49 research outputs found
The Galactic Center Black Hole Laboratory
The super-massive 4 million solar mass black hole Sagittarius~A* (SgrA*)
shows flare emission from the millimeter to the X-ray domain. A detailed
analysis of the infrared light curves allows us to address the accretion
phenomenon in a statistical way. The analysis shows that the near-infrared
flare amplitudes are dominated by a single state power law, with the low states
in SgrA* limited by confusion through the unresolved stellar background. There
are several dusty objects in the immediate vicinity of SgrA*. The source G2/DSO
is one of them. Its nature is unclear. It may be comparable to similar stellar
dusty sources in the region or may consist predominantly of gas and dust. In
this case a particularly enhanced accretion activity onto SgrA* may be expected
in the near future. Here the interpretation of recent data and ongoing
observations are discussed.Comment: 30 pages - 7 figures - accepted for publication by Springer's
"Fundamental Theories of Physics" series; summarizing GC contributions of 2
conferences: 'Equations of Motion in Relativistic Gravity' at the
Physikzentrum Bad Honnef, Bad Honnef, Germany, (Feb. 17-23, 2013) and the
COST MP0905 'The Galactic Center Black Hole Laboratory' Granada, Spain (Nov.
19 - 22, 2013
A change in the optical polarization associated with a gamma-ray flare in the blazar 3C 279
It is widely accepted that strong and variable radiation detected over all
accessible energy bands in a number of active galaxies arises from a
relativistic, Doppler-boosted jet pointing close to our line of sight. The size
of the emitting zone and the location of this region relative to the central
supermassive black hole are, however, poorly known, with estimates ranging from
light-hours to a light-year or more. Here we report the coincidence of a
gamma-ray flare with a dramatic change of optical polarization angle. This
provides evidence for co-spatiality of optical and gamma-ray emission regions
and indicates a highly ordered jet magnetic field. The results also require a
non-axisymmetric structure of the emission zone, implying a curved trajectory
for the emitting material within the jet, with the dissipation region located
at a considerable distance from the black hole, at about 10^5 gravitational
radii.Comment: Published in Nature issued on 18 February 2010. Corresponding
authors: Masaaki Hayashida and Greg Madejsk
A fast radio burst in the direction of the Virgo Cluster
The rate of fast radio bursts (FRBs) in the direction of nearby galaxy
clusters is expected to be higher than the mean cosmological rate if
intrinsically faint FRBs are numerous. In this paper, we describe a targeted
search for faint FRBs near the core of the Virgo cluster using the Australian
Square Kilometer Array Pathfinder telescope. During 300 hr of observations, we
discovered one burst, FRB 180417, with dispersion measure DM
cmpc. The FRB was promptly followed up by several radio telescopes for
27 h, but no repeat bursts were detected. An optical follow-up of FRB 180417
using the PROMPT5 telescope revealed no new sources down to an -band
magnitude of 20.1. We argue that FRB 180417 is likely behind the Virgo cluster
as the Galactic and intracluster DM contribution are small compared to the DM
of the FRB, and there are no galaxies in the line of sight. The non-detection
of FRBs from Virgo constrains the faint-end slope, (at 68\%
confidence limit), and the minimum luminosity, erg s (at 68\% confidence limit), of the FRB luminosity
function assuming cosmic FRB rate of FRBs sky day with
flux above 1 Jy located out to redshift of 1. Further FRB surveys of galaxy
clusters with high-sensitivity instruments will tighten the constraints on the
faint end of the luminosity function and, thus, are strongly encouraged
A census of baryons in the Universe from localized fast radio bursts
More than three quarters of the baryonic content of the Universe resides in a
highly diffuse state that is difficult to observe, with only a small fraction
directly observed in galaxies and galaxy clusters. Censuses of the nearby
Universe have used absorption line spectroscopy to observe these invisible
baryons, but these measurements rely on large and uncertain corrections and are
insensitive to the majority of the volume, and likely mass. Specifically,
quasar spectroscopy is sensitive either to only the very trace amounts of
Hydrogen that exists in the atomic state, or highly ionized and enriched gas in
denser regions near galaxies. Sunyaev-Zel'dovich analyses provide evidence of
some of the gas in filamentary structures and studies of X-ray emission are
most sensitive to gas near galaxy clusters. Here we report the direct
measurement of the baryon content of the Universe using the dispersion of a
sample of localized fast radio bursts (FRBs), thus utilizing an effect that
measures the electron column density along each sight line and accounts for
every ionised baryon. We augment the sample of published arcsecond-localized
FRBs with a further four new localizations to host galaxies which have measured
redshifts of 0.291, 0.118, 0.378 and 0.522, completing a sample sufficiently
large to account for dispersion variations along the line of sight and in the
host galaxy environment to derive a cosmic baryon density of (95% confidence). This independent
measurement is consistent with Cosmic Microwave Background and Big Bang
Nucleosynthesis values.Comment: Published online in Nature 27 May, 202
Biophysical Factors Affecting the Distribution of Demersal Fish around the Head of a Submarine Canyon Off the Bonney Coast, South Australia
We sampled the demersal fish community of the Bonney Canyon, South Australia at depths (100–1,500 m) and locations that are poorly known. Seventy-eight species of demersal fish were obtained from 12 depth-stratified trawls along, and to either side, of the central canyon axis. Distributional patterns in species richness and biomass were highly correlated. Three fish assemblage groupings, characterised by small suites of species with narrow depth distributions, were identified on the shelf, upper slope and mid slope. The assemblage groupings were largely explained by depth (ρw = 0.78). Compared to the depth gradient, canyon-related effects are weak or occur at spatial or temporal scales not sampled in this study. A conceptual physical model displayed features consistent with the depth zonational patterns in fish, and also indicated that canyon upwelling can occur. The depth zonation of the fish assemblage was associated with the depth distribution of water masses in the area. Notably, the mid-slope community (1,000 m) coincided with a layer of Antarctic Intermediate Water, the upper slope community (500 m) resided within the core of the Flinders Current, and the shelf community was located in a well-mixed layer of surface water (<450 m depth)
A new class of glycomimetic drugs to prevent free fatty acid-induced endothelial dysfunction
Background: Carbohydrates play a major role in cell signaling in many biological processes. We have developed a set of glycomimetic drugs that mimic the structure of carbohydrates and represent a novel source of therapeutics for endothelial dysfunction, a key initiating factor in cardiovascular complications. Purpose: Our objective was to determine the protective effects of small molecule glycomimetics against free fatty acidinduced endothelial dysfunction, focusing on nitric oxide (NO) and oxidative stress pathways. Methods: Four glycomimetics were synthesized by the stepwise transformation of 2,5dihydroxybenzoic acid to a range of 2,5substituted benzoic acid derivatives, incorporating the key sulfate groups to mimic the interactions of heparan sulfate. Endothelial function was assessed using acetylcholineinduced, endotheliumdependent relaxation in mouse thoracic aortic rings using wire myography. Human umbilical vein endothelial cell (HUVEC) behavior was evaluated in the presence or absence of the free fatty acid, palmitate, with or without glycomimetics (1µM). DAF2 and H2DCFDA assays were used to determine nitric oxide (NO) and reactive oxygen species (ROS) production, respectively. Lipid peroxidation colorimetric and antioxidant enzyme activity assays were also carried out. RTPCR and western blotting were utilized to measure Akt, eNOS, Nrf2, NQO1 and HO1 expression. Results: Ex vivo endotheliumdependent relaxation was significantly improved by the glycomimetics under palmitateinduced oxidative stress. In vitro studies showed that the glycomimetics protected HUVECs against the palmitateinduced oxidative stress and enhanced NO production. We demonstrate that the protective effects of preincubation with glycomimetics occurred via upregulation of Akt/eNOS signaling, activation of the Nrf2/ARE pathway, and suppression of ROSinduced lipid peroxidation. Conclusion: We have developed a novel set of small molecule glycomimetics that protect against free fatty acidinduced endothelial dysfunction and thus, represent a new category of therapeutic drugs to target endothelial damage, the first line of defense against cardiovascular disease