378 research outputs found
The Formation of Broad Line Clouds in the Accretion Shocks of Active Galactic Nuclei
Recent work on the gas dynamics in the Galactic Center has improved our
understanding of the accretion processes in galactic nuclei, particularly with
regard to properties such as the specific angular momentum distribution,
density, and temperature of the inflowing plasma. This information can be
valuable in trying to determine the origin of the Broad Line Region (BLR) in
Active Galactic Nuclei (AGNs). In this paper, we explore various scenarios for
the cloud formation based on the underlying principle that the source of plasma
is ultimately that portion of the gas trapped by the central black hole from
the interstellar medium. Based on what we know about the Galactic Center, it is
likely that in highly dynamic environments such as this, the supply of matter
is due mostly to stellar winds from the central cluster. Winds accreting onto a
central black hole are subjected to several disturbances capable of producing
shocks, including a Bondi-Hoyle flow, stellar wind-wind collisions, and
turbulence. Shocked gas is initially compressed and heated out of thermal
equilibrium with the ambient radiation field; a cooling instability sets in as
the gas is cooled via inverse-Compton and bremsstrahlung processes. If the
cooling time is less than the dynamical flow time through the shock region, the
gas may clump to form the clouds responsible for broad line emission seen in
many AGN spectra. Clouds produced by this process display the correct range of
densities and velocity fields seen in broad emission lines. Very importantly,
the cloud distribution agrees with the results of reverberation studies, in
which it is seen that the central line peak responds slower to continuum
changes than the line wings.Comment: 22 pages, 5 figure
New Constraints on the Nature of Radio Emission in Sagittarius A*
The mm to sub-mm spectrum of Sgr A* at the Galactic center, as well as its
polarization characteristics, are consistent with the inner 10 Schwarzschild
radii of a tight Keplerian emitting region of hot, magnetized, orbiting gas.
This plasma may also be the source through self-Comptonization of the X-rays
detected by Chandra. It has long been suspected that the circularization region
between the quasi-spherical infall at large radii, and this inner zone, is
responsible for producing the rest of Sgr A*'s spectrum. In this paper, we
report the results of a detailed study of this region. First, the combination
of existing cm and X-ray data preclude the possibility of producing the
observed strong 1.36 GHz radio flux via thermal synchrotron within a bounded
flow. If Sgr A*'s radio spectrum is produced by accreting gas, it appears that
a non-thermal particle distribution is a necessity. Second, if this is the
correct picture for how Sgr A*'s spectrum is produced, it appears that the
Chandra-detected X-rays may originate either from self-Comptonization in the
inner Keplerian region, or from optically-thin nonthermal synchrotron emission
in the much larger, circularization zone, extending up to 500 Schwarzschild
radii or more. This is a question that should be answered by upcoming broadband
observations, since the mm-bump and X-rays are strongly correlated in the
former case, whereas the X-rays are strongly correlated to the cm-radio flux in
the latter. In addition, X-rays produced in the circularized gas could show
periodic or quasi-periodic variations, but not those produced via nonthermal
synchrotron emission much farther out.Comment: 15 pages, 2 figures, ApJ Letters, In pres
Region of magnetic dominance near a rotating black hole
This is a brief contribution in which a simplified criterion of the relevance
of the test-particle approximation describing motion of material near a
magnetized black hole is discussed. Application to processes of the dissipative
collimation of astronomical jets (as proposed by de Felice and Curir, 1992) is
mentioned.Comment: 11 pages, to appear in General Relativity and Gravitation, also
available (with additional illustrations) at
http://otokar.troja.mff.cuni.cz/user/karas/au_www/karas/papers.ht
A relativistic model of the radio jets in NGC 315
We apply our intrinsically symmetrical, decelerating relativistic jet model
to deep VLA imaging of the inner 140 arcsec of the giant low-luminosity radio
galaxy NGC 315. An optimized model accurately fits the data in both total
intensity and linear polarization. We infer that the velocity, emissivity and
field structure in NGC 315 are very similar to those of the other
low-luminosity sources we have modelled, but that all of the physical scales
are larger by a factor of about 5. We derive an inclination to the line of
sight of 38 degrees for the jets. Where they first brighten, their on-axis
velocity is approximately v/c = 0.9. They decelerate to v/c = 0.4 between 8 and
18 kpc from the nucleus and the velocity thereafter remains constant. The speed
at the edge of the jet is roughly 0.6 of the on-axis value where it is best
constrained, but the transverse velocity profile may deviate systematically
from the Gaussian form we assume. The proper emissivity profile is split into
three power-law regions separated by shorter transition zones. In the first of
these, at 3 kpc (the flaring point) the jets expand rapidly at constant
emissivity, leading to a large increase in the observed brightness on the
approaching side. At 10 kpc, the emissivity drops abruptly by a factor of 2.
Where the jets are well resolved their rest-frame emission is
centre-brightened. The magnetic field is modelled as random on small scales but
anisotropic and we rule out a globally ordered helical configuration. To a
first approximation, the field evolves from a mixture of longitudinal and
toroidal components to predominantly toroidal, but it also shows variations in
structure along and across the jets, with a significant radial component in
places. Simple adiabatic models fail to fit the emissivity variations.Comment: 20 pages, 17 figures, MNRAS (in press
Rapid and accurate broadband absorption cross-section measurement of human bodies in a reverberation chamber
A measurement methodology for polarization and angle of incidence averaged electromagnetic absorption cross-section using a reverberation chamber is presented. The method is optimized for simultaneous rapid and accurate determination of average absorption cross-section over the frequency range 1–15 GHz, making it suitable for use in human absorption and exposure studies. The typical measurement time of the subject is about 8 min with a corresponding statistical uncertainty of about 3% in the measured absorption cross-section. The method is validated by comparing measurements on a spherical phantom with Mie series calculations. The efficacy of the method is demonstrated with measurements of the posture dependence of the absorption cross-section of a human subject and an investigation of the effects of clothing on the measured absorption which are important considerations for the practical design of experiments for studies on human subjects
The structure of Chariklo's rings from stellar occultations
Two narrow and dense rings (called C1R and C2R) were discovered around the
Centaur object (10199) Chariklo during a stellar occultation observed on 2013
June 3. Following this discovery, we planned observations of several
occultations by Chariklo's system in order to better characterize the physical
properties of the ring and main body. Here, we use 12 successful occulations by
Chariklo observed between 2014 and 2016. They provide ring profiles (physical
width, opacity, edge structure) and constraints on the radii and pole position.
Our new observations are currently consistent with the circular ring solution
and pole position, to within the km formal uncertainty for the ring
radii derived by Braga-Ribas et al. The six resolved C1R profiles reveal
significant width variations from to 7.5 km. The width of the fainter
ring C2R is less constrained, and may vary between 0.1 and 1 km. The inner and
outer edges of C1R are consistent with infinitely sharp boundaries, with
typical upper limits of one kilometer for the transition zone between the ring
and empty space. No constraint on the sharpness of C2R's edges is available. A
1 upper limit of m is derived for the equivalent width of
narrow (physical width <4 km) rings up to distances of 12,000 km, counted in
the ring plane
Rapid X-ray flaring from the direction of the supermassive black hole at the Galactic Centre
Most galactic nuclei are now believed to harbour supermassive black holes.
Studies of stellar motions in the central few light-years of our Milky Way
Galaxy indicate the presence of a dark object with a mass of about 2.6 million
solar masses. This object is spatially coincident with Sagittarius A* (Sgr A*),
the unique compact radio source located at the dynamical centre of our Galaxy.
By analogy with distant quasars and nearby active galactic nuclei (AGN), Sgr A*
is thought to be powered by the gravitational potential energy released by
matter as it accretes onto a supermassive black hole. However, Sgr A* is much
fainter than expected in all wavebands, especially in X-rays, casting some
doubt on this model. Recently, we reported the first strong evidence of X-ray
emission from Sgr A*. Here we report the discovery of rapid X-ray flaring from
the direction of Sgr A*. These data provide compelling evidence that the X-ray
emission is coming from accretion onto a supermassive black hole at the
Galactic Centre, and the nature of the variations provides strong constraints
on the astrophysical processes near the event horizon of the black hole.Comment: 4 pages, 3 figures (Figs 1 and 3 in color), LaTe
Coordinated multi-wavelength observations of Sgr A*
We report on recent near-infrared (NIR) and X-ray observations of Sagittarius
A* (Sgr A*), the electromagnetic manifestation of the ~4x10^6 solar masses
super-massive black hole (SMBH) at the Galactic Center. The goal of these
coordinated multi-wavelength observations is to investigate the variable
emission from Sgr A* in order to obtain a better understanding of the
underlying physical processes in the accretion flow/outflow. The observations
have been carried out using the NACO adaptive optics (AO) instrument at the
European Southern Observatory's Very Large Telescope (July 2005, May 2007) and
the ACIS-I instrument aboard the Chandra X-ray Observatory (July 2005). We
report on a polarized NIR flare synchronous to a 8x1033 erg/s X-ray flare in
July 2005, and a further flare in May 2007 that shows the highest sub-flare to
flare contrast observed until now. The observations can be interpreted in the
framework of a model involving a temporary disk with a short jet. In the disk
component flux density variations can be explained due to hot spots on
relativistic orbits around the central SMBH. The variations of the
sub-structures of the May 2007 flare are interpreted as a variation of the hot
spot structure due to differential rotation within the disk.Comment: 15 pages, 7 figures, contribution for the conference "The Universe
under the Microscope" (AHAR 2008), to be published in Journal of Physics:
Conference Series by Institute of Physics Publishin
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