1,012 research outputs found
Determining the optimal locations for shock acceleration in magnetohydrodynamical jets
Observations of relativistic jets from black holes systems suggest that
particle acceleration often occurs at fixed locations within the flow. These
sites could be associated with critical points that allow the formation of
standing shock regions, such as the magnetosonic modified fast point. Using the
self-similar formulation of special relativistic magnetohydrodynamics by
Vlahakis & K\"onigl, we derive a new class of flow solutions that are both
relativistic and cross the modified fast point at a finite height. Our
solutions span a range of Lorentz factors up to at least 10, appropriate for
most jets in X-ray binaries and active galactic nuclei, and a range in injected
particle internal energy. A broad range of solutions exists, which will allow
the eventual matching of these scale-free models to physical boundary
conditions in the analysis of observed sources.Comment: 9 pages, 4 figures, accepted for publication in Ap
What is the Hidden Depolarization Mechanism in Low Luminosity AGN?
Millimeter wavelength polarimetry of accreting black hole systems can provide
a tomographic probe of the accretion flow on a wide range of linear scales. We
searched for linear polarization in two low luminosity active galactic nuclei
(LLAGN), M81 and M84, using the Combined Array for Millimeter Astronomy (CARMA)
and the Submillimeter Array (SMA). We find upper limits of
averaging over the full bandwidth and with a rotation measure (RM) synthesis
technique. These low polarization fractions, along with similar low values for
LLAGN M87 and 3C84, suggest that LLAGN have qualitatively different
polarization properties than radio-loud sources and Sgr A*. If the sources are
intrinsically polarized and then depolarized by Faraday rotation then we place
lower limits on the RM of a few times for the full
bandwidth case and for the RM synthesis
analysis. These limits are inconsistent with or marginally consistent with
expected accretion flow properties. Alternatively, the sources may be
depolarized by cold electrons within a few Schwarzschild radii from the black
hole, as suggested by numerical models.Comment: Accepted for publication in ApJ
Neutrino flares from black hole coronae
We present a model for neutrino flares in accreting black holes based on the
injection of a non-thermal population of relativistic particles in a magnetized
corona. The most important products of hadronic and photohadronic interactions
at high energies are pions. Charged pions decay into muons and neutrinos; muons
also decay yielding neutrinos. Taking into account these effects, coupled
transport equations are solved for all species of particles and the neutrino
production is estimated for the case of accreting galactic black holes.Comment: 13 pages, 8 figures, accepted for publication in Advances in Space
Researc
The Fundamental Plane of Black Hole Accretion and its Use as a Black Hole-Mass Estimator
We present an analysis of the fundamental plane of black hole accretion, an
empirical correlation of the mass of a black hole (), its 5 GHz radio
continuum luminosity (), and its 2-10 keV X-ray power-law
continuum luminosity (). We compile a sample of black holes with primary,
direct black hole-mass measurements that also have sensitive,
high-spatial-resolution radio and X-ray data. Taking into account a number of
systematic sources of uncertainty and their correlations with the measurements,
we use Markov chain Monte Carlo methods to fit a mass-predictor function of the
form . Our best-fit results are , , and
with the natural logarithm of the Gaussian intrinsic scatter in the log-mass
direction . This result is a
significant improvement over our earlier mass scaling result because of the
increase in active galactic nuclei sample size (from 18 to 30), improvement in
our X-ray binary sample selection, better identification of Seyferts, and
improvements in our analysis that takes into account systematic uncertainties
and correlated uncertainties. Because of these significant improvements, we are
able to consider potential influences on our sample by including all sources
with compact radio and X-ray emission but ultimately conclude that the
fundamental plane can empirically describe all such sources. We end with advice
for how to use this as a tool for estimating black hole masses.Comment: ApJ Accepted. Online interactive version of Figure 7 available at
http://kayhan.astro.lsa.umich.edu/supplementary_material/fp
Ethics of a partially effective dengue vaccine: Lessons from the Philippines
Dengvaxia, a chimeric yellow fever tetravalent dengue vaccine developed by SanofiPasteur is widely licensed in dengue-endemic countries. In a large cohort study Dengvaxia was found to partially protect children who had prior dengue virus (DENV) infections but sensitized seronegative children to breakthrough DENV disease of enhanced severity. In 2019, the European Medicines Agency and the US FDA issued licenses that reconciled safety issues by restricting vaccine to individuals with prior dengue infections. Using revised Dengvaxia efficacy and safety data we sought to estimate hospitalized and severe dengue cases among the more than 800,000 9 year-old children vaccinated in the Philippines. Despite an overall vaccine efficacy of 69% during 4 years post-vaccination we project there will be more than one thousand vaccinated seronegative and seropositive children hospitalized for severe dengue. Assisting these children through a program of enhanced surveillance leading to improved care deserves widespread support. Clinical responses observed during breakthrough dengue infections in vaccinated individuals counsel prudence in design of vaccine policies. Recommendations concerning continued use of this dengue vaccine are: (1) obtain a better definition of vaccine efficacy and safety through enhanced phase 4 surveillance, (2) obtain a valid, accessible, sensitive, specific and affordable serological test that identifies past wild-type dengue virus infection and (3) clarify safety and efficacy of Dengvaxia in flavivirus immunes. In the absence of an acceptable serological screening test these unresolved ethical issues suggest Dengvaxia be given only to those signing informed consent.Marie Skłodowska-Curie grant agreement No 79249
The SED of Low-Luminosity AGNs at high-spatial resolution
The inner structure of AGNs is expected to change below a certain luminosity
limit. The big blue bump, footprint of the accretion disk, is absent for the
majority of low-luminosity AGNs (LLAGNs). Moreover, recent simulations suggest
that the torus, a keystone in the Unified Model, vanishes for nuclei with L_bol
< 10^42 erg/s. However, the study of LLAGN is a complex task due to the
contribution of the host galaxy, which light swamps these faint nuclei. This is
specially critical in the IR range, at the maximum of the torus emission, due
to the contribution of the old stellar population and/or dust in the nuclear
region. Adaptive optics imaging in the NIR (VLT/NaCo) together with diffraction
limited imaging in the mid-IR (VLT/VISIR) permit us to isolate the nuclear
emission for some of the nearest LLAGNs in the Southern Hemisphere. These data
were extended to the optical/UV range (HST), radio (VLA, VLBI) and X-rays
(Chandra, XMM-Newton, Integral), in order to build a genuine spectral energy
distribution (SED) for each AGN with a consistent spatial resolution (< 0.5")
across the whole spectral range. From the individual SEDs, we construct an
average SED for LLAGNs sampled in all the wavebands mentioned before. Compared
with previous multiwavelength studies of LLAGNs, this work covers the mid-IR
and NIR ranges with high-spatial resolution data. The LLAGNs in the sample
present a large diversity in terms of SED shapes. Some of them are very well
described by a self-absorbed synchrotron (e.g. NGC 1052), while some other
present a thermal-like bump at ~1 micron (NGC 4594). All of them are
significantly different when compared with bright Seyferts and quasars,
suggesting that the inner structure of AGNs (i.e. the torus and the accretion
disk) suffers intrinsic changes at low luminosities.Comment: 8 pages, 5 figures. To appear in the proceedings of "Astrophysics at
High Angular Resolution" (AHAR 2011
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