82 research outputs found
Accelerating AGN jets to parsec scales using general relativistic MHD simulations
Accreting black holes produce collimated outflows, or jets, that traverse
many orders of magnitude in distance, accelerate to relativistic velocities,
and collimate into tight opening angles. Of these, perhaps the least understood
is jet collimation due to the interaction with the ambient medium. In order to
investigate this interaction, we carried out axisymmetric general relativistic
magnetohydrodynamic simulations of jets produced by a large accretion disc,
spanning over 5 orders of magnitude in time and distance, at an unprecedented
resolution. Supported by such a disc, the jet attains a parabolic shape,
similar to the M87 galaxy jet, and the product of the Lorentz factor and the
jet half-opening angle, , similar to values found from very
long baseline interferometry (VLBI) observations of active galactic nuclei
(AGN) jets; this suggests extended discs in AGN. We find that the interaction
between the jet and the ambient medium leads to the development of pinch
instabilities, which produce significant radial and lateral variability across
the jet by converting magnetic and kinetic energy into heat. Thus pinched
regions in the jet can be detectable as radiating hotspots and may provide an
ideal site for particle acceleration. Pinching also causes gas from the ambient
medium to become squeezed between magnetic field lines in the jet, leading to
enhanced mass-loading of the jet and potentially contributing to the
spine-sheath structure observed in AGN outflows.Comment: 18 pages, 24 figures, submitted to MNRAS, revised version. See our
Youtube channel for accompanying animations:
https://www.youtube.com/playlist?list=PLjldVlE2vDFzHMGn75tgc2Lod0kcTWZd
Corona, Jet, and Relativistic Line Models for Suzaku/RXTE/Chandra-HETG Observations of the Cygnus X-1 Hard State
Using Suzaku and the Rossi X-ray Timing Explorer, we have conducted a series
of four simultaneous observations of the galactic black hole candidate Cyg X-1
in what were historically faint and spectrally hard low states. Additionally,
all of these observations occurred near superior conjunction with our line of
sight to the X-ray source passing through the dense phases of the focused wind
from the mass donating secondary. One of our observations was also simultaneous
with observations by the Chandra-High Energy Transmission Grating. These latter
spectra are crucial for revealing the ionized absorption due to the secondary's
focused wind. Such absorption is present and must be accounted for in all four
spectra. These simultaneous data give an unprecedented view of the 0.8-300 keV
spectrum of Cyg X-1, and hence bear upon both corona and X-ray emitting jet
models of black hole hard states. Three models fit the spectra well: coronae
with thermal or mixed thermal/non-thermal electron populations, and jets. All
three models require a soft component that we fit with a low temperature disk
spectrum with an inner radius of only a few tens of GM/c^2. All three models
also agree that the known spectral break at 10\,keV is not solely due to the
presence of reflection, but each gives a different underlying explanation for
the augmentation of this break. Thus whereas all three models require that
there is a relativistically broadened Fe line, the strength and inner radius of
such a line is dependent upon the specific model, {thus making premature
line-based estimates of the black hole spin in the Cyg X-1 system. We look at
the relativistic line in detail, accounting for the narrow Fe emission and
ionized absorption detected by HETG. Although the specific relativistic
parameters of the line are continuum-dependent, none of the broad line fits
allow for an inner disk radius that is >40 GM/c^2.Comment: 22 pages, 16 figures. Uses emulateapj style. Final three tables
inserted as a figure to avoid issues with astro-ph's version of latex
mangling the use of lscape. To be published in the Astrophysical Journal,
January, 201
Going with the flow: can the base of jets subsume the role of compact accretion disk coronae?
The hard state of X-ray binaries (XRBs) is characterized by a power law
spectrum in the X-ray band, and a flat/inverted radio/IR spectrum associated
with occasionally imaged compact jets. It has generally been thought that the
hard X-rays result from Compton upscattering of thermal accretion disk photons
by a hot, coronal plasma whose properties are inferred via spectral fitting.
Interestingly, these properties-especially those from certain magnetized corona
models-are very similar to the derived plasma conditions at the jet footpoints.
Here we explore the question of whether the `corona' and `jet base' are in fact
related, starting by testing the strongest premise that they are synonymous. In
such models, the radio through the soft X-rays are dominated by synchrotron
emission, while the hard X-rays are dominated by inverse Compton at the jet
base - with both disk and synchrotron photons acting as seed photons. The
conditions at the jet base fix the conditions along the rest of the jet, thus
creating a direct link between the X-ray and radio emission. We also add to
this model a simple iron line and convolve the spectrum with neutral
reflection. After forward-folding the predicted spectra through the detector
response functions, we compare the results to simultaneous radio/X-ray data
obtained from the hard states of the Galactic XRBs GX339-4 and Cygnus X-1.
Results from simple Compton corona model fits are also presented for
comparison. We demonstrate that the jet model fits are statistically as good as
the single-component corona model X-ray fits, yet are also able to address the
simultaneous radio data.Comment: Accepted to the Astrophysical Journal. 14 pages, emulateapj.st
Smart Panking : uma aplicação para estacionamento em cidades inteligentes
The world is experiencing two major transformations: the number of people living in urban areas surpassing the number of people in rural areas and a technological revolution changing the lives of billions of people. With more people living in cities, the problems they face are intensified and the response to these problems increasingly involves the use of a lot of technology, from which emerges the concept of Smart City. In order to deal with one of the most critical problems of cities, urban mobility, this work seeks to understand its causes and what interventions could be undertaken. One of the most prominent problems that impacts the urban mobility is the high number of vehicles circulating in cities and one of the challenges is to manage this fleet of vehicles in order to reduce congestion, facilitating mobility and consequently reducing the amount of exhaust gases in the atmosphere. Since one of the causes of this problem is the time spent by drivers looking for parking spaces, one solution is to minimize it. For this purpose a Systematic Literature Review of primary studies that were inserted in this context was carried out and through a Product Research in the Market, followed by the application of a questionnaire to stakeholders, relevant characteristics were extracted for the development of a solution for this scenario. Therefore, the objective of this work was to create a tool that intelligently optimizes the process of supply and demand of parking spots, considering the growing number of vehicles and the limited space of cities as a contribution to the development of Smart Cities.O mundo está vivenciando duas grandes transformações: o número de pessoas vivendo em zonas urbanas ultrapassando o número de pessoas em zonas rurais e uma revolução tecnológica mudando a vida de bilhões de pessoas. Com mais pessoas vivendo nas cidades, os problemas que elas enfrentam são intensificados e a resposta a esses problemas envolve cada vez mais o emprego de muita tecnologia, de onde emerge o conceito de Cidade Inteligente. Com o intuito de lidar com um dos problemas mais crucias das cidades, a mobilidade urbana, este trabalho procurou levantar as suas causas e quais intervenções poderiam ser realizadas. Um dos problemas mais proeminentes que gera impacto na mobilidade urbana é o crescimento da quantidade de veÃculos circulando nas cidades e um dos desafios é gerenciar esta frota de veÃculos buscando diminuir o congestionamento, facilitando a mobilidade e consequentemente reduzindo a quantidade de gases expelidos na atmosfera. Uma vez que uma das causas deste problema é o tempo gasto pelos motoristas na procura por vagas para estacionar, uma solução é minimizar esta causa. Para este fim foi realizada uma Revisão Sistemática dos estudos primários que estão inseridos neste contexto e por meio de uma Pesquisa de Produtos no Mercado, seguido da aplicação de um questionário para stakeholders foram extraÃdas caracterÃsticas relevantes para o desenvolvimento de uma solução para este cenário. Sendo assim o objetivo deste trabalho foi criar uma ferramenta que otimiza de forma inteligente o processo de oferta e procura de vagas de estacionamento, tendo em vista o número crescente de veÃculos e o espaço limitado das cidades como contribuição para o desenvolvimento das Cidades Inteligentes.São Cristóvã
Short Timescale Evolution of the Polarized Radio Jet during V404 Cygni's 2015 Outburst
We present a high time resolution, multi-frequency linear polarization
analysis of Very Large Array (VLA) radio observations during some of the
brightest radio flaring (~1 Jy) activity of the 2015 outburst of V404 Cygni.
The VLA simultaneously captured the radio evolution in two bands (each with two
1 GHz base-bands), recorded at 5/7 GHz and 21/26 GHz, allowing for a broadband
polarimetric analysis. Given the source's high flux densities, we were able to
measure polarization on timescales of ~13 minutes, constituting one of the
highest temporal resolution radio polarimetric studies of a black hole X-ray
binary (BHXB) outburst to date. Across all base-bands, we detect variable,
weakly linearly polarized emission (<1%) with a single, bright peak in the
time-resolved polarization fraction, consistent with an origin in an evolving,
dynamic jet component. We applied two independent polarimetric methods to
extract the intrinsic electric vector position angles and rotation measures
from the 5 and 7 GHz base-band data and detected a variable intrinsic
polarization angle, indicative of a rapidly evolving local environment or a
complex magnetic field geometry. Comparisons to the simultaneous,
spatially-resolved observations taken with the Very Long Baseline Array at 15.6
GHz, do not show a significant connection between the jet ejections and the
polarization state.Comment: 24 pages, 9 figures, accepted by MNRA
A Multiwavelength Study of GRS 1716-249 in Outburst: Constraints on Its System Parameters
We present a detailed study of the evolution of the Galactic black hole transient GRS 1716-249 during its 2016-2017 outburst at optical (Las Cumbres Observatory), mid-infrared (Very Large Telescope), near-infrared (Rapid Eye Mount telescope), and ultraviolet (the Neil Gehrels Swift Observatory Ultraviolet/Optical Telescope) wavelengths, along with archival radio and X-ray data. We show that the optical/near-infrared and UV emission of the source mainly originates from a multi-temperature accretion disk, while the mid-infrared and radio emission are dominated by synchrotron emission from a compact jet. The optical/UV flux density is correlated with the X-ray emission when the source is in the hard state, consistent with an X-ray irradiated accretion disk with an additional contribution from the viscous disk during the outburst fade. We find evidence for a weak, but highly variable jet component at mid-infrared wavelengths. We also report the long-term optical light curve of the source and find that the quiescent i'-band magnitude is 21.39 +/- 0.15 mag. Furthermore, we discuss how previous estimates of the system parameters of the source are based on various incorrect assumptions, and so are likely to be inaccurate. By comparing our GRS 1716-249 data set to those of other outbursting black hole X-ray binaries, we find that while GRS 1716-249 shows similar X-ray behavior, it is noticeably optically fainter, if the literature distance of 2.4 kpc is adopted. Using several lines of reasoning, we argue that the source distance is further than previously assumed in the literature, likely within 4-17 kpc, with a most likely range of similar to 4-8 kpc
First M87 Event Horizon Telescope Results. IV. Imaging the Central Supermassive Black Hole
We present the first Event Horizon Telescope (EHT) images of M87, using observations from April 2017 at 1.3 mm wavelength. These images show a prominent ring with a diameter of similar to 40 mu as, consistent with the size and shape of the lensed photon orbit encircling the "shadow" of a supermassive black hole. The ring is persistent across four observing nights and shows enhanced brightness in the south. To assess the reliability of these results, we implemented a two-stage imaging procedure. In the first stage, four teams, each blind to the others' work, produced images of M87 using both an established method (CLEAN) and a newer technique (regularized maximum likelihood). This stage allowed us to avoid shared human bias and to assess common features among independent reconstructions. In the second stage, we reconstructed synthetic data from a large survey of imaging parameters and then compared the results with the corresponding ground truth images. This stage allowed us to select parameters objectively to use when reconstructing images of M87. Across all tests in both stages, the ring diameter and asymmetry remained stable, insensitive to the choice of imaging technique. We describe the EHT imaging procedures, the primary image features in M87, and the dependence of these features on imaging assumptions
First M87 Event Horizon Telescope Results. VI. The Shadow and Mass of the Central Black Hole
We present measurements of the properties of the central radio source in M87 using Event Horizon Telescope data obtained during the 2017 campaign. We develop and fit geometric crescent models (asymmetric rings with interior brightness depressions) using two independent sampling algorithms that consider distinct representations of the visibility data. We show that the crescent family of models is statistically preferred over other comparably complex geometric models that we explore. We calibrate the geometric model parameters using general relativistic magnetohydrodynamic (GRMHD) models of the emission region and estimate physical properties of the source. We further fit images generated from GRMHD models directly to the data. We compare the derived emission region and black hole parameters from these analyses with those recovered from reconstructed images. There is a remarkable consistency among all methods and data sets. We find that >50% of the total flux at arcsecond scales comes from near the horizon, and that the emission is dramatically suppressed interior to this region by a factor >10, providing direct evidence of the predicted shadow of a black hole. Across all methods, we measure a crescent diameter of 42 +/- 3 mu as and constrain its fractional width to be <0.5. Associating the crescent feature with the emission surrounding the black hole shadow, we infer an angular gravitational radius of GM/Dc(2) = 3.8 +/- 0.4 mu as. Folding in a distance measurement of 16.8(-0.7)(+0.8) gives a black hole mass of M = 6.5. 0.2 vertical bar(stat) +/- 0.7 vertical bar(sys) x 10(9) M-circle dot. This measurement from lensed emission near the event horizon is consistent with the presence of a central Kerr black hole, as predicted by the general theory of relativity
Monitoring the Morphology of M87* in 2009–2017 with the Event Horizon Telescope
The Event Horizon Telescope (EHT) has recently delivered the first resolved images of M87*, the supermassive black hole in the center of the M87 galaxy. These images were produced using 230 GHz observations performed in 2017 April. Additional observations are required to investigate the persistence of the primary image feature—a ring with azimuthal brightness asymmetry—and to quantify the image variability on event horizon scales. To address this need, we analyze M87* data collected with prototype EHT arrays in 2009, 2011, 2012, and 2013. While these observations do not contain enough information to produce images, they are sufficient to constrain simple geometric models. We develop a modeling approach based on the framework utilized for the 2017 EHT data analysis and validate our procedures using synthetic data. Applying the same approach to the observational data sets, we find the M87* morphology in 2009–2017 to be consistent with a persistent asymmetric ring of ~40 μas diameter. The position angle of the peak intensity varies in time. In particular, we find a significant difference between the position angle measured in 2013 and 2017. These variations are in broad agreement with predictions of a subset of general relativistic magnetohydrodynamic simulations. We show that quantifying the variability across multiple observational epochs has the potential to constrain the physical properties of the source, such as the accretion state or the black hole spin
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