1,278 research outputs found
Constraints on jet formation mechanisms with the most energetic giant outbursts in MS 0735+7421
Giant X-ray cavities lie in some active galactic nuclei (AGNs) locating in
central galaxies of clusters, most of these cavities are thought to be inflated
by jets of AGNs. The jets can be either powered by rotating black holes or the
accretion disks surrounding black holes, or both. In this work, we choose the
most energetic cavity, MS 0735+7421, with stored energy ~ 10^62 erg, to
constrain the jet formation mechanisms and the evolution of the central massive
black hole in this source. The bolometric luminosity of the AGN in this cavity
is ~ 10^(-5) L_Edd, however, the mean power of the jet required to inflate the
cavity is estimated as ~ 0.02 L_Edd, which implies that the source has
experienced strong outbursts previously. During outbursts, the jet power and
the mass accretion rate should be significantly higher than its present values.
We construct an accretion disk model, in which the angular momentum and energy
carried away by jets is properly included, to calculate the spin and mass
evolution of the massive black hole. In our calculations, different jet
formation mechanisms are employed, and we find that the jets generated with the
Blandford-Znajek (BZ) mechanism are unable to produce the giant cavity with ~
10^62 erg in this source. Only the jets accelerated with the combination of the
Blandford-Payne (BP) and BZ mechanisms can successfully inflate such a giant
cavity, if the magnetic pressure is close to equipartition with the total
(radiation+gas) pressure of the accretion disk. For dynamo generated magnetic
field in the disk, such an energetic giant cavity can be inflated by the
magnetically driven jets only if the initial black hole spin parameter a_0 >
0.95. Our calculations show that the final spin parameter a of the black hole
is always ~ 0:9 - 0.998 for all the computational examples which can provide
sufficient energy for the cavity of MS 0735+7421.Comment: 25 pages, 8 figures, accepted by Ap
A strong negative correlation between radio loudness and optical-to-X-ray spectral index in low-luminosity AGNs
It has been argued for years that the accretion mode changes from bright
active galactic nuclei (AGNs) to low-luminosity AGNs (LLAGNs) at a rough
dividing point of bolometric Eddington ratio . In this
work, we strengthen this scenario through investigation of the relationship
between the radio loudness and the optical-to-X-ray spectral index
in LLAGNs with .
We compile from literature a sample of 32 LLAGNs, consisting 18 LINERs and 14
low Eddington ratio Seyfert galaxies, and observe a strong negative -- relationship, with large scatter in both
and . We further demonstrate that this negative correlation,
and the additional two negative relationships reported in literature (-- and -- correlations), can be
understood consistently and comprehensively under the truncated accretion--jet
model, the model that has been applied successfully applied to LLAGNs. We argue
that the scatter in the observations are (mainly) due to the spread in the
viscosity parameter of a hot accretion flow, a parameter that
potentially can serve as a diagnose of the strength and/or configuration of
magnetic fields in accretion flows.Comment: 8 pages, 3 figures, 2 tables. Accepted by MNRA
Stochastic pole expansion method
In this paper, we propose a new analytic continuation method to extract real
frequency spectral functions from imaginary frequency Green's functions of
quantum many-body systems. This method is based on the pole representation of
Matsubara Green's function and a stochastic sampling procedure is utilized to
optimize the amplitudes and locations of poles. In order to capture narrow
peaks and sharp band edges in the spectral functions, a constrained sampling
algorithm and a self-adaptive sampling algorithm are developed. To demonstrate
the usefulness and performance of the new method, we at first apply it to study
the spectral functions of representative fermionic and bosonic correlators.
Then we employ this method to tackle the analytic continuation problems of
matrix-valued Green's functions. The synthetic Green's functions, as well as
realistic correlation functions from finite temperature quantum many-body
calculations, are used as input. The benchmark results demonstrate that this
method is capable of reproducing most of the key characteristics in the
spectral functions. The sharp, smooth, and multi-peak features in both
low-frequency and high-frequency regions of spectral functions could be
accurately resolved, which overcomes one of the main limitations of the
traditional maximum entropy method. More importantly, it exhibits excellent
robustness with respect to noisy and incomplete input data. The causality of
spectral function is always satisfied even in the presence of sizable noises.
As a byproduct, this method could derive a fitting formula for the Matsubara
data, which provides a compact approximation to the many-body Green's
functions. Hence, we expect that this new method could become a pivotal
workhorse for numerically analytic continuation and be broadly useful in many
applications.Comment: 26 pages, 20 figure
Reconstructing lattice QCD spectral functions with stochastic pole expansion and Nevanlinna analytic continuation
The reconstruction of spectral functions from Euclidean correlation functions
is a well-known, yet ill-posed inverse problem in the fields of many-body and
high-energy physics. In this paper, we present a comprehensive investigation of
two recently developed analytic continuation methods, namely stochastic pole
expansion and Nevanlinna analytic continuation, for extracting spectral
functions from mock lattice QCD data. We examine a range of Euclidean
correlation functions generated by representative models, including the
Breit-Wigner model, the Gaussian mixture model, the resonance-continuum model,
and the bottomonium model. Our findings demonstrate that the stochastic pole
expansion method, when combined with the constrained sampling algorithm and the
self-adaptive sampling algorithm, successfully recovers the essential features
of the spectral functions and exhibits excellent resilience to noise of input
data. In contrast, the Nevanlinna analytic continuation method suffers from
numerical instability, often resulting in the emergence of spurious peaks and
significant oscillations in the high-energy regions of the spectral functions,
even with the application of the Hardy basis function optimization algorithm.Comment: 14 pages, 8 figure
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