183 research outputs found
On the nature of the ultraluminous X-ray transient in Cen~A (NGC 5128)
We combine 9 ROSAT, 9 Chandra, and 2 XMM-Newton observations of the Cen~A
galaxy to obtain the X-ray light curve of 1RXH J132519.8-430312 (=CXOU
J132519.9430317) spanning 1990 to 2003. The source reached a peak 0.1-2.4
keV flux F_X>10^{-12} ergs cm^{-2} s^{-1} during a 10~day span in 1995 July.
The inferred peak isotropic luminosity of the source therefore exceeded 3
10^{39} ergs s^{-1}, which places the source in the class of ultra-luminous
X-ray sources. Coherent pulsations at 13.264 Hz are detected during a second
bright episode (F_X >3 times 10^{-13} ergs cm^{-2} s^{-1}) in 1999 December.
The source is detected and varies significantly within three additional
observations but is below the detection threshold in 7 observations. The X-ray
spectrum in 1999 December is best described as a cut-off power law or a
disk-blackbody (multi-colored disk). We also detect an optical source, m_F555W
~ 24.1 mag, within the Chandra error circle of 1RXH J132519.8-430312 in HST
images taken 195~days before the nearest X-ray observation. The optical
brightness of this source is consistent with a late O or early B star at the
distance of Cen A. If the optical source is the counterpart, then the X-ray and
optical behavior of 1RXH J132519.8-430312 are similar to the transient Be/X-ray
pulsar A 0538-66.Comment: 7 pages, 8 figures. ApJ (accepted
The role of rs2237781 within GRM8 in eating behavior
Introduction: The glutamate receptor, metabotropic 8 gene (GRM8) encodes a G-protein-coupled glutamate receptor and has been associated with smoking behavior and liability to alcoholism implying a role in addiction vulnerability. Data from animal studies suggest that GRM8 may be involved in the regulation of the neuropeptide Y and melanocortin pathways and might influence food intake and metabolism. This study aimed to investigate the effects of the genetic variant rs2237781 within GRM8 on human eating behavior. Methods: The initial analysis included 548 Sorbs from Germany who have been extensively phenotyped for metabolic traits and who completed the German version of the three-factor eating questionnaire. In addition, we analyzed two independent sample sets comprising 293 subjects from another German cohort and 430 Old Order Amish individuals. Genetic associations with restraint, disinhibition, and hunger were assessed in an additive linear regression model. Results: Among the Sorbs the major G allele of rs2237781 was significantly associated with increased restraint scores in eating behavior (P = 1.9 9 10?4; b =+1.936). The German cohort and the Old Order Amish population revealed a trend in the same direction for restraint (P = 0.242; b =+0.874; P = 0.908; b =+0.096; respectively). A meta-analysis resulted in a combined P = 3.1 9 10?3 (Z-score 2.948). Conclusion: Our data suggest that rs2237781 within GRM8 may influence human eating behavior factors probably via pathways involved in addictive behavior
High Energy gamma-rays From FR I Jets
Thanks to Hubble and Chandra telescopes, some of the large scale jets in
extragalactic radio sources are now being observed at optical and X-ray
frequencies. For the FR I objects the synchrotron nature of this emission is
surely established, although a lot of uncertainties - connected for example
with the particle acceleration processes involved - remain. In this paper we
study production of high energy gamma-rays in FR I kiloparsec-scale jets by
inverse-Compton emission of the synchrotron-emitting electrons. We consider
different origin of seed photons contributing to the inverse-Compton
scattering, including nuclear jet radiation as well as ambient, stellar and
circumstellar emission of the host galaxies. We discuss how future detections
or non-detections of the evaluated gamma-ray fluxes can provide constraints on
the unknown large scale jet parameters, i.e. the magnetic field intensity and
the jet Doppler factor. For the nearby sources Centaurus A and M 87, we find
measurable fluxes of TeV photons resulting from synchrotron self-Compton
process and from comptonisation of the galactic photon fields, respectively. In
the case of Centaurus A, we also find a relatively strong emission component
due to comptonisation of the nuclear blazar photons, which could be easily
observed by GLAST at energy ~10 GeV, providing important test for the
unification of FR I sources with BL Lac objects.Comment: 39 pages, 6 figures included. Modified version, accepted for
publication in Astrophysical Journa
Probing photo-ionization: simulations of positive streamers in varying N2:O2 mixtures
Photo-ionization is the accepted mechanism for the propagation of positive
streamers in air though the parameters are not very well known; the efficiency
of this mechanism largely depends on the presence of both nitrogen and oxygen.
But experiments show that streamer propagation is amazingly robust against
changes of the gas composition; even for pure nitrogen with impurity levels
below 1 ppm streamers propagate essentially with the same velocity as in air,
but their minimal diameter is smaller, and they branch more frequently.
Additionally, they move more in a zigzag fashion and sometimes exhibit a
feathery structure. In our simulations, we test the relative importance of
photo-ionization and of the background ionization from pulsed repetitive
discharges, in air as well as in nitrogen with 1 ppm O2 . We also test
reasonable parameter changes of the photo-ionization model. We find that photo-
ionization dominates streamer propagation in air for repetition frequencies of
at least 1 kHz, while in nitrogen with 1 ppm O2 the effect of the repetition
frequency has to be included above 1 Hz. Finally, we explain the feather-like
structures around streamer channels that are observed in experiments in
nitrogen with high purity, but not in air.Comment: 12 figure
Ultrahigh Energy Cosmic Rays: The state of the art before the Auger Observatory
In this review we discuss the important progress made in recent years towards
understanding the experimental data on cosmic rays with energies \agt 10^{19}
eV. We begin with a brief survey of the available data, including a description
of the energy spectrum, mass composition, and arrival directions. At this point
we also give a short overview of experimental techniques. After that, we
introduce the fundamentals of acceleration and propagation in order to discuss
the conjectured nearby cosmic ray sources. We then turn to theoretical notions
of physics beyond the Standard Model where we consider both exotic primaries
and exotic physical laws. Particular attention is given to the role that
TeV-scale gravity could play in addressing the origin of the highest energy
cosmic rays. In the final part of the review we discuss the potential of future
cosmic ray experiments for the discovery of tiny black holes that should be
produced in the Earth's atmosphere if TeV-scale gravity is realized in Nature.Comment: Final version. To be published in Int. J. Mod. Phys.
Elasticity of iron at the temperature of the Earth's inner core
Seismological body-wave(1) and free-oscillation(2) studies of the Earth's solid inner core have revealed that compressional waves traverse the inner core faster along near-polar paths than in the equatorial plane. Studies have also documented local deviations from this first-order pattern of anisotropy on length scales ranging from 1 to 1,000 km (refs 3, 4). These observations, together with reports of the differential rotation(5) of the inner core, have generated considerable interest in the physical state and dynamics of the inner core, and in the structure and elasticity of its main constituent, iron, at appropriate conditions of pressure and temperature. Here we report first-principles calculations of the structure and elasticity of dense hexagonal close-packed (h.c.p.) iron at high temperatures. We find that the axial ratio c/a of h.c.p. iron increases substantially with increasing temperature, reaching a value of nearly 1.7 at a temperature of 5,700 K, where aggregate bulk and shear moduli match those of the inner core. As a consequence of the increasing c/a ratio, we have found that the single-crystal longitudinal anisotropy of h.c.p. iron at high temperature has the opposite sense from that at low temperature(6,7). By combining our results with a simple model of polycrystalline texture in the inner core, in which basal planes are partially aligned with the rotation axis, we can account for seismological observations of inner-core anisotropy.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/62959/1/413057a0.pd
High energy radiation from Centaurus A
We calculate for the nearest active galactic nucleus (AGN), Centaurus A, the
flux of high energy cosmic rays and of accompanying secondary photons and
neutrinos expected from hadronic interactions in the source. We use as two
basic models for the generation of ultrahigh energy cosmic rays (UHECR) shock
acceleration in the radio jet and acceleration in the regular electromagnetic
field close to the core of the AGN. While scattering on photons dominates in
scenarios with acceleration close to the core, scattering on gas becomes more
important if acceleration takes place along the jet. Normalizing the UHECR flux
from Centaurus A to the observations of the Auger experiment, the neutrino flux
may be marginally observable in a 1 km neutrino telescope, if a steep UHECR
flux \d N/\d E\propto E^{-\alpha} with extends down to
eV. The associated photon flux is close to or exceeds the observational data of
atmospheric Cherenkov and -ray telescopes for \alpha\gsim 2. In
particular, we find that already present data favour either a softer UHECR
injection spectrum than for Centaurus A or a lower UHECR flux than
expected from the normalization to the Auger observations.Comment: 12 pages, 6 figures; v2: revised version to appear in a special issue
of New Journal for Physic
BARRA v1.0: the Bureau of Meteorology Atmospheric high-resolution Regional Reanalysis for Australia
The Bureau of Meteorology Atmospheric high-resolution
Regional Reanalysis for Australia (BARRA) is the first atmospheric regional
reanalysis over a large region covering Australia, New Zealand, and Southeast
Asia. The production of the reanalysis with approximately 12 km horizontal
resolution – BARRA-R – is well underway with completion expected in 2019.
This paper describes the numerical weather forecast model, the data
assimilation methods, the forcing and observational data used to produce
BARRA-R, and analyses results from the 2003–2016 reanalysis. BARRA-R
provides a realistic depiction of the meteorology at and near the surface
over land as diagnosed by temperature, wind speed, surface pressure, and
precipitation. Comparing against the global reanalyses ERA-Interim and MERRA-2,
BARRA-R scores lower root mean square errors when evaluated against
(point-scale) 2 m temperature, 10 m wind speed, and surface pressure
observations. It also shows reduced biases in daily 2 m temperature maximum
and minimum at 5 km resolution and a higher frequency of very heavy
precipitation days at 5 and 25 km resolution when compared to gridded
satellite and gauge analyses. Some issues with BARRA-R are also identified:
biases in 10 m wind, lower precipitation than observed over the tropical
oceans, and higher precipitation over regions with higher elevations in south
Asia and New Zealand. Some of these issues could be improved through
dynamical downscaling of BARRA-R fields using convective-scale (<2 km) models.</p
Antecedents and consequences of effectuation and causation in the international new venture creation process
The selection of the entry mode in an international market is of key importance for the venture. A process-based perspective on entry mode selection can add to the International Business and International Entrepreneurship literature. Framing the international market entry as an entrepreneurial process, this paper analyzes the antecedents and consequences of causation and effectuation in the entry mode selection. For the analysis, regression-based techniques were used on a sample of 65 gazelles. The results indicate that experienced entrepreneurs tend to apply effectuation rather than causation, while uncertainty does not have a systematic influence. Entrepreneurs using causation-based international new venture creation processes tend to engage in export-type entry modes, while effectuation-based international new venture creation processes do not predetermine the entry mod
The Soft Gamma-Ray Spectral Variability of Cygnus X-1
We have used observations of Cygnus X-1 from the Compton Gamma-Ray
Observatory (CGRO) and BeppoSAX to study the variation in the MeV gamma-ray
emission between the hard and soft spectral states, using spectra that cover
the energy range 20 keV up to 10 MeV. These data provide evidence for
significant spectral variability at energies above 1 MeV. In particular,
whereas the hard X-ray flux decreases during the soft state, the flux at
energies above 1 MeV increases, resulting in a significantly harder gamma-ray
spectrum at energies above 1 MeV. This behavior is consistent with the general
picture of galactic black hole candidates having two distinct spectral forms at
soft gamma-ray energies. These data extend this picture, for the first time, to
energies above 1 MeV. We have used two different hybrid thermal/non-thermal
Comptonization models to fit broad band spectral data obtained in both the hard
and soft spectral states. These fits provide a quantitative estimate of the
electron distribution and allow us to probe the physical changes that take
place during transitions between the low and high X-ray states. We find that
there is a significant increase (by a factor of ~4) in the bolometric
luminosity as the source moves from the hard state to the soft state.
Furthermore, the presence of a non-thermal tail in the Comptonizing electron
distribution provides significant constraints on the magnetic field in the
source region.Comment: 19 pages, 9 figures, accepted for publication in ApJ. Final version
includes expanded discussion sectio
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