown away by radiation pressure, forming the observed outflows. The LIL regions are episodic due to the mass cycle of clouds with the CAS in response to continuous injection by the SF disk, giving rise to different types of AGNs. Based on SDSS quasar spectra, we identify a spectral sequence in light of emission line equivalent width from Phase I to IV. A key phase in the episodic appearance of the BLRs is bright type II AGNs with no or only weak BLRs. We discuss observational implications and tests of the theoretical predictions of this model.

astro-ph.CO↗

arXiv2011

Star formation in self-gravitating disks in active galactic nuclei. I. Metallicity gradients in broad line regions

It has been suggested that the high metallicity generally observed in active galactic nuclei (AGNs) and quasars originates from ongoing star formation in the self-gravitating part of accretion disks around the supermassive black holes. We designate this region as the star forming (SF) disk, in which metals are produced from supernova explosions (SNexp) while at the same time inflows are driven by SNexp-excited turbulent viscosity to accrete onto the SMBHs. In this paper, an equation of metallicity governed by SNexp and radial advection is established to describe the metal distribution and evolution in the SF disk. We find that the metal abundance is enriched at different rates at different positions in the disk, and that a metallicity gradient is set up that evolves for steady-state AGNs. Metallicity as an integrated physical parameter can be used as a probe of the SF disk age during one episode of SMBH activity. In the SF disk, evaporation of molecular clouds heated by SNexp blast waves unavoidably forms hot gas. This heating is eventually balanced by the cooling of the hot gas, but we show that the hot gas will escape from the SF disk before being cooled, and diffuse into the BLRs forming with a typical rate of $\sim 1\sunmyr$. The diffusion of hot gas from a SF disk depends on ongoing star formation, leading to the metallicity gradients in BLR observed in AGNs. We discuss this and other observable consequences of this scenario.

astro-ph.GA↗

arXiv2010

Accretion Disks in Active Galactic Nuclei: Gas Supply Driven by Star Formation

Self-gravitating accretion disks collapse to star-forming(SF) regions extending to the inner edge of the dusty torus in active galactic nuclei (AGNs). A full set of equations including feedback of star formation is given to describe the dynamics of the regions. We explore the role of supernovae explosion (SNexp), acting to excite turbulent viscosity, in the transportation of angular momentum in the regions within 1pc scale. We find that accretion disks with typical rates in AGNs can be driven by SNexp in the regions and metals are produced spontaneously. The present model predicts a metallicity--luminosity relationship consistent with that observed in AGNs. As relics of SF regions, a ring (or belt) consisting of old stars remains for every episode of supermassive black hole activity. We suggest that multiple stellar rings with random directions interact and form a nuclear star cluster after episodes driven by star formation.

astro-ph.CO↗

arXiv2010

The variability of optical \feii emission in PG QSO 1700+518

It is found that \feii emission contributes significantly to the optical and ultraviolet spectra of most active galactic nuclei. The origin of the optical/UV \feii emission is still a question open to debate. The variability of \feii would give clues to this origin. Using 7.5 yr spectroscopic monitoring data of one Palomer-Green (PG) quasi-stellar object (QSO), PG 1700+518, with strong optical \feii emission, we obtain the light curves of the continuum \lv, \feii, the broad component of \hb, and the narrow component of \hb by the spectral decomposition. Through the interpolation cross-correlation method, we calculate the time lags for light curves of \feii, the total \hb, the broad component of \hb, and the narrow component of \hb with respect to the continuum light curve. We find that the \feii time lag in PG1700+518 is $209^{+100}_{-147}$ days, and the \hb time lag cannot be determined. Assuming that \feii and \hb emission regions follow the virial relation between the time lag and the FWHM for the \hb and \feii emission lines, we can derive that the \hb time lag is $148^{+72}_{-104}$ days. The \hb time lag calculated from the empirical luminosity--size relation is 222 days, which is consistent with our measured \feii time lag. Considering the optical \feii contribution, PG 1700+518 shares the same characteristic on the spectral slope variability as other 15 PG QSOs in our previous work, i.e., harder spectrum during brighter phase.

astro-ph.CO↗

arXiv2009

Active Galactic Nuclei with Double-peaked Narrow Lines: Are They Dual AGNs?

Double-peaked [O III]5007, profiles in active galactic nuclei (AGNs) may provide evidence for the existence of dual AGNs, but a good diagnostic for selecting them is currently lacking. Starting from $\sim$ 7000 active galaxies in SDSS DR7, we assemble a sample of 87 type 2 AGNs with double-peaked [O III]5007, profiles. The nuclear obscuration in the type 2 AGNs allows us to determine redshifts of host galaxies through stellar absorption lines. We typically find that one peak is redshifted and another is blueshifted relative to the host galaxy. We find a strong correlation between the ratios of the shifts and the double peak fluxes. The correlation can be naturally explained by the Keplerian relation predicted by models of co-rotating dual AGNs. The current sample statistically favors that most of the [O III] double-peaked sources are dual AGNs and disfavors other explanations, such as rotating disk and outflows. These dual AGNs have a separation distance at $\sim 1$ kpc scale, showing an intermediate phase of merging systems. The appearance of dual AGNs is about $\sim 10^{-2}$, impacting on the current observational deficit of binary supermassive black holes with a probability of $\sim 10^{-4}$ (Boroson & Lauer).

astro-ph.CO↗

arXiv2009

Radiation Tolerance of CMOS Monolithic Active Pixel Sensors with Self-Biased Pixels

CMOS Monolithic Active Pixel Sensors (MAPS) are proposed as a technology for various vertex detectors in nuclear and particle physics. We discuss the mechanisms of ionizing radiation damage on MAPS hosting the the dead time free, so-called self bias pixel. Moreover, we discuss radiation hardened sensor designs which allow operating detectors after exposing them to irradiation doses above 1 Mrad

physics.ins-det↗

arXiv2009

Episodic Random Accretion and the Cosmological Evolution of Supermassive Black Hole Spins

The growth of supermassive black holes (BHs) located at the centers of their host galaxies comes mainly from accretion of gas, but how to fuel them remains an outstanding unsolved problem in quasar evolution. This issue can be elucidated by quantifying the radiative efficiency parameter ($η$) as a function of redshift, which also provides constraints on the average spin of the BHs and its possible evolution with time. We derive a formalism to link $η$ with the luminosity density, BH mass density, and duty cycle of quasars, quantities we can estimate from existing quasar and galaxy survey data. We find that $η$ has a strong cosmological evolution: at z~2, $η\approx 0.3$, and by $z\approx 0$ it has decreased by an order of magnitude, to $η\approx 0.03$. We interpret this trend as evolution in BH spin, and we appeal to episodic, random accretion as the mechanism for reducing the spin. The observation that the fraction of radio-loud quasars decreases with increasing redshift is inconsistent with the popular notion that BH spin is a critical factor for generating strong radio jets. In agreement with previous studies, we show that the derived history of BH accretion closely follows the cosmic history of star formation, consistent with other evidence that BHs and their host galaxies coevolve.

astro-ph.GA↗

arXiv2009

The starburst-AGN connection: the role of young stellar populations in fueling supermassive black holes

Tracing the star formation history in circumnuclear regions (CNRs) is a key step towards understanding the starburst-AGN connection. However, bright nuclei outshining the entire host galaxy prevent the analysis of the stellar populations of CNRs around type-I AGNs. Obscuration of the nuclei by the central torus provides an unique opportunity to study the stellar populations of AGN host galaxies. We assemble a sample of 10, 848 type-II AGNs with a redshift range of $0.03\le z\le 0.08$ from the Sloan Digital Sky Survey's Data Release 4, and measure the mean specific star formation rates (SSFRs) over the past 100Myr in the central $\sim1-2$ kpc . We find a tight correlation between the Eddington ratio ($λ$) of the central black hole (BH) and the mean SSFR, strongly implying that supernova explosions (SNexp) play a role in the transportation of gas to galactic centers. We outline a model for this connection by accounting for the role of SNexp in the dynamics of CNRs. In our model, the viscosity of turbulence excited by SNexp is enhanced, and thus angular momentum can be efficiently transported, driving inflows towards galactic centers. Our model explains the observed relation $λ\propto \rm SSFR^{1.5-2.0}$, suggesting that AGN are triggered by SNexp in CNRs.

astro-ph.GA↗

arXiv2007

Early Growth of Massive Black Holes in Quasars

Episodic activity of quasars is driving growth of supermassive black holes (SMBHs) via accretion of baryon gas. In this Letter, we develop a simple method to analyse the duty cycle of quasars up to redshift $z\sim 6$ universe from luminosity functions (LFs). We find that the duty cycle below redshift $z\sim 2$ follows the cosmic history of star formation rate (SFR) density. Beyond $z\sim 2$, the evolutionary trends of the duty cycle are just opposite to that of the cosmic SFR density history, implying the role of feedback from black hole activity. With the duty cycle, we get the net lifetime of quasars ($z\le 5$) about $\sim 10^9$yrs. Based on the local SMBHs, the mean mass of SMBHs is obtained at any redshifts and their seeds are of $10^5\sunm$ at the reionization epoch ($z_{\rm re}$) of the universe through the conservation of the black hole number density in comoving frame. We find that primordial black holes ($\sim 10^3\sunm$) are able to grow up to the seeds via a moderate super-Eddington accretion of $\sim 30$ times of the critical rate from $z=24$ to $z_{\rm re}$. Highly super-Eddington accretion onto the primordials is not necessary.

astro-ph↗

arXiv2007

Suppressed star formation in circumnuclear regions in Seyfert galaxies

Feedback from black hole activity is widely believed to play a key role in regulating star formation and black hole growth. A long-standing issue is the relation between the star formation and fueling the supermassive black holes in active galactic nuclei (AGNs). We compile a sample of 57 Seyfert galaxies to tackle this issue. We estimate the surface densities of gas and star formation rates in circumnuclear regions (CNRs). Comparing with the well-known Kennicutt-Schmidt (K-S) law, we find that the star formation rates in CNRs of most Seyfert galaxies are suppressed in this sample. Feedback is suggested to explain the suppressed star formation rates.

astro-ph↗

arXiv2005

Feedback Limits Rapid Growth of Seed Black Holes at High Redshift

Seed black holes formed in the collapse of population III stars have been invoked to explain the presence of supermassive black holes at high redshift. It has been suggested that a seed black hole can grow up to $10^{5\sim 6}\sunm$ through highly super-Eddington accretion for a period of $\sim 10^{6\sim 7}$ yr between redshift $z=20\sim 24$. We studied the feedback of radiation pressure, Compton heating and outflow during the seed black hole growth. It is found that its surrounding medium fueled to the seed hole is greatly heated by Compton heating. For a super-critical accretion onto a $10^3\sunm$ seed hole, a Compton sphere (with a temperature $\sim 10^6$K) forms in a timescale of $1.6\times 10^3$yr so that the hole is only supplied by a rate of $10^{-3}$ Eddington limit from the Compton sphere. Beyond the Compton sphere, the kinetic feedback of the strong outflow heats the medium at large distance, this leads to a dramatical decrease of the outer Bondi accretion onto the black hole and avoid the accumulation of the matter. The highly super-critical accretion will be rapidly halted by the strong feedback. The seed black holes hardly grow up at the very early universe unless the strong feedback can be avoided.

astro-ph↗

arXiv2005

Captures of Red Giant Stars by Black Holes in Elliptical Galaxies: Feedback to the Hot Gas

The highly disturbed hot gas in elliptical galaxies, as revealed in many {\em Chandra} X-ray images, implies a source of energy in the galactic nucleus. In some elliptical galaxies faint X-ray ``ghost'' cavities appear without corresponding radio lobes. It has been suggested that ghost cavities are caused by short-lived activity with a timescale of $\sim 10^3-10^4$ years, but this is difficult to understand within the popular paradigm of active galactic nuclei. We suggest an episode model for ghost cavities, invoking captures of red giant stars by the black hole located at the center of the elliptical galaxies at a typical rate of $10^{-5}$yr$^{-1}$ per galaxy. The accretion of tidally disrupted red giant stars onto the black hole powers activity in a timescale of a few years. The total energy channeled into the jet/outflow during the cooling time of the hot gas is $\sim 10^{56}$ erg, which is the typical work required to form the observed cavities. In this scenario, the faint cavities are produced by the feedback following accretion of the debris of the captured red giant stars onto the black holes. We apply the present model to several elliptical galaxies and find that it can explain the formation of the ghost cavities. This model can be tested in the future by comparisons between radio and X-ray observations.

astro-ph↗

arXiv1996

Thermalization and Lyapunov Exponents in the Yang-Mills-Higgs Theory

We investigate thermalization processes occurring at different time scales in the Yang-Mills-Higgs system at high temperatures. We determine the largest Lyapunov exponent associated with the gauge fields and verify its relation to the perturbatively calculated damping rate of a static gauge boson.

hep-th↗

arXiv1995

Wavepacket Dynamics in Yang-Mills Theory

We discuss the results of numerical simulations of colliding wavepackets in $SU(2)$ Yang--Mills theory. We investigate their behavior as a function of amplitude and momentum distribution. We find regions in our parameter space in which initial wave packets scatter into final configurations with dramatically different momentum distributions. These results constitute new classical trajectories with multiparticle boundary conditions. We explain their relevance for the calculation of scattering amplitudes in the semiclassical approximation. Finally, we give directions for future work.

hep-ph↗