287 research outputs found
Self Organization and a Dynamical Transition in Traffic Flow Models
A simple model that describes traffic flow in two dimensions is studied. A
sharp {\it jamming transition } is found that separates between the low density
dynamical phase in which all cars move at maximal speed and the high density
jammed phase in which they are all stuck. Self organization effects in both
phases are studied and discussed.Comment: 6 pages, 4 figure
Design and tests of the hard X-ray polarimeter X-Calibur
X-ray polarimetry promises to give qualitatively new information about
high-energy astrophysical sources, such as binary black hole systems,
micro-quasars, active galactic nuclei, and gamma-ray bursts. We designed, built
and tested a hard X-ray polarimeter X-Calibur to be used in the focal plane of
the InFOCuS grazing incidence hard X-ray telescope. X-Calibur combines a low-Z
Compton scatterer with a CZT detector assembly to measure the polarization of
10-80 keV X-rays making use of the fact that polarized photons Compton scatter
preferentially perpendicular to the electric field orientation. X-Calibur
achieves a high detection efficiency of order unity.Comment: 9 pages, 5 figures, conference proceedings: SPIE 2011 (San Diego
Analytical time-like geodesics
Time-like orbits in Schwarzschild space-time are presented and classified in
a very transparent and straightforward way into four types. The analytical
solutions to orbit, time, and proper time equations are given for all orbit
types in the form r=r(\lambda), t=t(\chi), and \tau=\tau(\chi), where \lambda\
is the true anomaly and \chi\ is a parameter along the orbit. A very simple
relation between \lambda\ and \chi\ is also shown. These solutions are very
useful for modeling temporal evolution of transient phenomena near black holes
since they are expressed with Jacobi elliptic functions and elliptic integrals,
which can be calculated very efficiently and accurately.Comment: 15 pages, 10 figures, accepted by General Relativity and Gravitatio
Gravitational-Wave Recoil from the Ringdown Phase of Coalescing Black Hole Binaries
The gravitational recoil or "kick" of a black hole formed from the merger of
two orbiting black holes, and caused by the anisotropic emission of
gravitational radiation, is an astrophysically important phenomenon. We combine
(i) an earlier calculation, using post-Newtonian theory, of the kick velocity
accumulated up to the merger of two non-spinning black holes, (ii) a
"close-limit approximation" calculation of the radiation emitted during the
ringdown phase, and based on a solution of the Regge-Wheeler and Zerilli
equations using initial data accurate to second post-Newtonian order. We prove
that ringdown radiation produces a significant "anti-kick". Adding the
contributions due to inspiral, merger and ringdown phases, our results for the
net kick velocity agree with those from numerical relativity to 10-15 percent
over a wide range of mass ratios, with a maximum velocity of 180 km/s at a mass
ratio of 0.38.Comment: 9 pages, 5 figures; to appear in Class. Quant. Gra
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
A new model for QPOs in accreting black holes: application to the microquasar GRS 1915+105
(abridged) In this paper we extend the idea suggested previously by Petri
(2005a,b) that the high frequency quasi-periodic oscillations observed in
low-mass X-ray binaries may be explained as a resonant oscillation of the
accretion disk with a rotating asymmetric background (gravitational or
magnetic) field imposed by the compact object. Here, we apply this general idea
to black hole binaries. It is assumed that a test particle experiences a
similar parametric resonance mechanism such as the one described in paper I and
II but now the resonance is induced by the interaction between a spiral density
wave in the accretion disk, excited close to the innermost stable circular
orbit, and vertical epicyclic oscillations. We use the Kerr spacetime geometry
to deduce the characteristic frequencies of this test particle. The response of
the test particle is maximal when the frequency ratio of the two strongest
resonances is equal to 3:2 as observed in black hole candidates. Finally,
applying our model to the microquasar GRS 1915+105, we reproduce the correct
value of several HF-QPOs. Indeed the presence of the 168/113/56/42/28 Hz
features in the power spectrum time analysis is predicted. Moreover, based only
on the two HF-QPO frequencies, our model is able to constrain the mass and angular momentum of the accreting black hole.Comment: Accepted for publication in Astrophysics & Space Scienc
Beyond undetectable: modeling the clinical benefit of improved antiretroviral adherence in persons with human immunodeficiency virus with virologic suppression
BACKGROUND: Incomplete antiretroviral therapy (ART) adherence has been linked to deleterious immunologic, inflammatory, and clinical consequences, even among virally suppressed (<50 copies/mL) persons with human immunodeficiency virus (PWH). The impact of improving adherence in the risk of severe non-AIDS events (SNAEs) and death in this population is unknown. METHODS: We estimated the reduction in the risk of SNAEs or death resulting from an increase in ART adherence by (1) applying existing data on the association between adherence with high residual inflammation/coagulopathy in virally suppressed PWH, and (2) using a Cox proportional hazards model derived from changes in plasma interleukin 6 (IL-6) and D-dimer from 3 randomized clinical trials. Comparatively, assuming 100% ART adherence in a PWH who achieves viral suppression, we estimated the number of persons in whom a decrease in adherence to <100% would need to be observed for an additional SNAE or death event to occur during 3- and 5-year follow-up. RESULTS: Increasing ART adherence to 100% in PWH who are suppressed on ART despite imperfect adherence translated into a 6%-37% reduction in the risk of SNAEs or death. Comparatively, based on an anticipated 12% increase in IL-6, 254 and 165 PWH would need to decrease their adherence from 100% to <100% for an additional event to occur over 3- and 5-year follow-up, respectively. CONCLUSIONS: Modest gains in ART adherence could have clinical benefits beyond virologic suppression. Increasing ART adherence (eg, via an intervention or switch to long-acting ART) in PWH who remain virally suppressed despite incomplete adherence should be evaluated
Advanced localization of massive black hole coalescences with LISA
The coalescence of massive black holes is one of the primary sources of
gravitational waves (GWs) for LISA. Measurements of the GWs can localize the
source on the sky to an ellipse with a major axis of a few tens of arcminutes
to a few degrees, depending on source redshift, and a minor axis which is 2--4
times smaller. The distance (and thus an approximate redshift) can be
determined to better than a per cent for the closest sources we consider,
although weak lensing degrades this performance. It will be of great interest
to search this three-dimensional `pixel' for an electromagnetic counterpart to
the GW event. The presence of a counterpart allows unique studies which combine
electromagnetic and GW information, especially if the counterpart is found
prior to final merger of the holes. To understand the feasibility of early
counterpart detection, we calculate the evolution of the GW pixel with time. We
find that the greatest improvement in pixel size occurs in the final day before
merger, when spin precession effects are maximal. The source can be localized
to within 10 square degrees as early as a month before merger at ; for
higher redshifts, this accuracy is only possible in the last few days.Comment: 11 pages, 4 figures, version published in Classical and Quantum
Gravity (special issue for proceedings of 7th International LISA Symposium
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