24 research outputs found
The Bending Feature of the Fermi Bubbles: A Presumed Horizontal Galactic Wind and Its Implication on the Bubbles' Age
There are two spectacular structures in our Milky Way: the {\it Fermi}
bubbles in gamma-ray observations and the North Polar Spur (NPS) structure in
X-ray observations. Because of their morphological similarities, they may share
the same origin, i.e., related to the past activity of Galactic center (GC).
Besides, those structures show significant bending feature toward the west in
Galactic coordinates. This inspires us to consider the possibility that the
bending may be caused by a presumed global horizontal galactic wind (HGW)
blowing from the east to the west. Under this assumption, we adopt a toy shock
expansion model to understand two observational features: (1) the relative
thickness of the NPS; (2) the bending of the {\it Fermi} bubbles and NPS. In
this model, the contact discontinuity (CD) marks the boundary of the {\it
Fermi} bubbles, and the shocked interstellar medium (ISM) marks the NPS X-ray
structure. We find that the Mach number of the forward shock in the east is
1.9-2.3, and the velocity of the HGW is ~ 0.7-0.9 . Depending on
the temperature of the pre-shock ISM, the velocity of the expanding NPS in
Galactic coordinates is around 180-290 km/s, and the HGW is ~ 110-190 km/s. We
argue that, the age of the NPS and the {\it Fermi} bubbles is about 18-34 Myr.
This is a novel method, independent of injection theories and radiative
mechanisms, for the estimation on the age of the {\it Fermi} bubble/NPS.Comment: 6 pages, 4 figure, accepted by ApJ
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
On the Efficiency of Thermal Conduction in Galaxy Clusters
Galaxy clusters host a large reservoir of diffuse plasma with
radially-varying temperature profiles. The efficiency of thermal conduction in
the intracluster medium (ICM) is complicated by the existence of turbulence and
magnetic fields, and has received a lot of attention in the literature.
Previous studies suggest that the magnetothermal instability developed in outer
regions of galaxy clusters would drive magnetic field lines preferentially
radial, resulting in efficient conduction along the radial direction. Using a
series of spherically-symmetric simulations, here we investigate the impact of
thermal conduction on the observed temperature distributions in outer regions
of three massive clusters, and find that thermal conduction substantially
modifies the ICM temperature profile. Within 3 Gyr, the gas temperature at a
representative radius of typically decreases by ~10 - 20% and the
average temperature slope between and drops by ~ 30 -
40%, indicating that the observed ICM would not stay in a long-term equilibrium
state in the presence of thermal conduction. However, X-ray observations show
that the outer regions of massive clusters have remarkably similar
radially-declining temperature profiles, suggesting that they should be quite
stable. Our study thus suggests that the effective conductivity along the
radial direction must be suppressed below the Spitzer value by a factor of 10
or more, unless additional heating sources offset conductive cooling and
maintain the observed temperature distributions. Our study provides a
smoking-gun evidence for the suppression of parallel conduction along magnetic
field lines in low-collisionality plasmas by kinetic mirror or whistler
instabilities.Comment: Slightly revised version, accepted for publication in ApJ. 11 pages,
7 figure
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
An Asymmetrical Model for High Energy Radiation of Cassiopeia A
Cassiopeia A (Cas A) supernova remnant shows strong radiation from radio to
gamma-ray bands. The mechanism of gamma-ray radiation in Cas A and its possible
contribution to PeV cosmic rays are still under debate. The X-ray imaging
reveals an asymmetric profile of Cas A, suggesting the existence of a jet-like
structure. In this work, we propose an asymmetrical model for Cas A, consisting
of a fast moving jet-like structure and a slowly expanding isotropic shell.
This model can account for the multi-wavelength spectra of Cas A, especially
for the power-law hard X-ray spectrum from 60 to 220 keV. The GeV to TeV
emission from Cas A should be contributed by both hadronic and leptonic
processes. Moreover, the jet-like structure may produce a gamma-ray flux of
at TeV, to be examined by
LHAASO and CTA.Comment: 7 pages, 7 figures. MNRAS in pres
The peculiar filamentary HI structure of NGC 6145
In this paper, we report the peculiar HI morphology of the cluster spiral
galaxy NGC 6145, which has a 150 kpc HI filament on one side that is nearly
parallel to its major axis. This filament is made up of several HI clouds and
the diffuse HI gas between them, with no optical counterparts. We compare its
HI distribution with other one-sided HI distributions in the literature, and
find that the overall HI distribution is very different from the typical tidal
and ram-pressure stripped HI shape, and its morphology is inconsistent with
being a pure accretion event. Only about 30% of the total HI gas is anchored on
the stellar disk, while most of HI gas forms the filament in the west. At a
projected distance of 122 kpc, we find a massive elliptical companion (NGC
6146) with extended radio emission, whose axis points to an HI gap in NGC 6145.
The velocity of the HI filament shows an overall light-of- sight motion of 80
to 180 km/s with respect to NGC 6145. Using the long-slit spectra of NGC 6145
along its major stellar axis, we find that some outer regions show enhanced
star formation, while in contrast, almost no star formation activities are
found in its center (less than 2 kpc). Pure accretion, tidal or ram-pressure
stripping is not likely to produce the observed HI filament. An alternative
explanation is the jet-stripping from NGC 6146, although direct evidence for a
jet-cold gas interaction has not been found.Comment: 12 pages, 6 figures; Accepted for publication in A
Radio emission of tidal disruption events from wind-cloud interaction
Winds can be launched in tidal disruption event (TDE). It has been proposed
that the winds can interact with the cloud surrounding the black hole, produce
bow shocks, accelerate electrons, and produce radio emission. We restudy the
wind-cloud interaction model. We employ the properties of winds found by the
radiation hydrodynamic simulations of super-Eddington circularized accretion
flow in TDEs. We can calculate the peak radio emission frequency, the
luminosity at the peak frequency, and their time-evolution based on the TDEs
wind-cloud interaction model. We find that the model predicted peak radio
emission frequency, the luminosity at peak frequency, and their time evolution
can be well consistent with those in TDEs AT2019dsg and ASASSN-14li. This
indicates that in these two radio TDEs, the wind-cloud interaction mechanism
may be responsible for the radio emission.Comment: 11 pages, 10 figures, accepted to MNRA
Infrared echo and late-stage rebrightening of nuclear transient PS1-10adi: exploring Torus by tidal disruption event in active galactic nuclei
Tidal disruption events (TDEs) in active galactic nuclei (AGNs) have been
overlooked for a long time but tentatively been investigated recently. We
report the discovery of a long-lasting luminous mid-infrared (mid-IR) flare in
PS1-10adi, which is a newly-identified highly energetic transient event
occurred in AGN. The IR luminosity of PS1-10adi, as well as other analogous
events, are at least one order of magnitude higher than all known supernova,
but can be well interpreted as the dust echoes of TDEs, whose ultra-high IR
energy is reprocessed from the dusty torus around the black hole. The torus
dust is sublimating during the early stage of the outburst and probably lead to
the observed rapid emergence of Fe II lines. Moreover, the UV-optical
rebrightening and contemporaneous X-ray onset after rest-frame days
since the optical peak is also an intriguing feature of PS1-10adi, which could
be attributed to the interaction between the high-velocity outflow and torus.
We suggest that the luminous IR echo is a very typical phenomenon of TDEs in
AGNs and may provide us an ideal opportunity to explore the torus properties.Comment: Accepted by ApJ, 1 table, 8 figure