739 research outputs found
Discovery of a redshifted X-ray emission line in the symbiotic neutron star binary 4U 1700+24
We present the spectral analysis of an XMM-Newton observation of the X-ray
binary 4U 1700+24, performed during an outburst in August 2002. The EPIC-PN
spectrum above 1 keV can be modeled by a blackbody plus Comptonization model,
as in previous observations. At lower energies, however, we detect a prominent
soft excess, which we model with a broad Gaussian centered at ~0.5 keV. In the
high resolution RGS spectrum we detect a single emission line, centered at
19.19^{+0.05}_{-0.09} \AA. We discuss two possible interpretations for this
line: O VIII at redshift z=0.012^{+0.002}_{-0.004} or Ne IX at redshift z~0.4.Comment: 5 pages, 2 figures, A&A accepte
Saving a World Treasure: Protecting Florence from Flooding
The Committee Firenze 2016, on the occasion of the 50th anniversary of the tragic 1966 flood, invited six engineers and scientists to form an International Technical Scientific Committee (ITSC) to assess the current status of flood protection for the city of Florence and identify steps to reduce the risk of flooding facing the city. In this final Report, ITSC concludes that Florence remains at risk to significant flooding and this risk grows each day. It is not a question of whether a flood of the magnitude of 1966 or greater will occur, but when. In fact, the level of protection that exists in Florence now is not on a level appropriate to the citizens and treasures that rest within the city. If, under current conditions, a 1966-like flood occurred, the consequences to human lives, treasures, properties and community infrastructure could be much more catastrophic than they were in 1966
Recent Technological Developments on LGAD and iLGAD Detectors for Tracking and Timing Applications
This paper reports the last technological development on the Low Gain
Avalanche Detector (LGAD) and introduces a new architecture of these detectors
called inverse-LGAD (iLGAD). Both approaches are based on the standard
Avalanche Photo Diodes (APD) concept, commonly used in optical and X-ray
detection applications, including an internal multiplication of the charge
generated by radiation. The multiplication is inherent to the basic n++-p+-p
structure, where the doping profile of the p+ layer is optimized to achieve
high field and high impact ionization at the junction. The LGAD structures are
optimized for applications such as tracking or timing detectors for high energy
physics experiments or medical applications where time resolution lower than 30
ps is required. Detailed TCAD device simulations together with the electrical
and charge collection measurements are presented through this work.Comment: Keywords: silicon detectors, avalanche multiplication, timing
detectors, tracking detectors. 8 pages. 8 Figure
Are Causality Violations Undesirable?
Causality violations are typically seen as unrealistic and undesirable
features of a physical model. The following points out three reasons why
causality violations, which Bonnor and Steadman identified even in solutions to
the Einstein equation referring to ordinary laboratory situations, are not
necessarily undesirable. First, a space-time in which every causal curve can be
extended into a closed causal curve is singularity free--a necessary property
of a globally applicable physical theory. Second, a causality-violating
space-time exhibits a nontrivial topology--no closed timelike curve (CTC) can
be homotopic among CTCs to a point, or that point would not be causally well
behaved--and nontrivial topology has been explored as a model of particles.
Finally, if every causal curve in a given space-time passes through an event
horizon, a property which can be called "causal censorship", then that
space-time with event horizons excised would still be causally well behaved.Comment: Accepted in October 2008 by Foundations of Physics. Latex2e, 6 pages,
no figures. Presented at a seminar at the Universidad Nacional Autonoma de
Mexico. Version 2 was co-winner of the QMUL CTC Essay Priz
XMM-Newton Finds That SAX J1750.8-2900 May Harbor the Hottest, Most Luminous Known Neutron Star
We have performed the first sensitive X-ray observation of the low-mass X-ray
binary SAX J1750.8-2900 in quiescence with XMM-Newton. The spectrum was fit to
both a classical black body model, and a non-magnetized, pure hydrogen neutron
star atmosphere model. A power law component was added to these models, but we
found that it was not required by the fits. The distance to SAX J1750.8-2900 is
known to be D = 6.79 kpc from a previous analysis of photospheric radius
expansion bursts. This distance implies a bolometric luminosity (as given by
the NS atmosphere model) of (1.05 +/- 0.12) x 10^34 (D/6.79 kpc)^2 erg s^-1,
which is the highest known luminosity for a NS LMXB in quiescence. One simple
explanation for this surprising result could be that the crust and core of the
NS were not in thermal equilibrium during the observation. We argue that this
was likely not the case, and that the core temperature of the NS in SAX
J1750.8-2900 is unusually high
Further X-ray observations of EXO 0748-676 in quiescence: evidence for a cooling neutron star crust
In late 2008, the quasi-persistent neutron star X-ray transient and eclipsing
binary EXO 0748-676 started a transition from outburst to quiescence, after it
had been actively accreting for more than 24 years. In a previous work, we
discussed Chandra and Swift observations obtained during the first five months
after this transition. Here, we report on further X-ray observations of EXO
0748-676, extending the quiescent monitoring to 1.6 years. Chandra and
XMM-Newton data reveal quiescent X-ray spectra composed of a soft, thermal
component that is well-fitted by a neutron star atmosphere model. An additional
hard powerlaw tail is detected that changes non-monotonically over time,
contributing between 4 and 20 percent to the total unabsorbed 0.5-10 keV flux.
The combined set of Chandra, XMM-Newton and Swift data reveals that the thermal
bolometric luminosity fades from ~1E34 to 6E33 (D/7.4 kpc)^2 erg/s, whereas the
inferred neutron star effective temperature decreases from ~124 to 109 eV. We
interpret the observed decay as cooling of the neutron star crust and show that
the fractional quiescent temperature change of EXO 0748-676 is markedly smaller
than observed for three other neutron star X-ray binaries that underwent
prolonged accretion outbursts.Comment: Moderate textual revisions according to referee report, accepted for
publication in MNRA
On the nature of the "radio quiet" black hole binaries
The accretion/ejection coupling in accreting black hole binaries has been
described by empirical relations between the X-ray/radio and
X-ray/optical-infrared luminosities. These correlations were initially supposed
to be universal. However, recently many sources have been found to produce jets
that, given certain accretion-powered luminosities, are fainter than expected
from the correlations. This shows that black holes with similar accretion flows
can produce a broad range of outflows in power. Here we discuss whether typical
parameters of the binary system, as well as the properties of the outburst,
produce any effect on the energy output in the jet. We also define a jet-toy
model in which the bulk Lorentz factor becomes larger than ~1 above ~0.1% of
the Eddington luminosity. We finally compare the "radio quiet" black holes with
the neutron stars.Comment: in the Proceedings of IAU Symposium 275, "Jets at all Scales", Buenos
Aires, Argentina, 13-17 September 2010, eds. G.E. Romero, R.A. Sunyaev and T.
Bellon
What determines growth potential and juvenile quality of farmed fish species?
Enhanced production of high quality and healthy fry is a key target for a successful and competitive expansion of the aquaculture industry. Although large quantities of fish larvae are produced, survival rates are often low or highly variable and growth potential is in most cases not fully exploited, indicating significant gaps in our knowledge concerning optimal nutritional and culture conditions. Understanding the mechanisms that control early development and muscle growth are critical for the identification of time windows in development that introduce growth variation, and improve the viability and quality of juveniles. This literature review of the current state of knowledge aims to provide a framework for a better understanding of fish skeletal muscle ontogeny, and its impact on larval and juvenile quality as broadly defined. It focuses on fundamental biological knowledge relevant to larval phenotype and quality and, in particular, on the factors affecting the development of skeletal muscle. It also discusses the available methodologies to assess growth and larvae/juvenile quality, identifies gaps in knowledge and suggests future research directions. The focus is primarily on the major farmed non-salmonid fish species in Europe that include gilthead sea bream, European sea bass, turbot, Atlantic cod, Senegalese sole and Atlantic halibut
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