17,122 research outputs found
Terrorist Attacks On Public Bus Transportation: A Preliminary Empirical Analysis, MTI Report WP 09-01
This report provides data on terrorist attacks against public bus transportation targets and serious crimes committed against such targets throughout the world. The data are drawn from the MTI database of attacks on public surface transportation, which is expanded and updated as information becomes available. This analysis is based on the database as of December 17, 2009. Data include the frequency and lethality with which buses, bus stations, and bus stops are attacked; the relationship between fatalities and attacks against bus targets and the relationship between injuries and attacks against those targets; how often, relative to other surface transportation targets, buses are attacked, first with all weapons and then with only explosive and incendiary devices; the relative lethality of attacks; and the distribution of attacks. It then presents some preliminary observations drawn from those data that can help stakeholders governments, transit managers, and employees to focus on the ways the most frequent and/or most lethal attacks are carried out as they consider measures to prevent or mitigate attacks that may be considered likely to happen in the United States
A 20 Thousand Solar Mass Black Hole in the Stellar Cluster G1
We present the detection of a 2.0(+1.4,-0.8)x10^4 solar mass black hole (BH)
in the stellar cluster G1 (Mayall II), based on data taken with the Space
Telescope Imaging Spectrograph onboard the Hubble Space Telescope. G1 is one of
the most massive stellar clusters in M31. The central velocity dispersion (25
kms) and the measured BH mass of G1 places it on a linear extrapolation of the
correlation between BH mass and bulge velocity dispersion established for
nearby galaxies. The detection of a BH in this low-mass stellar system suggests
that (1) the most likely candidates for seed massive BHs come from stellar
clusters, (2) there is a direct link between massive stellar clusters and
normal galaxies, and (3) the formation process of both bulges and massive
clusters is similar due to their concordance in the M_BH/sigma relation.
Globular clusters in our Galaxy should be searched for central BHs.Comment: 4 pages, accepted in The Astrophysical Journal Letters, October 200
The Ages of Dwarf Ellipticals
We present narrow band photometry of 91 dwarf ellipticals in the Coma and
Fornax clusters taken through the Stromgren (uvby) filter system. Dividing the
sample by dwarf morphology into nucleated (dEN) and non-nucleated (dE) dwarfs
reveals two distinct populations of early-type systems based on integrated
colors. The class of dEN galaxies are redder in their continuum colors as
compared to bright cluster ellipticals and dE type dwarfs, and their position
in multi-color diagrams can only be explained by an older mean age for their
underlying stellar populations. By comparison with the narrow band photometry
of the M87 globular cluster system (Jordan et al. 2002), we find that dENs are
a higher metallicity continuation of the old, metal-poor color sequence of
galactic globulars and the blue population of M87 globulars. Bright ellipticals
and dE dwarfs, on the other hand, follow the color sequence of the metal-rich,
red population of M87 globulars. A comparison to SED models, convolved to a
simple metallicity model, finds that dENs and blue globulars are 3 to 4 Gyrs
older than cluster ellipticals and 5 Gyrs older than dE type galaxies. The
implication is that globulars and dEN galaxies are primordial and have
metallicities set by external constraints such as the enrichment of their
formation clouds. Bright ellipticals and dE galaxies have metallicities and
ages that suggest an extended phase of initial star formation to produce a
younger mean age, even if their formation epoch is similar to that of dENs and
blue globulars, and an internally driven chemical evolutionary history.Comment: 13 pages AAS LaTeX, 6 figures, accepted for publication in A
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Measuring Dark Matter Profiles Non-Parametrically In Dwarf Spheroidals: An Application To Draco
We introduce a novel implementation of orbit-based (or Schwarzschild) modeling that allows dark matter density profiles to be calculated non-parametrically in nearby galaxies. Our models require no assumptions to be made about velocity anisotropy or the dark matter profile. The technique can be applied to any dispersion-supported stellar system, and we demonstrate its use by studying the Local Group dwarf spheroidal galaxy (dSph) Draco. We use existing kinematic data at larger radii and also present 12 new radial velocities within the central 13 pc obtained with the VIRUS-W integral field spectrograph on the 2.7 m telescope at McDonald Observatory. Our non-parametric Schwarzschild models find strong evidence that the dark matter profile in Draco is cuspy for 20 = 20 pc is well fit by a power law with slope alpha = -1.0 +/- 0.2, consistent with predictions from cold dark matter simulations. Our models confirm that, despite its low baryon content relative to other dSphs, Draco lives in a massive halo.NSF-0908639Astronom
Picoheterotroph (Bacteria and Archaea) biomass distribution in the global ocean
We compiled a database of 39 766 data points consisting of flow cytometric and microscopical measurements of picoheterotroph abundance, including both Bacteria and Archaea. After gridding with 1° spacing, the database covers 1.3% of the ocean surface. There are data covering all ocean basins and depths except the Southern Hemisphere below 350m or from April until June. The average picoheterotroph biomass is 3.9 ± 3.6 ”g Cl-1 with a 20-fold decrease between the surface and the deep sea. We estimate a total ocean inventory of about 1.3 à 1029 picoheterotroph cells. Surprisingly, the abundance in the coastal regions is the same as at the same depths in the open ocean. Using an average of published open ocean measurements for the conversion from abundance to carbon biomass of 9.1 fg cell-1, we calculate a picoheterotroph carbon inventory of about 1.2 Pg C. The main source of uncertainty in this inventory is the conversion factor from abundance to biomass. Picoheterotroph biomass is ? 2 times higher in the tropics than in the polar oceans
Probing the qudit depolarizing channel
For the quantum depolarizing channel with any finite dimension, we compare
three schemes for channel identification: unentangled probes, probes maximally
entangled with an external ancilla, and maximally entangled probe pairs. This
comparison includes cases where the ancilla is itself depolarizing and where
the probe is circulated back through the channel before measurement. Compared
on the basis of (quantum Fisher) information gained per channel use, we find
broadly that entanglement with an ancilla dominates the other two schemes, but
only if entanglement is cheap relative to the cost per channel use and only if
the external ancilla is well shielded from depolarization. We arrive at these
results by a relatively simple analytical means. A separate, more complicated
analysis for partially entangled probes shows for the qudit depolarizing
channel that any amount of probe entanglement is advantageous and that the
greatest advantage comes with maximal entanglement
Logical equivalence between generalized urn models and finite automata
To every generalized urn model there exists a finite (Mealy) automaton with
identical propositional calculus. The converse is true as well.Comment: 9 pages, minor change
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