75 research outputs found
Fast jet proper motion discovered in a blazar at z = 4.72
High-resolution observations of high-redshift () radio quasars offer a
unique insight into jet kinematics at early cosmological epochs, as well as
constraints on cosmological model parameters. Due to the general weakness of
extremely distant objects and the apparently slow structural changes caused by
cosmological time dilation, only a couple of high-redshift quasars have been
studied with parsec-scale resolutions, and with limited number of observing
epochs. Here we report on very long baseline interferometry (VLBI) observations
of a high-redshift blazar J1430+4204 () in the 8 GHz frequency band at
five different epochs spanning 22 years. The source shows a compact core--jet
structure with two jet components being identified within 3 milli-arcsecond
(mas) scale. The long time span and multiple-epoch data allow for the kinematic
studies of the jet components. That results in a jet proper motion of = 0.0170.002~mas\,yr and =0.1560.015~mas\,yr, respectively. For the fastest-moving
outer jet component J2, the corresponding apparent transverse speed is . The inferred bulk jet Lorentz factor and
viewing angle indicate highly
relativistic beaming. The Lorentz factor and apparent proper motion are the
highest measured to date among the jetted radio sources, while the jet
kinematics is still consistent with the cosmological interpretation of quasar
redshifts.Comment: Science Bulletin, submitte
A method for checking high-redshift identification of radio AGNs
In large-scale optical spectroscopic surveys, there are many objects found to
have multiple redshift measurements due to the weakness of their emission lines
and the different automatic identification algorithms used. These include some
suspicious high-redshift (z >= 5) active galactic nuclei (AGNs). Here we
present a method for inspecting the high-redshift identification of such
sources provided that they are radio-loud and have very long baseline
interferometry (VLBI) imaging observations of their milli-arcsec (mas) scale
jet structure available at multiple epochs. The method is based on the
determination of jet component proper motions, and the fact that the
combination of jet physics (the observed maximal values of the bulk Lorentz
factor) and cosmology (the time dilation of observed phenomena in the early
Universe) constrain the possible values of apparent proper motions. As an
example, we present the case of the quasar J2346+0705 that was reported with
two different redshifts, and , in the
literature. We measured the apparent proper motions () of three components
identified in its radio jet by utilizing VLBI data taken from 2014 to 2018. We
obtained mas yr, mas yr, and mas yr. The
maximal proper motion is converted to an apparent transverse speed of
. if the source is at redshift 5.063. This
value exceeds the blazar jet speeds known to date. This and other arguments
suggest that J2346+0705 is hosted by a low-redshift galaxy. Our method may be
applicable for other high-redshift AGN candidates lacking unambiguous
spectroscopic redshift determination or having photometric redshift estimates
only, but showing prominent radio jets allowing for VLBI measurements of fast
jet proper motions.Comment: accepted for publication in Monthly Notices of the Royal Astronomical
Societ
An Emergency Disposal Decision-making Method with Human--Machine Collaboration
Rapid developments in artificial intelligence technology have led to unmanned
systems replacing human beings in many fields requiring high-precision
predictions and decisions. In modern operational environments, all job plans
are affected by emergency events such as equipment failures and resource
shortages, making a quick resolution critical. The use of unmanned systems to
assist decision-making can improve resolution efficiency, but their
decision-making is not interpretable and may make the wrong decisions. Current
unmanned systems require human supervision and control. Based on this, we
propose a collaborative human--machine method for resolving unplanned events
using two phases: task filtering and task scheduling. In the task filtering
phase, we propose a human--machine collaborative decision-making algorithm for
dynamic tasks. The GACRNN model is used to predict the state of the job nodes,
locate the key nodes, and generate a machine-predicted resolution task list. A
human decision-maker supervises the list in real time and modifies and confirms
the machine-predicted list through the human--machine interface. In the task
scheduling phase, we propose a scheduling algorithm that integrates human
experience constraints. The steps to resolve an event are inserted into the
normal job sequence to schedule the resolution. We propose several
human--machine collaboration methods in each phase to generate steps to resolve
an unplanned event while minimizing the impact on the original job plan.Comment: 15 pages, 16 figure
Imaging and variability studies of CTA~102 during the 2016 January -ray flare
The -ray bright blazar CTA 102 is studied using imaging (new 15 GHz
and archival 43 GHz Very Long Baseline Array, VLBA data) and time variable
optical flux density, polarization degree and electric vector position angle
(EVPA) spanning between 2015 June 1 and 2016 October 1, covering a prominent
-ray flare during 2016 January. The pc-scale jet indicates expansion
with oscillatory features upto 17 mas. Component proper motions are in the
range 0.04 - 0.33 mas/yr with acceleration upto 1.2 mas followed by a slowing
down beyond 1.5 mas. A jet bulk Lorentz factor 17.5, position angle of
128.3 degrees, inclination angle 6.6 degrees and intrinsic half opening
angle 1.8 degrees are derived from the VLBA data. These inferences are
employed in a helical jet model to infer long term variability in flux density,
polarization degree, EVPA and a rotation of the Stokes Q and U parameters. A
core distance of = 22.9 pc, and a magnetic field
strength at 1 pc and the core location of 1.57 G and 0.07 G respectively are
inferred using the core shift method. The study is useful in the context of
estimating jet parameters and in offering clues to distinguish mechanisms
responsible for variability over different timescales.Comment: 20 pages, 7 figures, 3 tables; accepted for publication in Ap
Concentration Polarization of High-Density Lipoprotein and Its Relation with Shear Stress in an In Vitro Model
The purpose of this study was to determine the concentration polarization of high-density lipoprotein (HDL) at the surface of the carotid artery under conditions of steady flow and to establish its relationship with shear stress using an in vitro vascular simulation model of carotid bifurcation. Shear stress, HDL concentration at the surface, and the ratio of HDL concentration at the surface to concentration in bulk flow were measured at different locations within the model under high-speed (1.451 m/s) and low-speed (0.559 m/s) flow. HDL showed concentration polarization at the surface of the carotid artery model, particularly in the internal carotid artery sinus. With decreasing flow velocity, the shear stress at the surface also decreased, and HDL concentration polarization increased. The concentration polarization of HDL was negatively and strongly correlated with shear stress at both low- (r = −0.872, P < .001) and high-speed flow (r = −0.592, P = .0018)
Relationship between Concentration Difference of Different Density Lipoproteins and Shear Stress in Atherosclerosis
Previous research has observed concentration polarization in LDL and HDL in the arterial system. However, there is no report that links this concentration polarization to the development of vascular atherosclerosis (AS). Therefore, the purpose of this study is to establish the relationship between concentration difference of LDL and HDL and shear stress using a carotid bifurcation vascular model. PTFE was employed to create the carotid bifurcation model. Endothelial cells were coated on the inner wall of the graft. In a recirculation system, HDL and LDL concentration were measured under two different ICA flow velocities at 5 different locations within our model. We report the following: (1) LDL and HDL concentration difference was observed in both high flow and low flow environments; (2) the degree of LDL and HDL concentration polarization varied depending of high flow and low flow environment; (3) absolute values of concentration difference between LDL and HDL at the inner wall surface decreased with the increase in shear stress when shear stress was more than 1.5 Pa. This variation trend would be more pronounced if shear stress were less than 0.5 Pa. Our study suggests that under the action of shear stress, concentration differences of LDL or HDL create a disturbance in the balance of atherogenic factors and anti-As factors, resulting in the occurrence of AS
Radio Jet Proper-motion Analysis of Nine Distant Quasars above Redshift 3.5
Up to now, jet kinematic studies of radio quasars have barely reached beyond
the redshift range at . This significantly limits our knowledge of
high-redshift jets, which can provide key information for understanding the jet
nature and the growth of the black holes in the early Universe. In this paper,
we selected 9 radio-loud quasars at which display milliarcsec-scale jet
morphology. We provided evidence on the source nature by presenting
high-resolution very long baseline interferometry (VLBI) images of the sample
at 8.4~GHz frequency and making spectral index maps. We also consider Gaia
optical positions that are available for 7 out of the 9 quasars, for a better
identification of the jet components within the radio structures. We find that
6 sources can be classified as core--jet blazars. The remaining 3 objects are
more likely young, jetted radio sources, compact symmetric objects. By
including multi-epoch archival VLBI data, we also obtained jet component proper
motions of the sample and estimated the jet kinematic and geometric parameters
(Doppler factor, Lorentz factor, viewing angle). Our results show that at
, the jet apparent transverse speeds do not exceed 20 times the speed of
light (). This is consistent with earlier high-redshift quasar measurements
in the literature and the tendency derived from low-redshift blazars that fast
jet speeds () only occur at low redshifts. The results from this paper
contribute to the understanding of the cosmological evolution of radio AGN.Comment: 28 pages, 4 figures, 8 tables. To appear in Ap
Decelerated non-relativistic expansion in a tidal disruption event with a potential neutrino association
A tidal disruption event (TDE) involves the tidal shredding of a star in the
vicinity of a dormant supermassive black hole. The nearby (230
mega-parsec) radio-quiet (radio luminosity of erg s)
AT2019dsg is the first TDE potentially associated with a neutrino event. The
origin of the non-thermal emission in AT2019dsg remains inconclusive;
possibilities include a relativistic jet or a sub-relativistic outflow.
Distinguishing between them can address neutrino production mechanisms. High
resolution very long baseline interferometry monitoring provides uniquely
constraining flux densities and proper motion of the ejecta. A non-relativistic
(outflow velocity of 0.1 ) decelerated expansion in a relatively
dense environment is found to produce the radio emission. Neutrino production
may be related to the acceleration of protons by the outflow. The present study
thus helps exclude jet-related origins for the non-thermal emission and
neutrino production, and constrains non-jetted scenarios.Comment: 40 pages, 3 figures, 2 tables. Submitted after revisio
Status and progress of China SKA Regional Centre prototype
The Square Kilometre Array (SKA) project consists of delivering two largest
radio telescope arrays being built by the SKA Observatory (SKAO), which is an
intergovernmental organization bringing together nations from around the world
with China being one of the major member countries. The computing resources
needed to process, distribute, curate and use the vast amount of data that will
be generated by the SKA telescopes are too large for the SKAO to manage on its
own. To address this challenge, the SKAO is working with the international
community to create a shared, distributed data, computing and networking
capability called the SKA Regional Centre Alliance. In this model, the SKAO
will be supported by a global network of SKA Regional Centres (SRCs)
distributed around the world in its member countries to build an end-to-end
science data system that will provide astronomers with high-quality science
products. SRCs undertake deep processing, scientific analysis, and long-term
storage of the SKA data, as well as user support. China has been actively
participating in and promoting the construction of SRCs. This paper introduces
the international cooperation and ongoing prototyping of the global SRC
network, the construction plan of the China SRC and describes in detail the
China SRC prototype. The paper also presents examples of scientific
applications of SKA precursor and pathfinder telescopes completed using
resources from the China SRC prototype. Finally, the future prospects of the
China SRC are presented.Comment: T. An, et al. Status and progress of China SKA Regional Centre
prototype. Sci. China-Phys. Mech. Astron. 65: 129501 (2022
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