11,571 research outputs found
Points of General Relativisitic Shock Wave Interaction are "Regularity Singularities" where Spacetime is Not Locally Flat
We show that the regularity of the gravitational metric tensor in spherically
symmetric spacetimes cannot be lifted from to within the
class of coordinate transformations in a neighborhood of a point of
shock wave interaction in General Relativity, without forcing the determinant
of the metric tensor to vanish at the point of interaction. This is in contrast
to Israel's Theorem which states that such coordinate transformations always
exist in a neighborhood of a point on a smooth single shock surface. The
results thus imply that points of shock wave interaction represent a new kind
of singularity for perfect fluids evolving in spacetime, singularities that
make perfectly good sense physically, that can form from the evolution of
smooth initial data, but at which the spacetime is not locally Minkowskian
under any coordinate transformation. In particular, at such singularities,
delta function sources in the second derivatives of the gravitational metric
tensor exist in all coordinate systems of the atlas, but due to
cancelation, the curvature tensor remains uniformly bounded.Comment: This article has been withdrawn since the main result is wrong due to
an computational error. See arXiv:1506.04081 and arXiv:1409.5060 for a
correction of this error and a proof of the opposite statemen
The Discovery of 8.7s Pulsations from the Ultrasoft X-Ray Source 4u0142+614
We discovered a periodicity at about 8.7s from the X--ray sources 4U0142+61,
previously considered a possible black hole candidate on the basis of its
ultrasoft spectrum. The pulsations are visible only in the 1--4 keV energy
range, during an observation obtained with the EXOSAT satellite in August 1984.
A search for delays in the pulse arrival times caused by orbital motion gave
negative results. In the same data, periodic oscillations at 25 minutes had
been previously found in an additional hard spectral component dominating above
4 keV which arises from the X-ray transient RX J0146.9+6121, discovered with
ROSAT and identified with a Be star. Though the very high (>10^4) X--ray to
optical flux ratio of 4U0142+61 is compatible with models based on an isolated
neutron star, the simplest explanation involves a low mass X--ray binary with a
very faint companion, similar to 4U1626--67. The discovery of periodic
pulsations from 4U0142+61 weakens the phenomenological criterion that an
ultrasoft spectral component is a signature of accreting black holes.Comment: plain LaTeX v3.1, 14 pages + 2 PostScript figures available upon
request to [email protected] . To appear on The Astrophysical Journal,
Letters. SISSA ref.: 106/94/
On the Orbital Period of the Intermediate Polar 1WGA J1958.2+3232
Recently, Norton et al. 2002, on the basis of multiwavelength photometry of
1WGA J1958.2+3232, argued that the -1 day alias of the strongest peak in the
power spectrum is the true orbital period of the system, casting doubts on the
period estimated by Zharikov et al. 2001. We re-analyzed this system using our
photometric and spectroscopic data along with the data kindly provided by Andy
Norton and confirm our previous finding. After refining our analysis we find
that the true orbital period of this binary system is 4.35h.Comment: 4 pages, 5 figures, Accepted for publication in A&A Letter
HCN versus HCO+ as dense molecular gas mass tracer in Luminous Infrared Galaxies
It has been recently argued that the HCN J=1--0 line emission may not be an
unbiased tracer of dense molecular gas (\rm n\ga 10^4 cm^{-3}) in Luminous
Infrared Galaxies (LIRGs: ) and HCO J=1--0
may constitute a better tracer instead (Graci\'a-Carpio et al. 2006), casting
doubt into earlier claims supporting the former as a good tracer of such gas
(Gao & Solomon 2004; Wu et al. 2006). In this paper new sensitive HCN J=4--3
observations of four such galaxies are presented, revealing a surprisingly wide
excitation range for their dense gas phase that may render the J=1--0
transition from either species a poor proxy of its mass. Moreover the
well-known sensitivity of the HCO abundance on the ionization degree of the
molecular gas (an important issue omitted from the ongoing discussion about the
relative merits of HCN and HCO as dense gas tracers) may severely reduce
the HCO abundance in the star-forming and highly turbulent molecular gas
found in LIRGs, while HCN remains abundant. This may result to the decreasing
HCO/HCN J=1--0 line ratio with increasing IR luminosity found in LIRGs, and
casts doubts on the HCO rather than the HCN as a good dense molecular gas
tracer. Multi-transition observations of both molecules are needed to identify
the best such tracer, its relation to ongoing star formation, and constrain
what may be a considerable range of dense gas properties in such galaxies.Comment: 16 pages, 4 figures, Accepted for publication in the Astrophysical
Journa
Linking the X-ray timing and spectral properties of the glitching AXP 1RXS J170849-400910
Previous studies of the X-ray flux and spectral properties of 1RXS
J170849-400910 showed hints of a possible correlation with the spin glitches
that occurred in 1999 and 2001. However, due to the sparseness of spectral
measurements and the paucity of detected glitches no firm conclusion could be
drawn. We retrieved and analysed archival XTE pointings of 1RXS J170849-400910
covering the time interval between January 2003 and June 2006 and carried out a
detailed timing analysis by means of phase fitting techniques. We detected two
relatively large glitches Delta nu / nu of 1.2 and 2.1 10^-6 occurred in
January and June 2005. Interestingly, the occurrence times of these glitches
are in agreement with the predictions made in our previous studies. This
finding strongly suggests a connection between the flux, spectral and timing
properties of 1RXS J170849-400910.Comment: Submitted to A&A, 4 pages; results presented at the INT meeting "The
Neutron Star Crust and Surface: Observations and Models" on June 27; referee
comments adde
Interior of Distorted Black Holes
We study the interior of distorted static axisymmetric black holes. We obtain
a general interior solution and study its asymptotics both near the horizon and
singularity. As a special example, we apply the obtained results to the case of
the so-called `caged' black holes.Comment: 12 pages, 16 figure
Maintaining a Wormhole with a Scalar Field
It is well known that it takes matter that violates the averaged weak energy
condition to hold the throat of a wormhole open. The production of such
``exotic'' matter is usually discussed within the context of quantum field
theory. In this paper I show that it is possible to produce the exotic matter
required to hold a wormhole open classically. This is accomplished by coupling
a scalar field to matter that satisfies the weak energy condition. The
energy-momentum tensor of the scalar field and the matter separately satisfy
the weak energy condition, but there exists an interaction energy-momentum
tensor that does not. It is this interaction energy-momentum tensor that allows
the wormhole to be maintained.Comment: 12 pages, LaTe
A Comment on Junction and Energy Conditions in Thin Shells
This comment contains a suggestion for a slight modification of Israel's
covariant formulation of junction conditions between two spacetimes, placing
both sides on equal footing with normals having uniquely defined orientations.
The signs of mass energy densities in thin shells at the junction depend not
only on the orientations of the normals and it is useful therefore to discuss
the sign separately. Calculations gain in clarity by not choosing the
orientations in advance. Simple examples illustrate our point and complete
previous classifications of spherical thin shells in spherically symmetric
spacetimes relevant to cosmology.Comment: (Tex file + PS file with a figure) Tex errors were correcte
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