52 research outputs found
The Mass Distribution of Quasars in Optical Time-domain Surveys
The determination of supermassive black hole (SMBH) masses is the key to
understanding the host galaxy build-up and the SMBH mass assembly histories.
The SMBH masses of non-local quasars are frequently estimated via the
single-epoch virial black-hole mass estimators, which may suffer from
significant biases. Here we demonstrate a new approach to infer the mass
distribution of SMBHs in quasars by modelling quasar UV/optical variability.
Our inferred black hole masses are systematically smaller than the virial ones
by dex; the dex offsets are roughly consistent with
the expected biases of the virial black-hole mass estimators. In the upcoming
time-domain astronomy era, our methodology can be used to constrain the cosmic
evolution of quasar mass distributions.Comment: 9 pages, 8 figures, accepted to MNRA
The Accretion Wind Model of the Fermi Bubbles (II): Radiation
In a previous work, we have shown that the formation of the Fermi bubbles can
be due to the interaction between winds launched from the hot accretion flow in
Sgr A* and the interstellar medium (ISM). In that work, we focus only on the
morphology. In this paper we continue our study by calculating the gamma-ray
radiation. Some cosmic ray protons (CRp) and electrons must be contained in the
winds, which are likely formed by physical processes such as magnetic
reconnection. We have performed MHD simulations to study the spatial
distribution of CRp, considering the advection and diffusion of CRp in the
presence of magnetic field. We find that a permeated zone is formed just
outside of the contact discontinuity between winds and ISM, where the
collisions between CRp and thermal nuclei mainly occur. The decay of neutral
pions generated in the collisions, combined with the inverse Compton scattering
of background soft photons by the secondary leptons generated in the collisions
and primary CR electrons can well explain the observed gamma-ray spectral
energy distribution. Other features such as the uniform surface brightness
along the latitude and the boundary width of the bubbles are also explained.
The advantage of this accretion wind model is that the adopted wind properties
come from the detailed small scale MHD numerical simulation of accretion flows
and the value of mass accretion rate has independent observational evidences.
The success of the model suggests that we may seriously consider the
possibility that cavities and bubbles observed in other contexts such as galaxy
clusters may be formed by winds rather than jets.Comment: 13 pages,6 figures, accepted for publication in Ap
Fermi Bubbles Inflated by Winds Launched from the Hot Accretion Flow in Sgr A*
A pair of giant gamma-ray bubbles have been revealed by the {\it Fermi} LAT.
In this paper we investigate their formation mechanism. Observations have
indicated that the activity of the supermassive black hole located at the
Galactic center, Sgr A*, was much stronger than the present time. Specifically,
one possibility is that while Sgr A* was also in the hot accretion regime, the
accretion rate should be times higher during the past
yr. On the other hand, recent MHD numerical simulations of hot accretion flows
have unambiguously shown the existence of strong winds and obtained their
properties. Based on these knowledge, by performing three-dimensional
hydrodynamical simulations, we show in this paper that the Fermi bubbles could
be inflated by winds launched from the ``past' hot accretion flow in Sgr A*. In
our model, the active phase of Sgr A* is required to last for about 10 million
years and it was quenched no more than 0.2 million years ago. The Central
Molecular Zone (CMZ) is included and it collimates the wind orientation towards
the Galactic poles. Viscosity suppresses the Rayleigh-Taylor and
Kelvin-Helmholtz instabilities and results in the smoothness of the bubble
edge. The main observational features of the bubbles can be well explained.
Specifically, the {\it ROSAT} X-ray features are interpreted by the shocked
interstellar medium and the interaction region between winds and CMZ gas. The
thermal pressure and temperature obtained in our model are in good consistency
with the recent {\it Suzaku} observations.Comment: 12 pages,8 figures, Accepted by Ap
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