1,614 research outputs found
Nonlinear electrodynamics and the gravitational redshift of highly magnetised neutron stars
The idea that the nonlinear electromagnetic interaction, i. e., light
propagation in vacuum, can be geometrized was developed by Novello et al.
(2000) and Novello & Salim (2001). Since then a number of physical consequences
for the dynamics of a variety of systems have been explored. In a recent paper
Mosquera Cuesta & Salim (2003) presented the first astrophysical study where
such nonlinear electrodynamics (NLEDs) effects were accounted for in the case
of a highly magnetized neutron star or pulsar. In that paper the NLEDs was
invoked {\it a l\`a} Euler-Heisenberg, which is an infinite series expansion of
which only the first term was used for the analisys. The immediate consequence
of that study was an overall modification of the space-time geometry around the
pulsar, which is ``perceived'', in principle, only by light propagating out of
the star. This translates into an significant change in the surface redshift,
as inferred from absorption (emission) lines observed from a super magnetized
pulsar. The result proves to be even more dramatic for the so-called magnetars,
pulsars endowed with magnetic () fields higher then the Schafroth quantum
electrodynamics critical -field. Here we demonstrate that the same effect
still appears if one calls for the NLEDs in the form of the one rigorously
derived by Born & Infeld (1934) based on the special relativistic limit for the
velocity of approaching of an elementary particle to a pointlike electron [From
the mathematical point of view, the Born & Infeld (1934) NLEDs is described by
an exact Lagrangean, whose dynamics has been successfully studied in a wide set
of physical systems.].Comment: Accepted for publication in Month. Not. Roy. Ast. Soc. latex file,
mn-1.4.sty, 5 pages, 2 figure
A study of nano-particle based silane consolidants for Globigerina limestone
This STEPS funded study focuses on the application of hybrid and nanoparticle loaded hybrid silane consolidants for the treatment of 'Franka' type Globigerina Limestone. Consolidants act by gluing the deteriorated stone material to the underlying healthy stone (Dukes, 1972; Gutt, 1973; Alessandrini et al., 1975; Garrod, 2001).
The consolidants evaluated in this work were a laboratory prepared hybrid silane based on a mixture of tetraethylorthosilicate (TEOS) and 3-(glycidoxypropyl) trimethoxysilane (GPTMS), the same hybrid consolidant loaded with silica nanoparticles and GPTMS-modified silica nanoparticles. In addition, a consolidant based on the hydrolysis product of TEOS was also tested. Prepared consolidants were applied to Globigerina Limestone test blocks by complete immersion. Untreated stone block were used as benchmarks. Following application of the consolidants, half of the treated limestone blocks were subjected to accelerated weathering.
Non-weathered and weathered limestone blocks were then characterised by optical and electron microscopy and the stone colour before and after treatment with consolidants was assessed by a colorimetric technique. The pore size distribution before and after application of the different consolidant treatments was assessed by Mercury Intrusion Porosimetry. A water absorption by capillarity technique was also carried out to assess how the water uptake rate into the limestone alters with the different treatments. The mechanical properties of the consolidated limestone blocks were assessed by resistance to sodium sulfate crystallisation.
Microscopy observations showed that the consolidants penetrated the stone to different extents depending on the consolidant and the method of application. The hybrid consolidant on its own led to yellowing of the limestone but the addition of nanoparticles to the hybrid (modified or not) appeared to help restore the original colour of the stone. The porosity of the limestone was only marginally affected by the different treatments but the somewhat hydrophobic nature of the consolidants led to a disruption in the capillary flow of water into the limestone.peer-reviewe
A new approach to measure reduction intensity on cores and tools on cobbles: the Volumetric Reconstruction Method
Knowing to what extent lithic cores have been reduced through knapping is an important step toward understanding the technological variability of lithic assemblages and disentangling the formation processes of archaeological assemblages. In addition, it is a good complement to more developed studies of reduction intensity in retouched tools, and can provide information on raw material management or site occupation dynamics. This paper presents a new methodology for estimating the intensity of reduction in cores and tools on cobbles, the Volumetric Reconstruction Method (VRM). This method is based on a correction of the dimensions (length, width, and thickness) of each core from an assemblage. The mean values of thickness and platform thickness of the assemblage’s flakes are used as corrections for the cores’ original dimensions, after its diacritic analysis. Then, based on these new dimensions, the volume or mass of the original blank are reconstructed using the ellipsoid volume formula. The accuracy of this method was experimentally tested, reproducing a variety of possible archaeological scenarios. The experimental results demonstrate a high inferential potential of the VRM, both in estimating the original volume or mass of the original blanks, and in inferring the individual percentage of reduction for each core. The results of random resampling demonstrate the applicability of VRM to non size-biased archaeological contexts.Introduction Methods - The Volumetric Reconstruction Method - Experimental design - Statistical procedures - Resamples Results - Geometric formulas - Reduction strategy and size - Resampling (randomly biased record) - Resampling (size bias) - Measuring the effect of number of generations Discussion and conclusion
A white dwarf-neutron star relativistic binary model for soft gamma-ray repeaters
A scenario for SGRs is introduced in which gravitational radiation reaction
effects drive the dynamics of an ultrashort orbital period X-ray binary
embracing a high-mass donor white dwarf (WD) to a rapidly rotating low
magnetised massive neutron star (NS) surrounded by a thick, dense and massive
accretion torus. Driven by GR reaction, sparsely, the binary separation
reduces, the WD overflows its Roche lobe and the mass transfer drives unstable
the accretion disk around the NS. As the binary circular orbital period is a
multiple integer number () of the period of the WD fundamental mode (Pons et
al. 2002), the WD is since long pulsating at its fundamental mode; and most of
its harmonics, due to the tidal interaction with its NS orbital companion.
Hence, when the powerful irradiation glows onto the WD; from the fireball
ejected as part of the disk matter slumps onto the NS, it is partially
absorbed. This huge energy excites other WD radial (-mode) pulsations
(Podsiadlowski 1991,1995). After each mass-transfer episode the binary
separation (and orbital period) is augmented significantly (Deloye & Bildsten
2003; Al\'ecyan & Morsink 2004) due to the binary's angular momentum
redistribution. Thus a new adiabatic inspiral phase driven by GR reaction
starts which brings the binary close again, and the process repeats. This model
allows to explain most of SGRs observational features: their recurrent
activity, energetics of giant superoutbursts and quiescent stages, and
particularly the intriguing subpulses discovered by BeppoSAX (Feroci et al.
1999), which are suggested here to be {\it overtones} of the WD radial
fundamental mode (see the accompanying paper: Mosquera Cuesta 2004b).Comment: This paper was submitted as a "Letter to the Editor" of MNRAS in July
17/2004. Since that time no answer or referee report was provided to the
Author [MNRAS publication policy limits reviewal process no longer than one
month (+/- half more) for the reviewal of this kind of submission). I hope
this contribution is not receiving a similar "peer-reviewing" as given to the
A. Dar and A. De Rujula's "Cannonball model for gamma-ray bursts", or to the
R.K. Williams' "Penrose process for energy extraction from rotating black
holes". The author welcomes criticisms and suggestions on this pape
Revealing the Structure of an Accretion Disk Through Energy Dependent X-ray Microlensing
We present results from monitoring observations of the gravitationally lensed
quasar RX J1131-1231 performed with the Chandra X-ray Observatory. The X-ray
observations were planned with relatively long exposures that allowed a search
for energy-dependent microlensing in the soft (0.2-2 keV) and hard (2-10 keV)
light curves of the images of RX J1131-1231. We detect significant microlensing
in the X-ray light-curves of images A and D, and energy-dependent microlensing
of image D. The magnification of the soft band appears to be larger than that
in the hard band by a factor of ~ 1.3 when image D becomes more magnified. This
can be explained by the difference between a compact, softer-spectrum corona
that is producing a more extended, harder spectrum reflection component off the
disk. This is supported by the evolution of the fluorescent iron line in image
D over three consecutive time-averaged phases of the light curve. In the first
period, a Fe line at E = 6.36(-0.16,+0.13) keV is detected (at > 99%
confidence). In the second period, two Fe lines are detected, one at E =
5.47(-0.08,+0.06) keV (detected at > 99% confidence) and another at E =
6.02(-0.07,+0.09) keV (marginally detected at > 90% confidence), and in the
third period, a broadened Fe line at 6.42(-0.15,+0.19) keV is detected (at >
99% confidence). This evolution of the Fe line profile during the microlensing
event is consistent with the line distortion expected when a caustic passes
over the inner disk where the shape of the fluorescent Fe line is distorted by
General Relativistic and Doppler effects.Comment: 20 pages, includes 10 figures, submitted to Ap
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