998 research outputs found
Hybrid Thermal-Nonthermal Synchrotron Emission from Hot Accretion Flows
We investigate the effect of a hybrid electron population, consisting of both
thermal and non-thermal particles, on the synchrotron spectrum, image size, and
image shape of a hot accretion flow onto a supermassive black hole. We find two
universal features in the emitted synchrotron spectrum: (i) a prominent
shoulder at low (< 10^11 Hz) frequencies that is weakly dependent on the shape
of the electron energy distribution, and (ii) an extended tail of emission at
high (> 10^13 Hz) frequencies whose spectral slope depends on the slope of the
power-law energy distribution of the electrons. In the low-frequency shoulder,
the luminosity can be up to two orders of magnitude greater than with a purely
thermal plasma even if only a small fraction (< 1%) of the steady-state
electron energy is in the non-thermal electrons. We apply the hybrid model to
the Galactic center source, Sgr A*. The observed radio and IR spectra imply
that at most 1% of the steady-state electron energy is present in a power-law
tail in this source. This corresponds to no more than 10% of the electron
energy injected into the non-thermal electrons and hence 90% into the thermal
electrons. We show that such a hybrid distribution can be sustained in the flow
because thermalization via Coulomb collisions and synchrotron self-absorption
are both inefficient. The presence of non-thermal electrons enlarges the size
of the radio image at low frequencies and alters the frequency dependence of
the brightness temperature. A purely thermal electron distributions produces a
sharp-edged image while a hybrid distribution causes strong limb brightening.
These effects can be seen up to frequencies ~10^11 Hz and are accessible to
radio interferometers.Comment: 33 pages with figures, to appear in the Astrophysical Journa
Sgr A* Polarization: No ADAF, Low Accretion Rate, and Non-Thermal Synchrotron Emission
The recent detection of polarized radiation from Sgr A* requires a
non-thermal electron distribution for the emitting plasma. The Faraday rotation
measure must be small, placing strong limits on the density and magnetic field
strength. We show that these constraints rule out advection-dominated accretion
flow models. We construct a simple two-component model which can reproduce both
the radio to mm spectrum and the polarization. This model predicts that the
polarization should rise to nearly 100% at shorter wavelengths. The first
component, possibly a black-hole powered jet, is compact, low density, and
self-absorbed near 1 mm with ordered magnetic field, relativistic Alfven speed,
and a non-thermal electron distribution. The second component is poorly
constrained, but may be a convection-dominated accretion flow with dM/dt~10^-9
M_Sun/yr, in which feedback from accretion onto the black hole suppresses the
accretion rate at large radii. The black hole shadow should be detectable with
sub-mm VLBI.Comment: 4 pages, 1 figure, accepted by ApJL, several changes from submitted
versio
What is the Accretion Rate in Sgr A*?
The radio source Sgr A* at the center of our Galaxy is believed to be a 2.6 x
10^6 solar mass black hole which accretes gas from the winds of nearby stars.
We show that limits on the X-ray and infrared emission from the Galactic Center
provide an upper limit of ~ 8 x 10^{-5} solar masses per year on the mass
accretion rate in Sgr A*. The advection-dominated accretion flow (ADAF) model
favors a rate < 10^{-5} solar masses per year. In comparison, the Bondi
accretion rate onto Sgr A*, estimated using the observed spatial distribution
of mass losing stars and assuming non-interacting stellar winds, is ~ 3 x
10^{-5} solar masses per year. There is thus rough agreement between the Bondi,
the ADAF, and the X-ray inferred accretion rates for Sgr A*. We discuss
uncertainties in these estimates, emphasizing the importance of upcoming
observations by the Chandra X-ray observatory (CXO) for tightening the X-ray
derived limits.Comment: to appear in ApJ Letter
Constraining the Accretion Rate Onto Sagittarius A* Using Linear Polarization
Two possible explanations for the low luminosity of the supermassive black
hole at the center of our galaxy are (1) an accretion rate of order the
canonical Bondi value (roughly 10^{-5} solar masses per year), but a very low
radiative efficiency for the accreting gas or (2) an accretion rate much less
than the Bondi rate. Both models can explain the broad-band spectrum of the
Galactic Center. We show that they can be distinguished using the linear
polarization of synchrotron radiation. Accretion at the Bondi rate predicts no
linear polarization at any frequency due to Faraday depolarization. Low
accretion rate models, on the other hand, have much lower gas densities and
magnetic field strengths close to the black hole; polarization may therefore be
observable at high frequencies. If confirmed, a recent detection of linear
polarization from Sgr A above 150 GHz argues for an accretion rate of order
10^{-8} solar masses per year, much less than the Bondi rate. This test can be
applied to other low-luminosity galactic nuclei.Comment: final version accepted by ApJ; references added, somewhat shortene
VLTI/VINCI observations of the nucleus of NGC 1068 using the adaptive optics system MACAO
We present the first near-infrared K-band long-baseline interferometric
measurement of the prototype Seyfert 2 galaxy NGC 1068 with resolution lambda/B
\~ 10 mas obtained with the Very Large Telescope Interferometer (VLTI) and the
two 8.2m Unit Telescopes UT2 and UT3. The adaptive optics system MACAO was
employed to deliver wavefront-corrected beams to the K-band commissioning
instrument VINCI. A squared visibility amplitude of 16.3 +/- 4.3 % was measured
for NGC 1068 at a sky-projected baseline length of 45.8 m and azimuth angle
44.9 deg. This value corresponds to a FWHM of the K-band intensity distribution
of 5.0 +/- 0.5 mas (0.4 +/- 0.04 pc) at the distance of NGC 1068) if it
consists of a single Gaussian component. Taking into account K-band speckle
interferometry observations (Wittkowski et al. 1998; Weinberger et al. 1999;
Weigelt et al. 2004), we favor a multi-component model for the intensity
distribution where a part of the flux originates from scales clearly smaller
than about 5 mas (<0.4 pc), and another part of the flux from larger scales.
The K-band emission from the small (< 5 mas) scales might arise from
substructure of the dusty nuclear torus, or directly from the central accretion
flow viewed through only moderate extinction.Comment: Accepted for publication in Astronomy and Astrophysics Letter
Measuring the Black Hole Spin in Sgr A*
The polarized mm/sub-mm radiation from Sgr A* is apparently produced by a
Keplerian structure whose peak emission occurs within several Schwarzschild
radii (r_S=2GM/c^2) of the black hole. The Chandra X-ray counterpart, if
confirmed, is presumably the self-Comptonized component from this region. In
this paper, we suggest that sub-mm timing observations could yield a signal
corresponding to the period P_0 of the marginally stable orbit, and therefore
point directly to the black hole's spin a. Sgr A*'s mass is now known to be
(2.6\pm 0.2)\times 10^6 M_\odot (an unusually accurate value for supermassive
black hole candidates), for which 2.7 min<P_0<36 min, depending on the value of
a and whether the Keplerian flow is prograde or retrograde. A Schwarzschild
black hole (a=0) should have P_0 ~ 20 min. The identification of the orbital
frequency with the innermost stable circular orbit is made feasible by the
transition from optically thick to thin emission at sub-mm wavelengths. With
stratification in the emitter, the peak of the sub-mm bump in Sgr A*'s spectrum
is thus produced at the smallest radius. We caution, however, that theoretical
uncertainties in the structure of the emission region may still produce some
ambiguity in the timing signal. Given that Sgr A*'s flux at mm is
several Jy, these periods should lie within the temporal-resolving capability
of sub-mm telescopes using bolometric detectors. A determination of P_0 should
provide not only a value of a, but it should also define the angular momentum
vector of the orbiting gas in relation to the black hole's spin axis. In
addition, since the X-ray flux detected by Chandra appears to be the
self-Comptonized mm to sub-mm component, these temporal fluctuations may also
be evident in the X-ray signal.Comment: 15 pages, 1 figures. Accepted for publication in ApJ Letter
Congenital bilateral knee dislocation: a case report
A luxação congénita bilateral do joelho, é uma patologia rara. Quando ocorre associada a um síndrome polimalformativo
e irredutível com tratamento conservador, a sua abordagem e tratamento tornam-se um desafio terapêutico.
Reportamos um caso de um recém-nascido, do sexo feminino com cromossomopatia e patologia multiorgânica associada,
com luxação grave bilateral do joelho. Foi submetida a tratamento conservador com imobilizações gessadas seriadas não se
obtendo, contudo, a redução do joelho.
Optámos, posteriormente, por tratamento cirúrgico e realizámos redução aberta e alongamento do aparelho extensor
com quadriceplastia V-Y de ambos os joelhos, obtendo-se uma redução estável.
O tratamento cirúrgico usado criteriosamente no caso de uma luxação grave do joelho, através do alongamento do aparelho extensor, foi um método eficaz, obtendo-se um joelho reduzido e estável, sem complicações no pós-operatório e com
bons resultados funcionais
Protecting vulnerable consumers in "Switching Markets''
This paper studies regulatory policy interventions aimed at protecting vulnerable consumers who are disengaged and thus exposed to exploitation. We model heterogeneous consumer switching costs alongside asymmetric market shares. This setting encompasses many markets in which established firms are challenged by new entrants. We identify circumstances under which such interventions can be counterproductive, both with regard to the stated consumer protection objective and the complementary aim to promote competition
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